European Network on Lateritic Weathering and Global Environment(EUROlAT)

International Summer School

6· 7 July, 1995BOlldy, France

organized by:1I R12

Geosciences de l'Environnement Tropicaland

Laboratoire des Formations Supedicielles"Terre Ocean Atmosphere" Department· ORSTOM

organizing Committee:Or. Francis SONOAG

Or. Torsten SCHWARZOr. Fruncols SOUBIES

Nicole ZERBIBllnnlele RAlllOT

TECHNICAL PROGRAMME/PROGRAMME

Thursday, July 6

8.30 RECiISTRA110N 1 INSCRImON

9.00 OPENINCi SESSION 1 StANCE D'OUVERTUREPo SOLER, Director of ïerre, Océan, Atmosphère" Department ORSTOM and1: SCHWJIRZ, Secretlry General of EUROlAT

LECTURE SESSION 1 - TROPICAL ENVIRONMENTS 1 ENVIRONNEMENTS TROPICAUX

9.30-10.00 C;. IbIRDOSSY· Hungalu Magyar AluminiumpariPaleoclimates and present tropical c1imate

10.00-10.30 J. L. PRoBST· CNRS-CqS. StrasbourgCO2uptake by continental erosion and carbon river transports ta the oceans

10.30-11.00 Discussion

Il.00-11.30 Coffee break/Pause café

Il.30~ 12.00 'Y. LuCJIS . Université de Toulon et du VarImpact of biological activity on soils functioning and formation in equatarial forest environment

12.00-12.30 B. DupR1- CNRS-D.M.P.. ToulouseUse of isotopes and traces elements for the study of lateritic systems.

12.30-13.00 Discussion

13.00-14.30 Lunch at ORSTOM lIe-de-France Research Center/Déjeuner au Centre ORSTOM

14.30-15.30 Poster session

15.30-16.00 Carree break/Pause café

16.00-18.00 Workshop 1Table RondeThe rates of weathering in tropical regions: available data and determination methodsChairman : 1. SOUllES - DRSTDM. Toulouse

19.30 Dinner and visit of Paris / Diner et visite de Paris3

Friday, July 7LECTURE SESSION 2 - MINERAL DEPOSITS 1 GJTES MINÉRAUX

9.00- 9.30

9.30-\0.00

10.00-10.45

10.45-11.30

11.30-12.00

12.00-12.30

12.30-14.00

14.00-14.30

14.30-15.00

15.00-15A5

15.45-16.15

16.15-16.45

16.45-17.15

17. 15-1 7.30

1. VJlLEroN· HamburgSupergene minerai and ore deposits

H. ZUqERS • BRGM. OrléansGeochemical exploration in lateritic areas

Discussion

Corree Break! Pause café

q. Jbumossy. Hungalu Magyar AluminiumpariPaleoenvironment of bauxite formation

Discussion

Lunch at ORSTOM lIe-de-France Research Center/Déjeuner au Centre ORSTOM

LECTURE SESSION 3 - MINERALOGY AND GEOCHEMISTRY 1 MINÉRALOGIE ET GÉOCHIMIE

Po ILDUONSE • Université Paris VI et VIICrystal chemistly of fine~ divided earth surface material

H. ST1JN/EIC • Technische Universitat MünchenIron oxides in lateritic soils

Corree break! Pause café

R. q. SCHWJIB • Friedrich Alexander Universitat ErlangenGeochemistry of phosphorus. sulfur. and arsenic in latentes

T: SCHWJIRZ· Technische Universitat BerlinParent rock identification of lateritic weathering products

Discussion

Closing session 1 Séance de clôture

CONTENTS

BARDOSSY GeorgeCarboniferous to jurassic bauxite depositsas paleoclimatic and paleogeographie indieators

COMBES Pierre-Jean, BARDOSSY GeorgeInfluence des bauxites et latéritessur la composition de l'atmosphère

PROBST Jean-LucCO2 uptake continental erosionand carbon river transports to the oceans

,... LUCAS YvesInteractions milieu vivant - milieu minéral dans les systèmes pédologiquesde la région de Manaus, Amazonie. f..l- ' ~ {'~ "1 .

GAILLARDET Jérôme, DUPRÉ Bernard, ALLÈGRE Claude J.A global chemical mass budget applied to the Congo Basin Rivers.Erosion rates and continental crust composition

ZEEGERS HubertApplying geochemical methods in the west tropics:more facts than figures

ILDEFONSE Philippe, MORIN G.Crystal chemistry of finely divided earth surface materials

STANJEK HelgeIron and aluminum (hydr)oxides in lateritie soils: Properties and processes

Il

27

37

63

69

98

102

lOS

5

SCHWAB Roland GottliebGeochemistry of phosphorus. sulphur and arsenic in laterites

SCHWARZ TorstenThe significance of parent rocks on the development of lateritic weathering products

109

117

6

LIST Of POSTERS DISPLAYED DUR/HG THE EUROLAT'95 SUMMER SCHOOL

BERTAUX J., FROLfCH F., ILDEFONSE Ph. 133Quantification of amorphous and crystal.line compounds by FTIR spectroscopy.Example of salitre sediments (Minasgerais. Brazil) (- ,~(,y)

Jo

BOEGLIN J.L., PROBST J.L. 135Continental weathering and river transport of suspended and major dissolvedelements in the Niger upper basin f \<tJ 660

!>

CECANTINI G., FIGUEIREDO A.M.G., SONDAG F., SOUBIÈS F. 137...-Biogeochemistry of Trace Elements (fi. REE•...) under Ncerrado" COlet in Central Brazil("Lagoa Campestre", Salitre. MG) t 12> 6'6'1

à

.,,- DJEMAï A., MALENGREAU N., LAUQUET G., MUUERJ.P.. 145Optical spectroscopy of kaolins from the amazon basin (Brazil){-A\ 366"7-

MONTOROI J.P. 149-- Water redistribution in lateritic soils of lower Casamance (Senegal\. \ g663

~

NIMPAGARITSE G. 153Geochemical exploration in lateritic tropical environment : a graphical interpretation method

PODWOJEWSKI P., BOURDON E. tA 155The hardening process of ferriginous accumulations in New Caledonian oxisols \ Bb6~

TROMBINO L., CREMASHI M. 157Micromorphological study of lateritic relict palaeosols from the Tadrat acacusmassif (Libya. Central Sahara)

7

LISTE DES PARTICIPANTS/LIST Of PARTICIPANTS

BALLAND M. Université Paris 6 et 7, Laboratoire de Minéralogie-eristallographie,Tour 16, 4, place Jussieu - F-7S2S2 Paris cedex os

BARBALA Varga Dep. or Geology, Eotvos University Muzeum Krl 4/H- 1088 Budapest. Hongrie

BARDOSSY G. Societe Hongroise d'Aluminium, Kossuth Ter 18 - H - 1055 Budapest

BELGITH R. ORSTOM Centre de Recherche d'Ile de France,.32, avenue Henri Varagnat - 93140 Bondy

BERTAUX Jacques ORSTOM Centre de Recherche d'Ile de France,32, avenue Henri Varagnat - 93140 Bondy

BOEGLIN CNRS - Centre de Geochimie de Surface, 1. rue BlessigF- 67084 Strasbourg Cedex

BOURMAN R. University or South Australia Holbrooks Raad - AUS - 5032 Underdale SA

BRASSET Thierry Université Paris VI et VII, Laboratoire de Minéralogie-Cristallographie,T. 26-4, place Jussieu, 75252 Paris cedex

DELAUNE Mireille ORSTOM Centre de Recherche d'Ile de France,32, avenue Henri Varagnat - 93140 Bondy

DJEMAi A. ORSTOM Centre de Recherche d'Ile de France,32, avenue Henri Varagnat - 93140 Bondy

DOHRMANN Henning Universitat Hamburg, Geol.-Palilol.-Inst Bundestr. SS0- 20146 Hamburg

DUPRË M. 18, avenue Edouard Belin, Observatoire Midi-Pyrénées, 31055 Toulouse

FAURE Paul ORSTOM Centre de Recherche d'Ile de France,32, avenue Henri Varagnat - 93140 Bondy

FREYSSINET Philippe BRGM - SGN/GRF, BP 6009 - F- 45060 Orléans Cedex 2

GI FFARD Isabelle ORSTOM Centre de Recherche d'Ile de France,32, avenue Henri Varagnat - 93140 Bondy

8

GOMES Celso de Sousa Universidade de Aveiro, Dep. de Geociencias - P- 3800 Aveiro

HACHMANN Wiebke Univ. Hambourg, Geol. Pal. Inst u. Museum. Bundesstr. 55/20146 Hambourg

ILDEFONSE Philippe Université Paris 6 et 7. laboratoire de Minéralogie-eristallographie.Tour 16. 4, place Jussieu - F-75252 Paris cedex 05

LE CORNEC Florence ORSTOM Centre de Recherche d'lIe de France.32. avenue Henri Varagnat - 93140 Bondy

UEBENBERG Léon GENCOR. P.O. Box 61820 Marshalltown. 2107 South Africa

LUCAS Yves laboratoire LEPI - Université de Toulon et du Var - BP 132 ­83957 la Garde Cedex

Mc ALISTER Joan Queens Univ.. Belfast. School of Geosciences. ELMWOOD ave.• Belfast

MELFI Adolpho José Inst Astronom. e Geofisico. Avenida Miguel Stefano. 4.200, C.P. 30627BR - 01051 Sao Paulo. Brazil

MONTOROI jean-Pierre ORSTOM Centre de Recherche d'Ile de France.32. avenue Henri Varagnat - 93140 Bondy

MOUTIE Jacques Ecole des Mines•. 158. Cours Faurier, 42023 Saint-~tienne

MULLER jean-Pierre ORSTOM. Centre de Recherche d'lIe de France, laboratoire des Formations Superficielles,32. avenue Henri Varagnat - 93140 Bondy

MUTAKYAHWA Univ. Hambourg. Geol. Pal. Inst u. Museum. Bundesstr. 55/20146Hambourg

NASRAOUI M. Ecole des Minesde Saint-Etienne, 158 Cours Fauriel -F - 42100 Saint-Etienne

NIMPAGARITSE G. Université Catholique de Louvain,laboratoire G.E.M.• 3 PI. L. Pasteur­B- 1348 \..ouvain-la-Neuve

PODWOjEWSKI Pascal CGS. 1. rue Blessig. 67084 Strasbourg cedex

PROBST jean-Luc CNRS - Centre de Geochimie ~ Surface. 1. rue BlessigF- 67084 Strasbourg Cedex

ROCHA Fernando Univ.Aveiro. Dep. Geociencias. Universidade de Aveiro, 3800 Aveiro

SAVIN Sam Department of Geological Science Case Westeme Reserve University. Cleaveland.OH 44106 USA

SCHUHMANN Univ. Hambourg. Geol. Pal. Inst u. Museum. Bundesstr. 55/20146Hambourg

9

SCHWAB Roland G. Universitàt Erlangen-Nürnberg Lehrstuhl r. Mineralogie, Schlossgarten 5 aD 91054 Erlangen

SCHWARZ Torsten Technical University of Berlin, BH 4, Inst of Mineral Deposits Research, Ernst-Reuter-Platz 1

D - 10587 Berlin -t5 c Ct. wC1 ~ 2-@ k'J. t (/ -be t-lt"'7 . cie.

SOLER Pierre Chef Dépt TOA; 213, rue La Fayette; 75480 Paris cedex la

SONDAG Francis ORSTOM, Centre de Recherche d'Ile de France. Laboratoire des Formations Superficielles,32, avenue Henri Varagnat - 93140 Bondy

SOUBIÈS François ORSTOM. Université Paul Sabatier, 39. Allée Jules Guesde, 31400 Toulouse

STANjEK Helge Technische Universitàt München. Lehrstuhl für Bodenkunde­D- 85354 Freising-Weihenstephan

TIETZ G.F. Universitàt Hamburg. Geol. Palàont Inst. u. Museum. Bundesstr. 55 ­D- 20146 Hamburg

TROMBINO Luca Universita Degli Studi Di Milano. Dip. di Scienze della Terra. Via Mangiagalli 34.1- 20133 Milano

TURENNE jean-François Directeur du Centre de Recherche d'Ile de France, ORSTOM,32, avenue Henri Varagnat - 93140 Bondy

VALETON Ida Auf dem Heinberg 56; D - 21438 Brackel

VERGARI Anne Faculté Polytechnique de Mons, rue de Houdain - 7000 Mons, Belgique

VICTOR janio Univ. Hambourg, Geol. Pal. Inst u. Museum. Bundesstr. 55/20146Hambourg

VOLKOFF Boris ORSTOM Centre de Recherche d'Ile de France.32. avenue Henri Varagnat - 93140 Bondy

WIECK Oliver Univ. Hambourg. Geol. Pal.lnst u. Museum. Bundesstr. 55/20146Hambourg

ZEEGERS Hubert BRGM - Département Exploration. BP 6009 - F- 45060 Orléans Cedex 2

10

sUlnlner schoolécole d'été

e~iG~~~

cuL~cuuI,

1'.UiUecJ.(;becJ.W'Ui4'J.'#UC'~

Gyorgy BARD055Y

Member of the Hungarian Acac/emy ofSciences, Buc/apestHungarian Aluiminium Inc/ustrial Buc/apest -Hongrie

Publishec/ in/paru dans: Canadian Sodety of Petro/eum Ge%giSfJ, 1992, Memoir 11:283-283Il

ABSTRACT

Recent bauxite deposits occur in areas characterised by tropical monsoon c1imate. Ali availablepaleoclimatic evidence suggests that similar c1imatic conditions prevailed where bauxite was formed inthe geologic pasto at least since the Devonian, when land vegetation spread over the continents. Thusweil dated bauxite deposits are reliable paleoclimatic and paleogeographie indicators.

ln the second part of the paper the locations and dimensions of the bauxite deposits are Iisted. fromthe Carboniferous to the end of the jurassic. The frequency of bauxite occurrence is compared with theglobal c1imatic conditions. A first expansion or bauxite deposits started in the Middle Devonian,reaching a peak during the Early Carboniferous. in good agreement with the globally warm and wetc1imate of this time. The global c1imate was unfavourable during the Triassic and Early and Middlejurassic, allowing the formation of only a restricted number of bauxite deposits.

ln the last part of the paper published paleogeographie and paleoclimatic maps are compared with thedistribution of bauxite deposits. Generally good accordance was found with these maps. In some areasadditional data were provided by the presence of bauxite deposits.

RÉSUMÉ

Des dépôts de bauxite récents se trouvent dans des régions caractérisées par un climat de moussontropical. Toute évidence paléoclimatique disponible suggère que des conditions climatiques semblablesprédominèrent où la bauxite se forma dans le passé géologique. au moins depuis le Dévonien, lorsquela végétation terrestre se répandit sur les continents. Il s'ensuit donc que des dépôts de bauxite biendatés peuvent constituer des indices paléoclimatiques et paléogéographiques fiables.

Dans la seconde partie de ce papier l'emplacement et les dimensions des dépôts de bauxite sonténumérés, du Carbonifère à la fin du jurassique. La fréquence des dépôts de bauxite est comparée auxconditions climatiques globales. Une première expansion des dépôts de bauxite débuta durant leDévonien moyen, atteignant un maximum durant le Carbonifère inférieur, en bon accord avec le climatglobal chaud et humide de cette période de temps. Durant le Triasique et le jurassique inférieur etmoyen le climat global était défavorable. permettant ainsi la formation de seulement un nombrerestreint de dépôts de bauxite.

Dans la dernière partie de ce papier des cartes paléogéographiques et paléoclimatiques publiées sontcomparées avec la distribution des dépôts de bauxite. Un accord généralement bon avec ces cartes aété trouvé. Dans certaines régions des données supplémentaires furent fournies par la présence dedépôts de bauxite.

GENERAL CONSIDERATIONS

Bauxite and laterite are residual rocks, products of intense continental subaerial weathering. They canbe best distinguished by their mineralogical composition, as shown on Figure 1. Laterite is a rockcomposed of equal parts or clay minerais (mainly kaolinite). iron minerais and alumina hydroxideminerais. Bauxite contains mainly alumina hydroxide minerais, less iron minerais and very few kaolinite.Lateritic ferrite and lateritic kaolin, also called saprolite, are the other compositional end-members of thediagram. Laterite is the most frequent member of the residual rocks. Three types of bauxite deposits canbe distinguished.

• Lateritic bauxite deposits occur on positive landforms. mainly on nat topped plateaux. They wereformed by the weathering of underlying alumosilicate rocks.

12

• Karst bauxite deposits are mainly fillings of karstic depressions on limestone and dolomite surfaces.The majority of karst bauxite deposits are allochthonous. produced by erosion of nearby bauxitieweathering profiles.

• Tikhvin-type deposits are allochthonous. overlying the eroded surface of alumosilieate rocks. They areproducts of the erosion of lateritic bauxite deposits. and have accumulated mainly in ancient erosionalvalleys (Bardossy and Aleva. 1990).

n.maIJI.go.ll'lli.

100°10

IIrlal.ril.

kooIiniliebowùtic

silic.ous bauxilitious100°10 ~:.:.::;,;;.;.;.~_;;.;ba;.:.U_XII_._......L. __ka_ot_'n_---,:....-""",,\__ 100%

glbtl5ll. B00lobxl 50 °1. bd 10°/. bal ~'~baflVnot.

_~~). ~~IS

Fig. 1- Classification of the lateritic rocks. After Bardossy and Aleva (1990)

According to my calculations. approximately 88% of the global bauxite belong to lateritie bauxitedeposits. 11% to karst bauxite deposits and less than 1% ta Tikhvin-type deposits.

Recent bauxite depo sits occur in are as c harac teri zed by the following c1imatic conditions:• Mean annuai temperature more than 22°C.• Annual precipitation more than 1200 mm. distributed over 9-1 1 rainy and 1-3 relatively dry months.

These conditions correspond ta a tropical monsoon climate. Of course. bauxite formation is controlledby the balance between precipitation and evaparation and not by precipitation alone.

Recent lateritie bauxite deposits preferentially occur on large scale continental planation surfaces.Bauxite can be formed only under continental subaerial conditions: underwater bauxitisation ischemically impossible. However. eroded bauxitic material can be sedimented in marshes. lakes. lagoonsand. exceptionally. even in littoral environments. Bauxitisation requires oxidizing and hydratingconditions. Bauxite is the most hydrated rock type on earth. with + H20 reaching 30-34%.

Recent laterite has a much broader geographic occurrence than bauxite. extending into some areascharacterized by subtropieal c1imate. Consequently the c1imatic limits of laterite formation are broaderthan those of bauxite formation.

Notwithstanding the above outlined. weil defined characteristies bauxite has rarely been applied tatesting paleogeographie and paleoclimatic models. An exception is a recent paper of Boucot (1993). inwhieh Paleozoie bauxite deposits were amply taken into account. Otherwise coals. coral reefs. eoliansediments. evaporites. phosphorites. red beds. tillites etc. are the commonly used indieator rocks. 1didnot find in the Iiterature any objection specifically against the paleogeographie and paleomorphologic

13

application of bauxite deposits. However. concerning paleoclimatic applications. the following doubtswere noted in the Iiterature:

Ooes lateritic bauxite in the geological record necessorily imply atropical c1irnate?

It was Owen (1954) who first postulated this idea. Recently Taylor et al. (1992) stated that the upperparts of basait nows were bauxitised during the Early Tertiary in the Monaro Region. New SouthWales. Australia. According to their research the c1imate was wet. cool and thermally seasonal whenthe weathering occurred. The area was weil forested and characterized by minimal erosion for a longperiod. Admitting the results of the above authors. 1would like to stress that the Monaro Region wasquite exceptional concerning its tectonic stability and very low rate of erosion. Even in thesecircumstances the weathering product was only bauxitic {aterite. according to our classification (seeFig. 1) because of its high silica and low alumina content. In addition. the weathering products have ahigh smectite content. which is alien to normal bauxite.

Genetic borders are never strict in nature. There exists. certainly. a broad transition interval. wherebelow 22°C temperatures gibbsite formation may begin if ail other conditions are favourable. Butweathering products formed in lower temperature are not real. low in silica (10%) bauxites. A largenumber of chemical laboratory tests have proved the strong increase of weathering rates anddissolution with temperature. Consequent/y. it seems improbable that low silica bauxite deposits ofnoticeable size - with tonnages over 1 million couId be formed under a temperate to cool c1imate. Itshould be mentioned that sorne authors have suggested a requirement for even higher temperabJresand more precipitation for bauxite formation than the values given above. (e.g.. Tardy et al. 1990.25°C temperature and more than 1800 mm/year rainfall). Thus moderate lateritic weathering underexceptionally favourable conditions at temperate to cool c1imate is just possible. and this is why lateriteis probably not a good paleoclimatic indicator rock.

(an be bauxite deposits accurately dated ?It is true that for surface deposits dating is a difficult task. However. exploration has revealed in recentdecades a large number of buried bauxite deposits throughout the Phanerozoic. Most of these depositsare situated in relatively short stratigraphic gaps (Iess than 10 My). The time interval of bauxiteformation can be further shortened in most cases by geomorphological and sedimentologicalresearches.

Are ancient bauxite deposits too rare for systetnatic evaluations?This is not true. Discoveries in recent decades have great/y increased the number of known bauxitedeposits even for the Paleozoic era. e.g.. in China (Liao Shifan and Liang Tongrong. 1991) and Russia.

Were the conditions ofbauxite formation during the Paleozoic and the Mesozoic the sorne as todoy?This objection questions the validity of a uniformitarian approach for continental weathering products.Ziegler et al. (1984) discussed the problem of c1imatic uniformitarianism for carbonate reefs.

They came to the conclusion that. at least for the Phanerozoic. paleoclimatic maps show carbonatedistribution patterns which conform to the model based on the present situation. The same can bestated for bauxite deposits. based on the physico-chemical properties of chemical elements involved.For example. it is improbable that the solubility or the pH and EH dependence of A1. Si. Fe and Ti inbauxite should have changed during the Phanerozoic. 14

During the Phanerozoic global cooling and warming occurred several times and the equator-to-poletemperature gradient also significantly changed. Globally dry and wet periods have been detected asweil (Parrish. 1993). Ali these changes must have influenced the extent of bauxite formation in thepast However. as outJined above. the c1imatic conditions for bauxite formation represent a threshold.not an interval. Thus. during globally warm and wet periods areas suitable for bauxite formation simplyexpanded and the intensity of the process increased. As a consequence. more bauxite was formed.Thus bauxite deposits. unlike coral reefs. are not paleolatitudinal indicators sensu stricto. rather theirpresence simply indicates a tropical monsoonal c1imate.

Another point of discussion is to determine to what degree bauxite formation was influenced bychanges of the atmosphere's composition. It is weil known from the studies of Budyko et al. (1987).Berner (1991) and Robinson (1991) that both the CO2 and the O2 contents of the atmosphere haveshown considerable fluctuation during the Phanerozoic. Peaks of the CO2 content had only an indirecteffect on bauxite forrnation. by creating the "greenhouse effect". and thus promoting warmer globalc1imate. The oxygen content probably had a direct effect. bauxite formation being favoured byoxidizing conditions. The curves of Budyko et al. (1987) and Berner (1991) show a rise of the oxygencontent starting in the Middle Devonian and ending in the Early Permian. with a peak in the EarlyCarboniferous. This period corresponds to the first global expansion of bauxite in the Phanerozoic. Asecond large oxygen peak is indicated in the Cretaceous. This was probably one of the most intenseperiods of bauxite for"mation in Earth·s history (D'Argenio and Mindszenty. 1992).

Global dimatic changes are also c10sely related to the eustatic sea level fluctuations (Haq et al. 1987).Intervals of large scale lateritic bauxite formation coincide with the highstands of the sea levelcharacterized by more humid c1imate. On the other hand. karst bauxite deposits were preferentiallyformed during short lived sea level drops. when shallow marine carbonate platforms emerged and wereexposed on coastal lowlands. e.g.. in southern France and the Dinaric Mountains. Karst depressionsserved as traps for eroded and transported bauxitic material (note that lowering of sea level enhancederosion of higher Iying lateritic bauxite deposits). Thus allochthonous karst bauxite deposits areindicators of tectonic instability.

Bauxite formation depended also on the vegetation coyer. The majority of recent bauxite deposits arecovered by tropical forest and. to a much lesser extent. by Savannah-type woodland. Bauxite was notonly protected by the vegetation coyer. but roots and soil bacteria actively contributed to thebreakdown of the initial minerais of the parent rocks. This is the main reason why the first expansionof bauxite formation started in the Middle Devonian. when vegetation spread over continental surfaces.

The above outlined c1imatic. tectonic and biologic factors had a joint. covariate effect on the globalextent and amount of bauxite formation.

It is generally agreed that shallow marine oolitic ironstone beds. consisting of goethite. berthierine andsiderite. were derived from continental laterite and bauxite. in which iron was preconcentrated (Hallam.1984). Valeton (1983) presumed that shallow marine carbonate-siderite sediments and glauconite bedsare also products of eroded lateritic material precipitated in the sea. Thus ail these sediments indicatelateritisation in adjacent continental areas and can be used as indirect paleoclimatic indicators.

THE LOCATION AND DIMENSIONS OF BAUXITE DEPOSITS FROM THE CARBONIFEROUS TO THE END OF THE JURASSIC

The location of ail the known bauxite deposits is presented on Figure 2. The oldest Phanerozoic depositis of Early Cambrian age. It is a small karst bauxite deposit in the Eastern Sayan Mountains. Siberia.Russia. Major karst bauxite deposits were formed in the Middle and Late Devonian in the UralMountains. and lateritic bauxite deposits in the Timan Mountains in Russia.

Early Carboniferous bauxite deposits occur in four areas:15

• on the Russian Platform and adjacent areas. lateritic and Tikhvin-type deposits (Belgorod. Tikhvin.Lake Oniega. Timan Mountains in Russia. Lublin and Nowa Ruda in Poland):

• small deposits on the Taymir Peninsula. Siberia. Russia;

• in the Turkestanian and Aliysk Ranges (Uzbekistan. Tadjikistan);

• in the Henan. Hunan. Hubei. Guizhou. Sichuan and Yunnan Provinces (China). Late Carboniferouskarst and Tikhvin-type bauxite deposits occur in China in Henan. Liaoning. Shanxi and ShandonProvinces. Minor bauxite deposits were discovered recently in Xing Jiang Province. close to thenorthwest border of China. Small lateritic bauxite deposits occur at Decazeville (France). karst typedeposits in Missouri (USA) and allochthonous bauxite deposits at Clearfjeld. Pennsylvania (USA). Thearea and the dimensions of Late Carboniferous bauxite deposits diminished considerably in comparisonwith those of the Early Carboniferous.

The extent of bauxite formation further diminished during the Early Permian. Karst and Tikhvin-typebauxite deposits occur in Guizhou. Sichuan and Yunnan Provinces (China). Small Tikhvin-type depositsoccur in the Phjongjang and Pukhang districts. North Korea. Small allochthonous bauxite deposits areknown in the Minor Caucasus. Nakhichevan Autonomous Territory. Boehmite-bearing latente profilesare found in the upper Hunter Valley. NSW. Australia. Parts of the profiles have been eroded andresedimented in the form of bauxite conglomerate. The late Permian is characterized by a new. butshort expansion of karst type bauxite in China in Guangxi. Shanxi. Sichuan and Yunnan Provinces. inthe northeastern part of Vietnam and in Cambodia (Sisophon. Battambang). Karst bauxite formationstarted at this time in the Tethys area. in the Taurus Mountains (Turkey).

30° 30°~3

0" 0"

1 II

2 :.:.~

30" 3 TT

4 C:> jJ506@

Fig. 2- Geographie position of the bauxite deposits. 1- lateritic bauxites. 2- karst bauxites. 3- Tikhvin-typebauxites. 4- lateritic bauxite provinces. 5- karst-bauxite provinces. 6- provinces of Tikhvin-type bauxites

16

With the end of the Paleozoic era bauxite formation completely ceased in eastern Asia: instead itspread along the coasts of the Tethys. During Early Triassic karst bauxite deposits were formed in theZagros and Alborz Mountains (Iran). in the Little Caucasus Mountains (Azerbaijan) and in the PamirMountains (Tadjikistan). Bauxite formation continued in Turkey in the Taurus Mountains and in theMenderes Massif. Ali these deposits are of minor dimensions and tonnage. The late Triassic ischaracterized by karst baumte deposits located in South Fergana (Kirgistan). jajarm. Yazd and Kerman(Iran). large karst bauxite deposits occur in the Dinaric Mountains (Slovenia. Croatia. Bosnia.Montenegro and Albania). but their bauxitisation was incomplete.

The Early jurassic is the epoch with the smallest number of known bauxite deposits. Small karst bauxitedeposits occur in North Fergana (Uzbekhistan). and on Skopelos Island (Greece). At Maktesh Ramon(Israel) ancient laterite plateaux occur together with their eroded material that accumulated in nearbykarstic depressions. There are very few deposits of Middle jurassie age: Rahov (Ukraine). in theWestern Alps (Switzerland and France). in the Betic Cordilleras (Spain). and at Campbellpore andMuzaffabad (Pakistan). Ferriferous lateritic bauxite was formed in the eastern Taurus Mountains(Turkey). The late jurassie can be considered as the beginning of a new global expansion of bauxiteformation. Major karst bauxite deposits were formed in central Greece and in the Dinaric Mountains.and some small ones on the Crimean Peninsula (Ukraine). Younger stratigraphie periods were nottopics of the Pangea Conference and their bauxite deposits are not discussed in this paper.

Summarizing the above outlined evolution. mainly karst and Tikhvin-type deposits were formed fromMiddle Devonian to late Permian time in East Asia. and. for a short period within that time span.lateritic bauxites on the Russian Platform. Bauxite formation completely ceased at the end of thePaleozoic in these areas. Beginning with late Permian. and having its maximum in Middle Cretaceous.karst bauxite formation occurred on coastal lowlands of the western Tethys (Bardossy and Dercourt.1990). Starting in the Middle Cretaceous a large scale expansion of lateritic bauxite formation teokplace on planation surfaces of several continents. e.g.. Brazil. West Africa. India. Western andNorthern Australia. In the same time karst bauxite formation continued in coastal areas of the Tethys.expanding more and more in a western direction to sorne of the Caribbean islands (Combes andBardossy. in press). Finally. bauxite formation strongly diminished with the end of the Pliocene. but incertain areas it continues up to the present.

1calculated the amount of bauxite found in the subsequent periods and epochs of the Phanerozoie.The time intervals being very different. 1 calculated for each one the relative amount of bauxiteexpressed by millions of tonnes per million years (Fig. 3). Of course. the amount of eroded bauxitecannot be determined. but in my opinion it must be related to the total amount of bauxite found in thegiven time interval.

100 1000Mt/Ma

17

Detailed descriptions and references on the above mentioned deposits can be found in Bardossy (1982)and Bardossy and Aleva (1990).

PALEOel.lMATIC AND PALEOGEOGRAPHie INfORMATION fURNISHED BY BAUXITE DEPOSITS AS eOMPAREDWITH OTHER GEOLOGIC INDICATORS AND EXISTING RECONSTRUCTIONS

On a global scale there is good agreement betwéen the amount of bauxite formation and the changesof global c1imate as evaluated by Parrish (1993) for the Pangea Supercontinent. Thus the EarlyCarboniferous expansion of bauxite is in coincidence with global warming and wet c1imate. plus anincrease of atmospheric oxygen content. The decline of bauxite formation in the late Carboniferousand its minimum in the Early Permian corresponds ta a global cooling and drying. and glaciation on theGondwana continent (Crowley et al.. 1991). The relatively short expansion during late Permian was acon sequence of a new global warming. but soon a global dry ing countered this favourable effectThat is why much less bauxite was formed during the dry Triassic and a new minimum reached duringthe Early Jurassic. This was also a time of minimum atmospheric oxygen content. A new expansionstarted in the late Jurassic. ta culminate in Middle and late Cretaceous - in good accordance with theglobal warming and increase of precipitation and atmospherie oxygen content during that time interval.

As a further step. 1 completed ail available paleogeographic and paleoclimatic reconstructions byincluding the bauxitic deposits of corresponding age. Except in a few cases 1 found an overall goodagreement with the paleoclimatic reconstructions. including the rainfall maps. As for thepaleogeographie reconstructions. 1have the impression that in several cases the emergent continentalsurfaces are underestimated. at least in areas where bauxite deposits occur. In the following someexamples will be discussed.

Bauxite deposits of Early Carboniferous age are located in the tropical and subtropical belt of thereconstruction of Ziegler et al. (1981). at less than 30° latitude from the paleoequator. The onlyexception is the small bauxite occurrence on the present Taymir Peninsula. situated at that time on thesouthern coast of the Siberia Plate (Fig. 4). Maybe warm ocean currents arriving from the southeastcompensated for the high latitudinal position of this deposit The rainfall pattern of Rowley et al.(1985) indicates a wet. humid c1imate for ail the areas where bauxite deposits are located.

The location of late Carboniferous deposits is also in good accordance with their paleoclimatic position(Parrish et al.. 1986)(Fig. 5). Note the warm ocean currents along the southeastern coast of the Tarimmieroplate. where bauxite deposits occur. According to the paleo-rainfall map of Rowley et al. (1985).the c1imate remained wet on ail the East Asian microplates where bauxite deposits are found. TheMissouri and Clearfield deposits are situated according to the reconstruction in the interior of theLaurasia continent However. Keller et al. (1954) indicated Pennsylvanian marine shale as a lateralfacies not far from the Missouri bauxite deposits. Similarly. Erickson (1963) emphasized that the bauxitelenses of the Clearfield deposit were sedimented in a coastal swamp environment Both statements arein contradiction with the presumed continental areas of the paleogeographie reconstruction.

There is a good paleoclimatic agreement concerning the position of the Early Permian bauxite deposits.except the laterite profile and the bauxite-Iaterite conglomerate described by loughnan (1975) from theupper Hunter Valley. NSW. Australia. Basait of Sakmarian age is the parent rock of the laterite. Thepartial erosion and redeposition of the laterite occurred during the Artinskian period. detected both inoutcrops and boreholes. When visiting this location 1 found that it is a real laterite profile. Theoverlying Artinskian coal measures put the age of lateritisation between the Sakmarian and Artinskian.These facts are in connïct with the generally accepted latitudinal position of Australia at that time andthe presumed glaciation in the Sydney basin (Crowell and Frakes. 1973: Ziegler. 1990). Maybe thelateritisation occurred in a temperate to cool c1imate as described by Taylor et al. (1992) for theMonaro Province. or during a short interglacial interval. This problem merits thorough re-examination.There is fairly good accordance with respect ta paleoclimate between the position of the late Permianbauxite deposits and the corresponding reconstruction (Fig. 6). The presence of bauxite deposits in theTaurus Mountains. Turkey indicates local emergence at least for this area.

18

-, ,-Fig. 4- Visean paleogeographie reconstruction (Early Carboniferous). After Rowley et al. (1985). Ocean currents after Zielger et al.

(1981). completed by the bauxite deposits. Legends: white - oceanic areas. Iight grey - shallow sea. grey-Iowland. black - mountainranges. dotted arrows ,. cool ocean currents. continuous arrows - warm ocean currents. dots - bauxite deposits

J

19

Fig. 5- Westphalian (Late Carboniferous) paleogeographie reconstruction. After Rowley et al. (1985). Ocean currents after Ziegler et al.(1981). completed by the bauxite deposits. Legend as in Figure 4.

1 rFig. 6- Kazanian (Late Permian) paleogeographie reconstruction. After Ziegler et al. (1981). completed by the bauxite deposits. Legend

as in Figure 4

20

Fig.7- Early Triassic paleogeographie reconstruction. After Parrish et al. (1982). completed by the bauxite deposits. Legend:white - oceanic areas. light - shallow sea. grey - lowland. black - mountain ranges. dots ~ bauxite deposits. Cs coal5. E~ evaporites

For the Early Triassic there is good paleoclimatic agreement. but the bauxite deposits indicateemergence where according to the reconstruction shallow epicontinental marine conditions shouldhave prevailed (Fig. 7). The paleo-rainfall map of Parrish et al. (1982) indicates high to medium rainfallfor the areas where the bauxite deposits occur. 1found no reconstruction for the Late Triassic. but onecan conclude from the foregoing and subsequent reconstructions that the agreement with the locationof bauxite deposits must be good.

Fig. 8- Pliensbachian (Early Jurassic) paleogeographic reconstruction. After Parrish et al. (1982).completed by the bauxite deposits. Legend as in Figure 7

Bauxite deposits of the Pliensbachian are ail situated in the tropical and subtropical belt. along thewestern and northwestern coast of the Tethys (Fig. 8). The paleo-rainfall map of Parrish et al. (1982) isalso in good agreement with the position of the bauxite deposits. Here additional insular emergence isindicated by bauxite deposits for the area of Greece.

The paleogeographie reconstruction of the Callovian. constructed by Dercourt et al. (1989) representsthe western part of the Tethys (Fig. 9). Bauxite deposits are ail situated in the presumedtropical-subtropical belt Their presence indicates quite a number of islands in the shallow part of thewestern Tethys. The 500 km long strip of ferriferous lateritic bauxite in the Eastern Taurus Mountains isof particular paleogeographie interest. as it represents lateritic weathering of ophiolitic rocks.

The most detailed paleoclimatic and paleogeographie information is available for the Kimmeridgian. Alarge number of weil dated major karst bauxite deposits is located at the western end of the Tethys.near the paleo-equator in the present Dinaric and Hellenic Mountains (Fig. 10). Unfortunately there isno good agreement with the annuai precipitation map of Moore et al. (1992). which shows onlymedium precipitation rates for the areas of the bauxite deposits. On the simulated convective rainfallmap of Valdes and Sellwood (1992) bauxite deposits are also situated just between two rainfallmaxima. The map of geologic paleoclimate indicators constructod by Valdes and Sellwood is one of thefew where bauxite deposits are also iIIustrated (Fig. Il). Unfortunately some of the deposits are not inthe right place and the map had to be completed by a number of missing deposits. Paleoclimaticagreement between the reconstruction and the bauxite deposits is good. The last availablereconstruction refers to the Portlandian (Volgian)(Fig. 12). It shows that bauxite formation continued

21

in the same areas as during the Kimmeridgian. but more bauxite was formed. The paleoclimatieaccordance is good. The bauxite deposits indieate a several hundred kilometres long insular emergencealong the present Dinarie and Hellenie Mountains.

Fig.9- Callovian area. After Bardossy and Dercourt (1990). Legend- white-emerged areas,lightgrey - shallow sea, dark grey - deeper sea over oceanie crust. dots - bauxite deposits.

o·ï------------I-~---____:===::::::=====r 90N:: '90N 1

60N

30N

o -----------------u..u--"'-J

30S

·60S

60N'

30N.

30S'

60S

90S L ~··~---J:::::======:::::::::=------------l- 90S.._." ...__.. -- ....._.... -----------_._-----y-

Fig. 10- KimmeridgianlTithonian (Late Jurassie) paleogeographie reconstruction. After Moore et al.(1992). completed by the bauxite deposits. Legend: dots - bauxite deposits. 22

The topics of the Pangea Conference ended with the Late jurassic. 1should mention that Cretaceousand Tertiary reconstructions show even better agreement with the bauxite deposits than thosediscussed above. A completely new feature of the Cretaceous and Tertiary is the development ofbauxite deposits and large lateritic surfaces in the interiors of the continents. such as in South America.West Africa. India. Southeast Asia. and the northern and western parts of Australia. Thesedevelopments were certainly due ta the breakup of the Pangea and to the favourable c1imatic changesrelated to it (Hallam. (984).

Until Middle Cretaceous the bauxite deposits were situated near the ancient coastJines. mainly on lowIying carbonate platforms. Gyllenhaal et al. (1991) demonstrated the importance of ïand-sea breezes"in some coastal belts. where precipitation is considerably enhanced. Such belts are generally up ta 50km wide. According ta those authors it is probable that this phenomenon had been active in the pastas weil from the eQuator up ta 40-45° latitude. at least for the Phanerozoic. This is a good additionalexplanation for the preferred location of bauxite deposits in the ancient coastal belts.

o'

---+-----+~.;:::,.;jI_f_-+--+_-_+_----r_7__+-____i 60·

Fig. 11- Map of paleoclimatic indicator rocks for the Middle and Late Jurassic. After Valdes andSellwood (1992). completed by the bauxite deposits. Legend: C- coals. E- evaporites. dots - bauxite

deposits (Middle jurassic). squares - bauxite deposits (late jurassic)

CONCLUSIONS

• Buried and stratigraphically weil dated bauxite deposits are reliable paleoclimatic and paleogeographieindieators.

• Their locations are. in most cases. in good accordance with the existing Phanerozoie reconstructions.The presence of bauxite deposits indicated in several places islands or additional coastal lowlands whereshallow marine conditions had been postulated.

• ln some places there is not full agreement between the location of bauxite deposits andcorresponding paleo-rainfall maps.

• With respect to paleoclimate. the only major problem is presented by the Early Permian laterite insoutheastern Australia. necessitating a thorough re-examination of the existing paleoclimatic model. atleast for this area.

23

Fig. 12- Portlandian (Volgian) paleogeographie reconstruction. After Parrish et al. (1982). completed bythe bauxite deposits. Legend as in Figure 7.

ACKNOWLEDGEMENTS

The author gratefully acknowledges the review of Professor j.T. Parrish. her helpful suggestions andremaries.

REFERENCES

Bardossy. Gy. 1982. Karst bauxites. Developments in Economie Geology 14. Elsevier ScientificPublishing Co.• Amsterdam-oxford-New York. 441 p.

Bardossy. Gy. and Aleva. G.j.j. 1990. Lateritie bauxites. Developments in Economie Geology 27.Elsevier. Amsterdam-Oxford-New York-Tokyo. 624 p.

Bardossy, Gy. and Dercout. j. 1990. les gisements de bauxites téthysiens (Méditerranée. Proche etMoyen Orient). cadre paléogéographique et controles génétiques. Bulletin de la Société Géologique deFrance. 8 série. tome VI. p. 869·888.

Berner. RA 1991. A model for the atmosphenc C02 over Phanerozoic time. Amencan journal ofScience. 291. p. 339-376.

Boucot. A.j. 1993. Paleozoic paleogeography and biogeography. Revista Espanola de Paleontologia. No.Extraordinario. p. 15-20.

Budyko. M.I.. Ronov. A.B. and Yanshin. A.l. 1987. History of the Earth's atmosphere. Springer. NewYork. 139 p.

24

Combes. RJ. and Bardossy. Gy. 1994. Geodynamics of bauxite in the Tethys area. Comptes Rendus.Acad. Sci .. Paris. U18. ser. Il. p. 359-366.

Crowell. J.c. and Frakes. L.A. 1973. The late Paleozoic glaciation. In: Gondwana Geology. K.S.W.Campbell (ed.). ANU Press. Canberra. p. 313-331.

Crowley. T.J .. Baum. S.K. and Hyde. W.T. 1991. Climate model.comparison of Gondwanan andLaurentide glaciations. Journal of Geophysical Research 96. p. 9217-9226.

D·Argenio. B. and Mindszenty. A. 1992. Tectonic and c1imatic control on paleokarst and bauxites.Giornale di Geologia. ser. 3. vol. 54/1. p. 207218.

Dercourt. J.. Ricou. LE.. Adamia. S.. Gzaczar. G.. Funk. H.. Lefeld. J.. Rakus. M.. Sandulescu. M..Tollmann. A. and Tehoumachenko. R 1989. Evolution of the northern margin of Tethys from Anisianto Oligocene. from Geneva to Baku. Mémoires de la Société Géologique de France. 154 p.

Enckson. E.S. jr. 1963 Mineralogical. petrographie and geochemical relationships in some high-aluminaand associated c1aystones from the Clearfield basin. Pennsylvania. Pennsylvania State University. AnnArbor. Michigan. 190p.

Gyllenhaal. LO.. Engberts. c.J .. Markwick. RJ .. Smith. L.H. and Patzkowsky. M.L 1991. TheFujita-Ziegler model: a new semi-quantitative technique for estimating paleoclimate frompaleogeographie maps. Palaeogeography. Palaeoclimatology. Palaeoecology. 86. p. 41-66.

Hallam. A. 1984. Continental humid and arid zones dunng the Jurassic and Cretaceous.Palaeogeography. Palaeoclimatology. Palaeoecology. 47. p. 195-223.

Haq. B.U.. Hardenbol. j. and Vai!. RR. 1987. Cbronology of f1uctuating sea level since the Triassic.Science. 235. p. 1152-1167.

Keller. W.D.. Westcott. J.E. Bledsoe. A.O. 1954. The origin of Missouri fire c1ays. Proceedings of the2nd National Conference on Clays nd Clay Minerais. p. 7-46.

Liao Shifan and Liang Tongrong. 1991. Bauxite Geology of China. Science and Technology PublishingHouse of Guizhou. China. 277 p.

Loughnan. EC. 1975. Laterites and flint c1ays in the Early Permian of the Sydney Basin. Australia. andtheir palaeoclimatic implications. Journal of Sedimentary Petrology. 45. No. 3. p. 591-598.

Moore. G.T.. Hayashida. O.N. Ross. C.A. and Jacobson. S.R. 1992. Paleoclimate of theKimmeridgian/Tithonian (Late Jurassie) world: 1. Results using a general circulation model.Palaeogeography. Palaeoclimatology. Palaeoecology. 93. p. 113-150.

Owen. H. B. 1954. Bauxite in Australia. Bureau of Mineral Resources. Geology and Geophysics BulletinNo. 24. Canberra. 234 p.

Parrish. j.T.• Ziegler. A.M. and Scotese. C.R. 1982. Rainfall patterns and the distribution of coals andevaporites in the Mesozoic and Cenozoic. Palaeogeography. Palaeoclimatology. Palaeoecology. 40. p.67-101.

Parrish. J.M .. Parrish. J.T. and Ziegler. A.M. 1986. Permian-Triassic paleogeography andpaleoclimatology and implieations for therapsid distribution. In: The Ecology and Biology orMammal-like Reptiles.j.H. Hotton III. RD. MacLean. j.j. Rotb and E.C. Roth (eds.). SmithsonianInstitution Press. Washington. D.C.. p. 109-13 J.

Parrish. J,T. '993. Climate of the Supercontinent Pangea. Journal of Geology 101. p. 215-233.

25

Robinson. j.M. 1991. Phanerozoic atmospheric reconstructions: a terrestrial perspective.Palaeogeography. Palaeoclimatology. Palaeoecology 97. p. 51 -62.

Rowley, D.B.. Raymond, A.. Parrish. j.T.. Lottes. A.L. Scotese, C.R. and Ziegler, A.M. 1985.Carboniferous paleogeographie. phytogeographie and paleoclimatie reconstructions. Internationaljournal of Coal Geology 5. p. 7-42.

Taylor. G.. Eggleton. RA. Holzhauer. C.C.. Maconachie. L.A.. Gordon. M.. Brown. M.C. andMcQueen. K.G. 1991. Cool c1imate lateritic and bauxitic weathering. journal of Geology. v. 100, p.669-677.

Tardy, Y.. Kobilsek. B.. Roquin. c.. and Paquet. H. 1990. Innuence of penatlantic c1imates andpaleoclimates on the distribution and mineralogical composition of bauxites and ferricretes. Proceedingsof 2nd International Symposium on Geochemistry of the Earth·s surface and minerai formation. Aix enProvence. p. 179-182.

Valdes. RJ. and SelJwood. B.W. 1992. A palaeoclimatic model for the Kimmeridgian. Palzogeography.Palaeoclimatology. Palzoecology 95. p. 47-72.

Valeton. 1. 1982. Klimaperioden lateritischer Verwitterung und ihr Abbild in den synchronenSedimentationsraumen. Zeitschnft der deutschen geologischen Gesellschaft 134. p. 413-452.

Ziegler. A.M. 1990. Phytogeographie pattems and continental configurations during the Permianperiod. Geological Society London Memoir 12. p. 363-379.

Ziegler. A.M .. Bambach. R.K., Parrish. J.T.. Barrett. S.E. Gierlowski. E.H .. Parker. W.c.. Raymond. A.and Sepkovski. J.J. 1981. Paleozoie Biogeography and c1imatology. In: N.J. Niklas (ed.). Paleobotany.Paleoecology and Evolution. p. 231-266.

Ziegler. A.M.. Hulver. M.L.. Lottes. A.L. and Schmachtenberg. W.E 1984. Uniformitarianism andpalaeoclimates: Inferences from the distribution of carbonate rocks. In: Brenchley. R (ed.). Fossils andClimate. p. 3-25.

26

sUlnlner schoolécole d'été

!J~~~et~

~1a~~L'~

Pierre-Jean COMBES' et Gyorgy BARDOSSY2

1Université Montpellier 1/, Géologie des GÎtes Minéraux,Montpellier et URA n° 1405 du CNRS2Hungarian Aluminium Industrial, Budapest -Hongrie

Published in/paru dans: CR. Acad. Sei. Paris, 1995, t.230, série 1/0:109-116

27

RÉUMÉ

Une évaluation globale au Crétacé moyen et supérieur. période optimale pour le développement del'altération ferrallitique, montre qu'une proportion notable de l'oxygène de l'atmosphère a étéaccumulée dans les bauxites et latérites sous forme d'oxy-hydroxydes et de kaolinite. Les bauxitespeuvent donc jouer un rôle dans le contrOle de la composition de l'atmosphère, selon la valeur de leurtaux de production. Pour une même quantité d'oxygène fournie par la photosynthèse, la teneur dansl'atmosphère s'élèvera si ce taux est faible et baissera s'il est fort Ainsi, la diminution des bauxites etlatérites au cours du cénozoïque a pu intervenir dans l'augmentation en oxygène relevée par certainsauteurs depuis l'Oligocène.

Mots-clés: Bauxites. Latérites, Paléoatmosphère, Oxygène, Crétacé, Cénozoïque.

Absfrad: Controlling influence of bauxites and laterites on the Earth's atmosphere

It has been demonstrated by geochemical calculations that bauxites and laterites are among the rocksmost enriched in oxygen. There is around 10% difference between the average oxygen content of theparent rocks and their lateritic-bauxitic weathering products. This difference is compensated by thefixation of atmospheric oxygen in bauxite and laterite. A non-negligible amount of atmospheric oxygenwas flXed by these processes during the Earth's history. particularly when the rate of weathering wasstrongest and the extent of the weathered surfaces largest As an example the Cretaceous Period isdiscussed in detail. from the Aptian to the Senonian ages. when very intense bauxitization andlateritization prevailed and executed its influence on the oxygen content of the atmosphere.

Keywords: Bauxites, Laterites, Palaeoatmosphere, Oxygen, Cretaceous, Cenozoic.

Bauxitization and lateritization lead to a significant concentration of oxygen in these rocks in the formof AI and Fe oxide-hydroxide minerais and kaolinite. Thus bauxite and laterite are among the most oxy­gen-rich rocks of the Earth's crust The weathering of the primary minerais involves the fIXation ofsome water-dissolved C02 as weil. The rate of bauxite and laterite formation was 50 intense duringsome periods of the Earth's history that it was able ta influence the composition of the atmosphereThis influence is discussed in this paper, taking as an example for detailed evaluation the CretaceousPeriod.

CRETACEOUS BAUXITES AND LATERITES

During the Cretaceous. particularly during the Aptian, Albian and Cenomanian ages, a favourableconvergence of different c1imatic, eustatic. tectonic and palaeogeographic factors allowed highly intenselateritization and bauxitization on the Earth's surface (Combes and Bardossy, 1994) . The greenhouseeffect related to the high atmospheric CO2 content, plus a very weak equator-to-pole temperature

gradient (Barron, 1983: Berner et al.. 1983: Arthur et al.. 1985: Budyko et al.. 1987: Crowley, (993)enlarged the tropical humid c1imatic belt, suitable ror bauxitization and lateritization up to the latitudesof 40-50°. Mainly karst bauxite deposits of the Tethys region were preserved from this time. On theother hand. most of the laterites and lateritic bauxite deposits were eroded, because of their position onpositive landforms of large intracontinental areas (Bardossy and Aleva, 1990). One of the rare remnantsof this type of deposits is at Az Zabirah, Saudi Arabia (Black et aL, 1984). A minor portion of theeroded bauxitic and lateritic material accumulated in karst depressions on pericontinental carbonateplatforms. The major part or the eroded material was washed away and was incorporated in f1ysch-typesediments of the Late Cretaceous to Paleocene age.

28

RELATIONSHIP Of CRETACEOUS BAUXITES AND LATERITES TO JHE ATMOSPHERIC C02 AND 02 CONTENT

The high atmospheric CO2 content facilitated the lateritic-bauxitic weathering (Pédro. 1964) and theprecipitation of the carbonate minerais from the surface waters. Thus there was a close relationshipbetween the laterites and lateritic bauxites and the formation of pericontinental carbonate platformswith the subsequent accumulation of karst bauxite deposits. The atmospheric CO2 content was tieddown by the photosynthetic activity of plants as weil. producing oxygen and organic matter. the latter.in most cases. being buried in sediments. This part of the Cretaceous was particularly rich in buriedorganic matter. such as black shales. as indicated by Larson (1991).

The oxygen produced by photosynthesis was chemically bound again by the oxidation of parts of theorganic materials. by incompletely oxidized volcanic gases. by the oxidation of sulfuric compounds tosulfates and by the lateritic and bauxitic weathering processes outlined above (Budyko et al.. 1987;Walker et al. 1988; Hoiser et al. 1988). The O2 and CO2 content of the atmosphere represented acontinuously changing equilibrium between the newly produced and the fixed oxygen and C02' In ouropinion. bauxitization and lateritization were important factors in the change of this equilibrium.

EVALUATION Of THE ROLE Of BAUXITE AND LATERrrE fORMATION IN THE REGULAll0NOf IHE ATMOSPHERIC OXYGEN CONTENT

Methods

The original mass of bauxite and laterite formed during the chosen time intenal can be calculated fromthe extent of the potential land surfaces and from the thickness of the weathered zone. the latter beingderived from a given rate of weathering. From the total mass of bauxite and laterite the amount ofoxygen flXed by the lateritic weathering can be calculated.

Time interva/sAptian. Albian and Cenomanian ages (from 114 to 91 Ma): significant amounts of mainly karst bauxiteand laterite was formed in the Tethys region (Combes and Bardossy. 1994). Tumnian and Senonianages (from 91 to 65Ma): extensive laterite and lateritic bauxite deposits were formed in largecontinental areas. such as South America. West Africa. India. etc. (Bardossy and Aleva. 1990).

(aleu/ated surfacesThe potential surfaces of lateritic weathering have been calculated from the Upper Cenomanian andfrom the Maastrichtian maps of the Tethys Atlas (Dercourt et al.. 1993). It is likely that not ail thepotential surfaces were weathered to bauxite and laterite. For this reason only half of the potential landsurface was taken into account for our calculations (see tablel).

Rate of weathering.

ln our calculations five rates of weathering have been applied from one to Sm/million years (tables 1and Il). We based our selection of these values on the results of observation data. experimentalweathering tests and on computer simulations carried out by several authors (Leneuf. 1959; Fritz andTardy. 1973; Trescases. 1975; Moreira-Nordemann. 1978).

Results (tables 1and II)

For the bauxites a global average of 55.7% oxygen content was applied. based on the calculations ofBronevoi et al. (1985). The same average was obtained by Bardossy and Aleva (1990) for the typical 29

laterites, rich in kaolinite. The average atmospheric oxygen content for the Cretaceous Period wasadopted from Budyko et al. (1987). The average oxygen content of the related parent rocks is 45,7%,according to our calculations. This means around 10% difference in the oxygen content of the parentrocks and the corresponding weathering products. The resulting relative percentages of fixedatmospheric oxygen, depending on the rates of weathering, are presented in tables 1and II. They varyfrom Z.Z to Il,Z%.

Interpretation and conclusions

The above outlined results demonstrate that a non-negligible part of the atmospheric oxygen contentwas fixed during the Earth's history by the processes oF laterite and bauxite formation. This activity wasmost significant during the peak time intervals of global bauxitization and lateritization, when theirsurface extent was largest and the rate of weathering was highest ln our opinion. this was one of themajor reasons for the decrease of atmospheric oxygen content from the Upper Cretaceous to thebeginning of the Oligocene and its increase from the Oligocene to the present (see figure 37, inBudyko et al., 1987).

INTRODUOION

les bauxites et latérites se forment au contact de l'atmosphère et de l'hydrosphère par altération d'unegrande variété de roches sédimentaires, métamorphiques et magmatiques. le processus d'altération, laferrallitisation (latéritisation), représente l'un des types d'évolution les plus rigoureux dans le domaineexogène dont J'aboutissement est une concentration en AI et Fe, éléments les moins mobiles dans cecontexte, par néoformation d'oxy-hydroxydes. les bauxites et latérites se classent ainsi parmi les rochesles plus riches en oxygène. leur genèse est donc susceptible d'influencer la composition del'atmosphère:

• par utilisation d'une partie du COZ dissous dans l'eau au cours de l'altération des minéraux primaires:

• par fixation et accumulation de quantités importantes d'oxygène. le but de cette note est un essaid'évaluation de cette interdépendance bauxites-latérites/composition de J'atmosphère en prenant surtoutcomme exemple le Crétacé, période au cours de laquelle la ferrallitisation a été particulièrementdéveloppée.

LES BAUXlUS ET LATÉRITES DU CRÉTACÉ

Au Crétacé, et notamment au Crétacé moyen (Aptien-Albien-eénomanien), tous les facteurs favorablesà la genèse des bauxites et latérites ont été réalisés (Combes et Bardossy, 1994):

• climat chaud et humide largement étendu, grâce à l'effet de serre dü à une teneur élevée en COZ et à

un faible gradient de températures entre l'équateur et les pOles:

• montée eustatique contribuant aussi à l'extension de ce type climatique et permettantl'envahissement des domaines péricontinentaux avec mise en place de plates-formes carbonatées:

• instabilité tectonique contrôlant, selon la valeur du taux de soulèvement. l'altération des rochesmères ou l'érosion des bauxites et latérites intracontinentales et le dépôt de produits détritiqueshyperalumineux sur les plates-formes carbonatées péricontinentales:

30

• paléogéographie favorable, liée à la disposition E-W de la Téthys et à la situation intertropicale d'unegrande partie du domaine émergé.

le large développement des platesformes carbonatées en bordure de la Téthys a permis un importantdépàt de bauxites karstiques. le matériel alumineux provenait de l'érosion des couvertures latéritiquesdes domaines intracontinentaux qui étaient probablement largement altérés, mais dont l'évaluationdirecte n'est plus possible. Seuls de rares témoins subsistent dans les zones peu tectonisées, assez peusoulevées. qui ont pu être recouvertes et protégées par les transgressions du Crétacé supérieur, commeen Sardaigne (Combes. 1990) et en Arabie Saoudite (Black et al.. 1984). De manière générale, lesbauxites latéritiques et latérites, en position haute intracontinentale, sont restées à l'écart destransgressions et ont été érodées. dès que l'instabilité tectonique est devenue prépondérante, Elles ontpu. de cette manière. alimenter les bauxites karstiques détritiques sur les plates-formes carbonatéespéricontinentales. Par la suite. l'accentuation des mouvements tectoniques a provoqué la dispersioncomplète des couvertures de bauxites latéritiques et de latérites qui ont contribué à alimenter les sériesf1yschoides (exemple: Crétacé supérieur des Pyrénées et des Hellénides). Ainsi. le dépàt des bauxitesdétritiques karstiques et d'une partie des sédiments de type f1ysch. représentant une masse considérable,témoignent indirectement de l'importance majeure des domaines latéritisés intracontinentaux durant leCrétacé et maintenant disparus.

Aptien Turonienà Cénomanien à Sénonien(91 à 65 Ma)

23 Ma 26 Ma

23 m 26 m

7740000 km2 7260000 km2

445. 10 12 t 471,9. 10 12 t

0,2478.. 1021 g 0,2628.. 1021 g

1,135. loZl g 1,176. 1021 g

Période considéréeEvaluated period(114à91 Ma)

DuréeTime interualEpaisseur altérée (taux d'altération 1 mIMa)Thickness of weathered zone frate of weathering 1 miMa)Estimation de la surface altérée 0.5Extent of the weathered sufface x 0.5Masse de bauxites et latérites (2.5t1m3)

Tonnage of bauxite and laterite (2.,5 Um3)

Oxygène total fixé dans les bauxites et latérites (0.557 .1 06g1t)Total quantity of oxygen fixed by bauxite and lateriteQuantité d'oxygène atmosphérique durant la période(calculée d'après les données de Budyko et al., 1987. tabl 12)

Quantity of atmospheric oxygen in the evaluated period(calculated from data of Budyko et al 1987. table 12)Pourcentage de l'oxygène fixé dans les bauxites et latéritesloxygène atmosphérique x 0,1

Percentage of oxygen fixed by bauxite andlaterite/atmospheric oxygen x 0,1

2,2 % 2,3 %

Tableau 1Calcul du pourcentage de l'oxygène fixé dans les bauxites et latérites par rapport à l'oxygéneatmosphérique au Crétacé moyen et supérieur pour un taux d'altération de 1miMa.

Calculation of the percentage of oxygen fixed by bauxites and laterites/atmospheric oxygen duringAptian to Cenomanian and Turonian-Senonian. Rate of weathering 1miMa

LES BAUXITES ET LATÉRITES DU CRÉTACÉ ET LA TENEUR EN COr02 DE L'ATMOSPHÈRE

les valeurs élevées en CO2 de l'atmosphère au Crétacé (Barron, 1983: Berner et al.. 1983: Arthur et al.,31

1985; Budyko et al., 1987; Berner, 1992; Crowley, 1993) augmentent la teneur en acide carbonique deseaux, ce qui favorise l'altération ferrallitique (Pédro, 1964) et la formation des bicarbonates solubles àpartir desquels précipitent les carbonates. Cette relation explique l'interdépendance entre les bauxiteslatéritiques et latérites intracontinentales et les plates-formes carbonatées péricontinentales et ensuite,après déstabilisation des couvertures latéritiques, le dépôt de bauxites karstiques détritiques. déjàsignalé. Le CO2 est également consommé par la réaction photosynthétique fournissant de l'oxygène.dont c'est la source principale, et de la matière organique en partie enfouie dans les sédiments,particulièrement abondante au Crétacé moyen (black shales) (in Larson, 1991). Il Y a donc aussi,durant les périodes de haute teneur en CO2, une interdépendance bauxites et latéritesrochescarbonatées-matière organique des sédiments.

Pour ce qui concerne plus spécialement l'oxygène, les quantités fournies par la photosynthèse sont.comme on le sait (Budyko et al., 1987: Walker et al., 1988: Holser et al., 1988), consommées parplusieurs réactions: les plus importantes correspondent à l'oxydation de la matière organique, des gazvolcaniques incomplètement oxydés, des roches au cours de l'altération (formation des oxyhydroxydes)et des sulfures en sulfates. C'est à-dire que les bauxites et latérites, par leur teneur élevée en oxydes ethydroxydes de Fe et AI, participent à la consommation en O2 Comme la composition en oxygène del'atmosphère, pour chaque période considérée, exprime l'état d'équilibre entre ce qui est fourni et ce quiest consommé, la genèse des bauxites et latérites peut en partie contrôler la teneur en oxygène par lesquantités susceptibles de s'y trouver fIXées. Il nous a paru intéressant de tenter une évaluation de cecontrOle.

Taux d'altérationRate of weathering

2m/Ma

2

3m/Ma

2

4m/Ma

2

Sm/Ma

2

Pourcentage de l'oxygène fixé dans les bauxiteset latérites/oxygène atmosphérique x 0,1(colonne 1.114 à 91 Ma. 2,91 à 65 Ma) 4,4% 4,5% 6,5% 6,7% 8,7% 8,9% 10,9% Il,2%

Percentage of oxygen {lXed in bauxites and/aterites/atmospheric oxygen x 0,1(First co/umn. 114-91 Ma: secondco/umn. 91-65.Ma).

Tableau Il Pourcentage de l'oxygène fixé dans les bauxites et latérites par rapport 3 l'oxygèneatmosphérique au Crétacé moyen et supérieur pour des taux d'altération de 2 à 5 miMa Mêmes

éléments de calcul que pour le tableau 1Percentage of oxygen {lXed by bauxites and /aterites/atmospheric oxygen for 2 to 5 m/Ma weathering

rates. Same e/ements of ca/cu/ation as table L

ESSAI D'ÉVALUATION DU ROtE DE LA GENÈSE DES BAUXITES ET LATÉRITESDANS LA RÉGULATION DE L'OXYGÈNE A'rMOSPHÉRIQUECe premier essai est une quantification approximative, testée avant tout pour évaluer le niveau auquelse situe la fixation d'oxygène dans les bauxites et latérites. la comparaison avec la teneur en O2 del'atmosphère nous permettra ensuite de déterminer si la formation des bauxites et latérites peut jouerun rôle significatif dans le contrOle de cette teneur.

Méthodologie

Durant les périodes considérées, si "on prend en compte les surfaces susceptibles d'être ferrallitisées etle taux d'altération fournissant l'épaisseur des altérites, il est possible de calculer la masse totale desbauxites et latérites et celle de l'oxygène fixé. 32

Périodes considéréesAptien-Albien-eénomanien (114 à 91 Ma): constitution d'un stock important de bauxites et latéritesdans les régions téthysiennes (Combes et Bardossy, 1994), sur la Laurasie méridionale, depuis l'Asiejusqu'à l'Amérique du Nord, et sur le Seuil Méditerranéen où existent de nombreux gisementskarstiques dont certains indiquent l'existence d'arrière-pays intracontinentaux couverts de bauxiteslatéritiques. Ce type est également présent en Arabie Saoudite (Black et al., 1984), mais pas en Afriqueseptentrionale où le climat était trop aride (Combes et Bardossy, 1994). Turonien-Sénonien (91 à65Ma): la majeure partie des régions téthysiennes et de la Laurasie quittent la bande intertropicale enraison de la migration des plaques vers le Nord et deviennent trop instables (phase pyrénéo-alpine).Dans ces conditions, le stock de bauxites et latérites de la période précédente peut être recyclé dans desgisements détritiques ou dispersés dans les f1yschs. mais il n'y a plus un développement généralisé de laferrallitisation. Celle-ci se situe maintenant en Amérique du Sud, Afrique occidentale et orientale, Asie,Inde (Bardossy et Aleva. 1990).

Surfaces retenuesLes surfaces susceptibles d'être ferrallitisées sont estimées à partir des cartes de l'Atlas Téthys (Dercourtet al., 1993) en utilisant celles du Cénomanien supérieur pour l'Aptien-Albien-Cénomanien et duMaastrichtien pour le Turonien-Sénonien. Les bandes latitudinales choisies sont de ° à 400 pour leCrétacé moyen. en raison du faible gradient des températures et de la forte teneur en C02 permettantl'altération ferrallitique à des latitudes supérieures à celles observées actuellement (exemple: les bauxitesfrançaises). La diminution du CO2 et de la température au cours du Crétacé supérieur (Budyko et al.,1987) justifie la réduction de cet intervalle à 0-300 durant cette période. Par ailleurs, comme il estvraisemblable que toutes les surfaces retenues n'ont pas été ferrallitisées (relief trop élevé, zones aridesà l'abri de l'influence océanique. dépressions mal drainées, roches mères défavorables, altérationincomplète etc.). les valeurs obtenues ont été réduites de moitié.

Taux d'altérationLes études thermodynamiques et les données de la chimie minérale appliquées aux conditionsnaturelles de la ferrallitisation (Fritz et Tardy, 1973) sur roche mère granitique ou basique, donnent untaux d'altération de l'ordre de 30cm/l00 000 ans. Des estimations basées sur des mesures de terrain àpartir de roches variées, dans les conditions actuelles de la ferrallitisation, sont supérieures: 5 à50mm/lOOO ans (granites, Côte d'Ivoire) (Leneuf. 1959), 12,5 à 46.8 mml 1 000 ans (péridotites.Nouvelle-Calédonie) (Trescases, 1975), 1 m/25 000 ans (granulites, Brésil) (Moreira Nordemann.1978). Ces valeurs élevées intéressent des surfaces relativement limitées avec de bonnes conditions dedrainage, mais ne peuvent ëtre appliquées, en tant que taux d'altération moyen, à la totalité desdomaines émergés retenus. Elles nous indiquent toutefois une limite supérieure, c'est-à-dire que le tauxd'altération moyen doit se situer au-dessous de ces valeurs. Dans nos calculs. nous avons fait dessimulations (tableaux 1et Il) avec des taux de 1 à 5 mIMa qui nous semblent acceptables, d'autant plusque, durant le Crétacé, les teneurs en CO2 et les températures élevées favorisaient probablement uneaccélération de la vitesse moyenne d'altération.

Résultats (tableaux 1et III

La teneur en oxygène utilisée de 0,557.1 06g/t de bauxite et latérite est une valeur moyenne obtenue àpartir de 6 700 analyses chimiques provenant de 48 différents districts (Bronevoi et al., 1985). Lesquantités d'oxygène atmosphérique à l'Aptien-Albien-Cénomanien et au Turonien-Sénonien ont étécalculées en appliquant les valeurs de Budyko et aL (1987) pour le Crétacé inférieur (133 à 101 Ma) etle Crétacé supérieur (101 à 67 Ma) . Cela permet ensuite de déterminer, pour ces mêmes périodes, lepourcentage de l'oxygène fixé dans les bauxites et latérites par rapport à celui de l'atmosphère. Cettevaleur est calculée en introduisant un important facteur de correction. Il faut. en effet. tenir compteseulement de la différence entre l'oxygène des bauxites et latérites et celui fourni par la roche mère. Lateneur moyenne en 02 de la croüte terrestre est de 46,6 % (Mason, 1958), celle des

33

basalte-dolérite-gabbro, représentant près de 40 % des roches mères (Bardossy et Aleva, 1990), se situeautour de 44 % (Eugster, 1978). Il nous a paru raisonnable, et pour simplifier les caleuls, de choisir lavaleur intermédiaire de 45,7 % comme teneur moyenne, ce qui situe à 10 % le gain en oxygène fixédans les bauxites et latérites par rapport à la roche mère. Dans les tableauxl et Il, le pourcentage del'oxygène fixé dans les bauxites et latérites par rapport à l'oxygène atmosphérique a donc été multipliépar 0,1. Les résultats obtenus (tableaux 1et Il) varient entre 2,2 et 11,2 % selon le taux d'altération.Ces valeurs représentent la quantité d'oxygène prise à l'atmosphère au cours de la ferrallitisation.

Dans le calcul précédent. les principales causes d'erreur peuvent provenir: 1) de l'évaluation dessurfaces ferrallitisées, car il est difficile de prévoir les causes locales susceptibles de géner l'altération: 2)de l'intensité de l'altération, car les surfaces retenues ont vraisemblablement été recouvertes par desbauxites et des latérites à des degrés divers d'évolution, Il est permis toutefois de minimiser cette caused'erreur car. même s'il s'agit de latérites peu évoluées, elles contiennent de la Jcaolinite, minéral lui aussiparticulièrement riche en 02 (55,7 %). Enfin, on peut remarquer que les taux d'altération utilisés sontdes valeurs moyennes pour les périodes considérées. L'altération n'a probablement pas été constantepuisque, par exemple, la modification périodique des paramètres orbitaux de la Terre et l'existenced'épisodes glaciaires envisagés par certains auteurs au Crétacé (Weissert et Lini, 1991: Frakes et al.,1992) sont susceptibles de provoquer des variations à haute fréquence dont nous ne pouvons tenircompte dans l'état actuel des connaissances.

Interprétation: rôle de la genèse des bauxites et tatéritesdans le contrôle de la teneur en oxygène atmosphérique

Les données précédentes situent l'oxygène fixé dans les bauxites et latérites à un niveau qui peutinnuencer sensiblement la composition de l'atmosphère. Pour une certaine quantité d'oxygène d'originephotosynthétique, la teneur en 02 de l'atmosphère aura tendance à augmenter si le taux de productionen bauxites et latérites est faible et à diminuer s'il est fort. En supposant. évidemment que les autresréactions consommant de l'oxygène restent égales par ailleurs. Ainsi. il est possible de proposer que labaisse du C02 et de la température au cours du Cénozo;que (Berner et al.. 1983: BudyJco et al., 1987:Berner, 1992; Crowley, 1993) a provoqué une diminution globale de l'altération et du taux deproductivité en bauxites et latérites, par rapport aux conditions particulièrement favorables du Crétacé.Cette évolution pourrait expliquer que, malgré la tendance générale à la baisse du CO2, qui devraitentraîner une diminution corrélative en 02 fourni par photosynthèse, on enregistre une augmentationen oxygène à partir de l'Oligocène (BudyJco et al.. 198n. Cette augmentation serait due, au moinspartiellement, à la réduction du taux de production en bauxites: l'un des consommateurs en oxygèneétant déficient. l'oxygène peut alors augmenter dans l'atmosphère. D'autre part. la diminutiond'ensemble des bauxites et latérites au Cénozo:que a pu être accentuée par l'apparition de conditionspaléogéographiques moins favorables qu'au Crétacé, où la disposition E~W de la Téthys et la largeur dela bande latitudinale d'altération permettaient à des surfaces plus importantes d'être intéressées par laferrallitisation. Toutefois, il faut préciser que les conditions favorables à la ferrallitisation n'ont pasdisparu durant le Cénozoïque. Ce type d'altération s'est perpétué dans certaines zones privilégiées où apu s'installer un régime de mousson, principalement aux Caraïbes, en Amérique du Sud, en Afrique del'Ouest et dans le Sud-Est asiatique, avec même deux maximums relatifs au Miocène moyen et auPliocène supérieur (Bardossy et Aleva, (990).

CONCLUSION

Au cours du Crétacé moyen et supérieur, la haute teneur en CO2 et les températures élevées, répartiesavec un faible gradient latitudinal. ont permis un développement et une extension maximum del'altération ferrallitique. Un essai d'évaluation globale montre qu'une part non négligeable de l'oxygènede l'atmosphère, créé par la photosynthèse à partir du CO2 d'origine endogène, a pu être fixé dans lesbauxites et latérites sous forme d'oxy-hydroxydes et de Jcaolinite. Ce rôle d'accumulateur en oxygène se

34

situe à un niveau suffisant pour proposer que les bauxites et latérites participent. avec d'autres facteurs.au contrôle de l'atmosphère dont la composition pourra varier selon leur taux de production. On peutpenser que la diminution de l'altération moyenne au cours du Cénozoique (baisse du C02' destempératures. paléogéographie moins favorable qu'au Crétacé) et de la genèse des bauxites et latérites aété une des causes de l'augmentation en oxygène atmosphérique observée depuis l'Oligocène par unediminution importante d'un des accumulateurs significatifs de ce gaz dans la lithosphère.

BARDOSSY. G.• et AlEVA. G. j. j .• 1990. lateritic bauxites. Dev. in Econ. Geol.. 27. Elsevier.Amsterdam. 1vol.. 624 p.

BARRON. E. j .. 1983. A warm equable Cretaceous: the nature of the problem. Earth-Scienœ Rev.• 19.p.305-338.

BERN ER. R. A.• 1992. Palaeo-e02 and c1imate. Nature. 3S8.p.114.

BERNER. R. A.• LASAGA. A. C. et GARRElS. R.• 1983. The carbonate-silicate geochemical cycle and itseffect on atmospheric carbon dioxide over the past 100 million years. Amer. j. Sc.• 283. p.641483.

BLACK R V.• lOZEj. G. P. et MADDAH. S. S.• 1984. Geology and mineralogy of the Zabirah bauxite.Northem Saudi Arabia. In L. JACOB. ed•• Bauxite. AIME. Soc. Min. Eng.• p.619-638.

BRONEVO!. V. A.. ZHllBERMINC. V. A. et TENLXKOV. V. A.• 1985. Average chemical compositionof bauxites and their evolution in time (en Russe). Geokhimiya. Moscow. 4. p.435446.

BUDYKO. M. j .. RONOV. A. B. et YANSHIN. A. l.. 1987. History of theEarth's atmosthere.Springer-Yerlag. Berlin. 138 p.

COMBES. P. j .• 1990. Typologie. cadre géodynamique et genèse des bauxites françaises. GeodinamicaActa. 4. 2. p.91-1 09.

COMBES. P. j .• et BARDOSSY. G.• 1994. Typologie et contrôle géodynamique des bauxitestéthysiennes. C.R Acad. Sei. Paris. 318. série Il. p.3S9-366.

CROWlEY. T. j .• 1993. Geological assessment of the greenhouse effect. Bull. Amer. MeteorologicalSoc.• 74. 12. p.2363-2373.

DERCOURT. j .• RICOU. l. E. et VRIELYNCK B.• éd.• 1993. Atlas Tethys. Palaeoenvironnental Maps.Gauthier-Villars. Paris. 307p.• 14maps. 1pl.

EUGSTER. H. P.. 1978. Oxygen. Abundance in common igneous rocks. In WEDEPOHl K H.• éd.•Handbooh of geochemistry. 11-1. Springer-Verlag. Berlin.

FRAEXES. L. A.. FRANCIS. j. E. et SYKTUS. j. 1.. 1992. Climate modes of the Phanerozoi c. The h istory of the Earth·s c1imate oser the past 600 million years. Cambridge University Press. 274 p .

FRITZ. B. et TARDY. Y.• J973. Etude thermodynamique du système gibbsite. quartz. kaolinite. gazcarbonique. Application à la genèse des podzols et des bauxites. Sei. Géol.. Buns 26. 4. p.339-367.

HOLSER. W. T.• SCHIDlOWSKI. M.. MACKENZIE. F. T. et MAYNARD.j. B.. 1988. Geochemical cyclesof carbon and sulfur. in BRYAN GREGOR. C.. GARRELS. R. M.. MACKENZIE. F. T. et MAYNARD.j. B.éd.• ~Chemical cycles in the evolution of the Earth". John Wiley and Sons. New York. p.IOS-IB.

LARSON. R. L. 1991. Geological consequences of superplumes. Geology. 19. p. 963-966.

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LENEUF, N., 1959. L'altération des granites calco-alcalins et des granodiorites en Côte d'Ivoire forestièreet les sols qui en sont dérivés, Thése Fac. Sei., Paris. ORSTOM, 212p.

MASON, B.• 1958. Prineiples of geochemistry. John Wiley and Sons. New York, 310p.

MOREIRA-NORDEMANN. L. M.• 1978. Estimation de la vitesse d'altération des roches par l'utilisationde l'uranium comme traceur. Application au bassin de la rivière Preto. Brésil. CR Acad. Sei .. Paris. 287,série D. p.419422.

PEDRO. G.• 1964. Contribution à l'étude expérimentale de l'altération géochimique des rochescristallines. Ann. Agron. Fr.: 15. nQ2. p.85-191; n9 3. p.243-333: ng 4. p.339456.

TRESCASES, J. J.. 1975. L'évolution géochimique supergène des roches ultrabasiques en zone tropicale.Formation des gisements nickélifères de Nouvelle Calédonie, Thése, Univ. Strasbourg. Mém. ORSTOM.n° 78.259 p.

WALKER. j. C; G. et DREVER. j. 1.. 1988. Geochemical cycles of atmospheric gases. in BRYANGREGOR. c.. GARRELS. R. M.• MACKENZIE. F. T. et MAYNARD. j. B. éd.. Chemical cycles in theevolution of the Earth. p. 55-173.

WEISSERT. H. et L1NI. A.. 1991. !ce age interludes during the time of Cretaceous greenhouse c1imate ?ln MULLER. D. W .• MACKENZIEj. A. et WEISSERT. H.• éd.• Controvasies in modan Geology.Academie Press. New York.

36

~~ ffi® ~m LI U Q)$

sUlnlner schoolécole d'été

e(j:ï.~~~

and~~~ ta. tlte GCea/IU

Jean-Luc PROBSTCentre de Géochimie de /0 Surface, CNRS, 1rue B/essig, 61084 Strasbourg Cedex, France

To he pub/ishedin/à paraÎtre dans: Trends in Hydr%gy, 5.6. PANDAlAI Ed., Trivandrum, 1995

37

INTRODUŒON

The continental erosion combines both physical and chemical processes which release organic andminerai materials to be exported in suspension and in solution by rivers into the oceans. The chemicalerosion of inorganic materials consists in dissolving or hydrolysing primary minerais of rocks and soils.releasing in solution elements which are drained into groundwater and rivers.

Severa1authors have modelized the natural weathering pathway derived from carbonic acid reaction onminerais ta procedure dissolved inorganic carbon (mainly HC03-) (GARRELS and MACKENZIE. 1971 :HOLLAND. 1978 : BERNER et al.. 1983 : WOLLAST and MACKENZIE. 1983: MEYBECK. 1987 :PROBST. 1992 . PROBST et al.. 1992 and AM lOTTE-SUCHET. 1995). The reactions involvingweathering of silicate and carbonate minerais can be summarized as follows :

Silicate minerai hydrolysis :

2 NaAISi30 S + 2 C02 + 3 H20 ++ AI2Si205(OH)4 + Na + + 2 HC03- + 4 Si02(Albite) (Kaolinite)

Carbonate minerai disolution :CaC03 + C02 + H20 ++ Ca2 + + 2 HC03­(Calcite)

(1)

(2)

For almost the totality of dissolution or hydrolysis reactions. chemical erosion corresponds to asignificant carbon dioxide (C02) consumption. The tatality of bicarbonate ions (HC03-) produced bysilicate weathering (Eqn. 1) is derived from atmospheric/soil C02 via photasynthesis and mineralizationof organic matter in soils. Whereas for carbonate dissolution (Eqn 2). only haIf the bicarbonate ionsreleased originate from C02.

The chemical and mechanical erosion of soil organic material reJeases dissolved organic carbon (ooqand particulate organic carbon (poq which are exported respectively by subsurface f10w and surfacerunoff ta rivers. In both cases. ail organic carbon transported by rivers is originating from atmosphericC02. via photasynthesis and Iitter fall.

The main scientific objectives of this study are first ta quantify the different fluxes of carbon (organicand minerai) originating from atmospheric and soil C02 and transported by rivers from continents taoceans. Second. they are- to modelize continental erosion (chemical and mechanical erosion of mineraiand organic soil materials) and the "continent/ocean" carbon fluxes in relation ta c1imate. hydrologiccycle. vegetation and cultivation. soil and Iithology. and morphology of landscape. The modelsdevelopped in this study can be used to predict the influences of different c1imatic and humanperturbation scenarii on the carbon transfer from continents ta oceans.

38

Table 1 - Mean annual nuxes of dissolved major elements transported by world rivers (AMIOTTE SUCHET. 1995)

dissolved major elements fluxes

(106 tian)S Qt

106 km3.y-l n Si02 Ca + + Mg + + Na + K+ CI- S04- HC03- Réf. Dq*

km-2

AMAZON 5.91 6300 - 42.84 6.93 14,49 6.30 17.01 35.91 166.32 (1 ) 2AMOUR 1.92 325 - 3.29 0.84 2.65 0.37 0.75 1.64 13.88 (2) 1COLORADO 0.71 20 0.20 1.80 0.60 2.28 0.12 2.06 6.26 2.94 (3) 3COLUMBIA 0.67 25\ - 5.21 1.36 1.83 0.29 0.90 2.60 20.62 (3) 3CONGO 3.70 1300 13.39 3.08 1.79 2.59 1.82 1.82 1.52 17,46 (4) 3DANLIBE 0.78 206 1.03 10.09 1.85 4,45 0.52 4.02 4.94 39.14 (5) 2FRASER 0.25 112 0.55 1.79 0.24 0.17 0.08 0.01 0.90 6.72 (2) 1GANGES-BRAHM. 1.67 971 - 22.62 4.37 3.69 3.30 4.66 5.73 90,40 (5) 3GODAVARI 0.32 84 0.72 1.64 0,40 0.89 0.25 1.23 0.57 7.80 (6) 1HUANGHf 0.81 53 0.27 2.32 1.26 2.73 0.16 2.75 4.00 10,47 (7) 2HUNGHO 0.16 123 0.57 2.04 1.00 1.36 0.18 1.03 1.76 10.05 (8) 1INDIGIRKA 0.35 59 - - . - - - - -INDUS 0.92 238 0.73 7.89 2.27 6.58 1.62 5.55 7.58 36.24 (9) 2IRRAWADI 0,41 428 - - - - - - - -KOLYMA 0.66 135 - - - - - - - -LENA 2,44 533 - 9.59 2.03 8.80 1.22 8.10 Il.30 35.39 (10) 1L1AOHE 0.19 6 - - - - - - - -LIMPOPO 0.34 5 - 0.10 0.06 0.10 0.02 0.07 0.03 0.72 (10) 1MACKENZIE 1.62 249 0.87 8.96 1.94 1.74 0.20 1.89 7.82 26.39 (II) 1MAGDALENA 0.28 237 2.99 3.56 0.78 1.97 0.44 3.18 3,41 Il.68 (5) 1MEHANDI 0.19 67 0.61 0.70 0.64 0.68 0.10 0.98 2.10 4.10 (12) 3MEKONG 0.85 530 4.69 7.53 1.70 1.91 1.06 2.81 2.01 30.69 (5) 3MISSISSIPPI 3.26 580 - 23.64 6.58 12,48 1.82 14.58 31.35 72.95 (3) 3MURRAY 1.14 22 - 0,46 0.37 2.22 0.13 3.76 0.84 2.07 (13) 3NEGRO 0.18 30 0,49 0.65 0.13 0.85 0.05 0.60 1.31 2.13 (14) 1NIGER 1.55 192 2.67 1.09 0.44 0.38 0.19 0.15 0.13 6.57 (15) 2NILE 1.90 89 - 2.57 1.01 3.03 0.59 2.84 2,48 14.01 (\6) 2OB 2.25 419 - 7.98 1.61 1.80 0.38 0.85 3.10 32.72 (2) 1ORANGE 0.73 Il 0.18 0.20 0.09 0.15 0.02 0.12 0.08 1,18 (5) 3ORINOCO 1.02 1100 8,43 3,40 0.62 1.10 0.84 1.09 1,43 13.61 (17) 2PARANA 2.87 470 - 3.59 0.95 2.29 1.07 2.32 1.94 17.95 (18) 2PO 0.07 46 - 2.86 0.55 0.68 0.09 0.83 2.76 8.19 (19) 3RHONE 0.10 49 - 3,48 0.32 0.56 0.11 1.10 2.25 8.75 (18) 3RUFUI 0.18 9 - - - - - - - -SAO FRANCISCO 0.63 97 2.38 2.09 - . - 0.22 0.19 2.11 (20) 1SEVERNAIA DVINAO.26 109 - 4.51 1.02 1.28 0.18 1.53 5.13 13.30 (10) 1ST LAWRENCE 1.10 413 - 12.32 3.13 4.44 0.52 8.79 10.82 36.96 (21) 2SUSITNA 0.05 40 0.19 0.76 0.13 0.14 0.14 0.08 0.55 2,47 (3) 1TIGRIS 0.92 46 - 2.44 1.23 1.51 - 2,45 3.54 8.14 (2) 1XI JIANG 0,46 302 1.96 Il.63 1.38 0,40 0.26 0.37 2.32 40.20 (22) 1YANA 0.24 29 - 0.24 0.06 0.01 0.00 0.04 0.08 0.92 (10) 1YANGTZE 1.83 928 5.85 25.71 6.77 8.56 1.19 4.18 12.99 102,45 (23) 3YENISEI 2.55 562 - Il.80 2.30 1.13 0.16 5.06 4.83 41.59 (24) 2YUKON 0.83 210 - 6.92 1.31 0.64 0.31 0.22 2.76 23.71 (3) 3ZAMBEZI 1,42 223 2.74 2.16 0,49 0.89 0.27 0.22 0.67 5.55 (5) 2

• Dq: Data quality. 1 • poor (Iess than 6 samplinc): 2 • sufficient (between 6 and 12 sampling along one year): 3 • good (12 sampling pet year during at least 2years)

r~r~rences: (1) Probst et al. (1994) - (2) Meybeck (198-4) - (3) uses (annuel) - (,,) Probst et al. (1992b) (pIRAT and PEGI/CNRS Projects) - (5) Meybeck (1979) ­(6) Biksham et Subramanian (1988) • (7) Wei-Bin et al. (1983) • (8) Ming-hui et al. (1982) • (9) Anin (1988) • (10) üvinptone (1963) • (II) Reeder et al. (1972) ­(12) Subramanian (1979) - (13) Herczeg et al. (1993) - (14) Depetris (1980) - (15) Martins (1982) - (16) • Kempe (1983) - (17) Paoloni et al. (1987) • (18) Kempe(1982) - (19) Pettine et al. (1985) - (20) Paredes et al. (1983) • (21) Cossa et Tremblay (1983) - (22) Qunying et al. (1987) - (23) Wei-Bin (1985) • (24) Gitelson et.. 1 IlnOO\

39

MATERIALS

River transport doto

Most or the data concerning the river transport or dissolved major elements are rrom the literature.Sorne data. as ror the Congo and the Garonne rivers. have been obtained within the rramework or theFrench CNRS/INSU Scientific Programs (respectively PIRAT and PEGlIGBF. DBT 1"Fleuves et érosion").As seen in table 1. the quality or the data is good. except ror sorne rivers.

Particulate (POC) and dissolved (DOC) organic carbon fluxes are actually available ror 32 large rivers(table 2). Most or these data have been obtained during the execution or the SCOPE/UNEP Program"Transport or Carbon and Minerais in Major World Rivers" and published in the proceedings or theannual meetings (DEGENS. 1982 ; DEGENS et al.. 1983. 1985. 1987. 1988. 1991 a). Other data arerrom the literature and rrom the French CNRS/INSU Scientific Program. as e.g. ror the Garonne river(ONT Garonne. DBT 1·Fleuves et érosion". INSU/CNRS). As seen in table 2. most or the data are ofgood quality as weil.

Table 2: Fluxes or dissolved organic carbon (FOOC) and or particulate organic carbon (FPOC)or sorne world rivers. a): FPOC value rrom: Milliman et al. (1984): b): FPOC values rrom:

Subramanian and Ittekkot. 1991 Dq is an index ror the data quality (1 - poor. 2 - surficient. 3 • good)(LUDWIG et al.. submitted)

River Abrev. Dq FOOC FPOC POC% DOC POC Source(tlyr km2) % TSS (mg/I)

Amazon Amz 3 4.461 2.826 6.09 4.46 2.83 Richey et al.. 1990Zaire Zai 2 2.465 0.680 7.00 7.25 2.00 Nkounkou & Probst. 1987Mississippi Mis 2 1.319 0.320 8.79 2.14 Leenheer. 1982Nile Nil 3 0.089 0.116 2.95 3.85 Abu el Ella. 1993Parana Par 3 1.432 0.279 2.64 8.68 1.69 Depetris & Cascante. 1985Ob Ob 1 1.182 0.115 9.09 0.88 Romankev. & Artem.. 1985Changjiang Yts 2 5.690 6.144 2.52 12.37 13.36 Gan Wei Bin et al.. 1983 a)Mackenzie Mac 3 0.838 0.855 2.95 4.93 5.03 Telang et al.. 1991Ganges/ Brahm. GaBra 1 2.215 5.222 3.87 9.13 Safiullah et al.. 1987 b)Niger Nig 3 0.593 0.414 3.27 3.71 2.59 Martins & Probst. 1991St Lawrence SLa 3 1.632 0.326 5.98 3.75 0.75 Telang et al.. 1991Orinoco Ori 3 4.824 1.590 1.82 4.39 1.45 Lewis & Saunders. 1989Indus Ind 2 2.929 1.794 0.46 14.40 8.82 Arain. 1987 b)Orange Org 3 0.250 0.106 2.18 2.50 1.06 Hart. 1987Yukon Yuk 2 0.953 0.307 0.32 4.14 1.33 Telang et al.. 1991Huanghe Hua 2 0,481 14.678 0.70 6.25 190.63 Zhang et al.. 1992Columbia Col 3 0.795 0.098 2.12 0.26 Dahm et al.. 1981Don Don 1 0.600 0.252 8.81 3.69 Romankev. & Artem.• 1985N Dvina NDi 1 4.498 0.197 - 13.63 0.60 Romankev. & Artem.• 1985Senegal Sen 3 0.072 1.50 Degens. 1991 bRhine Rhi 2 1.013 0.575 5.60 5.33 3.03 Eisma et al.. 1982Rioni Rio 1 0.994 1.835 1.05 1.94 Romankev. &Artem.• 1985Loire Loi 3 1.379 0.733 7.91 5.63 2.99 Meybeck et al.. 1988Brazos Brz 3 0.243 0.281 0.82 3.74 4.32 Malcom & Durum. 1976Rhone Rho 3 0.885 0.440 1.67 0.83 Kempe et al.. 1991Gambia Garn 3 0.262 0.117 2.38 2.39 1.07 Lesack et al.. 1984Garonne Gar 3 0.892 1.001 3.60 2.79 3.13 Etcheber. unpubl.Po Po 1 2.065 0.881 3.08 1.31 Pettine et al.. 1985Tiber Tib 1 1.788 0.616 3.97 1.37 Pettine et al.. 1985Waikato Wai 3 4.576 1.132 10.11 5.46 1.35 Auckl. Reg. Auth.. unpubl.Ems Ems 1 3.026 0.970 7.03 8.12 2.60 Cadeé. 1987Severn Sev 3 2.001 5.27 Mantoura & Woodward. 1983

40

Characterislics of the river basins

Relief (altitude and slope). vegetation and cultivation intensity. population density. c1imate(temperature and precipitation). drainage intensity. geological substratum (rock erodibility andlithology) and soils (depth and carbon content) have been characterized by calculating average valuesfor each river basin (PROBST. 1992 ; AMIOTTE-SUCHET. 1995 ; LUDWIG et al.. submitted).

Altitudes over the continents have been published by GATE and NELSON (1975) in a grid resolution of10' x 10' latitude-longitude and were supplied by the NCAR (National Center for AtmosphericResearch. Boulder. USA). Mean slopes are from MOORE and MARK (1986) in the same resolutionthan altitudes.

Information on vegetation and cultivation were taken from the digitized vegetation map of OLSON(OLSON et al.. 1983. 1985) which exists in a resolution of 0.5 0 x 0.5 0 at the CDIAC (Carbon DioxideInformation Analysis Center. Oak Ridge. USA). Vegetation units were transfered to figures for biomassand net primary production according ta the values given in the original publications. The populationdensity of each drainage basin was calculated from a data set developped at the Institut furEnergieforschung (Graz. Autriche) (AHAM ER et al.. 1992).

Ail hydroclimatic parameters are available with a resolution of 10 or 0.5 0 latitude-longitude(WILLMOTT et al.. 1985: LEGATES and WILLMOTT. 1992).

A map of the continental lithology has been generated by AMIOTTE-SUCHET{199S) in a gridresolution of 10 x 10 using the simplified IithoJogical maps and the soil maps published by the FAO­UNESCO (1971. 1975. 1976. 1978. 1979. 1981) for each continent.

Concerning the soil organic carbon erosion. we considered two controlling factors:' soil depth andorganic carbon content. The mean soil depth data are from STAUB and ROSENZWEIG (1992): theyare based on the FAO soil map of the world digitalized by ZOBLER (1986) in a spatial resolution of 10

x 10• For the organic carbon stored in the soils. there is actually no global data set available.

Consequently. for this study we derived the soil carbon content from the vegetation types of OLSONfollowing the assignment published by ADAMS et al. (1990) who attributed a given carbon content inthe soil to each vegetation type.

INORGANIC CARBON RIVER FLUXES AND C02 CONSUMPl'lON BY CONTINENTAL ROCK WEA'rHERING

Calculation on major world river basins

A geochemical model (MEGA) was developped by AM lOTTE-SUCHET (1995) to calculate thecontribution of atmospheric C02 ta the total bicarbonate nuxes exported by rivers. This model (fig. 1)is based on the stoechiometriy of the chemical reactions which control the dissolution or thehydrolysis of the different minerais and on ionic ratios in water draining the major rock types.

After correction for the atmospheric inputs of cations and anions (see AM lOTTE-SUCHET. 1995 :AM lOTTE SUCHET and PROBST. 1994: PROBST et al.. 1994). the mineralogical origins of major

dissolved elements transported by river (table 1) are determined and allow to calculate the nux of C02consumed by minerai weathering as follows:

Feoz = 2FÛJ +MIl (silicate~+ FNa +K (silicate~+ FÛJ+Mg (carbonate)- 2Fso• (pyrit~

(3)

The nuxes of Ca + Mg and Na + K realeased by the weathering of silicates and the nuxes of S04realeased by the pyrite oxydation have been calculated by the model MEGA {AM lOTTE-SUCHET.

41

1995) for each river basin. using average ionic ratios in water draining each rock type (after the data ofMEYBECK (1986)) and outcrop proportions of major rock types together with their correspondingmean drainage intensity.

F"JMg 511)=Mg· 0.2 Cb

moles(2)

Fso,pyr/~te)=

Py mole R•.,(1)

FcJCII all)=

C. - (SQ- Py) - 0.8 Cb]moles

(2)

FKfK 511)=K moles

si (N'tC1)<O, Fclgypsum)

VKfsylvlte)= (S~~/:I)\ (CI-Na) (1)

moles

FNINa 5/1)=(Na-CI)moles

* equivalent to sulpuric acid coming from the oxidation of pyrite minerais and which take the place ofcarbonic acid in the weathering of silicate minerais.

(1) Py - molar flux of 504 coming from the oxidation of pyrite:Py - Rpyr(Na + K-el}( I/Rsil + 1)

with Rpyr - S04I(Na+ K+ Ca + Mg) and RsiJ - (Na + 1<)1(Ca + Mg) in water draining si6cate rodes.(2) Ch - molar flux of Ca and Mg coming from carbonate weathering:

Cb - Ca+Mg-(S04-Py)-I/Rsil(Na+K+CI)with Rsil - (Na + K)/(Ca + Mg) in water draining silicate rocks.

Fig. 1- The MEGA model: determination of the origin of the major elements dissolved in river watersand coming from rock weathering (ail fluxes in moles). AMIOTIE SUCHET (1995)

The results of these calculations are reported in table 3 for major world river basins. As seen in figure2. the fluxes of C02 consumed by chemical erosion are weil correlated ta the drainage intensity and.to a lesser extent, to the relative abundance of carbonates and shales in the river basin.

42

Table 3- Mean an nuai fluxes of total bicarbonate ions (FHC03). of consumed C02 (FC02)and of bicarbonate ions coming from carbonate minerai dissolution (FCaC03). transported by largeworld rivers. %C02 and %CaC03 are the respective contributions of FC02 and of FCaC03 to the

total bicarbonate nux (FHC03) (AM lOTIE SUCHET. 1995)

FHC03 FC02 FCaC03 %C02 %CaC03

103 moles.km-2.y- 1 %

AMAZON 462 333 129 72 28AMOUR 118 112 7 94 6COLORADO 67 67 a 100 aCOLUMBIA 505 346 159 69 31CONGO 77 59 19 76 24DANUBE 825 651 174 79 21FRASER 446 290 156 65 35GANGES-BRAHM. 888 517 371 58 42GODAVARI 397 272 125 68 32HUANGHE 211 149 62 71 29HUNGHO 999 646 353 65 35INDUS 649 501 147 77 23LENA 238 200 38 84 16LIMPOPO 35 34 a 100 aMACKENZIE 268 144 123 54 46MAGDALENA 694 400 294 58 42MEHANDI 345 240 105 70 30MEKONG 593 358 235 60 40MISSISSIPPI 366 239 127 65 35MURRAY 30 21 9 70 30NEGRO 196 196 a 100 aNIGER 69 50 19 72 28NILE 121 98 22 81 19OB 238 160 79 67 33ORANGE 27 25 1 95 5ORINOCO 219 154 65 70 30PARANA 103 89 13 87 13PO 1919 1173 746 61 39RHONE 1449 725 725 50 50SEVERNAIA DVINA 832 537 295 65 35ST LAWRENCE 551 276 276 50 50SUSITNA 793 574 219 72 28XI JIANG 1420 734 686 52 48YANA 63 31 31 50 50YANGTZE 919 564 356 61 39YEN ISSEl 267 134 133 50 50YUKON 467 268 199 57 43ZAMBEZI 64 36 29 55 45

Nevertheless. the scattering of points indicates that ether factors such as temperature also control thenuxes of C02. It is interesting to note in figure 2a that the rivers draining lateritic formations such asthe Amazon. Orinoco. Parana. Congo. Niger and Zambezi river basins have lower C02 nuxes thanother river basins with similar drainage intensities. In these basins. chemical erosion has developpeddeep weathering profiles (Iaterites) which are poer in alterable minerais and which protect thegeological substratum (fARDY. 1993).

43

ln figure 2b. the fluxes of C02 are low in the artic river basins such as the Mackenzie. Yukon. Yenisseiand Lena probably because of low temperatures which decrease the weathering rates and consequentlythe corresponding C02 fluxes.

3.5 1400

Iog(Fc~·78IoQ(OI+O~ ''''.1200 • a ",le.. Iimper.le

l) 3.0 J) C and. lemi-uid

~y / 10006 ltapieal conlraaled0 ~opOc:alllumid

Ir I(l 800~ 2.5

Ë0 ..

00 ..

'~0 600 01

3 m 2- •2.0

F. 400 t.00 .. .00

~ ~ ~·o ... •6

0 200 ••.........Vana ~~ .0 C

o ~ ~ ~ 00 100 1~

1cari>+1'hIl (%)

Fig. 2b - Relationship between the specifie nux ofeonsumed e02 (Fe02) and the intensity ofdrainage of carbonate rocks (lcarb) and of shales(Ishal). (AMIOm SUCHET. (995)

3.51.5

1.0 L..-_--L.__I.--_--L.__I.--_.....J

1.0 2.0 2.5 3.0

Jog(Q) (mm.y"')

Fig. 2a • ReJationship between the drainage intensity(Q) for the large river basins of the world. Emptycircle represents basins where extended lateritic

formations are outcroping and which are not takeninta aceount in the regression calculation.

(AMIOTTE SUCHET. 1995)

Empirical modelling on small wafersheds

Relationships between the flux of C02 consumed by chemical erosion (FC02) and the river discharge(0) were calculated using the data published by MEYSECK (1986) concerning runoff andconcentrations of the major dissolved elements for 232 French monolithologic drainage basins(AM lOTTE-SUCHET and PROSST. 1993a). The lithologies of these small watersheds are representativefor the major rock types outcropping on continents.

For each watershed. we have calculated the bicarbonate river flux using runoff and HC03­concentration. Atmospheric/soil C02 consumed by rock weathering was considered to be equivalent tothe whole HC03- flux for streams draining silicate rocks (see eq. (1» and to haIf of the HC03- flux forstreams draining carbonate rocks (see eq. (2)). Empirical reJationships between FC02 and 0 was thendetermined for 7 major rock types (AM lOTTE-SUCHET and PROSST. 1993a). This set of relationshipsconstitutes the basis of a Global Erosion Model (GEM-C02) developped to predict C02 consumptionover the continents (AM lOTTE SUCHET and PROSST. 1995). FC02 increases when the runoffincreases and as it can be seen in figure 3. for a given runoff. FC02 varies according to the rock type.Hence. Fe02 is 17 times higher on carbonate rocks than on plutonic and metamorphic rocks. whichconsume the lowest amount of atmosphericlsoil C02. The amount of C02 consumed by other rocktypes ranges between these two extremes. Thus. for a given drainage intensity and going from thesmallest FC02 to the highest. the rock types can be c1assified as follow : plutonic and metamorphicrocks. sands and sandstones. acid volcanic rocks. evaporite rocks. basalts. shales and carbonate rocks.Note that FC02 is twice as high for the weathering of basalts as for the weathering of acid volcanicrocks. which are structurally very similar.

44

b120

o CClr1Ianat. racko- 100 a plutan\c: andï rMlGmorpllÎC rack.

Il!.. 801

]60,..:;

0

... El 401C-< 20--o·... aro.

a160

0_"140 A .dd voI=nrc roc::b

C pfutanlc and120 m.taITlGIlJ"'C roc..

ï.. '! 100,~ I!O

Ô 60

! ..40

8"20ca.

0

o 50 100 150 200 250 JOO

Q (l.km-a.•-1)a 20 4{) 60 80 100

Q (l.km-:l.9-1)

fig.3 - Relationships between the flux of atmospheric C02 conumed by rock weathering (FC02)and the stream runoff (Q) for watersheds on (a) carbonate rocks and plutonic and metamorphic rocks.

and (b) for watersheds on basalts. acid volcanic rocks and plutonic and metamorphic rocks(AMIOm SUCHET and PROBST, 1993a).

Validation of GEM·C02 an large river basins

For validation. GEM-C02 have been tested on three different large river basins : the Garonne intemperate c1imate and the Congo and the Amazon in tropical-equatorial c1imate (AMIOTTE-SUCHETand PROBST. 1993b). Each drainage basin area is divided in small grid cells which are each treated as amonolithologic drainage basin and for which the drainage intensity and the Iithology are determinedfrom maps. For each cell. FC02 is calculated using the empirical relationships between FC02 and Qdetermined by AM lOTTE-SUCHET and PROBST (1993a) as described above (fig. 3).

These simulations show that the empirical modelling of C02 consumption by weathering recentlyproposed by AMIOTTE-SUCH ET and PROBST (1993a) can be applied successfully to large river basinsin tropical-equatorial c1imates as weil as in temperate c1imates. The results are of the same order ofmagnitude as estimates obtained using field measurements (table 4). Nevertheless.. sorne differencesappear which can be partly attributed to the disparity of drainage intensity corresponding to differentrecord periods. During a dry period. the flux of atmospheric C02 consumed by rock weathering islower than du ring a humid period. Moreover. sorne other sources of incertainty in our model have tobe pointed out. The first ones are the inevitable simplifications of the lithological maps of the basinsand the mineralogical and lithological heterogenities of rocks such as shales. Aiso. the presence of deeplateritic profiles in the Congo basin has probably induced an over-estimation of the model results, whilelarge outcrops of heterogeneous shales in the Garonne basin has led to an underestimation of themodel results.

Another source of uncertainty is the fact that the model does not take into account other factors suchas temperature. vegetation. elevation. slopes which are believed to influence chemicaJ erosion and C02consumption (GARRELS and MACKENZIE. 1971 : HOlLAND. 1978 : BERNER et al.. 1983 : TARDY.1986 : MEYBECK, 1986. (987). Nevertheless. at a global scale, ail these variables have a weak influencecompared to drainage intensity and to lithology, as shown by PROBST (1992) and AMIOTTE-SUCHET(1995). Moreover. on small watersheds. fluctuations of C02 consumption are weil explained bydrainage variations and Iithological diversity (fig. 3).

A quantitative appraisal of the error induced by ail sources of uncertainty is difficult but if the errorseems quite important for the Garonne basin (about 36 %), it is smaller for the Congo and Amazonbasins (5 te 15 %).

45

Comparing the different basins. the weathering C02 flux of the Amazon is 4 times higher than that ofthe Congo. mainly because of the different drainage intensities. The simulations also show that theabundance of carbonate areas is a major factor controlling the weathering C02 flux and itscontribution to the total bicarbonate river flux. For example. the weathering C02 flux is 3.4 timeshigher for the Garonne basin than for the Congo. mainly because carbonate rock outcrops are 3.5 timesmore abundant in the Garonne basin. Consequently. the contribution of weathering C02 flux to thetotal bicarbonate river flux appears to be a negative function of the drainage contribution of carbonateareas as a proportion of the total drainage basin.

Table 4. Comparison of simulated results obtained with GEM-C02 and MEGA for the different basinswith estimations made by different authors (1. STALLARD. 1980 : 2. PROBST. et al.. 1994 :

3. AMIOTTE-SUCHET. 1995 : 4. AMIOTTE-SUCHET and PROBST. in press: 5. ETCHANCHU. 1988 :6. SEMHI et al.. 1993) using field measurements. 0 - drainage intensity (mm y-I). 0% - % of total

drainage. S% - % of total basin area. FC02 - flux of atmospheric C02 consumed by rock weathering(103 moles km-2 y-I): F% - C02% of HC03- river flux

Carbonate outcrop Atmospheric C02 consumed

River basin 0 S% 0% FC02 F% Rer.

Amazon 1125 9.2 7.0 271 67.8 Gem-C02

1345 285 70.5 (1)

1080 310 67.4 (2)

1066 333 72.0 Mega (3)

Congo 370 10.6 2.5 65 79.8 Gem-C02

355 53 74.7 (2)

351 59 76 Mega (3) et (4)

Garonne 345 37.8 31.2 224 57 Gem-C02

415 441 55 (5)

155 113 55 (6)

376 411 53 Mega (3)

46

oLOo

oLO,

oLOT'-

ooT'-

oLO

oo1.0,ooT'­1

oLOT'­,

oLO

l'.

o

888~8~~2

".

on\li

N..;

..;ci

ci

ci

~'CIl

Cla

~a~~

2~

.e~N--OOO~

88:1iiiiiItilloLO1

oLOT'-,

ooT'-

oLO

ooLO1

ooT'­1

oLOT'­1

47

Fig.4-

Global

model

distributionof

mean

annualC

02consum

ptionby

chemical

weathering

ofcontinental

rocks(A

MlO

TIE

SU

CH

ET

andPR

OBST.

1994)

Aglobal model for C02 consumed by chemicol erosion of continental rocks

The validation of GEM-C02 for three large drainage basins in different bioclimatic zones allows us toapply the same modelling to the global scale. The first results were presented in AMIOTIE-SUCHETand PROBST (1995) and AMIOTIE-SUCHET (1995). The datasets on bicarbonate river fluxes and C02fluxes calculated by GEM-C02 are available From the data base of the CDIAC (Carbon DioxideInformation Analysis Center. Oak Ridge Nationallaboratary. MS. USA). The grid resolution is 1° x 10.

For each grid cell. a mean lithology has been determined using simplified lithological maps and soilmaps published by FAO-UNESCO (1971. 1975. 1976. 1978. 1979. 1981) for each continent. Thedrainage intensity has been calculated after the data of WlllMOTT (1985) on mean monthlyprecipitation and evapotranspiration over the continents. These c1imatological data have been suppliedby the NCAR (National Center for Atrnospheric Research. Boulder. Colorado. USA).

The global map of C02 consumption (FC02) resulting from the modelling (fig. 4) shows highvariations in FC02 (from 0 up to 7 000.103 moles km-2 y-I). High C02 consumption is observed invery humid regions such as the South East Asia and the Amazon basin. but also in temperate regionssuch as eastern USA and westem Europe. Whereas C02 consumption is less active in Africa. even intropical-equatarial latitudes. because of large outcrops of plutanic and metamorphic rocks and of sandsand sandstanes. The mean annual consumption of atmospheridsoil C02 on the whole continentalarea resulting from the modelling is 0.26 Gt C y-I. which is agreement with previous estimates varyingbetween 0.25 and 0.30 Gt Cy-I (BERNER et aL .. 1983 : MEYBECK. 1987 : PROBST. 1992).

As shown by AMIOTIE-SUCHET and PROBST (1995). the latitudinal distribution of the mean annualspecific C02 consumption has 3 maxima: (ij between 40° and 60· South. which corresponds ta highdrainage intensity over the tip of South America. (ii) between 0° and 10° South. which corresponds tavery humid c1imate of tropical-equatorial regions and (iii) between 30° and 40° North. where largecarbonate rock areas outcrop. This latitudinal distribution has been aise compared ta the mean annualatrnospheric C02 budget calculated by TANS et al. (1989) for different latitudes. High levels of C02consumption in the northern hemisphere coincide with a net sink of atmospheric C02 while thecontrary is observed for equatorial regions. C02 consumption occurs mainly in the northernhemisphere. with about 16.1012 moles.y-I of C02 (0.19 Gt C y-I). which corresponds ta 72 % of theglobal figure.

Concerning HC03- inputs ta oceans. it is calculated that the Atlantic receives 37 % of the total HC03­river flux. the Pacifie 34 %. the Indian ocean 18 %. the Mediterranean sea 6 % and the Arctic ocean 5%.

CONTINENTAL EROSION Of SOIL ORGANIC MArrER AND ORGANIC CARBON RIVER FLUXES

Factors controlling the river fluxes

Multiple regression analyses were performed between particulate (POC) or dissolved (DOC) organiccarbon river fluxes (table 2) and the different factors (see section 2) which control erosion and rivertransport of organic carbon. The best regression models were selected by the process of ail subsetsregression in which ail possible combinations of the independent variables are tested against thedependent variables. The best model is selected following the adjusted r2 and the Mallows Cp statistics(MALLOWS. 1973: SAS. 1986).

As seen in figure 5. the best single factor model to predict the flux of DOC is based on the riverdischarge as previously shown by ESSER and KOH lMA1ER (1991). The multiple regression analysisreveals that the model performance is significantly improved if one introduces the slope of the drainagebasins and the carbon content in the soils. Based on a set of 29 rivers. the equation calculated byLUDWIG et al. (submitted) is as follows :

48

Foce (1/km 2 .yr) =4x10-3 Q (mm)· 8.7581 Siope (radian)+95.4x10-3 Soil C (kg/m3 )(4)

with r - 0.90 significant at 0.1 % level.

DOC inputs to rivers are generally subdivided into allochtonous carbon produced by the terrestrialbiosphere on land and autochtonous carbon produced by phytoplankton in lakes and in the river itself.The results obtained by LUDWIG et al. (submitted) suggest that globally the soils are the majorcontributor to riverine DOC. what is in good agreement with most of the studies on this topic (SPITZYand LE EN HEER. 1991 : DEGENS et al.. 1991).

Fig. 5: Correlation between the fluxes of dissolved organic carbon (FDOC. upper boxes and solid lines).the fluxes of particulate organic carbon (FPOC.lower boxes and solid Iines) and the environmental

patterns characterizing the river basins (dashed boxes and dashed Iines). AT:mean annual temperature:APPT: annuai precipitation total. Q average drainage; NPP: net primary production:

SoilC: organic carbon in soils: VegC: biomass density: CuitA: percentage of cultivated area:PopD: population density: SoilH: mean soil depth: Siope: mean slope: Elev: mean elevation. Only

correlation coefficients < - 0.5 and > + 0.5 are depicted (LUDWIG et al.. submitted)

For POCo the variability of the fluxes is higher than for dissolved organic carbon. No c1ear correlationwith one or several of the c1imatic. biospheric or geomorphological parameters can be detected for theoverall river basins. Taking only a group of 17 rivers with the highest data quality index (excludingmost of the Himilayan rivers). the results are completely different Here. the best correlation occursbetween POC fluxes and river discharges. However it would be misleading ta predict the POC fluxesover the continents only from the discharge. MILI.IMAN and MEADE (1983) estimated that up to 70 %of world sediment transport to the oceans are supplied by rivers draining the South East Asia and thelarger islands in the Pacifie and Indian oceans. In these regions. mechanical erosion rates are very highand one risks to biase the results if one excludes these regions in the regressions.

As seen in table 2 and figure 6. there is a good relationship between river suspended sediment49

concentration and its POC percentage as previously shown by MEYBECK (1982). ITIEKKOT (1988)and PROBST (1992).

40

35 -

30 -

25 -

~ 20 -(10

~15 -

10 -

S- .0-

POC% = - 0.1596 log (CTSS)3 + 2.8305 log(CTSS)2

- 13.5960 Iog(CTSS) + 20.2802

r= 0.83. P > 0.001, n=19

•

•

" .,,~ . ."".. ......• •:-u-••1.:. .

'.~ J!S ••• • 'C!:q-.•••

··~·:,:!~W":~'::,::,... .•••. I..~ • .,. • •

: • ·1: ••• "'~I>'..;:..t~:.': r.5 •• .1 ~. OI.~' ....:.. ••-.~ • ".

· · ~----e----e.

11

101

100

TSS (mg/I)

1

10001

10000

fig. 6: Plot of POC percentage in total suspended solids (TSS) versus the TSS concentration.Cireles represent discharge weighted annual means for the rivers depicted inside the cireles.

The dots consist in about 450 individual measurements from the following rivers: Brz. Hua. Gar. Ind.Mac. Nig. Org. Par. SLa. Wai. Abreviations see table 2 (LUDWIG et al .. submitted).

This is the reason why LUDWIG et al. (submitted) proposed a modelling for the POC fluxes from theriver suspended sediment transports. Nevertheless mechanical erosion of the continents and riversediment transports into the oceans are difficult to estimate. Several empirical models have been alreadyproposed to predict the sediment fluxes to the oceans (FOURNIER. 19860 : JANSEN and PAINTER.1974: PINET and SOURIAU. 1988: PROBST and SIGHA. 1989 : PROBST. 1992) but the parametersretained in the models are mostly different and the correlation are often weak. On the basis of ourdatasets (table 2). the best model to predict the sediment fluxes (FTSS) on a global scales is thefollowing:

FTSS (t/km2 .yr}=0.0176 Q (mm) x Slope (radian) x IR (mm) +45.0109(5)

with r - 0.90 for n - 58 river basins. significant at 0.1 % level

IR is a rainfall index originally proposed by fOURNIER (1960) to characterize the rainfall variability. Inthis study. we used a modified version of the FOURNIER index as follows :

12IR - 2. P2m/Pa (6)

i-I

where Pm is the monthly precipitation and Pa. the total annual precipitation.50

Equation (5) explains mechanical erosion intensity as a product of parameters which is necessary toaccount for the highly variable sediment fluxes observed in the different river basins. The form issimilar to the Universal Soil Loss Equation (USLE) developped by WISCHMEIER and SMITH (1960)which includes also the rainfall intensity and the slope as the main factors controlling soil erosion.

Aglobal model for river transport of organic carbon

ln order te establish global and regional budgets for the export of organic carbon to the oceans. POCand DOC fluxes were calculated for each grid cell of a 0.5 0 x 0.5 0 latitudellongitude using differentmodels proposed by LUDWIG et al. (submitted) and the characteristics of each grid cell (fig. 7 and 8).

DOC fluxes over the continents (fig. 7) were obtained by applying equation (4). For POC fluxes (fig.8). we calculated first the average river suspended sediment concentration by dividing the results(sediment fluxes) of equation (5) by the mean drainage intensity of each grid cell. Then. the POCpercentage in the sediment and the POC fluxes were calculated using the model fitted to therelationship of figure 6.

On the whole. the models give a total figure of 0.378 Gt of organic carbon which are eroded andtransported by rivers te the oceans every year (LUDWIG et al.. 1994). About 55 % of this carbonenters the oceans in the dissolved form and about 45 % in the particulate form. These figures are veryclose to previous estimates. MEYBECK (1993) calculated for the DOC transport a figure of 0.198 GtC/yr and for the POCo a figure of 0.170 Gt C/yr. what is very closed to our values.. On a regional scale.however. there are large differences with MEYBECK's figures. He estimated for example for the DOCthat about 65 % of the global flux originate from the wet tropics (MEYBECK. 1982. 1987). Our modelcalculations reveal that these regions supplied only 45 % of the DOC fluxes. In facto MEYBECKobtained his estimates by the selection of one river which is supposed to be representative for oneclimatic type. The average concentration for this river were then multiplied with an estimated runoffvalue attributed te each climate. This is problematic because the big river systems rarely fall exclusivelyin one c1imatic type as shown by LUDWIG et al. (submitted). Moreover. the reported concentrationsfor rivers under similar c1imatic conditions vary often considerably and a selection of one river is alwayssubjective. MEYBECK considered an average DOC concentration of 8 mgll te be representative for thewet trepies. This is exclusively based on the value found for the Zaire river while the values of theAmazon and the Orinoco are both below 4.5 mgll (see table 2). Our models lead te an average DOCconcentration of 4.89 for tropical wet regions which is in good agreement with fields measurements.

For the tundra and taïga. the combination of high carbon contents in the soils and high drainageintensities leads te high DOC concentrations. what is also in good agreement with fjeld observations(ROMANKEVICH and ARTMYEV. 1985). The high concentrations in the dry c1imates are the results ofa very low drainage together with low. but still considerable soil carbon contents. For POCconcentrations. the trend is similar than for DOC concentrations. For most of the continental area.DOC/POC ratio is close to one. with a global value of 1.2. The increase of POC concentrations in thedesertic c1imate areas is stronger than for DOC. what points out the high mechanical erodibilityobserved in this zones. Here. DOC/POC ratio drops down te 0.4. Contrary te DOC. POC concentrationfor the tundra and taïga zone is low which reflects the weak suspended sediment transport rates typicalfor this region. This may be also due to a reduced agressivity of the precipitations. since a considerablepart of the runoff occurs as snowmelt. In the tundra and taïga bioclimates. DOC transport exceedsPOC transport nearly with a factor of three.

For the different oceans. one can note that the major part of the global DOC flux discharges into theAtlantic (51 %) while the bulk of POC discharges into the Pacific and the Indian ocean (59 %). Thisdiscrepancy reflects the high mechanical erosion rates on the southern and southeastern part of theAsian continent Our calculations reveal that 74 % of the global sediment input to oceans is erodedfrom this region while on the other hand from the same region only about 41 % of the global runoffenters the oceans. For the two hemispheres. one can estimate that on quarter to one third of the fluxes

51

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Fig.7:

Estim

atedD

OC

nuxesFrom

con

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nts

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oceans.E

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Fig.8:

Estim

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OC

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is going to the oceans in the southem hemisphere. Naturally. these figures can not account for thesedimentation and respiration processes taking place in the river estuaries which may restrict the furtheroffshore diffusion of the organie matter to the open ocean (EISMA and CADEE. 1991). On thecontinental scale. it is South America which has the highest specifie erosion rates of organie carbon.The rate for this continent is about three times higher than the rate for Afriea and about four timeshigher than the rate for Australia. the continents with the lowest erosion rate of organie carbon (notincluding here the AntarctiCi).

CONCLUSION

This study c1early shows that the main factor controlling the continental erosion of carbon and thetransfer of Cirbon from continents to oceans is the drainage which results from the difference betweenprecipitation and evapotranspiration. Carbon erosion and the corresponding atmosphericlsoil C02consumption increase when drainage intensity increases.

Other factors appear in this study to be important for the modelling of continental erosion. Uthology.i.e. nature and composition of continental rocks. is also a major factor controlling chernieal erosion ofinorganic Cirbon. The flux of C02 consumed by continental rock weathering illCrease when carbonaterock outcrops are more abundant

Dissolved organie carbon fluxes exported by rivers are also depending on Cirbon content in the soilsand on the slope of the drainage basins. Whereas continental erosion of particulate organic carbon iscontrolled by mechanical erosion of the soils. Consequently. POC transfer ta oceans is stronglyassociated with the river transport of suspended sediments which could be modelized in this study ona global scale on the basis of mean annual drainage intensity. seasonnal rainfa" variability and meanslope of river basins. On the whole. continental erosion of particulate organic carbon increases whenrainfall. drainage intensity and steepness of the relief increase. For DOC the flux increases whendrainage intensity and carbon content in the soils increase. whereas a steep basin morphologydecreases the flux.

Using the different modeJs deveJoÏ>ped in this study. the total consumption of C02 by continentalerosion of minerai and organic materials has been estimated to 0.638 Gt CJyr of which 40 % aretransported by rivers to the oceans as bicarbonate ions and 60 % as dissolved and particulate organiccarbon. Chemical erosion (DOC plus alkalinity) supply the major proportion (73 %) of the carbonwhile mechanical erosion of the soils produces only 27 % of the total C02 flux.

Nevertheless. as shawn by the model results. carbon fluxes (HC03- from C02. DOC and PCC) arevery variable over the continents. Consequently. the ratios chemical erosion/mechanical erosion andorganic erosionlinorganic erosion change according to the spatial distribution of the different factorscontrolling erosion over the continents.

The erosion models the CGS developped could be applied in the next years to simulate carbon erosionon the continents and carbon transfer to the oceans in response to different dimate or land use changescenarios or to any other natural or anthropogenic perturbations. The GEM-e02 model has beenrecently applied (AMIOrrE-SUCHET. 1995 : AM lOTTE-SUCHET et al.. in press) to estimate thedecrease of C02 consumed by rock weathering due to acid rain inputs over the continents. This modelis also used to simulate carbon erosion during the last glacial-interglacial period in collaboration withIAl group of Liege University.

Nevertheless. the performance of the Global Erosion Models could be improved by using drainage dataof better quality. Indeed on a global scale. drainage values resulting From the General CirculationModels are not in agreement with river discharge measurements. In the nen years. it is necessary tomake an effort to improve drainage data on a global scale. Moreover the spatial distributions of carboncontent and its turnover in the soils are still insufficiently known on a global scale. Since carbon

S4

content in the soils enters our organic carbon erosion model, more detailed informations on thisparameter could improve its performance.

Measurements of 13 Cl 12 C isotopie ratio on dissolved inorganie carbon in river water as we recent/ystarted at the CGS will be a usefull toc1to improve our erosion models.

Since minerai weathering rate is strongly depending on soil pC02. we plan ta refine our Global ErosionModel by coupling it with soil respiration models.

Finally, soil erosion consists in a permanent output of carbon from the biosphere. One has ta testwether this nux could help ta constrain NPP and soil respiration in vegetation models. On the otherhand it is now also possible ta deliver precise river carbon inputs ta oceans for the research groupswho deal with ocean models.

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61

~® lli® ~m LI 0 Q)@

sUlnlner schoolécole d'été

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Yves LUCAS

laboratoire des Echanges Parficulaires aux Interfaces {lEP/} -Université de Toulon et du Var

63

IffrRODUCTlON

Les syst~mes de sol de la r~gion de Manaus, en Amazonie Br~silienne, constituent un exempleparticuli~rement in~ressant de la ~dogen~ et du ronctionnement des sols en milieu ~quatorial. Lessols vont d'un pOle rerrallitique, ~ sols tr~ kaolinitiques dans lesquels la presque totalit~ des min~raux

primaires a disparu, à un pOle podzolique, à sols constitu~ presque exclusivement de quartz primairerelictuel. L'ensemble est d~veloppé ~ partir d'une m~me roche·m~re. Ces systèmes soul~vent desquestions rondamentales, en particulier les suivantes.

• Pourquoi les sols rerrallitiques pr6entent-ils des horizons supérieurs kaolinitiques, surmontant des. horizons plus gibbsitiques, allant localement jusqu'~ des bauxites? Sur ces rormations très anciennes,

on s'attendrait plutOt ~ trouver en sommet de profil des horizons constitu6 en majori~ de gibbsite, lemin~ral le moins soluble dans le systbne Si-AI. soit de gibbsite.

• Quels sont les racteurs d'une dirr~renciation de sols aussi contrastts, allant de podzols très sableux ~

des sols rerrallitiques tr~ argileux, alors que la roche-mère est la m~me?

De nombreuses ~tudes ont ft~ men~ sur ces systèmes au cours des dix dernières annw. Les r~ltats

permettent d'expliquer la stabili~ de la kaolinite dans les horizons su~rieurs des sols, donnent unehypothèse explicative de la gen~e et du ronctionnement de ces syst~es, et montrent "importance desinteractions milieu VÎVilnt • milieu minbal en milieu rorestier ~uatorial.

LE SYSTEME SOLS FERRALLrrlQUES •PODZOLS

La r~gion ~tudi~e est situ~e ~ une cinquantaine de kilomètres au nord de la ville de Manaus. Le climatactuel est du type Amazonien avec une pluviosit~ annuelle de l'ordre de 2100 mm et une saison s!chepeu marqu~. La ~gion est consti~ de plateaux supportant des sols rerrallitiques, et de versantssupportant des sols devenant progressivement ptus sableux vers t'aval. L'ensemble est~e1~ sur un~iment continental d'age Cr~tae~ (Putzer, 1984), form~ d'une alternance destrates sub-horizontalesde ma~riaux argilo-sableux ~ sableux constitu~ de quartz, de kaolinite et d'une petite quantit~

d'h~atite et d'anatase.

Les sols ferrallitiques de plateaux sont des sols tr~ argileux, con~ dans les horizons du m!tresup~rieur par environ 82% de kaolinite, 8% de gibbsite, 5% de quartz, 3% de goethite et 2%d'anatase. Ces sols ont une vingtaine de mètres de profondeur. Vers le milieu du profil, entre 5 et 12mètres de profondeur, on observe des horizons plus gibbsitiques et plus ferrugineux. La gibbsite estpr~te de manière diffuse dans les plasmas argileux, ainsi que dans une phase indur~, le plussouvent des nodules.. Localement, la phase gibbsitique indur~e peut ~tre tr~ abondante, allant jusqu'~

la formation de bauxites.

Les sols des versants deviennent très progressivement ptus sableux quand on descend vers l'aval. Al'aval des versants les plus longs, les sols sont uniquement constituts de sable quartzeux r6iduel: ils'agit alors de podzols g~ants. Les horizons ~Iuvi~ peuvent atteindre plus de 10 m~tres de profondeur.

Ces formations forment un système de sol appel~ système sols ferrallitiques-podzols, repr~entatif

d'importantes surfaces en Amazonie centrale.

LES ÉTUDES MENÉES

Etudes pétrographiques et cristallochimiques

Le système de sol a ~~ ~tudi~ de manière tr~ d~taill~ à l'<<helle des unit~ de relief(Lucas et al., 1984,1986, 1987: Chauvel. 1986: Lucas, 1989). Ces ~tudes ont montr~ que l'ensemble des sols sontdtveloppés in situ. ~ partir de la m~me roche-mère, et qu'il s'agit d'un système de transformation, dans 64

lequel les podzols de d~eloppent progressivement aux d~pends des sols rerrallitiques, au fur et ~

mesure du recul des versants. Parmi les nombreux r~ultats, on retiendra plus particulièrement lesr~ultats suivants.

L'~tude cristallochimique des kaolinites des sols de plateaux montrent leurs caract~ristiques (taille,rorme, cristallini~, substitution en rer) varient très progressivement de la base vers le haut des profils.La spectrom~trie infra-rouge (Lucas et al., 1986) montre par exemple une disparition très progressive dupic ~ 3670 cm-!. Ces r~ultats confirment l'absence de discontinuit~ d'ordre s~dimentaire dans leprofil: celui-ci est bien form~ in situ. Par ailleurs, ils montrent que les kaolinites sont dirr~rentes ~

chaque niveau du profil: elle ne peuvent donc pas être consid~r~es comme des reliques de processusplus profonds, amen~ plus près de la surface au cours de l'enfoncement g~ochimique du profil. Aucontraire. il faut consid~rer qu'au fur et ~ mesure de l'enfoncement g~ochimique du profil, il y aconstante dissolution-recristallisation des lcaolinites, aboutissant ~ des min~raux caract~ristiques dechaque niveau.ntude de la composition isotopique de ces kaolinites (Giral et al.. 1992) montre unecomposition isotopique variable sur l'ensemble du profil. mais toujours en ~quilibre avec la compositiondes eaux m~t~riques actuelles. Ceci confirme qu'~ chaque niveau, les minmux sont soumis ~ desph~nomènes de dissolution-pr~cipitation en relation avec les conditions de milieu. Cela signifie~galement soit une stabili~ du climat pendant le temps de formation des profils, soit un r~quilibrage

rapide des lcaolinites avec les conditions bioclimatiques actuelles.

L'ensemble de ces conclusions pose de manière p~cise la problème de la stabili~ des lcaolinites dans leshorizons de surface: pourquoi les kaolinites y apparaissent-elles stables. alors que les mod~isations

bas!es sur les stabilit!s thermodynamiques des min~raux laissent attendre de la gJbbsite?

Etudes de 10 dynamique de l'eau

La circulation de l'eau est essentiellement verticale dans les sols de plateau. A la base des versant, ilexiste ~galement une composante la~le, une partie des solutions percolantes est expo~ vers l'avalpar des circulations la~rales se situant vers 1 ~ 3 mètres de profondeur.

Dans les sols ferrallitiques de plateau, la dynamique de l'eau est entièrement ~gul~e par la v~g~tation

dans le mètre supbieur des profils (Grimaldi et al., 1989). Les deux tiers de l'eau de pluie arrivant ensommet de profil sont absorMs par les racines et ~vapotranspir~s. Au del~ de 50 ~ 60 cm deprofondeur, l'eau percole verticalement, lentement et r~gulièrement Des traçages isotopiques (Rozanskiet al., 1991) montrent que la vitesse moyenne d'infiltration de l'eau est de l'ordre de 2 mètres par ans.Ces r~ltats permettent d'karter deux hypothèse avanc~ pour expliquer la stabili~ de la gibbsitedans les horizons de surface: l'une (Kronberg et al., 1982) est que l'eau percolerait très rapidement ~

travers les horizons de surface, sans atteindre l'~quilibre avec la kaolinite, l'autre (Tardy et Novikoff,1985) est que les variations d'activit~ de l'eau dans les horizons de surface seraient suffisammentimportantes pour expliquer les variations minéralogiques observées.

Etudes du cycle des éléments

Le cycle des ~I~ments a ~~ approch~ par l'~tude des solutions percolantes dans le premier mètre du sol.pr~le~es par des Iysimètres. par l'~tude des eaux de sources, repr~ntative des solutions sortant dessystèmes, et par l'~tude du recyclage des ~I~ents par la litière rorestière.

Rôle de 10 litière

De pièges install~s sous forêt (Luizao, 1989) ont permis le prélèvement de la litière qui arrive 65annuellement en sommet de profil. Celle-ci comporte des ~I~ments dits nutritirs (Ca, Mg, K, P...) dont

le cycle est relativement bien connu en for~t ~quatoriale. elle comporte ~galement des ~I~ments

constitutifs des min~raux secondaires des sols ferrallitiques: Si. AI. Fe. Ti. Ces ~I~ments. dont lesconcentrations r~glent les interactions min~raux-solutions, sont donc injectés dans la solution du sol ensommet de profil par la minéralisation de la liti6e li~ ~ l'activi~ microbiologique. Le bilan r~alis~ surles sols ferrallitiques de plateau montre que la quantit~ de silicium recycl~ par la litiùe (environ 40kg/ha/an) est tr~ importante par rapport aux quantités expo~es annuellement dans les rivi~res quidrainent la zone ~tudi~e (Lucas et al., 1993) (environ Il kg/ha/an). Cela signifie que la forêt permet lemaintien d'un stock de silicium en sommet de profil. Si l'on dissous la quantité de Si et AI apportéannuellement par la liti~re par l'eau percolant annuellement ~ travers le profil, on obtient une solution ~

l'équilibre avec la kaolinite au pH entre 5 et 6. La conclusion est que la kaolinite, dans les sols deplateau, correpond ~ un état stationnaire entretenu par la vég~tation.

L'~tude géochimique des solutions qui percolent les premiers centim~tres. actuellement r~alisée par S.Cornu et C. Grimaldi, apporte des pr~cisions sur ce processus: la teneur totale ne Si et AI de la solutionen sommet de profil est en majeure partie contrOl~ par les apports provenant de la liti~re. et le pH est

contrOlé par l'iiCtivité microbiologique. en particulier l'iiCtivi~ nitrifiante et sulfatante. Ces param~tres

déterminent la position des solutions dans le diagrimme d'~uilibre thermodynamique Si-AI.

Exportation des éléments

Les solutions qui drainent le syst!me ont é~ prélevées au niveau des source, ~ l'issu de petits bassinsversant constitu~ exclusivement soit de sols femllitiques, soit de podzols. les fractions colloïdales ont~~ séparées par ultrafiltration tangentielle, et chaque fraction a ~~ analysée et son pouvoir complexantpar rapport à la mati~re organique déterminé par polarographie (Eyrolles. 1994). Ceci permet de séparerphase dissoute( < SkDalton), phase colloïdale et phase particulaire (> O.4SJ.UTl). Les r~ltats montrentque l'eau provenant des sols ferrallitiques a les concentrations les plus basses en mati~re organique, Feet AI, et les concentrations les plus ~Ievées en Si. La plupart des ~lérnents sont transpo~ dans laphase dissoute, sauf 10 de Si dans la phase particulaire, probablement sous forme de quartz, et 50% deAI dans la phase colloïdale, probablement sous forme d'hydroxyde. L'eau qui provient des podzols a desteneurs tr~ élevées en matim organique, Fe, AI et de faibles teneurs en Si. Le fer est transporté sousforme de complexes organo-métalliques dans la phase colloïdale. la fraction particulaire est importante.composée probablement de quartz, gibbsite, kaolinite.

CONCLUSIONS

Le fonctionnement général des syst!mes peut ~tre décrit de la mani~re suivante.

L'activi~ biologique dans les horizons supérieur du sol minéralise la liti~re et produit des compo~organiques agressifs dissolvant localement les minéraux pr~ents. Si, AI et Fe sont ainsi relach~ dans lasolution du sol.

Dans les sols ferrallitiques. cette solution migre lentement en profondeur. Le pH, tr~ variable dans leshorizons organiques de surface, est tamponn~, et la solution percolante est à l'équilibre avec lakaolinite. En profondeur, le prél~vement racinaire diminue progressivement la teneur en Si. produisantla précipitation de gibbsite, puis la teneur en Si augmente à nouveau sous l'effet de la lente dissolutiondes quartz. Les mati~res organiques complexantes restent bloquées par les surfaces argileuses ensommet de profil, et sont conden~es ou minéralistes. Le bilan est que la majeure partie de AI et Fe. etune partie de Si restent immobilis~ dans le profil sous forme de kaolinite. gibbsite et goethite. Les eauxde source sont ainsi pauvres en mati~re organique, Fe, AI et riches en Si. A chaque niveau du profil. ily a équilibre entre les solutions percolantes et les min~raux néoform~, "ensemble correspondant ~ unétat stationnaire.

66

Dans les podzols. la mati~re organique produite dans les horizons de surfaces peut migrer rapidementen raison de la haute perm~abilit~ et des faibles surfaces d'~change des horizons sableux. Lacomplexation du fer par la mati~re organique favorise la dissolution de la goethite, ce qui d~t1bilise lesagr~gats argileux et favorise leur dissolution. Ces processus agissent aux fronti~res des horizons sableux.permettant leur progression latérale. Les solutions sont exporttes rapidement hors du syst~me par lescirculations lat~rales. avant immobilisation de AI et Fe sous forme de min~raux n~oform~, ce quiexplique les teneurs ~Ievées en AI et Fe. Les temps de r~idence de l'eau sont faibles. ce qui explique lesfaibles teneurs en Si.

L'ensemble de ces r~ultlts montre que la plupart des processus p~ologiques qui agissent en milieu~quatorial ont une forte composante biologique. qui n'~tait pas toujours prise en compte dans lesmodèles et les thwries explicatives. Le recyclage v~g~t11 contrOle les flux d'~I~ments. en particulier deSi. donc la composition chimique et minmle des sols. Le type de matiùe organique d~termine parailleurs la structure du sol et sa stabilité. dont d~nd la dynamique de l'eau. Les transferts d'~I~ments

hors des syst~mes de sol et la stabilit~ des min~raux argileux d~pendent en grande partie de laformation et de la migration de complexes organo-m~talliques, donc des conditions d'activit~

microbiologique qui leur ont donn~ naissance.

Un grands nombres des ~tudes dkrite5 ont ~té supportées par des financements ORSTOM - TOA UR12 ET PEGI.

RÉFÉRENUS BIBUOGRAPHIQUES

CHAUVEL, A.• Y. LUCAS. AND R. BOULET. 1986. On the genesis of the soil mantle in the region ofManaus. Central Amazonia. Experientia 43, 3:285-299.

EYROLLE F.• 1994.La fraction colloïdale organique dans les processus de transport des m~t1ux dans leseaux de surface: application aux sy~mes d'a1tmtion en milieu tropical (Br~iQ. th~e de l'Universitéd'Aix-Marseilie III. 334P.

EYROLLE F.• FEVRIER D. and BENAIM j.Y.• 1993. Etude par DPASV de l'aptitude de la mati~re

organique colloïdale à fIXer et ~ transporter les m~t1ux: exemples de bassins versants en zone tropicale.Environmental Technology. 14, 701-717.

GIRAL. S., NAHON, O.• GIRARD. j.P.. (,. SAVIN, S. 1992. Variation in 180/160 ratios of kaolinite5within a lateritic profile: their significance for laterite genesis and isotopes paleoclimatology. GSAAbstract. 24. 7, MO

GRIMALDI. M.• SARRAZIN. M.. NUNES. N.• (,. CHAUVEL. A.. 1989. Caracterizao hydriea eporosim~triea de um latossel da Amazonia sob floresta e cultivo. Comm. XXII Congo Bras. Ci. Solo. 23­31/07/89. Recife. (un pub)

KRONBERG. B. 1.. W. S. FYFE. B. j. MCKJNNON, j. F. COUSTON. F. B. STILIANIDI. AND R. A.NASH. 1982. Model for Bauxite formation: Paragominas (BraziQ. Chemieal Geology 35:311-320.

LUCAS Y.. 1989. Syst~mes p~dologjques en Amazonie br~silienne. Equilibres. déséquilibres ettransformations. Thesis n·21 1.

LUCAS. Y.• A. CHAUVEL. R. BOULET. G. RANZANI. AND F. SCATOLINI. 1984. Transiçâo latossolos­podzois sobre a formaçâo Barreias na regiâo de Manaus. AmazOnia. Revista Brasileira de Ciencia deSolo 8:325-335.

LUCAS. Y.. A. CHAUVEL. AND J. P. AMBROS!. 1986. ·Processes of aluminium and iron accumulationin Latosols developed on quartz rich sediments from Central Amazonia (Manaus. Brazil).· in

67

Geochemistry and minerai formation in the earth surface: proceedings of the International MeetingGeochemistry of the Earth Surface and Process of Mineral Formation held in Granada. Spain, 16·22march 1986. R. Rodriguez-Oemente and Y. Tardy.pp. 289-299. CSIC (Madrid) - CNRS (Paris):

LUCAS. Y.. R. BOULET. AND L. VEILLON. 1987. ·Systèmes sols ferrallitiques • podzols en r~gion

amazonienne.· in Podzols et Podzolisation. D. Righi and A. Chauvel eds .pp. 53-65. AFES (Plaisir) ­INRA (Paris).

LUCAS. Y.• F. J. LUIZAO. A. CHAUVEL. J. ROUILLER. AND D. NAHON. 1993. The relation betweenbiological activity of the rainforest and minerai composition of the soils. Science 260:521-523.

LUiZAO. F. j. 1989. Litter production and minerai element input to the forest floor in a centralamazonian forest GeoJournal 19.4:407-417.

PUlZER, H.• 1984. Monographiae Biologicae. 56. p. 15-46.

ROZANSKI. 1<.. L ARAGUAS-ARAGUAS. A. PLATA BEDMAR. W. FRANKEN. A. C. TANCREDI. ANDA.. TUNDIS VITAL. 1991. International Symposium on the use of Stable Isotopes in Plant Nutrition.Soil Fertility and Environmental SbJdies. IAEA-SM-313. (UnPub).

TARDY. Y. AND A. NOVIKOFF. 1988. Activit! de l'eau et cUplacement des ~quilibres gibbsite-kaolinitedans les profils lat!ritiques. Comptes-Rendus de l'Académie de Sciences. Paris 306. Il:39-44.

68

sUlTllTler schoolécole d'été

1J~~mwU~~~

ttw etuUfO' BaM,n RÛWl/.L.c~~~~~~

Jérôme GAILLARDE~ Bernard DUPRÉand Claude J. ALLÈGRE

Laboratoire de Géochimie et Cosmochimie,URA CNRS 1758Institut de Physique du Globe, Université de Poris 1, Paris

To be published in/à paraître dans: Geochimica et Cosmachimica Ma 69

ABS'rRAa

Rivers carry the products of continental denudation either in a dissolved form (chemical erosion). or ina solid form (physical erosion). We focus in this paper on the relationship between physical erosionand chemical erosion. We establish the mass budget of the Congo Basin Rivers using chemiealcomplementarities between river suspended sediments. sandy bedload and dissolved load of the CongoBasin Rivers reported in a previous paper (Dupré et al.. 1994). A series of equations are presented.assuming that the physical and chemical erosion processes are in a steady state during one year. Thetotal mass of river-borne material (dissolved and particulate) transporte<! in the river over a given periodof time should balance the mass of upper continental crust eroded during this time.

We show that the local continental crust on each drainage basin cOIn be estimated and solve oursteady state weathering mode! using an inversion procedure. The very good agreement betweenmodelled and measured values of the river suspended sediment concentrations validates the steadystate hypothesis in this wet tropical area. Consequently. in this area. the sediment yie!d provide a goodestimate of the rates of mechanical denudation. This result also validates the calculation or thechemical and isotopie composition or the local continental upper crust using the bulk river load.Erosion rates for the silicate upper crust and thus independent or the lithological variability (silicates.evaporites and carbonates) of the drainage basins are calculated. Mechanical erosion rates and chemicalerosion rates for the Congo Basin at Brazzaville are 8 t1bn2/yr and S tIbn2/yr. The correspondingconsumption of atrnospheric C02 by weathering process is estimated te Six 103 mol/km2/an. Theseweathering and consumption rates are Iow in spite or the severity of the weathering conditions. of thehigh soir temperature and of the inœnsity of precipitations. These conclusions indicate the Iimitinginfluence the dynamic equilibrium of soils for silicate weathering. Finally. by estimating the localcontinental crust chemical composition before the onset of erosion processes. especially for the mostsoluble elements. we cOIn test the mode! ofTaylor and Mclennan.

INTRODurnON

The destruction of the continents by rainwater and transport by rivers is one of the main geologicalprocess reworking the surface of the hrth. large rivers integrate the chemical and isotopic diversity ofthe continental crust allowing us te assess the weathering proœsses on a large SCOlie.

Water attacks the continents through two major and competitive processes. First, the chemicalweathering partially dissolves continental rocks te form the soil. Second. rocks and soils are physicallybroken up (mechanical erosion). The produets of both chemical and mechanical denudation of rocksare transported from the continents to oceans by rivers. These principles allow the evaluation ofmechanical rates of denudation using measured stream sediment yie!ds (Milliman and Meade. 1983:Pinet and Souriau. 1988: Milliman and Syvitski. 1992). In geochemistry. previous studies have used themeasurements of modem sediment Ioad to calculate denudation fluxes (Martin and Meybeck. 1979) orchemical and isotopie analyses of the bulk river load (dissolved + particulate) to characterise thesource rocks in the drainage basin (e.g. Goldstein and Jacobsen. \987 or Négrel et al.. 1993).

Ali these studies depends on the tacit assumption or equilibrium between river material production(erosion) and exportation out or the drainage area. The impoftmce or this equilibrium. called ·streamequilibrium" by Trimble (1977. 1983) and ·steady state" by Martin and Meybeck (1979) has beenemphasised by several authors (Ahnert. 1970: Martin and Meybeck. 1979: Trimble. 1983: Milliman andMeade. 1983). Sediments transported by rivers as suspended or bottom particles mOlY not represent therates of soil erosion in the drainage basin because they are Iikely te be stored in soils. in large rivervalleys or in natural or artificial reservoirs. Consequently. the chemical and isotopic composition orcontinental crust for the elements fractionated by the erosion processes (e.g. Sr) depends critically onthe hypothesis of the steady state.

70

Prior to any study of continental crust composition or erosion processes inferred from rivers. thebalance between mechanical and chemical erosion must be established. This paper is an attempt to testthis concept of stream equilibrium. We propose a geochemical mm budget method. We show thatthe resolution of a simple mass balance equation determines the amount of suspended sedimentsrequired ta account for the chemical river load derived from weathering of silicates. After correcting therecycling of sedimentary rocks present in the Congo Basin using the results of Négrel et al. (1993). weconclude that the steady state hypothesis is valid for the Congo rivers. As a consequence. erosion ratesand C02 consumption can be estimated. Factors controlling the chemical weathering as weil as theway physical and chemical erosion are linked together. are of primary importance in particular forunderstanding the biogeochemical cycle of C02 and predicting its variations with time. Here. themechanical and chemical processes of erosion combine to control weathering and thus theconsumption of C02. Over the Congo Basin. mechanical break-up. controlled mainly by relief. andthus tectonics. strongly influences weathering rates.

Finally. we propose a composition of the local continental crust for the elements fractionated byerosion processes into dissolved and suspended phases.

SruDY AlEA AND PREVIOUS WORK

ln a previous paper. Oupr! et al. (1994). hereafter referred to as paper 1. we have investigated thechemical composition of the river borne material carried by the Congo Rivers and some of itstnbutaries. The Congo (also calle<! Congo-Zaire) is the second largest river in the world by waterdischarge and surface area. It runs 4700 km in Iength from the East African lakes ta the AtlanticOcean. draining one of the world's largest tropical forest Basic geomorphological and geologicalinformations about the drainage basin can be found in NlcounJcou et Probst (I987): Négrel et al. (1993)and in paper 1. The vital statistics of each sample and their location are listed in Table 1 and Fig. 1. TheAlima. ükouala and Sangha rivers f10w entirely in the tropical forest ecosysœm and exhibit the c1assicalfeatures of the so-called ·Black Rivers·: low pH. high dissolved and particulate organic matterconcentrations. low total inorganic suspended and dissolved loads (see Berner and Berner. 1987. pp221). The main result, as seen for the example of the Zaire river in Fig. 2. of the trace and majorelement systematics presented in paper 1is that chemical frictionations relative to the mean continentalcrust occur during erosion and (or) transport processes between the three main river-borne phases:dissolved (< 0.2 mm). suspended (> 0.2 mm). and bec! load:

• The elements Cs. Th. La. Ce. Ta. Nd. Sm. Eu. Tb. Yb. Sc. Fe. Co. Cr. Ni are uniformly enriched inthe suspended load relative to the mean continental aust of Taylar and McLennan (1985). except forthe Likouala and Alima rivers. two typical Black Rivers. The large particulate organic matterconcentrations of these rivers cause a dilution effect of inorganic material.

• The suspended load is depleted of U. Rb. Ba. K. Na. Sr and Ca relative to the continental oust whilethe dissolved load is enriched relative to the continental crust in each Congo Basin River. Thisobservation supports that these two reservoirs are complementary for the more soluble elements. Thedepletion of soluble elements in the suspended laad is smaller in the Black Rivers than in the non­organic dominated rivers.

• A depletion of Zr and Hf in suspended sediments correlates with an enrichment in the sandy bedloadsuggesting the same kind of complementarity for Hf and Zr between these two phases.

While the differences between dissolved and suspended phases for the soluble e1ements are likely to berelated to differential solubilities during the chemical weathering of the source rock. the differencesbetween bottom and suspended sediments fither reflect a sorting by gfiin size of mechanical erosionproducts during transport by the river. Indeed. heavy minerais like zircon (plus tourmaline. rutile.staurotide) and quartz minerais are likely to be separated from the other less heavy erosion products(e.g. kaolinite. illite) during transport.

71

THE STEADY STATE MODEl Of WEATHERING

Basic assumptians

We base our model on a number of principles which are described below.

• Chemical weathering acts in the transition zone between the soil layer and the pristine bedrock (theweathering front) to form weathering products. either in solution or as secondary minerais. Thesenewly formed minerais accumulate in the soils. together with the most stable primary minerais (such asheavy minerais). Conversely. mechanical erosion removes material from the soil. In this sense. chemicaland mechanical erosion are complementary processes for the production and destruction of soils.

• The mode! states that the dissofved load of rivers (corrected for atmospheric inputs) reRects theweathering process occurring at the weathering front We thus neglect water storage in soils or duringnuvial transport and any secondary chemical reactions such as interactions between suspended anddissolved load during transport (adsorption processes) or in-situ minerai precipitations. This laterassumption is justirted by the low pH values of the Congo basin rivers (Dup~ et al.. 1994).

• The model assumes the steady state of soils. During a given time interval. the amount of solidmaterial created by chemical erosion of the rocks of the drainage basin balances the amount exportedby mechanieal erosion to the river. It means that no storage of suspended (and sandy) material occurswithin the soils (of constant thickness) or within the hydrographie network. i.e. adynamie equilibriumexists between storage and remobilisation.

• The model applies on a time scale of one year of one year to span the seasonal cycle. During theseasonal cycle. variations of the dissolved concentrations occur for Ca. Na and to a lesser extend Sr. asexemplified at Bangui by Nkounlcou (1987). N~grel (1991). Dissolved concentrations decrease withincreasing discharge. and are weil described by a logarithmic function. Similar observations have beenreported for other rivers (Walling and Webb. 1983. Froelich. 1983). ConverseJy. K. Rb and Si seem taremain at constant concentrations over the hydrological cycle. The use of Iogarithmic laws ta calculatemean integrated annuai dissolved concentrations show that the concentrations measured during theNovember 1989 sampling cruise are representative of the annual concentrations. For the suspendedconcentrations. the variability over the sea.sonal cycle will be neglected because the mineralogy of thesuspended phase does not vary through the annual cycle occur (Martin and Meybeck. 1979 andreferences within).

Moss balance equations and corrections from the recyding of sedimentary rocks

With the basic principles described in the previous section. the complementarities reported in paper 1(Fig. 2) can be used to constrain a mass budget model. For any element X:

where

McCc(X) - MsCs(X) + MpCp(X) + MwCw(X) (1)

Mc is the annual mass of weathered original continental rocks on the drainage basin in units of massper unit of time (e.g. t1yr)

Mw is the annual water discharge at the sampling location point in unit of volume per unit of time(e.g. I/yr)

Mp is the annual mass of suspended sediments carried by the river in unit of mass per unit of time(e.g. t1yr)

72

Ms is the annual miSS of sands from the weathering of silicate rocks in units of mass per unit time(e.g. t/yr).

Cc(X) is the concentration of e!ement X in the continental crust (e.g. in ppm - g/t). Cp(X) and Cs(X)are the mean annual concentrations of the element X in the suspended and bottom loads in ppm andCw(X) is the concentration of the element X in the dissolved load in mgl!.

ln erosion processes. a distinction exist between rocks or minerais that dissolve entirely (congruentdissolution) and rocks that furnish bath dissolved and particulate materials by incongruent dissolution(Garrels and MacKenzie. 1971). We will consider that only the aluminosilicate rocks undergoincongruent dissolution and furnish solid materials ta the river. Conversely. carbonate and salty rocksdissolve completely. Recently. N~re! et al. (1993) have showed that the dissolved load of each CongoBasin river could be modelled by the multimixing of rain and the weathering products of threereservoirs (silicates. carbonates and evaporites). They determined the contribution in Na. Mg. Sr. Ca ofeach rock type ta the river dissolved Iaad. Using these proportions and their 3SSOciated errors we cancorrect the dissolve<! concentrations measured in the river Cw(X) from the recycling of carbonates andsalty rocks. Thus calculated. the dissolved concentrations (Cwsiij represent only the weathering of thesilicated part of the local crust. For each tributary. these proportions are given and are applied te correctthe bulk dissolved concentrations in Table 2. Only Sr. Na and Ca can be corrected in this manner andsorne difficulties arise for correcting Rb. Ba. U and Kdissolved concentrations. It is obvious that theseelements need to be corrected for inputs other than the weathering of silicates (Palmer and Edmond.1993 for U: Crozat 1979 for 1<). This problem will be discussed later with the results of the mode!. Asfor the suspended load concentrations. they do not need ta be corrected for non-silicate inputsbecause: (1) only the silicated rodes fumish particles ta the river via the erosion processes and (2)suspended sediments do not contain any calcium carbonate particles for the Congo Basin acidic watersare c1early undersaturated in calcium carbonate.

The miSS budget equations can then be written:

MsilCsil(X) - MsCs(X) + MpCp(X) + MwCwsil(X) (2)

with Cwsil(X) the concentration of X in the river dissolved phase due to the weathering of silicates.Dividing by Mw. we obtain

'tsilCsil (X) _ SCs(X) + PCp(X) + Cwsil(X) (3)

Mpwhere P - Mw is the essential parameter of the mode!. It denotes the suspended sedmentsconcentration predicted by the mode!. Comparison with measured values (SM, Table 1) will test thehypothesis of the steady state.

MsilThe ratio tsil - Mw is the total silicated denudation flux. It represents the mass of silicated crustchemically and mechanically annually eroded per litre of water.

Practically. because the presence of the Particulate Organic Matter on the filters causes a dilution effectwhich biases absolute suspended concentrations (see paper 1). only elemental ratios will be used. Aliconcentrations are normalised to Sr. Equation (3) becomes:

(~J =(~J a+(~) ~+(~J (l-a-~)Sr c Sr w Sr s Sr p(4)

where the subscripts c. w. s. p refer to the continental crust. the dissolved. sandy and suspended 73

phases respectively. By construction :

(P Cp(Sr)]

(l-Cl-~)= -.--tsil Cc(Sr) (

s Cs(Sr)]

(5) ~ = tSil' Cc(Sr) (Cw(Sr) ]

(6) Cl = t sil Cc(Sr) (7)

After determining the constants a and b. we solve these equations for the mode! parameters tsil. P andS.

Simplifications

Exportation fluxes calculated a priori using the suspended sediment concentrations measured during theNovember 1989 cruise and a bedload concentl'ition of 10% of the suspended load have beencalculated in paper 1for each element It tums out that equation (4) can he simplified depending onthe e!ements.

- if X is one of the most soluble e!ements Rb. U. Ba. K. Na. Ca. Sr : the dissolved and suspended phasecontnbutions must he bath taken inta account The sandy flux is negligJble. In this case. equation (4)becomes:

(~a=(:ctlX+(:C]p(l-a)(8)

This equation is similar ta the equation proposed for the Ge/Si l'itio in the Orinoco river by Mumaneand Stallard (1990) in which Ge and Si are partitioned between dissolved and suspended phases.

• if X is one of the elements Cs. Th. La. Ce. Ta. Nd. Sm. Eu. Tb. Yb. Sc. Fe. Co. Cr. Ni. only thesuspended phase flux is significant because the dissolved part of these elements is due ta colloidspassing through the filters as emphasised in paper 1. The sandy phase contribution ta the budget ofthese e!ements is also very small. For these -insoluble elements- elements. the mass budget equationbecomes:

(~) =(~) (l-a)Sr c Sr p

(9)

• For Hf and Zr: only sands and suspended sediments contribute ta for the mass budget

(~) =(~) ~+(~) (l-a-~)Sr c Sr s Sr p

(10)

Sandy phases complicate the calculation because of their highly variable concentrations. For Hf and Zr.the fractionations relative to the continental crust are due ta sorting of erosion products duringtransport. deposition and creeping in the rivers. Depending on local hydrodynamic conditions.winnowing and sorting effects will produce either an enrichment or a depletion of quartz relative taheavy minerais. These features preclude the use of the elements Hf or Zr in the model. limiting its useto soluble and insoluble elements undisturbed by post-weathering grain-size sorting. We mustthererore abandon the idea or determining the amount of sands carried on the river bottom. S. usingthe complementarity of hedloads and suspended loads for Zr and Hf. In the following. we will supposethat S represent 10 % of the suspended load. for reasons discussed in paper 1.

At this stage. the weathering mode! of includes for each sampie six equations ror the soluble elements74

and 15 equations for the insoluble elements. that constitute the mode! constraints. Their resolution isexpected ta give P. These equations are listed Table 3. The physical interpretation of a is extremely

[Cw(Sr) ]a=

simple: t silCc(Sr) _ a means no dissolve<! products (i.e. no chemical erosion of freshrocks). while a - 1 means that the totality of the rock is dissolve<! by chemical erosion (weathering)processes. Of course. natural processes are expected ta result from a combination of these two extremesituations. Note that our definition of 1- a is similar ta the ·chemical-weathering intensity. W· ofMumane and Stallard (1990) except that W refers to Si (while a refers to Sr) which is an elementsignificantly transported by sands and possibly perturbed by biogenic uptake.

The aim of the following sections is ta solve this system of equations by determining a .

SOlVING THE MODEl EQUATIONS Bl AN INVERSE METHOD

ln the following. this set of equations is soIved using the inverse theory as introduced in geochemistryby AlI~gre et al. (1983). In the formalism used. deduced from the general approach of Tarantola andValette (1982). no distinction is made between unknowns and data. Ali are parameters for which wehave more or less information. For each parameter. an a priori Vilue and an a priori error must beadopted. The magnitude of this enor indicates our knowIedge of the parameter. In the present case. ais the most unknown parameter. AIl we know about its a priori value is that a - 0.5 ± O.S. The crustalratios are poorIy known. but rough a priori estimates can be found. The best known parameters are thesuspended and dissolved ratios because they are calculated from measurements. Solving the set ofmodel equations by an inverse problem technique consists of finding the best set of Vilues for theparameters among those that satisfy the equations. The best-fitting solution is calle<! the a posteriori setof parameters and indudes estimates of their enors. Technically. an optimisation program computesthe a posteriori set of values and propagates the errors. The choice of the a priori values and a priorierrors is of aitical importance. The selected a priori values are listed in Table 4 with their errors andrepresented graphically in Fig. 3. This choice is now discussed in detaiJ.

Apriori river moterial chemical ratios: Partilioning of elements during weathering

The comparison of the chernical ratios in soIid and dissolved Ioads of each river is shown in Fig. 3. Forthe suspended load. the relative uncertainties are the analytical ones. say la % for ail X/Sr ratios. Forthe dissolved load. the Ca. Na and Sr concentrations due ta the dissolution of silicates and associatederrors from Négrel et al. (1993) (displayed in Table 2) provide estimates of (Na/Sr)w and (Ca/Sr)wratios with relative uncertainties of ± 20 and 80 %. respectively. For U. Rb. Ba and K. the differentcontributing sources are unknown. Nonetheless. we assign an uncertainty of 20 % to the ratios(UlSr)w. (Rb/Sr)w. (Ba/Sr)w and (K/Sr)w ratios even though silicate rocks are probably not the onlyreservoir that contribute ta their presence in the soluble phase in nvers. Palmer and Edmond (1993)emphasised the importance of carbonated inputs for dissolve<! U. In addition. a fraction of U may occurin a colloidal form. especially in the Black Rivers (paper 1). Rb. Ba and K. are probably involve<! thebiological turnover reservoir (Crozat 1979).

To test the influence of the a priori uncertainties assigne<! ta the a priori ratios of Rb. Ba and K to Sr.we performed severa1sensivity test Increasing the a priori errors from 20 % ta 60 % increases bypropagation the a posteriori errors of Rb/Sr. Ba/Sr and K/Sr ratios. The value and uncertainty of a arenot arrected.

Fig. 3 allow the partitioning of the different soluble e1ements between solids and dissolved phases ta beexamined. 75

Two typical Black Rivers (paper 1). the Likouala and Alima rivers exhibit U. Ba and Ca Sr-normalizedratios systematically lower relative te that of the other rivers (Fig. 3). It seems that the concentration ofSr in the suspended load of these Black Rivers are higher than those in the non organic-oominatedrivers. This increase may reflect the influence of organic complexes of Sr in the suspended phase.Moreover the Sr-normalized ratios in the dissolved phase are roughly in the range of variations of theother rivers. The exceptions are Rb. Ba and K (three elements involved in biological turnover) whichexhibit higher dissolved ratios. possibly due te the innuence of vegetation for these rivers nowing inthe tropical forest These specifie features indicate that the chemistry of the Likouala and Alima rivers isc1early biased by organic matter. We therefore exclude these two rivers from the inversion.

For the other rivers. the general trend is the enrichment of ail soluble and insoluble elements except Nain the suspended phase relative te the dissolved phase. In other words. Na is the only element moresoluble than Sr under the wet tropical conditions of the Congo basin rivers. The comparison of theseSr-normalized ratios with other soluble element ratios reveals the mobility sequence of the differentsoluble elements during silicate weathering. The decreasing order is Na > Sr > Rb > Ba > Ca - U.

Apriori crusfol rotios

ln companion papees (Ntgrel et al.. 1993 and AII~gre et al.. 1995). we have shawn that the 87Sr/86Sr.206Pb/204Pb. 207Pb/204Pb and 208Pb/204Pb isotopÎC ratios of the suspended material of each riversample an be used te ca\culate the Rb/Sr. LIIPb and Th/U ratios of the rocks of their drainage basin.The so-ca\cuJated chemiaJ ratios are close one te each other for the different drainage basins and dosete the Taylor and McLennan estimate. We extend here this result te the other soluble elements.affected by weathering processes. We graphically estimated. using the primitive mantle normaliseddiagram of each river (such a diagram is given in Fig. 1 for the laire river). the range of concentrationthat the suspended phase would have for any soluble element if no solubilisation during weatheringhad occurred. The so graphically determined abundances are marked with a star on Fig. 4. Moreformally for any Xsoluble e1ement :

tsilCc(X) - PCp(X) + Cw(X) - PCp*(X)

so :

(CP*(X») _( X)Cp*(Sr) - Sr c

Thus approximated. the crustal ratios normalised te Sr. for the soluble elements (Ca. Na. Ba. K. Rb. U)are given in table 4 and compared te the dissolved and suspended endmembers in Fig. 3. For theseelements. the a priori errors on each crustal ratio calculated by this technique are around 50-70%. Weobserve that ail crustal ratios are bracketed by the suspended and dissolved ratios. within the error bars.For the insoluble e1ements. the a priori errors on each crustal ratio calculated by the above techniqueare in the range of 40% for the insoluble elements. Equation (3) show that the ratio of two insolubleelements in the suspended load is equal to their ratio in the original continental crust :

(ihJ =(Jh)c P for any insoluble element Z.

ln other words. insoluble element ratios indicate only the rock source because they are unaffected byweathering processes. Consequently. the insoluble budget equations are not independent and only oneis necessary te constrain the mode!. In the following the ether insoluble element is Th. The inversionwas also performed using two or more insoluble ratios normalised to Sr. The results on the a posterioriparameters were indistinguishable from those obtained using only one insoluble ratio (Th/Sr).

76

RESULTS

Efficiency of the inversion procedure and strength of the model equations

The efficiency of an inversion procedure is evaluated by comparing the size of the a posteriori errorrelative to the a priori error (Allègre and Lewin. 1989). If the a priori error is greater than the aposteriori error. then a gain of precision occurred through the inversion of the data. No gain ofprecision is brought by the inversion procedure for the a priori river water parameters (Table 4) becausethese parameters are already relativeJy weil known and behave like constants in the algorithm. On theother hand. our precision of the erosion parameter a and of the crustal ratios is strongly improvedtypically by a factor of 1O.

The strength of an equation determine how it constrains the a posteriori values relative ta the otherequations. It can be measured relative to the other equations by means of the final distance betweenthe a priori and the a posteriori sets of parameters. When an equation is suppressed. then theconstraints will relax and the minimised distance will decrease. This property is used to quantify thestrength of an equation (in % of the standard final distance). In our case. the strongest equation is theequation using the insoluble element normalised to Sr (e.g. Th/Sr). The other mass budget equations.do not behave as strong constraints simply because none of their crustal mernbers is weil enoughprecisely known. As a result. the value of a for these equations is constrained by the other equationsand the a posteriori parameters are only determined by a simple calculation of errar propagation. Notefinally that an inversion procedure using Na as a normalisation has been performed. The results arequite identical to the results of the inversion runned using Sr. This conclusion is important because itshows that the results of the model resolution do not depends on the choice of the soluble e1ernentused in the normalisation.

Table 4 gives for each parameters the a posteriori values performed by the inversion calculation.

Erosion parameters

The a posteriori values of a for each sample are listed in Table 4 and S. The relative precision of a isabout 10% for ail the samples. The gain in precision is signifiant because the a posteriori error is 10times Iower than the a priori error. The value of a is fairly homogeneous across the different drainageareas with values ranging from 0.83 to 0.92. Values of a. close to 1. show that the intensity ofweathering on this wet tropical basin is very intense.

Although the values of tsil and P could in principle be derived directfy from the definition of a {7} andI-a (5). our poor knowledge of the continental concentration of Sr. Cc(Sr). precludes it. Theelimination of Cc(Sr) leads ta

1-a Cwsi1(Sr)p =--.----.,-:-'---'-

a Cp(Sr) (II)

The predicted values of P (mg/I) deduced from (II) are compared to the measured ones (SM) in Table5a and Fig. 5. These data were obtained during the Novernber 1989 sampling cruise on surface watersamples. one month before the peak discharge and were determined by filtration on 0.45 mm pre­weighed filters (paper 1).

The question of seasonal variations of the concentrations of suspended sediments must be addressed.ln other words. are the values measured in the field representative of mean annual sedimentconcentrations?

This question can be answered at Brazzaville and Bangui because data on water discharge andsuspended sediment concentrations have been collected daily by ORSTOM scientists since 1986

77

(Olivry et al.. 1989). At both locations, suspended sediment concentration (SM) and water discharge(Mw) are roughly correlated even though the plot of SM versus Mw rather form loops than straightlines. These data show at Bangui important variations of SM during the an nuai hydrological cycleranging from 5 mgll during the low water stage te 50 mgfl during the wet season. This tendency is notobserved at Brazzaville where the suspended concentrations remain fairly constant over thehydrological cycle.2. In bath cases, weighed average annual suspended sediment concentrations havebeen calculated (at Bangui 100 measurements available From April 1987 te April 1989, at Brazzaville 90measurements from January 1988 te July 1989). The results are displayed in Table 1 and show that theNovember 1989 cruise measurements are fairly representative of a mean annuai concentration within20% of uncertainty. Note. however, that these average calculations are based on surface watersuspended sediment samples. It has been shown by Dugas and Thiebaux (1989) that the sedimentconcentrations measured on depth-integrated samples are within 10 % of surface water sampledeterminations. We thus conclude that the determinations of suspended sediments for the whole set ofsampie during the cruise (Table 1 and 5) are good estimates of the annual sediment yields.

The comparison between the theoretical and measured sets of suspended sediments concentrations(Table Sa, Fig. 5) shows a very good agreement within mors. The main result of this wor!e is that theaveraged suspended sediments concentrations measured in the Congo Basin rivers are the same as theconcentrations predicted by assuming a steady-state erosion process.

APOSTERIORI CRUSTAL RATIOS

The a posteriori crustal ratios are listed Table 4. They show that our Icnowledge of the Sr-normalizedcrustal ratios is improved by the inversion scheme because precision is gained for these parameters(about a factor 2 between the a priori and posteriori errors). Because the intensity of the weathering ishigh (il close to 1) , it is not surprising that the continental CIUSt ratios are c10ser te the dissolvedendmember than te the suspende<! one. This conclusion is in agreement with the conclusions ofStallard and Edmond (1983) in Amazonian IowIand rivers and Nesbitt et al. (1980) in Australia. whoconclude that the ratios of alkalis and alkali-earths in stream waters in areas of intense chemicalweathering are close ta those of the bedrodc.

SANDS

Although. as stated earlier. the elernents Hf and Zr can IlOt be used in the inversion calculation becauseof their highly variable concentrations in the river bedloads. we can simply examine whether theassumption S - 0.1 P is consistent with the concentrations measured in the bedload. Equation (3)applied ta Zr gives:

MsilCsil(Zr) - MpCp(Zr) + MsCs(Zr).

As for the soluble elements. we cali Cp*(Zr) the Zr concentration that would have the suspendedphase if no depletion for this element (compared to the continental crust) was observed. Thenequation (9) becomes:

Cp(Zr) + S/P.Cs(Zr) - Cp*(Zr).

Using the Zaire sample. Cp*(Zr) can he determined graphically (Fig. 4) and S/P calculated for theextreme values of Cs(Zr). Cs(Zr) nuctuates from 30 te 473 ppm (paper 1). It leads to values of SfPbetween 0.47 to 7. far From the supposed value of SfP - 0.1.

We therefore conclude that the Zr concentrations measured in the sandy phase of the Congo Riversare underestimated. Possible explanations include an important dilution by quartz or the tendency ofzircon te remain in soils or in the upstream areas of the catehments. 78

DISCUSSION

Steady state

The river dissolved and particulate concentrations reported in paper 1 are therefore consistent. withinthe errors inherent in the method. with a steady state model of erosion based on the dynamicequilibrium between mechanical and chemical erosion.

This result implies that weathered material does not accumulate within the basins. at a broad scale. Iffluvial sediments are neither stored nor remobilised by the rivers. or these competing processes are inequilibrium. this result indicates that soils keep a constant thickness. In other words. the amount ofparticles removed from the top of the soif by mechanical erosion is balanced by approximately the sameamount of residual particles created by the penetration of the weathering front into the pristinebedrock. In our large-scale approach. recent man-induced environmental changes such as deforestationor forest fires do not greatfy disturb the equilibrium soils and erosion processes. in this tropical area.This concept of present-day equilibrium between chemical and physical erosion is close to the conceptof weathering-limited regime of denudation descnbed in the lowIand rivers of Amazonia by Stallard andEdmond (1983) or Stallard (1985). In such a regime. the ~te at which weathered material is removedfrom soils is equal to. or slightfy lower than the ~te at which it is produced This allows a thick soillayer to accumulate and ta remain stable. The river chemistries of bath Amazonian Iowlands andCongo Basin are very similar. reflecting the similarities of their erosional regimes and of theirenvironment (tectonie history. topography. c1imate. vegetation).

Because a steady state holds on the Congo Basin rivers. we conclude that the sediment yieldmeasurements provide a good estimate of the Central African continent denudation rates. Thedemonstration of steady state also validates the calculation of a bulk river Ioad using the measuredsuspended concentrations to reconstruct the chemical and isotopic composition of the continentalcrust of each ~inage basin. This is applied in detail for the Congo and Amazon river systems byN~grel et al. (1993) and Allègre et al. (1995).

Erosion rates and fluxes

The total rate of silicate erosion tsil. for each river includes bath chemieal and physical processes in thefollowing mass balance eqUition:

tsil - P + S + TOSsil

where P+S is the mechanical erosion rate derived from the model calculations (as discussed earlier weassume S - 0.1 .P) and TDSsil is the silicate chemical erosion rate. The mechanical. chemical and totalerosion rates are given in Table 5 and shown in Fig. 6. These rates are expressed in mgll and to allowcomparisons between rivers. they are also expressed in t1km2/yr using the vital statistics of Table 1.

The mechanical erosion rates nuctuate from 18 mgll to 43 mgll (6 t1km2/yr to 14 t1km2/yr) for thewhole series of rivers. The higher mechanical erosion rates are observed in the Zaire and Oubanguirivers. Nonetheless. compared to other major river system. the mechanical erosion rates of the Congorivers are low. The world average mechanical specific flux of erosion is 150 t1km2/yr (Berner andBerner. 1987).

The chemical silicate erosion rate. TDSsil. sums the concentrations of Ca. Na. Mg. K and S04 derivedfrom the silicate weathering and Si02 (Table 2). The uptake of Si02 by biogenic opal is neglected.Neither a nor HC03 originate rrom the dissolution of silicate continental rocks whereas Si02 cornesentirely from silicate weathering. We supposed that 33% of the S04 cornes from the weathering ofsilicate minerais (Berner and Berner. 1987). These corrections are not of critical importance because

79

Si02 accounts for at least 75 % of TDSsil (Table 2). Since the proportions of the different rock types inoutcrop are unknown. we can not calculate. in principle. the specific erosion fluxes (in t1kml/an) ofthe different lithologies. Nevertheless. we used the total basin areas to calculate specific fluxes for thesilicates. This approximation is justified by the sparceness of carbonate and evaporite outcrops amongthe basins (Boulvert and Salomon. 1988).

We observe (Table 5a) that TDSsil is remarlcably constant From one river to another (between 5 and 6tlkm2/yr) and Iower than the mechanical erosion rate in each river. Summerfield and Hulton (1994)estimated the silicate chemieal denudation rates of the major world drainage basins by attempting tacorrect the river disso/ved Ioads for the atmospheric and sedimentary recycled components. Our rangeof chemieal denudation is similar to their estimated value for the Congo River at Brazzaville (6t1km2/yr) and among the smallest when compared to chemical denudation rates of other majordrainage basins (from 3 t/krn21yr to 72t/krn21yr ). The world average estimations of silicate weatheringrates of Meybeck (1987) also show that the Congo river weathering rates are low. Bluth and Kump(1994) have reported log-log linear relationships between runorr and HC03- and Si02 fluxes of riversdraining only granitic catchments of variable c1imates. Their empirieal relations predict 4.7 molSi02Ikm2Iyr and 5.0 mol HC03-/km2/yr. For the Congo river at Brazzaville. we estimated. respectively4.7 mol Si021lan2lyr and 4.8 mol HC03- tIIcm2/yr (HC03- generated by silicate weathering). in goodagreement with the predictions of Bluth and Kump (1994).

It thus appear that the Congo drainage basins are characterised by an intense chemieal weathering (asshawn by the values of a of 0.8-0.9) and conversely by Iow chemical weathering rates. close to 5t:Ikm2Iyr.

The total specifIC fluxes of denueJation of the silicated part of the crust tsil (Table 5a) are also expresse<!in mmlkyr using a mean crustal density r of 2500 IcgIm3. The observed values of tsil are fairly constantwithin the 20 % uncertainties calculated by propagation. The silicated erosion rates vary From 30 mgllon the Lobaye and. Sangha catehments to 60 mg/Ion the Zaire drainage basin. Among the rivers. thespecifie denudation of the Congo shield varies From 6.8 tIlcm2/yr (3 mmllcyr) in the Sangha river ta18.4 tJlcm2Jyr (8 mm/kyr) for the zaire. For comparison. the average total denudation rate of thecontinents is 175 tJlcm2Jyr (Berner and Berner. 1987) but this rate is not corrected for the dissolutionof carbonates or evaporites and of atmospheric inputs.

Our calculated silicated erosion rates for the Congo Basin are Iow but in the same range as thosedetermined by Edmond et al. (1995) by measuring the dissolved concentrations of the major elementsand. the suspended sediment concentrations of rivers draining only silicates From the Guyana shield.This agreement is important because the river catchments investigated by Edmond et al. (1995) containonly silicate rodes. while the present values are deduced From data corrected for the dissolution ofsedimentary rocles. Our estimates are also consistent with the rates determined by independentmethods using cosmogenie isotopes (lOBe. 210Pb): la mmlkyr in the Iowlands rivers of Amazonia andin the Congo Basin (Algharib. 1991). 3 to 8 mm/kyr on the West African Craton in Burkina Faso(Brown et al.. 1994).

The existence of a dynamic equilibrium between chemical and physical erosions over each drainageBasin gives a main importance to the ratio of mechanical to chemical erosion rates. These ratios arecalculated in Table 5a:

mec _ P+Schem - TDSsil

The rivers of the Congo Basin appear to be characterised by mechanical erosion rates which arebetween 1.2 and 2.3 times higher than the chemical rates. In the Lobaye and Sangha drainage basins.chemical and mechanical erosions are about equal. while in the Kasaï. Zaire and Oubangui Basins.mechanical denudation is twice as strong. Note that these rivers drain the western borders of the RiftValley whose e1evations often exceed 2000rn. The relatively high physical erosion rates observed forthese catchments supports recent worles (Pinet and Souriau. 1988; Milliman and Syvitski. 1992. 80

Summerfield and Hulton. 1994) in which it appears that. at a continental scale. mechanical denudationis mainly controlled by relief.

ln Fig. 7. we plot this ratio versus total denudation rates fOI the Iivers of this study and the rivers ofthe silicated part of the Orinoco Basin (Edmond et al.. 1995). A fair IineaI relationship is obseIVedbetween the two paIameteIs (I - 0.97 for the Congo Basin). This consistency indicates thehomogeneity of chemical denudation relative to mechanical denudation thIoughout the differentdrainage basins. The constancy supports the idea that chemical denudation is mainly controlled byrunoff (Pinet and Souriau. 1988. Summerfield and Hulton. 1994). The similari.ry of the wet tropicalc1imates found in the Congo Basins can explain the overall constancy of chemical denudation amongthe samples.

Carbonate OI evaporite weathering rates can also be estimated using Table 2 and the Iesults of NégIelet al. (1993). These rates aIe listed for comparïson with the silicate weathering rates in Table 5b andcompaIed graphically with the silicate eIosion rates Fig. 6. As already noticed. the pIoportions of thesedimenta.ry Iocks outeIop aIeas aIe unknown but probably ve.ry smalt compaIed to the outeroppinga.reas of silicates. fOI this .reason can only calculate fluxes (in t/YI) of dissolved solids generated by thechemical weathering of sedimenta.ry rocks (Table Sb). The carbonate weathering rates f1uctuate fIom 2to 11 mgll depending on the amount of carbonate terrains found in the basins. The cOlTespondingfluxes range from 0.1 ta 6x106 t/yI (at Brazzaville). At Brazzaville fOI comparison. the flux of silicateweathering is 19X106 t/yI. The influence of the dissolution of evaporites p.resent in the basins onlyappear in the zaire river (2x106 t/yr).

(02 consumption

The proportions reported in Table 2 allow us ta distinguish between the consumption of atmosphericC02 by carbonate weathering and that caused by silicate weathering. Only the silicate weatheringrepresents a net consumption of atmospheric C02 because the flux of C02 consumed by theweathering of limestones on the continents is cJosely matched by the C02 delive.ry of calcite andaragonite precipitation in the ocean (Broecker and Peng. 1994).

Using the proportions of Mg and Ca in the dissolved laads. originating from the dissolution ofcarbonates (Table 2) we can detelTnine the concentration of HC03- (and fluxes in mol/yr) derivedfrom carbonate dissolution and the concentration of HC03- (and fluxes) due ta the weathering ofsilicates. Ail the bicarbonate ions produced by silicate weathering are derived from atmospheric C02.but only half of the bicarbonate ions derived from carbonate dissolution originate in the atrnosphere.The C02-specific fluxes consumed by carbonate and silicate weathering for the Oubangui. zaïre.Sangha. Kasai and Congo Rivers and the contributions of silicate and carbonate weathering to theHC03- river concentration are given Table 6. The proportion of C02 derived from silicate weathering isclose to 50-60 % except in Sangha r:ïver (9%) in which the importance of carbonate weathering isgreater. The fluxes of atmospheric/soils C02 consumed by rock weathering fluctuates from 15 to1OOX 103 mollkm2/yr with a mean value at Brazzaville of 51 moI1km2/yr. This value is in agreementwith the previous estimate of Probst et al. (1994) of 53xl03 moI1km2/yr. By contrast. their value forthe Oubangui 1 (74x103 mol/1an2/yr) is about twice the value calculated here (42x103 mollkm2/yr).Fïnally. the nuxes consumed by silicate weathering range from 2 to 76x 103 mol/km2/yr with a meanvalue in Brazzaville of 3Oxl03 molllcm2/y. This value is 25 % Iower than the value determined atBrazzaville by Probst et al. (1994) for silicate weathering (40x 103 mol/km2/y). Like the chemicalweathering rates. the consumption of atrnospheric C02 by weathering processes over the Congo Basinis low.

81

Residence time of soils and weathering

According to our model. the soils of the Congo Basin have reache<! a dynamic equilibrium betweenproduction and destruction. Using a mean soil thickness of 15 m and a mean soil density of 2500g/cm3. residence times of the soil materials close to 3x106 yr can be calculated. This residence time ishigh and allows an intense weathering to take place. Moreover. tropical areas are characterised by hightemperatures. heavy precipitations. and have one of the higher biological productivity in the world(rain forest). These three factors also contribute to chemical weathering by increasing the kineticdissolution rates (lassaga et al.. 1994) and by producing high C02 and organic acid concentrations(Drever. 1994 ) in soils. The suspende<! phase is mainly constituted of kaolinite (paper 1) indicatingsevere weathering conditions in the Congo river system. This observation explains the high depletionof suspended sediments for the most soluble e1ements (Na. Ca. Sr. Ba. K. Rb and U) relative to thecontinental crust and the a posteriori values of a close to 1. In this sense. the thick soils of the CongoBasin act positively on weathering by allowing an intense weathering to take place. leaving cation-freeresidues of kaolinite. quartz. gibbsite. ironhydroxides and non-weatherable heavy minerais.

On the other hand. the dynamic equilibrium indicated by our calculations for the Congo riverdrainages. suggest that very low rates of mechanical processes are an intrinsic self limitation to highweathering rates in these nat tropical areas. These concept of shielding influence of soils andconsequent negative feed back has also been suggesœd by Edmond et al. (1995) for the Guyana Shieldand Bluth and Kump (1994) for sman rivers in areas of variable lithology and c1imate.

The chemical weathering rates (and consequent C02 consumption) of silicate thus depends on thebalance between mechanical erosion of soils and minerai dissolution. High mechanical erosion rates(due to a tectonic uplift. for examp\e) would enhance the chemical erosion rates and the tumover ofthe soil. but presumably reduce the intensity of chemical silicate minerais weathering (Le. decrease a'values). depending on the Iànetic of the weathering reactions.

More intensive studies under different c1imate and relief conditions are c1early needed to betterunderstand the influence of soils (and their steady state) in controlling the silicate weathering rates andin tum. the net continental consumption of C02. The type of question we want to address in thefuture is : are erosion processes in a steady state in the other major river system of various relief andc1imate. is the relation between the ratio of the mechanical to chemical erosions and total denudation(Fig. n valid at a more global sca!e. is it limited to shield areas. or what are the relationships betweenthe weathering intensity (a) and mechanical erosion rates?

Continental crust composition

To determine the composition of the continental crust in each drainage basin using the a posterioricrustal ratios. we need the absolute Sr concentration of each drainage Basin. The steady state oferosion processes shown by the results of this study allow the Sr continental crust composition to becalculated by summing the dissolved and suspended phase contributions. Using the previouslycalculated values of P and tsil in the following relation:

tsil.Cc(Sr) - P.Cp(Sr) + Cwsil(Sr) (3)

the calculate<! concentrations of Cc(Sr) are calculated (Table Sa). They show a range of variation of200-300 ppm within propagated errors of 10-20%. These estimates are slightly lower than Taylor andMcLennan's estimate of 350 ppm. Furthermore. trace and major element concentrations in each localcontinental crust segment can be calculated for soluble elements (using the a posteriori ratios) and ailinsoluble e1ements (by multiplying the suspende<! concentrations by P/tsil). These values are listed inTable 7 and ail primitive mantle-normalised diagrams are plotted on Fig. 8 and compared to thepatterns of the suspended material (after weathering). As emphasised in paper 1. this diagram isdesigned in order to obtain a monotonic decrease of Taylor and Mclennan's model of continental 82

crust. For insoluble elements. the patterns of the reconstructed continental crust are deduced from thepattems of the suspended material by an upward translation. The ratio between them is P/tsil < 1.The enrichment of the insoluble elements in the residual phase (suspended phase) relative to thepristine continental crust is due to the removal of the most soluble elements during the weathering. Asfor Zr and Hf. they have not been corrected for their depletion in the suspended Ioad and are notplotted in Fig. 8.

Two caveats apply to the crustal patterns (Fig. 8 and Table 7). First. the propagated uncertaintiespropagated on soluble elements are greater (in the range of 30-40%) than the uncertainties reported oninsoluble concentrations (20%). Second. the soluble elements U. Rb. Ba and Kare tacitly assumed tocome only from silicated sources. If this is probably true for U. e1ements such K. Ba and Rb (N~grel etal.. 1993: Dupr~ et al.. 1994: Crozat. 1979) may be perturbed by the biomass reservoir. possiblyengiged in a susanal turnover. As a consequence. these e1ements probably have greater residencetimes in the river system than the other elements implying that their measured concentration isoverestimated. Consequently. the positive anomalies observed on the reconstructed upper crusts ofLobaye. Zaire. Sangha Kasai and Congo Basins for K and Ba have should be taken carefully. Withthose caveats in mind. we note that these positive anomalies are largest for the Sangha River. whasedrainage area is entirely covered by the rain forest The discrepancy of our calculated Rb/Sr values(mean: 0.46 ± O.On with Taylor and McLennan's (0.32) may indicate the same phenomenon. BycontriSt the UlPb (mean : 0.16 ± 0.(4) and Th/U ratios (3.2 ± O.n are constant within the errors andclose to the Taylor and McLennan's values (0.14 and 3.8 respective/y).

The overall homogeneity of the different crusts shown by the constancy of some inter elements ratiosdisplayed in Table 7 (lalYb. ThISe. Sm/Nd). is in agreement with the uniformity of suspendedsediments Nd mode) ages determined by AII~re et al. (1995). In contrast. the Sr. Na and Ca crustalconcentrations do not confirm this homogeneity of crustal composition in spite of the propagateduncert3inties. AbsoIute Sr concentrations nuctuate from 200 to 300 ppm and sorne of the pattems (Fig.8) exhibit negitive anomalies (Oubangui. lobaye. Congo and Sangha) for Na and Sr. Such depletion ofNa and Sr indicates that the pristine continental crust of these drainage basins has already beendepleted by andent weathering cycles (e.g. constituted by shales or shales-derived granites).

CONaU510N

The mass budget attempted in this paper using the chemicaJ complementarities of suspended sedimentsand soluble phases in the rivers of Central Africa provides three main results:

- The hypothesis of a steady state between mechanical and chemical erosion applies for each of theCongo Basin rivers studied here. Only the Black Rivers have specifie features that prevent the use ofthe measured concentrations in the dissolved and suspended phase in a simple mass budget

- ln this wet tropical area. the chemical weathering intensity is important (a - 0.8-0.9). but silicatedenudation rates (bath chemical and mechanical) are low. close to 5 mm/ky in agreement with therates determined by others in tropical regions of silicated lithology. The ratio of the mechanical erosionto chemical erosion is close to 1.5 indicating the relative importance of chemical processes compared toother world drainage basins. Future investigations are necessary to extend the method presented hereand in N~grel et al. (1993). in the Congo Rivers. to other major drainage areas in order to e1icit thecontrois of silicate erosion rates and C02 consumption over the earth 's surface.

• The assumption of uniformity of the continental crust used to solve the mass budget equations is notcontradicted by the inversion calculation. and a mean upper continental crust for the central AfricanShield is proposed.

Acknowledgements. This 'Hork was supported by the INSUlPIRAT/ -Dynamique et bilan de la Terre­Programme. The computer program used to performed the calculations is from Eric Lewin. He is

83

gratefully thanked for its mathematical and statistical assistance for the inversion computation. Wethank F. Seimbille, P. N~grel. S. Roy and P. Sarda for heJpful discussion and comments. We gratefullyacknowledge K. Feigl for its comments and substantial English corrections. The critica! comments of S.M. McLennan. R. F. Stallard. D. K. McDaniel and an anonymous reviewer helped significant/y inimproving the manuscript

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85

Stallard R. F. and Edmond j. M. (1983) Geochemistry of the Amazon: The influence of geology andweathering environment on the dissolved load. j. Geophys. Res. 88. 9671-9688.

Stallard R. F (1985) River chemistry. geology. geomorphology and soils in the Amazon and OrinocoBasins. In The chemistry of weathering (ed Drever j. 1.). pp 293-316. D. Reichel Publi. Co.

Summerfjeld M. A. and Hulton N. j. (1994) Natural controls of fluvial denudation rates in major worlddrainage basins. j. Geophys. Res. 99. 13.871-13.883.

Tarantola A. and Valette B. (1982) Generalised non-linear inverse problem solved using the leastsquares criterion. Rev. Geophys. Space. Physics. 20. n-2. 219·232.

Taylor S. R. and Mclennan S. M. (1985) The continental crust its Composition and Evolution. pp.312. Blackwell. Oxford.

Trimble W. S. (1977) The fallacy of the stream equilibrium in contemporary denudation studies. Am.joum. Sei. 277. 876-887.

Trimble W. S. (1983) A sediment budget for Caon Creek Basin in the driftless area. WlSCOf1sin. 1853·1977. Am. joum. Sei. 283. 4S-i-474.

U.N.E.S.C.O. (1974. 1979) D~bits de certains cours d'eaux du monde. Il et III. 1. A. S. H..L1.N.E.S.C.O.I04. 124.

Walling D. E. and Webb B. W. (1983) The dissolved Ioad of rivers: a global overview. Proceedings ofthe Hambourg symposium. August 1983. IAHS Publ. n- 141.

15'

30"

SUDAN

500 km\ .v.....

25'

ZAMBIA

-- .-.. .. --;r

20'·

t.1

15',

O"

.....'- ,..'

'1

""-'r' --.- . -. ."-' r__ ~.

-.- - Vele '- "-'/!1

\ .,.J

. ,r'UGANDA( 0"

(

1 {'''', (1 JI'.,,1 ) '. l, .

1 \ ~, • 1 ....

- 1\ ". ~~~\ l (.')'.:-.'... ) .• - - 'r 5<- ~ .j., .J .. - t., \-.,' i -J'-'-,J '--~.l 'l, Tan ~fYl~;'\' _ '.' .:; ~~.

) ,r '. ',1 \\ "" \ ,1 1", ,\.

]';, \..J ,S MœrV / (j\.;"

,J~ C~ f \"~' \~ J} 10c

. ..... _. 1 ~ \ ", ,-' J./ - .•.. .r;'r.' l . ...... ' .. 1,... _.' •

I--~l--I:-!-r---,,'---r ('~ - . \." )' 1.. - ..... " ..-- '- ,...

.lIl

,'..

IlAN~U~f..... Outt.n,,,, r

"~'" rt',,: ••

..-..~

Figure 1. Mëlp showing the locations of the sëlmpling sites on the Congo Basin Rivers studied here.

1000

eu+-' 100cCV

E 10 omodel of Taylor and McLennanupper crust

eu 1> • suspended pattern+-' 0.1 of the Zaire sampleEL..

0.01 o filtered pattern0-"- of the Zaïre sampleQ.

U 0.001-~

U 0.0001

0.00001Cs U Pb Ba Ce Hf Zr Na Eu Yb Sc Co Ni

Rb Th K La Ta Nd Sr Sm Tb Ca Fe Cr

Figure 2: Primitive mantle normalise<! diagram of the Zaire River suspended (Cp) and filtered (Cw)laads (location and sampling are described in paper I). This diagram is designed in order ta obtain amonatomic decrease of crustal abundances using the mode! of Taylor and McLennan (1985) uppercrust (open circles) The suspended and primitive mantle concentrations (from Hofmann. 1988) are

expressed in ppm and the filtered concentrations in IO-5g11 water. For each sampie. the e!ements Cs.Th. Pb. REE. Sc. Co. Cr. Ni roughly exhibit continental crust-like patterns in both filtered and

suspended phases. Ali other elements are depleted in the suspended load and enriched in the filteredlcad with the exception of Hf which is depleted in the filtered phase as weil.

87

OU.ANGUt LO'AVE lAIAIE LIJCOUA~A SANCHA AU"A KASAI CONGO

11o111

+ 1••'1

1••:t

••.LT•.L•••

l 1 l• l rl _

•....

200

~

Cl).......mZ 100

o cru•••1

• dis&OI•

• ,u'~.

T•1-,-lli

t-i;J'

~•

T•l

••

T•lT•.L

T•J.T•J.

f î1 -

100

300

200

600

~

en.......mo

3001

T•.Ll=l

T lrit~ JJJ'

oL-...L-_...J----J..----''---...I.----'--OUIANCUI LOIAYE lAME UXOUALA SANGHA A~Ya KASA: CONGO!

l•100 ~

1

200

Figure 3: A priori Sr-normalized ratios of the soluble elements for the river borne material (ealculatedusing Table 2) and the erustal estimates (calculated using the primitive mande normalised patterns of

the suspended sediments).ln general. the erustal estimate is braeketed by the dissolved and suspended endmembers. Note the

differences between Likouala and Alima rivers (Black Rivers) and the other rivers.

88

o cru.ll' OUBANGUI ~OBAYE lAI ilE UKOUAL& SANGHA ALiMA KASAI CONGO

• dtllOi.1.5f- ~ -

• lUI••

1.3 ~ T lT

1

T -• T ! -~ 1.1 ~ 1 • •en l .1 0 T

T •1~

....... 0.9 - + lJJ •

t 1c: q ~ ~•

~0.7 +• o -0.5 0

! .-• t ~1 10.3~. • •:: • ! -:t1 "'i 1 1 1

T60- T • -• 1

~ 1 Ten .... •....... 40 -. l _

::J....

.l- • T

.l-

~.•.L.

r20

-~ 9 9 ï

~~0 l•• • •10

• -8- 1 -

TT -

~ T •en 6- T • l -....... • • .l-

m .l- i -te ....

4- "ï -• I T

2- ~i l • .-~ • T Il

~ î-• 0 ëIl 0 1l l , 1

1 1 1 10

OUBANGUI ~OBA'E lAlAE UIitOUALA SANGHA AUMA IItASAI CONGO

Figure 3: A priori Sr-normalized ratios of the soluble elements for the river bome material (calculatedusing Table 2) and the crustal estimates (calculated using the primitive mantle normalised patterns of

the suspended sediments).ln general. the crustal estimate is bracketed by the dissolved and suspended endmembers. Note the

differences between Likouala and Alima rivers (Black Rivers) and the other rivers.

89

c: 10000o6Jnsen.- 100-nsE~

0c:

10CI)

Cp(Sr)IPMo6Jc:nsE 1CI)

>o6J

E 0.1~

c..

0.01Rb

CsU K La Ta Nd Sr Sm Tb Ca Fe Cr

Th Ba Ce Hf Zr Na Eu Yb Sc Co Ni

Figure 4: Method for estimating the priori crustal ratios using the primitive mantle (PM) norrnalisedpatterns of the suspended material. The order of elements on the X-axis is designed ta have a

monotonie decrease of the continental crust of Taylor and McLennan. The pattem represented hue isfor the Zaire river. Marked with a star are the concentrations that would have the suspended sediments

if no solubilisation of the soluble elements had occurred. These theoretical concentrations aredetermined graphically. The relative uncertainties of the crustal ratios are close ta 50-60%.

-.........ClECi)""CoED.

S1VI mg/lFigure 5: Comparison between- the mean an nuai river suspended concentrations and the predicted

value calculated using a steady state model of erosion. The two quantities are equal on the straight line.A 20% uncertainty is assigned to the measured concentrations while uncertainties close ta 40% are

orooae:ated thrOlJ!!h the caleulations of the r.odel v;JJues

CongoKasaïSanghaZaireOubangui Lobayeo

40 • mechanical erosion: P

- 0 silicate weathering: TOSsil

::::::: rn carbonate wGathering: TOSearb0') 0 evaporite weath.ring: TOSevE 30-U)

eu-ca....20

Îc:0.-U)

0...UJ 10

Figure 6: Mechanical (P) and chemieal (fOS) erosion fluxes in mg of crust per litre of river water overthe different drainage area of the Congo Basin rivers. P values are calculated using the inversion

results. TOSsil. carb. ev (refering ta sil~tes. carbonates and evaporites) are Cilculated using Table 2.Among the different drainage basins. only the mechanial denudation flux f1uctuate significant/y.

Chemical denudation is constant The influence of carbonates dissolution appears throughout the riversand is maximum in the Sangha Rivers. The influence of evaporites is detected only in the zaïre and

Congo waters.

Edmond et al. (1995): Orinoco basin

This study: Congo Basin o

8~ 3 -.: 0~.......... •~ •~ 2 - •8 0 • 0

8 0 •1 00

oo 20 40 60 80

1sil (mgll)Figure 7: ROItios of mechanical (P + S) to chemical (fOSsil) rates versus total erosion fluxes tsil _ P + 1

S + TOSsil in mg of crust per litre of water. The good correlation between the two parametersindicates the constancy of the silicated erosion rates From one basin ta another. The data of Edmond et

al. (/995) on the Grinoco Basin (constituted only of silicates) are plotted for comparison.

,l

01"1 LOBAYEOUBANGUI 1

11

10 1

01 1

1000 1 :: .'.. .~-.::::::. .......

100 ~ -""",r -

~~~~-

0.01 -,-...,.----,----~---...i....,'r__.,__-__r_

Cs U Pb Ba Ce HI Z1 Na Eu Yb SC Co NoRb Th K La Ta NO Sr Sm Tb e;, Fe Ct

a al -,-----r.'T'---:----:".....,,--r"",--,.--.,......--C. U Pb Ba Ce Hl Zr Na Eu Yb SC Co NI

Rb Th K La Ta NO Sr Sm Tb e;, Fe Cr

0.01 " ..• i i , 1Cs U Pb Ba Ce .. Zr Na Eu Yb SC Co Ni

Rb Th K La Ta Nd Sr Sm Tb e;, F. Cr

10001;100 ::

~

~~':i

0.1 -il ZAIRE

~0.01 . , 'i .. , i i i ..

Cs U PllBaCe HI Zr Na Eu Yb SC Co NiRb Th K La Ta Nd Sr Sm Tb e;, Fe Cr

- :

1000 "1

..'\''P-o.....

CONGO 64

'1

o., 1

la "1

a al -c-.""'·"T~""""'Pb-r,Ba"T"''''''''''''Ce-'-'Hl"""--'b-'N""'a~Eu-''''''Yb''''' 'T'''''Sc-Co-Nt­

Rb Th K La Ta Nd Sr Sm Tb e;, Fe Cr

KAS AI

la "1

o 1 ~

a01. i • i i , , . i • , •

Cs U Pb Ba Ce Hl Zr Na Eu Yb SC Co NiRb Th K La Ta NO Sr Sm Tb Ca F. Cr

100';' ~'4~"'"= \

• pristine contin~ntal crust (befor~ ~rosion).

o SUspende<! conc~ntrations (after ~rOSlOll: paper 1)

Figure 8: Primitive mantle normalised diagrams of the continental oust before erosion calculated afterthe inversion procedure (1) and of the suspended material (0) collected in the rivers for each drainageBasin. For insoluble elements. the two patterns are identical but translated. This factor of translation

is P/tsil and accounts for the concentration effect of ail insoluble elements that occur duringweathering when soluble e1ements are released from the pristine bedrock. The negative anomalies ofthe soluble elements have been corrected through the inversion procedure. Only Zr and Hf have not

been plotted because we are not able to calculate the continental crust for these elements.

1 Now at CNES/GRGS. 18 avenue EBelin. 31055 Toulouse. france.2 The hydrogrilph of the Oubilngui River at Bangui. lilce ilny hydrograph of the Basin nowing inta the Northernhemisphere. is fairly simple. consisting of one major rise and fall centered on November. The hydrograph of theCongo ilt BraZZiVllle consists of one major high water stage in December and one minor water peak in MilY.

92

Table 2: Measured dissolved concentrations. Estimates of the concentrations derived from carbonate i1nd silicilte weathering,

sampIe Oubanpl Lobaye Z&lre Ukauala Sancha AUma kaAl CantoRb 2.7 %1.1 2%0.8 3.9 %1.6 3.4% 1.4 3.6% 1.4 3.8% 15 2.7%0.55 3.1 %0.62IOOO·U 55%22 34% 14 86%34 11%5 69%28 25% 10 49%20la 18%3 14%3 25%4 17%2 28%4 24%4 20%3K 1100%220 782% 160 2190%440 1173% 235 1486%300 1330%270 1290%260 1408%282Sr 15%3 1/ %2 21 %4 7.9%2 13%3 9.6%2 105%2 11.5%3"'Sr iii CI) 76%7 58%8 73% 12 -H%13 59%7 66%6 71 %6 56%6Sr. 11%25 6%1 17%3 3%1 8%2 6.5%2 8%2 6.4% 15

.NI 1449 529 3100 299 782 782 1219 1/27

"'Neill CI) 80%2 64%7 66%4 67%23 55%5 5-f%6 56%5 41 :5Nad 1154%231 344%69 2046%410 200%40 430%86 422%84 6n% 134 462% 130

.Ca 2600 1920 2no 1600 2520 1520 1760 2080

: '"Cc ceri (.2) 55% 18 80%8 37%35 88%6 n%8 55% 12 35%17 43%16:Ca carb 1430 1536 1010 1424 1814 839 606 998"'Cc III C') 30% 18 8%7 25%29 6%6 9%7 13% Il 23%16 " %8Ca Il' 780%450 136% 120 896%900 96%96 227 %179 192% 160 405%280 229% 170'Mc 996 923 2381 948 753 899 923 136/:"'AI~ca (.2) -12%20 72%8 45% 19 87%7 45%/3 47% 12 23% 14 49%11;M&c:arb 418 665 1071 82S 338 422 212 667,"'AI~ iii CI) 30%20 10%8 31%18 87%7 12%/1 14% 10 23%13 Il %10;M, sil 295% 194 91 %73 729%423 57%47 89%82 16:90 350%250 150%90

:504 1248 864 2784 768 960 1152 1248 1056

504.u 412 285 919 253 317 380 412 348'HC03 14335 nd 22875 nd 7930 rxI 94S5 1124a 770 315 1645 S60 630 630 910 11555102 13.61 1254 11.38 9.04 12.57 10.05 12 12

TDS III 17 14 18 1/ 15 12 15 15

1DSsi1: Total dissolved solids ori&inilting from the weathering of the silicates (mg/I). Ail other concentrations in ppb.~1}Proportion of dissolved Nil. Û. Mg iII1d Sr originiltini from the silicated reservoir.Results from NegreJ i1nd i11. 1993.2} Proportion of dissoIved û and Mi coming from the carbonate dissolution (NegreJ et al. 1993.)

93

Table 3: Equations and parameters of the steady state model resoIved in this paper.

Mus buqet equations for -soluble- elemenb

For X - Na. Ca. K. Ba. Rb. U

(~) =(~) a+(~) (l-a): 6 equationsSr c Sr w Sr p .

Mas budpt equations for -insoluble- elemenb

For Z - Cs. Th. la. Ce. Ta. Nd. Sm. Eu. lb. Yb. Sc. Fe. Co. Cr. Ni

(:r )c =(~ )p(1-a): 1S equations

Definition of a:

[Cwsil (Sr) J

a = \ilCc(Sr)

p =l-a. Cwsil(Sr)a Cp(Sr)

The subsaipts c. w and p denoœ respectiveJy the continental crust, the ri'm waw and the suspended sediments.cwSil (Sr) is the dissofved Sr concentration originating from siticaœ wuthering (ppb). Cp(Sr) is the Sr concentation in

the river suspended sediments (ppm).Cc(Sr) is the Sr concentration in the pristine bedrock (ppm).P is the mode!suspended sediment concentration (gll) and tsil the total amount of bedrodt eroded per li~ of river water (glI).

94

T.... 4: "priori and 1 poJterlorl values and tflOrs 01 th. InYllrtcd paramctcrs.

DI - • locI 1posterion tllOlI a pllOlI tflOf) qUlnlJrllœ the pin 01 rnlonnation throuCh the HlYll/'Slon proccdu~. FOI c1arity. DI b ollly hted lOf the Zalle SlImple.C. p and w ttlets ltJjlet1ivdy to continenllll crwt. suspended sediments lInd dissolved plwe.

- LoMye %lire LIUuIIe SanahI lUIIu lCMaI e:-..'Priori a PO'ltflorl a Driorl a POIlIrI«I , DrioII a DOIltriorI a aIlflorl , oriorI a pOStIrlorl , priori • priori a /IOlltriorl apriori a IIOStIriorl

li 0.SiO.5 0.I2hO.OS O.StOJ 0.'21:1:0.OSJ 005:1:0.5 O.Sofa :1: O.OSJ lJ 005:1:005 0.StO.5 U2J:l:0.029 0.5:1:005 0.5:1:005 US1:1: 0.046 005:1:0.5 0.IS2:1:0./).f9J

CalS~c 151:1:94 127:1:39 ISI:l:9S 9H27 118:1:71 102:1:)8 0.3 111:1: 61 158:1: 95 7S:l:26 22J:I: 145 210:1: 126 '28:1:J6 160:1:92 106:1:J3(CalStlp 361:1:37 J6IJ:I: 37 414:1:41 4'1t41 m:l:14 JJltJ4 0 145:1: 15 560:1:56 S62:1:S6 151:1: 15 S37 :1:54 SJ9:l:54 411:1:50 ofC :1:50IICalSI)w 71 :1:45 n:l:(1 23:1:21 26:1:21 53:1:50 6O:I:.f3 0.07 32:1:30 29:1:24 Jof:l:23 30:1:26 51 :1:33 56:1:J2 36:1:30 41 :1:29

(NalSr)c 1lS:I: 67 H:I: 17 136:1:'7 5J:l:J 136:1;67 '" :1:20 0.52 132:1: 71 5hlO 129Z61 170:1: 101 7ltl5 161 :1:93 70:1: 12NaI5r)p 3St4 15:1:4 26:1:3 26:1:J 42:1:5 (1:1:5 0 65:1:7 6SH 29:1:4 29:1:3 2ft3 SO:l:5 SO:l:5

(NalSIIW 1oSt 21 1011 :1:20 57:1: Il 59:1: " 120:1: 24 l2J :l:2J 0.02 67:1: 14 54:1: Il 56:1: " 65:1: 13 14:1: 17 86:1:/7 7h 15 74:1:/J,

(KISIIc 124:1:. 101:1: 17 124:1:19 127:1:21 19:1:53 IJH21 0..04 13:1:40 19:1:54 15h29 121:1: 71 116:1:73 159:1:26 114:1: 76 197:1:JI(KlStIP 14et 15 '''':l:1S III :1: Il "':1: " 210:1:20 209:1:20 ° 130:1: 13 200:1:20 19h20 11):1: Il 1110:1:20 190:1:20 17St20 174:1: 20KJSllw 100:1:20 101 :1:20 130:1:26 UO:l:25 12h26 120:1:24 0.03 390:1:71 116:1:37 15hJI 205:1:4' 161 :1:32 154:1:JO 220:1:40 200:l:J7

8alSr)c 2.1:l:IJ 2.1 :1:0.4 2.0:1: 1.3 2.4:1:0.4 1.6:1:0.' 2.21 :1:0.3 GAI 1.33:1:0.51 l.hU J.I :1:0.5 1.77:1:0.93 2.1:l:IJ 3.J:l:0.5 2.0:1: 1.3 3.4:1:0.5h/Sr\p 4.2:1: 0.40 4.2:1:0.4 3:1: 0.3 HO.J 5.1:1:0.6 5.1t0.6 0 1.I6:l:OJ 1.7:1:0.' U:l:D.J 2AStO.3 6.12:1:0.60 6.1 :1:0.' 605:1:0.7 1.5:1:0.78lIISIIw 1.610.4 I.ItO." 2.3:1: 0.5 2.J:l:0.5 105:1:0.3 I.5:1:0.J 0 5.7:1: 1.1 JJ:l:0.7 HO.' 3:1:0,6 2.8:1:0.' 3.1 :1:0.6 2.9:1:0.6

103.(lUSr)c 14:1:1 " :l:J 14:1:9 " :1:2 14:1:9 U:l:J OAi 9.9:1:5.' 14.3:1: 1.7 12.' :1:2.2 22:1: '3 IJ.I:l:1J 7.1t1..6 11:1;/2 14:1:2

103.IUlStlP 41:1:4 4/:1:4 )6:1:4 31t4 ,St, 5St6 0 Iltl.5 62:1:6 Q:I:' Il:1:2 34:1:4 34:1:J 46:1:5 46:1: S

103.IUlSt)w St, 5:1:1 5.7:1: 1.1 5.7:1: 1.1 5.1:1: 1 S./:I: 1 0 4:1:0.8 1.6:1: 1.7 '.6:1: 1.7 3.1 :1:0.6 3.1:1:0.6 7.7:1: I.S 7.':1: 1.5

RbISr)c OAStO,) 0.42:1:0.08 OJSiO.3 O,ofO:l:O.06 0052:1:0.36 MI :1:0.08 0.65 0.5:1: 0,) OJStO.JJ 0.50:1:0.08 1:1:0.74 0..0491.0.2' 0..f3 :1:0.06 0.61 :1:0.37 0.6:1: 0.1IRb/Sr)p IJI:l:O.1 1.21 :1:0.1 0.17 :l:O.OS 0.17:1:0.09 1.42:1: o. 14 1.42:1:0.14 0 0.5:1:0.05 1.\:1:0.1 1.1 :1:0.1 0.1:1:0.1 0.96:1: 0.1 U6:I; O. 1 I.IStO.12 1.15:1:0./2Rb/Sllw 0.25 :1:005 0.25 :1: o.OS 0.3 :1:0.06 0.30:1:0.06 OJ3:1:0.05 0.2J:l:0.05 0 1.1 :l:OJ 0.45:1:0.09 0.45:1:0.09 0.6:1:0.12 0.3 :1:0.06 O.HO.06 005:1:0.1 0.5:1:0.1

103.(Th/51Ie 39:1: 17 (1:1: 15 37:1: 12 39:1: 12 32.1:1: 10.7 J/:I: 10 0.03 31:1:6 31:1: Il 29:1: /1 57:1: 19 37:1: 12 3h/2 )1:1: 13 J9:I: 12

103.fThlSllp 244:1: 20 244:1: 24 221:1: 20 217:1:20 200:1:20 201 :1:20 0 70 :1:1 377:1:31 379:1: 31 136:1: 14 260:1:26 25h26 265:1:30 264:1: JO. . . .

Table 5: Erosion panmeters deduced from the inversion procedure.

sampJes a • modcI SM '+$ TOS.n TDSID TDS carb TDSev t.a h mccldlemlDOCIeI

mcJI mcJI t1bD2Jy mcJI tlkrn2Jy mcJI mcJI mcJI mmJky

Oubmllli 0.828:i: 0.053 32:i: IS 30 82 17.4 4 S.I 0 52 4 2.1

Labaye 0.828:i: 0.OS3 16:i:7 16 6 142 4.8 62 0 3Z 3 1.2

lIJn 0.848:i: 0.OS3 39± 18 31 13.9 18.4 S.9 6.3 4 61 • 2.3

Saacba 0.923:i: 0.029 17:i:9 19 4 IS.I 3.2 11.4 0 34 3 l.2

J(.-.J 0.8S 1:i: 0.046 Z7± 12 17 11.4 IS S.8 2.3 0 4S 7 2

Conp 0.8S2 :i: 0.049 21:i: 10 21 a.t IS S.3 S 0 3t 6 1.5

[he values of a cornes directIy From the inversion I1!SUlts (Table 4). Pis the steady state mode! suspended concentntion. tsil the toaI silicated erosion::hemial and mechanical). h when converted in mmlltyr and Cc(Sr) is the caJculated Sr austaJ concentration. The. values of SM are the mesaured;uspended sediments concentration. ms is achemical erosion rate. Sil. cam and r:v denoœ the silicated. carbonated and evaporite reservoirs. The meclc:he ratio of chemical erosion relative ta mechanicaJ erosion.

Tibia 6: C02 consumption by chemicaI erosion of silicates and carbonates of the Con&o River catchments.

sampIe He°3- HC03- C07 flux C07f1ux C02 specific nux C02 specirlC flux total specili

He°3- from carbonates from silicates consumed by consumai by consumed by consumed by consumec'carbonates silicates carbonates silicates rockswutt

mmolll mmolll mmoIlI I09mo11yr I09mol/yr 103molJlan2/yr 103mo1J1cm2/yr I03mo111cn.

Oubangui 1 23S 106 129 6 14 12 30 42 rzm 37S 139 236 37 127 22 76 98SM\&ha 130 119 Il 3 1 13 2 ISt<asai ISS 6S 90 \1 31 13 3S qConlo 64 184 106 78 n 106 21 30 SI

96

Tabla T. Reconstructed continental oust of each drairla&'! Basin.

uppers;mple Oubanp L.obiye ZaR Sançh! I<isai Coo&O ~ sd oustCs 2.7 2.4 4.4 2.4 1.6 2.4 2.7 0.9 3.7Rb 105 89 126 120 16 120 101 17 112U 3 2.S 3.9 3.1 1.8 2.8 2.9 0.7 2.1Th 10.1 S.9 10 7.8 8 7.4 S.I 1.3 10.7Pb 20 IS.7 20.2 12.7 17 IS.S 17.9 2.S 20K 2S3S1 28321 40131 38160 31100 39400 33977 6403 21000Bi S27 535 64S 746 660 680 632 86 SSOLa 3S.3 27.9 35.5 21.1 28.1 22.5 29.7 5.0 30Ce Il.1 S9.4 69.1 SS.6 S&.9 S 1.2 63.1 13.4 64Ta 1 0.9 1.3 0.6 0.9 0.1 0.9 0.2 2.2Hf 2.2 1.9 2.7 1.6 2.2 2.2 2.1 0.4 5.8Nd 44 3S 33 12 17 18 27 13 26Zr IS 92 106 75 91 84 89 10 190Sr 251 223 307 240 200 200 237 40 350Na 24096 11119 34077 1l44Q IS600 14000 18839 8621 21900Sm 7.6 6.2 S.6 2.1 2.9 3.1 4.6 2.2 4.S&.1 1.32 1.16 1.17 0.91 1.03 0.86 1.09 0.16 0.11Tb 0.6S 0.S9 O.SS 0.4S 0.41 0.39 O.SI 0.11 0.64Yb 2.03 LIS 1.91 1.49 2.4 1.39 1.15 0.37 2.2Ca 31177 20739 31314 11000 25600 21200 24711 SI Il 30000Fe 52864 S976S 44OS1 S6316 4S606 42346 SOISI 7164 35000Sc 12.6 10.6 Il 9.4 10.6 1.7 10.S 1.4 IlCo 13 12 17 13 10 13 13 2 10Cr 12 71 73 76 79 64 74 6 35Ni 61 40 47 73 44 35 SO 14 20

Th/U 3.6 3.6 2.6 2.S 4.4 2.6 3.2 0.7 3.1UlPb O.IS 0.13 0.19 0.24 0.11 O.IS 0.16 0.04 0.14Rb/Sr 0.42 0.40 0.41 o.so 0.43 0.60 0.46 0.07 0.32Sm/Nd 0.17 0.11 0.17 0.11 0.17 0.17 0.17 0.00 0.17LaIYb 17.4 IS.I Il.6 IS.9 12.0 16.2 16.4 2.3 13.6Th/Sc 0.16 0.&4 0.91 0.83 0.7S 0.15 0.&4 O.OS 0.97

An concentrations in ppm. Upper austs values of Taytor and Maclennan. I98S.Insoluble and soluble elements are CCllTeCtEd &cm the weatheme processes. Onty Zr and Hf are not corncl!d.

97

summer schoolécole d'été

IJ~~~

mtltewed~:

mO/l,e !fïcti~~

Hubert ZEEGERS

BRGM, Mineral ExploroHon and Technologies Division -Orléans

98

Keywords: weathering. exploration. gold. base metals. mapping

INTRODUCIlON

The intense chemical weathering processes that occur in wet tropical environments. such as are foundin a large part of Africa and South America. result in drastic changes te the mineralogical and chemicalcompositions of the bedrok above the weathering front The geochemical signais obtained from thesealtered sampling media can therefore be considered as coded and it is up te the exploration geochemist.aiming te trace primary mineralization and bedrok characteristics. to decode them correct/y. Thedecoding. or interpreting. of geochemieal data from wet tropical environments cannat rely on a purelyquantitative statistical approach: the analytical results have ta be weighted by parameters describing thesample itself and its position relative te both the landscape and the weathering profile. In order words.certain facts need te be observed if one is te obtain a good understanding of the analytical figures.

As wet tropical environments cover a large variety of landscape.s and correspond te distinct degrees ofweathering intensity. and as the present situation at any Iocality is the result of a complex evolutionunder the influence of successive paleoclimates and tectonics. geochemical dispersion halos aroundmineralized bodies vary in shape and intensity from place to place. Moreover. the geochemicalsignatures obtaines from a giver coundry rock can show signifteant variations according te the intensityand local conditions of the weathering profile.

Applying geochemical methods ta minerai exploration. geologjcal mapping and environmental studiesin such extreme environments can be very effective as long as their present limitations are c1earlyrecognized. Improvement in this field still represents a major challenge for earth scientists.

MINERAL EXPLORATION

The use of geochemical methods for precious -and base-metal exploration in the wet tropics bringdistinct problems at each exploration stage. The major problem is always te find the best fit betweenthe geochemieal dispersion mode! and the sampling pattern with respect ta the sampling and anaJyticalprocedures. Economie considerations also play a very important role. for althought exploration costsshould always be kept ta a minimum. the cost of non-discovery must also be talœn inte consideration.

As a general rule it can be said that regional geochemical exploration in the wet tropics will reveal fewcharacteristies of the target mineralization. and the corresponding signature(s) will general1y be of veryIow intensity and contrast. The dispersion model will depend on the balance between chemical andmechanical processes for a given element. i.e. on the morphoclimatic conditions. When selecting themethodology for a regional geochemical survey. particularly the sampling density and type of analysis.account must also be taken of the high cost involved in following up poorly defined regional anomalies(planning and costing of an exploration program should be considered as a whole and not separatelyaccording te the different stages). In the more detailed phases of the geochemical exploration program(semi-regional. follow up). the geochemical signature of the derived sampling media (stream-sediments.soils) will give a much better reflection of the target mineralization and gangue. The target can now bedescribed by several quantitative and qualitative attributes. making it possible ta assess its potentialeconomic interest. and te proportion the effort required fOI furtheI assessment Nevertheless. even atthe detailed phase. the proceduIe can be seriously hampered by the severe conditions pIevailing in theweatheIing profile and theiI consequences on the geochemical behavioI of taIget OI pathfindeIelements.

For gold it has been shown that present c1imatic conditions in the tropies can result in a distinctbehavior in the upper part of the weathering profile. with significant enrichment undeI rainforestconditions and a strong leaching under dryeI savanna conditions (Zeegers and Freyssinet. 1993). 99

Organic matter present in the topsoil of wet tropical areas is shown ta play an important role on themobility of gold (Freyssinet 1995). Neglecting such information could result in overestimating theinterest of prospects showing strong Au anomalies in soil samples (or underestimating others where Auat the surface is strongly leached and no longer renects the Au grades at depth).

For base metals. the weathering profile in the wet tropies represents nothing more than a fullscalechromatograph. where each element is transported over a certain distanœ before it (partly)reprecipitates or combines with amorphous minerais or organic maner. This is fairly weil iIIustrated byan example from French Guiana Œutt and Zeegers. 1992} where Zn. which is dominant in the primaryore. is tatally leached in the soir overlying the mineralization and can only be traced in the organicphase of stream-sediment samples downstream of the sowce. Pb however. which is a minorcomponent of the primary ore. shows up as the main metal present in the overlying soir geochemicalanomaly.

GEOLOGICAL MAPPING

The scarcity and poor quality of outcrops in the wet tropies malce reliable geological mapping difflCUltand expensive. Obviously stream-sediment multi-element geochemistry. combined with othertechniques such as satellite imagery. side-looking radar and airbome geophysies. may considerablyenhance the field geologist's work. Swprisingly. in deeply weathered environments such as are fowndin Gabon or French Guiana. active mechanical erosion along the minor drainage axes gives a mineraiassemblage (and therefore a geochemical signatwe) in the stream sediments that is not so differentfrom that of underlying bedrock. except for a few poorly resistant minerais: this is because. along thedrainage axes only. quite strong erosion results in truncating most of the weathering profile. so thatalmost fresh bedrock material is mechanically dispersed in the streans. The geochemical data from suchsamples should no longer be considered as anaIytical figures but as depicting minerais and. possibly.rocks. Obviously. an improved knowledge of regional geology through multielement geochemistry willenable regional exploration programs ta be more reliable and selective. and also cheaper when oneconsiders that fewer poorly defined targets will need ta be followed up.

ENVIRONMEHrAL STUDIES

ln temperate cl i mates. the geochemical techniques developed for mi neral exploration already pl ayan important role in environmental studies. In the wet tropies the weathering profile represents apossible Non Saturated Zone having a thichness of between 20 and 100 m (compared with 1-10 m intemperate regions) and having a speciflC surface ranging from 20 ta 120 g1m2 (compared with 10 40g/m2 in temperate zones). The weathering profile therefore represents an active interface betweensurface and groundwater. and tracing metallic contaminations and their possible confinement bynatural processes is another challenge for geochemists.

CHALKNGES fOR THE fUTURE

Optimizing regional geochemical exploration techniques is still a priority for the future. this being oneof the most effective ways for generating new targets in the wet tropies where bedrock is generallycovered by a thick weathering profile. But 100 per cent exhaustiveness remains in the realm of wishfulthinking rather than scientific reality. The surprisingly good geochemiC31 response ta lithology observedin stream sediments along drainage axes of strongly erosional zones in the wet tropies. althoughadequate for geological mapping purposes. is tao restrictive from the exploration standpoint where thewhole of the exploration area must be investigated; Le. including the interfluves where most of theweathering profile is preserved and where mechanical erosion is far Jess active than chemical 100

weathering. Even where mechanical erosion does occur in these interfluve areas. the upper horizons ofthe profile that are concerned are severely leached and carry little information about fresh bedrockcharaeteristics. Moreover. considering that the target metals can be easily solubilized and transported insolution. the question is to be able to decode the corresponding contribution to the total chemicalcomposition as obtained by conventional geochemical analysis. It is of note that few major base-metaldiscoveries in the wet tropics are recorded in the literature as resulting From geochemical surveys; theexceptions correspond to large porphyry-type deposits.

1 would like to suggest that the problem of exploring for massive sulfide mineralization in the wettropics be reconsidered in view of the considerable experienœ gained. over the last 10-15 years. ingeochemical exploration techniques and in ow fundamental knowledge of weathering processes inthese c1imatic zones. Modeling geochemical dispersion and designing innovative sampling andanalytical procedwes should improve the efficiency and reliability of geochemical techniques for base­metal exploration.

We also face a major challenge in optimizing the Very Low Density geochemical approach forprecious-metal exploration in the wet tropics. Can we reliably Iocate gold mineralization which is onlya fraction of a km2 in size. sometimes hardly outcropping. by coIlecting samples in large catchmentareas (> la km2)? And what is the most appropriate sampling and analytical procedure? What we lackat present are comparative studies over large areas. where the performances of different techniques (e.g. BLEG. heavy concentrates. partial dissolution) should be tested. Of course. modeling precious-metaldispersion aver large distances should be a prerequisite.

CONaUSIONS

Notwithstan~ng the major changes in the chemical composition of rocJcs and ores brought about byweathering conditions in the wet tropics. geochemical techniques have proved to be efficient in thisenvironment when used for minerai exploration or geological mapping pwposes. Their effective use forenvironmental pwposes will certainly increase in the near futwe. Nevertheless. qualitative andquantitative data interpretation can only be achieved if careful account is taken of the regional andlocal parameters descnbing the morphology and weathering profile. Otherwise. misleading results maylead to wrong decisions in the minerai exploration process. with projeet averoost and/or failwe as aconsequence.

At least two major problem areas relating to minerai exploration in the wet tropics need to beaddressed by research dwing the coming years. The first is to improve the efficiency of regionalgeochemical techniques for base-metal vcploration. and the second is to develop a reliable Very LawDensity geochemical methodology for precious-metal exploration.

REFERENCES

Butt. C.RM.. and Zeegers. H. (editors). 1992. Regolith Exploration Geochemistry in Tropical andSubtropical Terrains. pp. 262-265. Handbook of Exploration Geochemistry. Vol. 4. Elsevier. Amsterdam

Freyssinet Ph. (1995). Gold mobility under rainforest conditions: example of the Yaou deposit 17thIGES Meeting. Townsville. Australia. May 15-19. 1995. (extended abstract).

Zeegers H. and Freyssinet Ph. (1993). A quantitative approach to gold signal evolution in differentlateritic contexts. 16th Int Symp. on Exploration Geochemistry. Beijing. China. Sept 1993. AbstractVolume. 87-188.

101

~~ ill® ~m LI U Q)$

sUlR1lR1er schoolécole d'été

Philippe ILDEFON5E and G. MORIN

Laboratoire de Minéralogie-Cristallographie, Universités Paris VI et VII

102

At Earth surface. minerai components which form by weathering in soils or by precipitation in streamsare finely divided. They can be either amorphous or crystalline. In tropical countries. these solids aremainly constituted of Si. AI and Fe incorporated in alumino-silicate or iron short ordered compounds(allophanes and ferrihydrite) and in crystalline oxides and oxyhydroxides. Their chemical compositionyields quite large variations in terms of AI-Si or AI-Fe substitutions. The detailed knowledge of theirstructure and composition is difficult to assess because these phases are intimately mixed together orbecause they are amorphous when using X-ray diffraction (XRD). Recent advances in spectroscopiemethods and in powder diffraction refinements allow ta derive quantitative and structural informationon these minerai components. Two kinds of results will be presented depending on the nature of thesolids considered. For XRD amorphous components. X-ray absorption spectroscopy (XAS) is apowerful method which allows ta investigate their local and medium range order. This spectroscopy isan element specifie probe and yields information on the coordination state of the elements. on the sitesymmetry and on the medium range order (i.e. interpolyedrallinlcages). For AI and Si. nuclear magneticresonance (NMR) is also a sensitive local probe for the determination of AI coordination number and Sipolymerization rate. It has been used in combination with XAS in the study of aluminosilicates. Thederived structural information may be used ta discuss the formation conditions and the transformationmechanisms between arnorphous precursors and crystalline equivalents. For crystalline components insoils. the main diffkulty is ta extract quantitative and structural data because they are always intimatelyassociated.

Recent developments in XRD powder refinement by using Rietveld method will be presented onexamples from bauxites. This refinement procedure yieJds quantitative weight fractions of each mineraiin the mixture provided that a structural model is known for every component Besides. for the mostabundant phases. structural information can be derived: cell parameters. site occupancies. meancoherent domain sizes. Magnetic properties measured by Mossbauer spectroscopy are also verysensitive ta the actual crystal-ehemistry of iron oxides and oxyhydroxides in terms of AI-substitutionrates and stoechiometry. This quantitative information may then be used ta distinguish betweenvarious generations of iron oxides minerais (goethite and hematite) in complex weathering profiles andta discuss their formation conditions.

103

~® ill® [1m Li DQ)g

summer schoolécole d'été

!JlUmauJ~(~m

~ d.tJi,Û:p~anJ~

Helge 5TANJEK

Lehrsluhl {ür BoJenkunde, TU München, D·85350 . freising-Weihenslephan

105

The mineralogy of iron and aluminum (hydr)oxides in laterite.s has been described in detail in manypublications. Various properties of the iron oxides (cf. Table 1) should reflect the environment inwhich these minerais have bt;en formed. Due to subsequent changes in the environment (e.g. shift inc1imate). the link between properties and factors has been destroyed. however. It is thererore necessaryto reconstruct environmental conditions from rather static parameters. This complex task can beapproached by applying inrormations gained rrom synthesis experiments and thermodynamiccalculations to reconstruct possible pathWiYs of Fe/AI oxide formation in laterites. Valuable inrormationis also drawn rrom observations on recent soils. In this lecture. c1imate. cations. pH, Eh and wateractivity will be addressed.

Table 1 lists static properties and dynamic factors:

Properties (static)

Phases. association of phasesConcentration of phasesCrystal size. crystal morphologyIsotopie compositionMicromorphological occurrenceLocation within profileChemical composition(metal substitution)

Factors (dynamic)

Oimate (temperature. water balance)Cations. anionsComplexing & organie speciespH. redox potentialWater activityAdsorbents (phosphate. silicate)Kinetic factors

The main iron minerais occurring in laterites are goethite (Gt) and haematite (Hm). which areassociated with lcaolinite. gibbsite. quarz and minor accessories. e.g. Ti oxides and spinells. Additionalto these mineraIs. boehmite. maghernite. and corundum. have been found. but other phases commonin recent soils are absent micas. feldspars. Iepidocrocite. ferrihydrite. This mineralogy is the result ofintense weathering. during which the chemical composition has been reduced essentially ta the systemAI203 - FelO - Si02-H20. Very long weathering times suggest that thermodynamic equilibrium mayoverride kinetic factors during minerai formation. Thermodynamic calculations by Y. Tardy andcoworkers expIain at least in a qualitative way the mineralogy in dependence of chemical composition.water activity. and temperature. This will be shawn by several examples.

THE INFLUENCE Of ANIONS AND CATIONS

ln laboratory mode! systems anions exert major influence upon the phases formed. High chlorineconcentrations enable the formation of akaganeite. Sul rate and carbonate promote goethite. whereaslepidocrocite may be suppressed quantitatively by these anions. The dominant cations are AI3 + andFe3 +. which Jead to AI substitution in goethite and haernatite. Apart rrom this chemical influence,AI3 + seems to promote haematite relative to goethite and decreases the crystal size of bath minerais.

THE INFLUENCE Of PH AND EH

pH exerts multiple influences on the phase composition and on crystal size and morphology.respectively. In laboratory experiments as weil as in recent soils the ratio or Hm 1 Hm + Gt increaseswithin the pH range rrom 4 to 6. Possibly due to changing growth mechanisms. crystal sizes ofgoethite and haematite are affected. tao. 80th the ratio. and the crystal size. however. are influencedby severa1other factors such as temperature or speed or crystal growth: their diagnostic capability,thererore. seems to be limited.

106

The translocation of iron within a lateritic profile is an essential process. The question is. whether ironis transported in its trivalent or divalent state. The trivalent way could be only achieved by complexingorganic agents because of the very Iow solubility of iron oxides at soil pH values. The divalent form.however. is highly soluble. and. hence. highly mobile. An indicatar for a transport in the reduced statecould be vanadium incorporated occasionally in iron oxides. Dissolution studies on natural lateriticsamples (Schwertmann. in preparation) show that V3 + may substitute for Fe3 + in haematite andgoethite. Additional to V3 +. Cr3 + substitution may occur (Stanjek. unpublished. Trolard et al..1995). Because of almost identical ionic radii of Fe3 + and V3 +. it is not possible to detect Vsubstitution by XRD. Dissolution experiments in Ha. however. yield V02 + and Fel + in identicalamounts. Thermodynamic calculations show that during dissolution. Fe3 + is reduced by V3 + . whichitself oxidizes ta the vanadylic form. Additional calculations show. that V-substituted iron oxides mayform at near-neutral pH from solutions containing Fe2 + and V02 + .

CONCLUSIONS

Almost ail of the solid phase parameters are innuenced by more than one environmental factor. Onlyfew parameters can be interpreted uniquely (e.g. the pair corundum + maghemite indicates fires. V3 +indicates redox procresses). The reconstruction of processes requires therefore the combination of asplenty information as possible from field work. model systems. and theoretical calculations.

107

SOME REFERENCES ON IRON AND ALUMINUM (HYDR)OXIDES

Bigham jM. Schwertmann U. Carlson L. Murad E(1990) A poorly crystallized oxyhydroxysulfate of ironformed by bacterial oxidation of Fe(lI) in aeid mine waters. Geochim Cosmochim Acta 54: 2743-2758

Fassbinder jWE. Stanjek H. Vali H (1990) Occurrence of magnetic bacteria in soil. Nature 343: 161-163

Schwertmann U (1991) Solubility and dissolution of iron oxides. Plant and Soil 130: 1-25

Schwertmann U (1992) Sorne aspects of fertility assoeiated with the mineralogy of highly weatheredtropical soils. Soil Sei Soc Am 29: 47-59

Schwertmann U (1993) Realtions between iron oxides. soil color. and soil formation. Soil Sci Soc Am31: 51-69

Schwertmann U. Cornell RM (1991) Iron Oxides in the Laboratory. VCH Verlagsgesellschaft mbH.Weinheim.. p 137 pp.

Schwertmann U. Fitzpatrick RW (1992) Iron minerais in surface environments. In: Skinner. HCW.Fitzpatrick. RW (eds) Biomineralization Processes of Iron and Manganese.- Catena Suppl.. vol 21.Catena. Cremlingen-Destedt pp 7-30

Stanjek H. Murad E. Schwertmann U (1990) Influence of AI substitution upon crystal size and roomtemperature Môssbauer spectra of natural hematites. Chem Geai 84: 292-293

Stanjek H. Schwertmann U (1991) Hydroxy1and aluminum substitution in synthetic hematites Eurolat'91 -Supergene Ore Deposits and Mineral Formation-. 5th Internat. Meeting. Berlin 1991. TU- Berlin.pp 184-185

Torrentj. Barr~n V. Schwertmann U (1990) Phosphate adsorbtion and desorption by goethites differingin crystal morphology. Soil Sci. Soc Am J 54: 1007·1012

Torrent J. Schwertmann U. Barron V (1991) Phosphate adsorption by natural goethite-rich materials.Proc 7th EUROCLAY Conf Dresden'91: 1095-1100

Torrent j•. Schwertmann U. Barron V (1994) Phosphate sorption by natural hematites. Europ J SoilScience 45

Zeese R. Schwertrnann U. netz GF. Jux U (1994) Mineralogy and stratigraphy of three deep lateriticprofiles of the Jos Plateau (Central Nigeria). In: Schwarz. T. Germann. K (eds) Lateritization Processesand Supergene Ore Formation.- Catena Spec.issue. vo121. E1sevier. Amsterdam. pp 195-214

Schwertmann U. Taylor RM (1989) Iron oxides. In: Dixon. jB. Weed. SB (eds) Minerais in SoilEnvironments. 2. edn.. chapt 8. Soil Sei. Soc. Am.. Madison. pp 379-438

Schwertmann U (1988) Occurrence and formation of iron oxides in various pedoenvironments. ln:Stucki. jW. Goodman. BA.

Schwertmann. U (eds) Iron in Soils and Clay Minerals.- NATO ASI Series - Series C: Mathematical andPhysical Sciences. vol 217. chapt Il. Reidel. Dordrecht - Boston - Lancaster· Tokyo. pp 267-302

Schwertmann U (1988) Sorne properties of soil and synthetic iron oxides. In: Stucki. jW. Goodman.BA. Schwertmann. U (eds) Iron in Soils and Clay Minerals.- NATO ASI Series - Series C: Mathematicaland Physical Sciences. vol 217. chapt 9. Reidel. Dordrecht - Boston· Lancaster - Tokyo. pp 203-250

Stucki jW. Goodman BA. Schwertmann U (1988) Iron in Soils and Clay Minerals.- NATO ASI Series ­Series C: Mathematical and Physical Sciences. vol 217. Reidel. Dordrecht - Boston - Lancaster· Tokyo 1Iron in Soils and Clay Minerais. p 893 pp.

108

~~ ffi® [1œ Li 0 Q)$

summer schoolécole d'été

Roland Gottlieb SCHWAB

Lehrstuhl {ür Mineralogie Schlobgarten, Erlangen

109

The best way ta discuss the geochemistry of phosphorus. sulphur and arsenic in laterites is to startfrom the so calied lateritic standard soil profile. where ail products of lateritic weathering are presentAs can be seen from fig. 1 the standard profile is composed of two parts:

100

0' ethers

f+++~ .+++ wavelhle

IIIIllIlU crandallite

D kaolinite

50(wt.%)

E:::] hornblende

W goethite + hematite

r;-;-,L!.-J quartz

_ gibbsite

o

~ wardite

~ apatite

~ smectite

o

deplh

(m)

l

B

A

Fig. 1- The complete lateritic soil profile from Itacupin with its four horizons CI - "transitionalhorizon", C2 - ·kaolinitic horizon~. B - "horizon of AI-phosphates" and A - iron crust".

ln the lateritic standars profile in Bthe "horizon of AI-phosphates" is replaced by a "horizon of bauxite"with gibbsite and boehmite instead of AI-phosphates. 1and Il . Weathering front Il is separating thesaprolitic part from the lateritic part of the profile. The profile from ltacupin is a typical crandallite-

bearing phosphate laterite.

• the lower saprolitic part, which is composed of argillaceous material with a transitional horizon ofsmectitic composition -this horizon can be very small or even be left out- and a normally thick"kaolinic horizon".

• the latertitic part of the profile is composed of a "horizon of bauxite" (with gibbsite and boehmite)110

overlain by a -ferruginous crust- (also called .iron crust/cap- or -ferricrusn of mainly hematite andgoethite. The material in this part of the soil profile are laterites ·sensu stricto·.

The geochemistry of the standard profile is normally quite monotonous and mainly governed by thepresence of residual minerais like chromite. zircon or monazite and by adsorption processes onkaolinites and on Fe-AI-Mn-oxihydrates.

The geochemistry of the standard profile is changed considerably. however. if the parent rocks containhigher concentrations of phosphorus (> 1000 ppm). sulphur (> 1000 ppm). or arsenic (> 2 ppm).

ln ail these cases the deciding factor is the existence of minerais of the alunite resp. crandallite type:the reason is. that these minerais are able to incorporate a wide spectrum of other ions. which arenormaly not present in laterites. Whenever minerais of the crandallite group can form. thegeochemistry of the respective laterite is quite different from the standard distribution pattern.

The structure of crandallites can be represented by the formula

[121M2+ H+ [6]AI3 ([4jp0.v2(OH)6

"-v-JM3+

and it is characterized by an extremek high capacity for ion substitutions.. Inta the 12-eoordinatedlattice site big M2 + /3 + -ions can enter and also [6]AI3 + and [4]pS + may be substituted by a lot ofother ions Iike Fe3+. Cr3+. and Ga3+ or Ass +. BiS + • S6 +. Se6+. and M0 6+. By thesesubstitutions the thermodynamic stabilities of the crandallites are significantly influenced. An essentialaspect for natural soil profiles is that pure crandallite CaHAI3(P0.v2(OH)6 is not stable under naturallateritic conditions and therefore natural crandallites can only form if other stabilizing ions are available.A high concentration of P03- alone is not sufficient to form minerais of the crandallite type with their

4

high variety of geochemical compositions. However.. some crandallitic mixed crystals are of an extremethermodynamic stability and are easily formed under lateritic conditions: they are very resistant againstany form of natural or chemical decomposition.

Four cases have ta be discriminated ilS far as the geochemistry of the lateritic standard profile isaffected:

• the parent rock is only enriched in phosphate

• the parent rock has normal P04-eoncentrations but is enriched in large Mel + /3 + ions like Sr. Ba.Pb or REEs

• the parent rock is enriched in arsenic

• the parent rock is enriched in sulphur.

(1JIf only phosphorus is enrichetf. the main feature of the laterite is the non-existence of minerais of thecrandallite type. as weil in the saprolitic part as in the lateritic part of the profile. Therefore mainly pureAI-Fe-phosphates Iike

augeJitewa rditewavellitevariszitesenegalitelazuliteberaunite

(AI.Feh(P0.v(OHhNaAI3(P°.v2(OH)42H20Al3(P04h(OHh 5H20(AI.Fe)(P0.v 2H20AI2{P0.v{OHh H20AI2(Mg..Fe){OHh(P°.v2Fe2 + Fe3+ s(OH)s{P0.v4 6H20 III

can form. mainly in the zone of kaolinite decay (weathering front Il): in this way the -horizon ofbauxite- is transformed into a -horizon of AI-phosphates- and the -iron crust- is phosphatized. In thesaprolitic part of the profile relatively high P04-eoncentrations are necessary to form wavellite besideswardite. In this way normal phosphate laterites are formed which are characterized by AI-phosphatesinstead of bauxite and by an .ron crust- enriched in AI-Fe-phosphates which cause its high weatheringresistance.

Besides the enrichment of phosphorus the remaining geochemistry of the profile is not affected.

ln this case the thermodynamic stability of crandallite is markedly increased by uptake of Me -ions. Theneven average crustal concentrations of P04 are sufficient to form the corresponding crandallites(fig. 2). The sequence of thermodynamic stabilities (and enrichment)

is Pb> Sr> Ba> Ca for Me2 + • and

La>Nd>Ce> Sm>Pr>Eu>Gd>Bi for Me3 +

-8 -7 -6 -5 -4 -3 -2 -1

10;) a H3P~

wavellite

nmonazite"

gibbsite1

pH

hydraxiapctite

-8 -7 -6 -5 -4 -3 -2 -1

10;) aH3P~

1

pH

Fig? 2- The pH-log a H3P04 -diagram ror Me2 + -and Me3 + - crandallites

With exeption of Pb2 + the Me3+ -crandallites are more stable than the Mel + -members. The mostcommon stabilizing Me2+ (3 + -ions in crustal rocks are Sr2 + and REE3 + (Ce3 +). ThereFore

crandallite-bearing phosphate laterites are easily formed on sr2 +-rich diorites and on carbonatites. Theprofiles of ltacupimoand Trauira with up to 9.9% srO and of Maicuru with up to 16.9% REE are goodexamples.

(3) fnrichment ofarsenic in the parent rock has to be considered under the following aspects:

112

• Arsenic is commonly present in form of AsO/As3- and it is at first oxidized ta As3+ in weatheringfront 1. As3+ is not able ta form lateritic compounds and only after oxidation ta AsS + in weathering

front Il it is able ta substitute pS + or ta form arsenates (fig. 3).

9

8

7

6

pH 5

4

3

2

1

arsenocrandallite

gibbsite

-8 -7 -6 -5 -4 -3 -2

Fig. 3- The Eh-pH-diagram of arsenic

• The concentrations are not high enough ta form pure arsenates: the substitution of pS + by AsS +in pure AI-phosphates is low. Of some phosphates. like wardite. the As-equivalents do not even exist

• However. substitution of pS + by AsS + in crandallite is strongly stabilizing its structure. Withexception of Pb-arsenocrandallite (arsenoplumbogummite) ail arsenocrandallites are more stable (moreinsoluble) than the corresponding phosphates (fig. 4). For the

Me2+ -ions the sequence of stabilities is Ba ~ Sr > Ca - Pb

and for the M3 + -ions it is Ce> La == Nd > Eu == Pr> Sm> Bi

113

3­As04

-----------2­HAs04

---- .... --------------

As

---------------------

P H2 =1atm.- ....­-­ ....-....----------------__ Ht'sO;

-----------è!AS02-

----:- /AsOr-

Ht\s04

-- ....-........---

0.8

0.4

-0.8

-1.2

-0.4

0.0EhM

o 2 4 6 8 10 12 14pH

Fig. 4-The pH-log a H3AS04 - diagram for Mel + IMe3 + -arsecocrandallites

Of considerable geochemical importance is the high thermodynamic stability of pureCaarsenocrandallite: whenever arsenic is present in sufficient quantity no other Mel + 13 + cations areneeded to form crandallite: the Ca-ions Iiberated from the parent rocks are then able to form(arseno-)crandallite. Therefore any elevated concentration of arsenic in the parent rock leads ta theformation of (arseno-)crandallite-bearing phosphate laterites. Relatively low concentrations of As (> 20ppm) are sufficient ta stabilize the crandallite structure. Sorne of these compounds are even insolublein concentrated minerai acids.

Pure sulfates of the alunite/crandallite type are weil known. but not stable under lateritic conditions: theyare too soluble to be preserved in laterites. However. compounds with mixed P04-S04 orAs04-S04-anions crystallizing in the crandallite structure. that means (arseno-)woodhouseites. formextremely stable compounds including the Cal + -ion. The most stable cornpounds of the crandallitetype known so far are the arseno-woodhouseites: they are charaeterized by an extreme chemical andthermie stability. The sequence of stabilities is

114

for Me2+ and

for Me3+Pb> Ba >Sr> Ca

La > Ce > Nd > Pr > Bi > Sm > Eu in the woodhouseites (fig. 5)

11~-----------------"

1

9

and for Me2+and for Me3+

c(HS04-)=10-10mol/lc(Me2+13+)=10-3mol/l

-8 -7 -6 -5 -4 -3 -2 -1

log aH3P04

Fig. 5- The pH-log a H3P04 -diagram for MeZ + ,Me3+ - woodhouseites

Sr>Ba>Pb>CaNd > La > Bi > Sm > Pr > Eu > Ce in the arsenowoodhouseites (fig. 6)

Ils

8c(HS04-)=10-1Omolll

7 c(Me2+13+)=10-3mol/l

6

5pH

4

3

2

1

-9 -8 -7 -6 -5 -4 -3 -2 -1

CONCLUSIONS

log a H:As04

Fig. 6- The pH-log aH3As04-diagramfor Mel/Me3+- arsenowoodhouseites

The bearing of compounds of the crandallite type on the geochemical distribution pattern in lateritescan easily be deduced from the thermodynamic stabilities of the respective (arseno-) crandallites and{arseno-)woodhouseites: high concentrations of phosphorus in the parent rocks for them alone are notable to form lateritic crandallites: to form them the phosphorus must be associated with largerMe2 + -ions like 5r2 + or with Me3 + -ions Iike REEs3 +. especially Ce3+ . The main lateriticcrandallites are therefore goyazites (5r2 +) or f10rencites (Ce3 +). Whenever arsenic is present inelevated concentration or when sulphur is present together with phosphorus or with arsenic. then verystable {arseno-)crandallites-woodhouseites will forro. either disseminated within the laterites or underformation of an own horizon of aluminious (arseno-)phosphates/sulfates. Many lateritic elementassociations Iike Ce with As can be interpreted by the formation of (arseno-)crandallites and(arseno-)woodhouseites. That these minerais with their relatively high concentrations of As and 504could not be detected so far in natural samples is due ta the lack of crystallographic data and their highresistance against normal procedures of chemical decomposition. The normal concentrations ta beexpected for As and 5°4 in crandallite bearing phosphate laterites are in a range of 10-150 ppm for As

and of 0.1-3.5% for 504'

116

~~ lli® [1œ il UQ)$

summer schoolécole d'été

<J~~ 01~ 'UJCkL cm lite

~entol~~~

Torsten SCHWARZTechniche Universifiit, Berlin, Germany

117

Weathering processes are governed by c1imate (mainly precipitation rate and atmosphericcomposition). topography and by the charader of the parent material (Loughnan 1969). Differentparent rocks produce different weathering products under the same c1imatic conditions (Chesworth1973). Given enough time and intensity. however. weathering processes will produce laterites whichhave only very Iittle in common with their parent rock (Schellmann 1982). In geological mapping orminerai exploration. however. information is required on the composition of the parent rock underlyingthe weathering mantle. The main aim of this paper is to give an overview of the role of different parentrocks during weathering and to provide detailed information about the Iiterature on more specifiesubjects. Most books related to chemical weathering stress the importance of parent materials (Aleva1994. Bardossy & Aleva 1990. Butt & Zeegers 1992. Carroll 1970. Colman & Dethier 1986. Drever1985. Lerman & Meybeck 1988. Loughnan 1969. Martini & Chesworth 1992. McFarlane 1976. Nahon1991. Tardy 1993).

WEATHERABILITY OF DIFFERENT PARENT ROCKS

The rate of minerai dissolution depends on the type of minerai. the availability of reactive mineraisurfaces and the rate at which pore solutions are f1ushed by more dilute rain-derived waters (Kronberg& Fyfe 1989). Ali factors are strongly controlled by the type of parent rock. Goldich (1938) set up theorder of increasing weathering stability: olivine - augite - hornblende - biotite - K-feldspar - muscovite ­quartz and Ca-plagioclase - Ca-Na-plagioclase - Na-plagioclase - K-feldspar (for more details see Nahon1991). Lasaga (1984) calculated the mean Iifetime of a 1mm crystal: this is 112 years for anorthite and34 ma for quartz. Detailed descriptions on the weathering pathways of single minerais are available for

oliuine: Casey et al. 1993. Colin et al. 1980. Delvigne et al. 1979. Eggleton 1984. Grandstaff 1978.1986. Gleuher 1990. Nahon et al.. 1982.

pyroxenes and amphiboles: Berner & Schott 1982. Berner et al. 1980. Colin et al. 1990.. Goodman &Wilson 1976. Hall & Martin 1986. Locke 1986. Mogk & Locke 1988. Nakajima & Ribbe 1980. Noack etal. 1983. Petit et al.. 1987. Proust & Velde 1978. Schott & Berner 1985. Schott et al.. 1981). Velbel1989. 1992).

mica (Bisdom et al. 1982. Brilha et al. 1991. Churchman 1980. de Castro 1983. Eswaran & Heng 1976.Gilkes 1973. 1979. Lin & Clemency 1981. Norrish 1973. Pereira 1993. Proust 1982. Ramero et al. 1992.Scott & Amonette 1988. Sequeira Braga & Paquet 1987. Wilson 1970.

feldspars: Anand et al. 1985. Berner & Holdren 1977 1979. Delvigne & Martin 1970. Eswaran & Bin1978. Gardner 1983. Graham 1949. Holdren & Berner 1979. Petersen etaI. 1992.VelbeI1983.

quartz: Borger 1993. Brantley et al. 1986. Burger & Landmann 1988. Durham & Martin 1975. Eswaran& Stoops 1979. Pye & Mazzullo 1994. Schnütgen & Spàth 1983. Tietz 1987.

heauy minerais: Akimoto et al. 1984. Anand & Gilkes 1984. Bailey et a1.l956. Basu & Molinaroli 1989.Carroll 1953. Colin et al. 1993. Dimanche & Bartholomé 1976. Grimm 1973. Friedrich et al. 1981.Hartmann 1959) •. Kanig et al. 1984. Mücke 1991. Nickel et al. 1973. Schmidt & Schwarz 1991

Weathering rates for rocks at about 34 mm per 1000 years (Tardy 1969). 37mm/ka (Velbel 1985) and30 mm/ka (Dethier 1986) have been estimated. The main factor which controls the rate of weatheringis the average rainfall. Depending on the topography this affects the rate at which a weathering profileis nushed by ion-free water. thereby removing the dissolved weathering products. Furthermore. thereactivity of a parent rock also affects the weathering rate: chemical weathering of mafic rocks is on theaverage 2.5 times faster than that of acidic rocks (Nahon 1991).

118

Several papers have described weathering processes affeding different parent rocks:basait gabbro: Abreu 1991. Altemüller & Poetsch 1984. Basham 1974. Boiter 1961. Carvalho et al.1983. Cawsey & Mellon 1983. Chesworth et al. 1981. Fritz 1988. Groke et al. 1983. Loughnan 1969.Nabiar 1981. Nesbitt & Wilson 1992. Prudencio et al. 1993. Schirrmeister & Stôrr 1994. Siefferman &Millot 1970. Schwarz 1987. 1988. Sobanski & Valetan 1994. Valetan et al. 1989)

granite: Anand & Gilkes 1984. Eswaran & Bin 1978. Gardner et al. 1978. Kanig et al. 1987. McCrea1990. Murray et al. 1978. Pavich 1986. Pereira 1993. Sequeira Braga & Paquet 1987

sediments: Chauvel at al. 1983. Parron & Nahon 1980. Trescases et al. 1987. Germann et al. 1994.

Supergene enrichment on different parent rocksSupergene minerai deposits are often connected ta a preconcentration of a given element present in theparent rock (Valeton 1984. Schellmann 1971). Banded iron ores. for example. can experience asupergene enrichment Ieading ta a beneficiation of the ore (Morris 1983. 1987. Valeton & Weggen1990). The same is true for manganese (Beauvais et al. 1987. Nahon et al. 1985. Nahon & Parc 1990.Perseil & Grandin 1978). gold (Freyssinet 1994. Grey et al. 1992). nickel (Barras et al. 1992. Golightly1981). and copper (Thornber 1985. 1992. Taufen & Marchetto 1989).

Kaolin Formatian an different parent raclesCommercial kaolin deposits are almost exIusively formed on acidic parent rocks (Petrov et al. 1980).Stable quartz grains are preserved. while large kaolinite flakes arise in the place of mica by epitaxialgrowth on mica sheets.. Iron and titanium are preserved in minerais formed after biotite... K-Feldsparchanges into smaU newly formed kaolinite flakes. On acidic plagioclase halloysite appears•. while onandesine-Iabrador montmorillonite arises.. On volcanic rocks smectite forms in the lower parts ofweathering profiles. while further upwards better drainage conditions lead first to the formation ofhalloysite and. under conditions of seasonal drying. meta-halloysite and finally kaolinite forms in theuppermost parts of the profiles (Rodriguez 1982).

Baume on diHerent parent rocksLateritic bauxite forms on every parent rock containing alumina (Bardossy & Aleva 1990). Globalstatistical calculations (Bardossy 1983) showed that the abundance of different parent rocks frombauxite is similar to the overall crustal abundance of these rocks. The following percentages oftonnages of bauxite derived from different parent rocks are given by (Bardossy & Aleva 1990):

19 % from basait17 % from dolerite13 % from arkoses11 % from kaolinitic sandy clay10 % from shale or slate9 % from granite8 % from granulite

Bauxite formation from basait is described by Belinga (1972). Bali & Gilkes (1987) and Sadleir & Gilkes(I976). and on granite by Boulangé (1983). Hieronymus et al. (1989) compare bauxite formation ondifferent parent rocks in Cameroon. Haiti and Amazonia. In southeast Nigeria there is also an expressed

119

lithodependence of bauxite formation (Schwarz 1994. in print). Schellmann (1994) demonstrated thaton acidic rocks generally laterite formation by strong leaching of bath Si and AI will prevail over bauxiteformation.

GEOCHEMICAL PARENT ROCK IDENTIFICATION

During intense chemical weathering primary differences between parent rocks will be progressivelyerased. From the mixture of quartz. iron oxides and clay minerais that comprise laterite it is extremelydifficult to determine the parent rock from which it derived. However, bath weathering resistantminerais and chemically less mobile elements can record information inherited from the parent rock(Basu & Molinaroli 1989). Both the abundance and the properties of relict minerais provide informationabout their possible source. Even more information can be drawn from the ratios of chemically lessmobile elements. In the non-weathering environment such immobile elements are widely used todifferentiate volcanic material of different origins and tectanic settings (Floyd & Winchester 1978.Morrison 1978, Pearce & Cann 1973, Pearce & Norry 1979). In studies of weathering products suchmethods provide useful tools for parent rock reconstruction. Hallberg (1984) described the parent rockidentification using mainly the ZrlTi-ratio.. Beuge & Muchangos (1994) took ratios of Fe. AI. Si. and Ti.and EdUn & Tenyakov (1993) chose a triangular plot of Zr-V-cr. Parent rock identification methody arewidely applied in bath research and minerai exploration.

APPLICATION IN RESEARCH

Schroll (1964. 1968. 1979) was probably one of the first who systematically used trace elements taunravel the origin of bauxites. Differences in the parent rock composition of lateritic weatheringproducts express themse/ves also in the content of different trace e1ernents. This has been shown bySchellmann 1986. Mosser et al.. (1974•. 1979. 1985) and Boski (1990. 1991). Fischer & Germann (1987)used trace element contents to investigate the parent rocks of Egyptian laterites. Valetan (1987) appliedthis method in Greece ta correlate karst bauxites with their ultrilmaflC parent rocks. Siad (1994) usedmultivariate statistical analyses of trace element contents ta deduce the nature of the parent rocks in alaterite covered area in Nigeria. Kimura & Swindale (1967) and Lecomte & Colin (1989) usedchromium and nickel as key e/ements.

APPLICATIONS IN EXPlORAllON

Methods of parent rock identification are very important for minerai exploration in deeply weatheredterrains (Butt & Zeegers 1989. 1992: Friedrich et al. 1981. 1992. Matheis 1981. Smith & Anand 1991)Mineralizations are often covered not only by a weathering mantle but also this weathering mantleconsists of a mixture if in-situ-formed and reworked material (Lecomte 1988. Marker et al. 1994.Zeegers 1991).

PROBlEMS Of GEOCHEMICAl PARENT ROCK IDENTifiCATION

Geochemical parent rock identification depends on two assumptions which are not always given: theweathering system is a c10sed system and that weathering processes are îsovolumetric. Element ratiosare quite reliable as long as there is no selektive dissolution of one of the elements. no preferredenrichment of one of the elements and no import of one of the elements inta the weathering system.These questions can partly be answered by taking isovolumetric samples (Millot & Bonifas 1955) andthus reconstructing mass balances. However. also the volume is not always a constant in weathering 120

products.

VOLUME CHANGE DURING WEATHERING

Many metasomatic processes are associated also with volume-changes (Gresens 1967). In theweathering environment Brimhall et al. (1987. 1991) showed that volume expansions of up to 200%can occur. This has to be regarded when drawing conclusions about element mobility fromisovolumetric geochemical data (Bocquier 1983. Gardner et al. 1978. 1980. Kaster 1961. Velbel 1990).Instead of the volume. geochemicaJ data can also be normalized ta "immobile" index-elements such asZr (Schwarz 1994). Ti (Cramer & Nesbitt 1983. Katalenets & Pirogova 1981). AI (Menegotto &Formoso 1983). Fe (Schellmann 1986).

MOBILITY Of IMMOBilE ElEMENTS

Also elements which are regarded as stable will be mobilized under certain conditions (Finlow-Bates &Stumpfl 1981). Lateritic weathering leads ta a relative enrichment of relatively less mobile elements(ADAMS & RICHARDSON 1960 and many others). However. in the weathering environment noelement is completely immobile (Anderson & Hawkes 1958). Braun et al. (1993) and Boulangé & Colin(1994) show this for REE. Schellmann (1994) and Farmer & Milnes (1990) give examples of descendingAI-migration. Gold is mobile (Mann 1984).. Aise iIImenite and zircon can display strong corrosionfeatures (Schmidt & Sehwarz 1991. Grimm 1961). The question whether mobilization of elements canoccur. depends on the stability of the minerai grain it occurs in and on the specific geochemicalconditions within the weathering profile.

CONCLUSION

Although neither volume nor stable element contents can be regarded as absolute constants.geochemical methods nevertheless provide valuable information on possible parent rocks. Simple ratiossuch as ZtfTi (Hallberg 1984) are a useful tocl for broadly describing the geochemical character ofparent rocks of lateritic weathering products.

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132

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J. DERTAUX, EFROLICH, Ph. ILDEFDN5E

ORSTOM, . Laboratoire des Formotions Superficielles -UR 12133

The study of weathering and erosion processes requires to establish a mass balance of minerais fromthe soils towards the area of sedimentation. This caIls for the development of quantitative tools thatcan be applied for mixtures of very fine and sometimes poorly crystallized minerais found in soils. Theresults presented here concern the use of FTIR spectroscopy as a quantitative method in suchenvironments.

Samples were prepared using the KBr disc method. This ensures that Beer's law is valid. A quantitavedetermination of the minerai content from various blends was performed by making a lineardecomposition of the mixtures spectrum by the spectra of their constituents. To check the validity ofthe procedure. the linear decomposition was performed on spectra from artificial mixtures of standardminerais (quartz. kaolinite. gibbsite, calcite. amorphous silica). Good agreements were obtained in therange 1315 - 315 cm-1.

FTIR spectroscopy was used to quantify the minerai constituents of a lacustrine sediment cored atSalitre. Minas Gerais. Brazil. The major phases are organic matter + kaolinite + gibbsite + quartz +amorphous silica (phytoliths and sponge spicules). 53 samples of the core were analyzed. The Iineardecomposition can be applied for this sedimenl provided the IR spectral response of the organic matteris known.. ·Standard spectra· of organic matter were obtained by analyzing pure organic portion of thesediment. or by subtracting the minerai fraction from the whole spectrum of an organo-mineral sample.

The comparison of quantitative re.sults from FTIR spectroscopy with chemical. analyses by ICP-AESdemonstrates the pertinence of the method.

134

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J.L. BOEGLIN', J.L. PROBST2

1ORsrOM, temporary ot CGS (CNRS), Strasbourg2Centre de Géochimie de la Surface (CNRS), Strasbourg

135

During the period january 1990-july 1993. 85 water samples were collected twice a mon th on the Nigerriver at Bamako (Mali). Concentrations of major dissolved elements were determined at the CGS usingc1assical techniques. The study period appears to be ve~ dry compared ta the 90 last years. Thedrainage area of the Niger upper basin is about 117 000 km at Bamako.

Variations of dissolved elements concentration seem very similar for the different species (Na + . K+ ,Mg2 + . Ca2 +. HC03-) and show the same seasonal pattern for the 3 years 1990-91-92 . Highestconcentrations approximatively correspond to lowest river discharges, and vice versa. For dissolvedsilica. a similar pattern can be noticed. but the range of variations is less important. For TSS otherwise,maximum concentrations (50-60mgll) occur about 6 weeks before the peak discharge.

Specifie fluxes of dissolved or suspended matters (in kglkm2 . year) could be calculated using a partialbudget method. A correction for atmospheric contribution (rainwaters) could be made using the molarratio given by MEYBECK (1984) for the different major elements, corresponding ta wet tropical andsavanna areas. Corrected values are very close ta crude values (see table below). showing that theatrnospheric imports must be very low.

Mg2+ Ca2 +fluxes

crude

Na+

461 249 200 439 3637

S02­4

72.8 59.7

Si02 TDS TSS

2957 8075 4867

correeted. 406 223 186 406 3637 o o 2957 78154867

fluxes exprimed in kglkm2 . year

Major dissolved species are bicarbonate and silica. TSS appear here very low compared. to other tropicalrivers.. For dissolved iron and aluminium. specifie fluxes could be approximatively estimated. ta 20 and15 kglkm2 . year. Chemical compositions of cations (Na + • K+. Mg2 + + Ca2 +) and anions(HC03-, CI-, SOil only show minor seasonal and annuai variations in the Nigers' waters.

According to values obtained for the weathering coefficient RE tTARDY. 1969). withRE-3{Na+) +3{K+) + 1.2S{Mg2+) +2{ea2+HSDVI05{Na+) +05{K+) +o.7S{Mg') +{ea2+)the dominanttype of weathering in the study area belongs to the kaolinisation (mean annual RE - 1.83).

The mechanical erosion rate is estimated to 2,4 rn/My (if soil density - 2.0). The chemical weatheringrate may be calculated from the total flux of silica exported by the Niger river. considering thatweathering profiles are mainly composed of kaolinite (neoformed) and of quartz (residual). Differentcalculations of weathering rates have been performed using different proportions of quartz andkaolinite. When the ratio quartzlkaolinite increases. the weathering ratio increases also: for example. ifone considers 30% of quartz ,the weathering rate can be estimated ta 3.9m/My . In this case. thethickness of soil profiles increases From about 1.5 rn/My. which means that even in the present ratherdry conditions. laterites continue ta be developed in the upstream part of the Niger basin.

136

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B~oI~'U:iœ~(~j" RCC,...)~ ,'ceIlIltUio."~ in eeubtcJB~(".e~ e~", SalJbœ, JWq)

G. CECCANTINI', A.M. G. FIGUEIRED02,

ESONDAG3, ESOUBIÈS3,41Instituto de Biodencias, University ofS60 Poulo (USP), Brazjl

2Instituto de Pesquisas Energeticas eNucleares OPEN), Soo Paulo, Brazi{J3Laboratoire des Formotions Superficielles· UR 12, TOA Dept. ORSTOM, Bondy, France

4Laboratoire de Minéralogie, UPS, Toulouse, France137

AB5TRAO

The ·cerrado· is a savanna-like formation of Central Brazil characterized essentially by the tortuosity ofthe trees and the leaf stiffness. The role of this vegetation on the secondary dispersion of some traceelements was investigated in a tropical area where carbonatitic complexes. rich in Ti. Rare Earthelements (REE). Zr. Nb.... occurs and plant samples analysis was combined with surface wateranalysis.

The REE absolute content of vegetal ashes is high. as might be expected due to the high levelsobserved in the soils. but falls within the usual range found by other workers and no relation betweenthe plant habit (tree. shrub. herb) and the REE content is found. A weak fractionation occurs for somespecies: it concerns Ce and HREE and is similar ta that observed in the surface waters collected in aplate Iysimeter where roots are active.The high Ti content in some species may indicate that thesespecies are Ti accumulators .

INTRODUCJlON

The ·cerrado· is a savanna-Iike formation. that covers wide areas in Central Brazil (- I.S x 106 km2).The most conspicuous features of this vegetation is the tortuosity of the trees and the leaf stiffness.The fires are very frequent but the plants are weil adapted against it : barks are very thick. roots aredeep and there exist many kinds of underground organs for carbohydrates and water storage (Rizzini.1979). The role of this vegetation on the secondary dispersion of sorne trace elements was investigatedin a area where carbonatitic complexes occur.

138

ENVIRONMENT

Several ultramafic-alkaline complexes accur in the Alto Paranaiba area. in the West of Minas GeraisState (ULBRICH and GOMEZ. 1981). One of these. the Salitre complex. comprising mainly micaceouspyroxenites ("bebedourite") or peridotites and some carbonatites. has a central. dolina-shaped.depression Clagoa Campestre") partially filled by sediments and occupied by a peaty lake (Fig. 1). Athick. loose latosolic cover (hematite. gœthite. gibbsite. kaolinite. quartz. anatase. crandallite familyphosphates) surrounds the depression. overlying a thick saprolite (50-100 m). Due to the particularmineralogy of the bedrock. where perovskite. apatite and calzirtite are often essential minerais. thesealterites have high contents in Ti02 (up to 30%). REE (up to 5% LR203). Zr02 (up ta 1%) andNb205 (up to 1%).

The c1imate is tropical. characterized by a mean annual temperature of ± 20·C and a rainfall of ±1500 mm; the length of the dry season reaches five months. from July ta November. The originalvegetation consists of semideciduous forest and arboreal savanna ("cerradiio") but nowadays the coveris a mosaic of largely modified physionomy of cerrado (human aetivity : corree and ranch).

The sampled area (plants. soils and waters) is situated at the border of the Lagoa Campestre. in a zoneof secondary cerrado cover (Fig. 1).

~­---CJ'~-o.:s ~ CJ=,.;I<S'="'--'-.......~""-_.. ...1:_:::-'----'-----'

..N1

: -..... - .... , .

, ­,-N'---l ~"""................ .c__

~..--.~

O ......~--

139Trxk

Figure 1: Localisation of the Salitre area. geological context. drainage system and location ofsampled zone and of the Iysimetric experiment. (+ 1137 - altitude in meters)

MAJERIAL AND MEIHOD5

Plants

Plant samples were collected from the cerrado area at the neighborings of a Iysimeter (see below)during the the dry season (August 1993). The 14 most frequent species (7 trees. 5 shrubs. 2 herbs ­each c1ass with variable root deep) were choosen in a broad variation of tropical plant families (1 fem,12 angiosperm families). Fresh leaves and other organs (stem/wood and root not yet analysed) werecollected (300-500 g) to obtain at least 50-100 g when dry. Each sample was washed many times usingabundant deionized water. dryed. and homogenized with a mixer. The powder was put into ceramiccapsules. dryed at 105 oC for 24 h. weighted and ashed in a electric oyen at 450 oC for 12 h. Theashes were divided and each plant sample was analysed by INAA and ICP-AES. For the analysis byINAA. each ash sample or standard were sealed in a polyethylen bag and irradiated during 8 hours in1012n.cm-2.s-1 termic neutron fJow. in the IEA-R 1 reactor of IPEN-Sao Paulo. The countings ofinduced gama radiation were made by a high purity Ge detector. with resolution of 1,90 keV for the1332 keV 6OCo peak, conected to a ACE 8K ORTEC plate. For analysis of the obtained spectra wasemployed the V1SPECT software developed in the Radiochemistry Supervision. Two series (2-4 h) ofmeasures were taken 5 and 20 days after the irradiation. The reference materials used were -Citrusleaves-, -Pine Needles- (NIST) and -BE-W (ANRT).

Water and soil

Water samples were collected during and at the end of the dry seasen (lune 93 and November 93respectively) and at the end of the rain sea50n (March 94). A pan-type Iysimetric experiment wasinstalled under weakly disturbed cerrado. It consists of i) cylindrical Iysimeters containing undisturbedsoil samples and ii) collectors in form of plates settled in a pit at 10. 25. 45 and 85 cm depth. On theone hand, the cylindrical Iysimeters are isolated from the surrounding soil: 50•. they are not penetratedby roots and the collected leachates represent 80 to 95 % of the rainwater drained vertically. On theother hand.. the plate collectors do not prevent the root penetration and their adsorption. activity. andthey collect only rapid circulating water representing 2. to 10 % of rainwater (Sondag et al. 1995).

The samples were immediately filtered (0,2 J.UTl) and a subsample was acidified for traces. and REEanalysis. The REE were analysed by ICP-MS using 1151n and 187Re as internai standards.. The SlRS 2standard water sample was taken as reference material. using REE values obtained by neutron activationanalysis.

Soils were sampled in the vicinity of the Iysimetric experiment Major elements were determined byICP-AES after melting with liB02 and dilution in 2N HN03: trace e1ements. including REE. were al50determined using ICP-AES after a mixed HCI04 - HF digestion.

Species La Ce Nd Sm Eu Tb Yb Lu

Alibertia concolor 12,0 25.00 13.00 1,40 0,34 0,18 0,30 0.05Bauhinia rufa 5,3 10.00 0.57 0.14 0,11 0.24Blepharocalyx salicifolia 13,0 33,00 13,00 1.30 0,35 0,12 0.30 0,06Dalbergia mischolobium 21.5 36,70 14,00 1,90 0,43 0,15 0.29 0,04Diandrostachia chrysotrix 41.0 115,00 33,00 5,00 1.03 0,45 0,41 0.08Erythroxylum sp 12.0 24,00 9.00 1.50 0.30 0.11 0.12 0.02Gochnatia polymorpha 11.0 30.00 7.00 1.30 0.27 0.24 0,05lamanonia temata Il,6 29.00 10.00 1.30 0,31 0,13 0.39 0,01Leandra aurea 21.0 44.00 3,00 0,64 0,18 0.24 0.04Pteridium aquilinum 11.0 21,00 12,00 1,60 0,40 0.60 0.51 0.10Qualea grandifJora 5.3 14,00 0.65 0.21 0,28 0,08

Table 1: REE content (ppm) in leaf ashes of plants from the cerrado formation around the Iysimeter 140

RESUlTS

Rare Earth Elements

REE values in the different plant samples are Iisted at Table 1. The REE absolute content of vegetalashes is high. as might be expected due ta the high levels observe<! in the soils. but falls within theusual range found by other workers (Yliruokanen. 1975. Ichiashi et al.. 1992). Diandrostachia chrysotrixhas the highest REE content. This specie is a grass. about 60 cm high. with a weil developped rootsystem that extends within a radius of about 1 m and till 50 cm depth. It is one of the dominants inthe herbaceous stratum in the cerrado areas around the Lagoa campestre. thus its role may be veryimportant in the cycling of REE. Its biomass is not yet estimated.

No relation between the plant habit (tree. shrub. herb) and the REE content is found (Fig. 2). Thisresult desagrees with those found by Ichiashi et al. (1992).

0.10 ,..-------------------------------------------.

0.01

---- La--0-- Ce

••••<)-... Nr1

----a-- Sm

'--EB--- Eu

_._.~... Eu

--~_. Yb

~..i!

~ § .!! :;.

f.!! ~ § ~2

~~

§ ~ ::. oS.. 13 0 .. '2c:~ ~ Je '§0 == ~ i.. :.'l "5 t 0' :!!'§ .c; .!! ..

i! .!!! ..~ s ct

&l ~ e .!!~ .1 c: .. § J@ i "5 ~

...:li!.!!

1 ~S!' .!! ..~"'t

~... ...

l ~ ~iïi .!!Q

Fig. 2 REE abundance in the different plant species (values normalised ta the mean REE abundance insoil samples)

Owing to the great homogeneity of RH values in the different soil horizons of the Iysimeter (Table Il).REE contents of the plants and of the waters collected in the Iysimeters were normalised to the averagevalue of these soil samples (Fig. 3). For this study. only the superficial waters are of concern because ofthe high density of roots and mycorhizes in the surface horizon.

For the waters. the sum of RH concentrations ranges from 0.2 ta 0.8 ppb: the REE abundancenormalised ta average REE value of the surrounding soils shows an enrichment in HREE (Fig. 3)

ln the plants. a weak fractionation occurs for some species: it concems Ce and HREE and is similar tathat observed in the surface waters collected in the plate Iysimeter where roots are active.

141

la Ce Nd Sm Eu Dy Er Yb

0-10 653 1204 487 85 19 28 26 510-25 677 1260 516 89 20 30 27 525-45 715 1319 550 93 22 31 28 545-85 729 1352 551 96 22 34 29 585-120 727 1329 545 88 21 32 33 5Mean of surroundingsoils (N - 20) 725 1370 563 97 22 36 33 6

Table Il : REE content (ppm) in the different horizons (depth in cm) of the Iysimetric profileand in the surrounding soils (mean value)

10-r------------------------------------------,

0,1

•••-0•••. Alibertia concolor ....•... Diandrostachia chrysotrix

--0-- Lamanonia ternata --1&0- Leandra aurea

---e--' Dalbergia mischolobium

...~... Pteridium aquilinum••0-' Plate 10cm depth

-....0- Cylinder 10 cm depth

Figure 3- REE abundance in plant and water samples (values .ormalised to the mean REE abundance in

soil samples)

TItanium

The high Ti content in Alibertia concolor. Gochnatia polymorpha. lamanonia ternata. and Qualeagrandinora may indicate that these species are Ti accumulators (see Ti content in dry leaves at TableIII). In fact the genus Qualea • as weil the other Vochysiaceae. are known as aluminium accumulators.An anatomie study of these species is being conducted in such a way to understand some aspects ofthe physiology of metal accumulations in plant tissues. 142

1N. K Rb Cs MI C. Sr 8. Se TI Zn Co P Th U Ash TI SI

Spedu '*' '*' ppm ppm '*' '*' ppm ppm ppm ppm ppm ppm '*' ppm ppm '*' dm ppmdry mat

JUibertia c:onc:olor 0.12 12.20 217 15.0 4.65 13.57 2591 4919 2.5 278 192 1.6 Il.30 2.5 5.44 15 0.98

Bauhinla ru/a 0.08 3.52 1.78 10.23 624 6122 94 569 7.48 5.44 5 0.79

Blepharoealyx ,allel/olla 0.20 17.87 214 18.0 4.78 10.10 1955 153 2.3 666 3.4 3.78 2.3 0.64 0.97 6 1.03

Dalbergla mlseholobium 0.21 14.87 451 28.0 7.61 18.30 5186 1671 2.3 668 631 7.8 5.42 2.4 0.32 1.03 7 1.03

Dlandrosraehla ehrylotrbc 0.26 6.64 286 41.0 1.51 3.12 221 930 4.4 138 230 6.7 4.28 7.4 1.30 3.33 5

Erythroxylum IP 0.09 2.06 72 12.0 1.63 4.74 137 1918 1.5 46 109 1.6 4.40 2.3 0.32 1.21

Qoehnaria polymorpha 0.23 18.44 540 13.4 4.52 9.20 466 2337 1.6 876 980 2.2 3.20 2.1 0.40 3.09 27 1.21

q. polymorpha . stem 0.17 21.90 4.73 7.42 1124 2734 1310 4.79

Lamanonia temata 0.23 15.92 251 6.0 6.45 17.38 4211 2101 2.6 971 114.0 8.32 2.7 1.30 3.28 32 0.64

uandra aurea 0.05 4.47 2.40 10.82 1583 4593 75 2.28 5.44 4

Picridium aqulUnum 0.14 5.82 761 0.4 2.22 4.33 245 1487 1.3 44 199 2.3 3.76 1.6 0.37 2.52

~lea grantll/lora 0.13 Il.34 274 33.0 1.96 7.14 3211 997 0.5 469 1.4 6.04 1.4 0.48 4.23 20 1.03

Rapanea el lanellolla 0.24 25.64 6.52 12.66 1394 470 433 2.95 3.28 14 0.74

Stryphnod.mdron IP 0.18 22.95 7.93 9.81 908 334 376 6.69 3.23 12 0.70

Zanthoxylum obleurum 0.13 22.32 5.65 9.64 2524 154 292 8.13 6.67 19 0.97

Table 111- Major and trace elements content in leaf ashes of plants from the cerrado formationaround the Iysimeter and scleromorphism index (SI)

Sderomorphism index

The scleromorphism index is a variable that measures the leaf stiffness. usualy applied for comparingthe level of xerophytism among ecosystems. It is calculated with the formula:

leaf dry weight/2 x leaf area.

We determine this index (average of 25 leaves) for 10 species (Table III).

Usualy. this index is high for dry and poor in nutrients formations (cerrado. savanna) and low for wetand richer vegetations (rain forests. semideciduous forests). For the moment. we only found referencesabout relation between the scleromorphysm index and the main nutrients in the soil (N. P. 1<) (Rizzini.1976) but none about the trace elements in the plants. Therefore. we investigate the possiblecorrelation of this index with the trace elements content of the cerrado plants. We round that only aweak correlation appears between the scleromorphism index and the values of Cs (r - 0.8. N - 6).The meaning of this correlation remains unexplained. Its confirmation requires the analysis of moresamples.

Acknowledgments - The authors wish ta thank B. Dupré for support in waters anlysis. M.. Meguro forthe use of her laboratory to prepare plant analysis and P. Magat and H.. L Ozorio F.. for field assistance.This work was supported by French ECOFIT Program (ORSTOM-eNRS) and. in Brazil. by FAPESP:Projeta Ternatico. processo 91/3518-0.

BIBLIOGRAPHY

ICHIASHI. H.. MORITA. H. and TATSUKAWA. R.. 1992. Rare earth elements (REEs) in naturallygrown plants in relation ta their variations in soifs. Environ. Pollut. 76:. 157-162.

RIZZINI. C. T.. 1976. Tratado de fitageografia do Brasil:. aspectas sociol6gicos e norfsticos. Vol. 1.Hueitec-edusp•. Silo Paulo. 327 p.

R1ZZINI. C. T.. 1979.. Tratado de fitageografia do Brasil: aspectas sociol6gicos e norfsticos.. Vol. 2.Hueitec-edusp. Silo Paulo. 376 p.

Sondag. F.• Soubiès. F.. FORTUNË. J. P.• DUPRt B.• Magat. P. and MELFJ. A. 1995.Hydrogeochemistry in the soils of the -Lagoa Campestre- basin (Salitre. MG. Bruil): dynamics of Rareearth. elernents. Appl. Geochem. (in press).

lILBRICH H. H. and GOMEZ C. B.. 1981. Alkaline rocks from continental Brazil. Earth Sei. Rev. 17. 135- 154.

YLIRUOKANEN. 1.. 1975. A chemical study on the occurence of rare earths in plants. Ann. Acad.Scient Fenn.. Ser. Al. 176: 28 p.

144

~~ ill® [1m u0 Q)$

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/;wm~4~ kvuu (B~)

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A. DJEMA/l, N. MALENGREAU2,G. LAUQUET2, J.~ MULLERl)

1ORSTOM, UR 12 "Géosciences de l'Environnement Tropicol': Bondy2laboratoire de Minéralogie-Cristallographie, UA CNRS 09 and IPGP, Universités de Paris VI et VII

145

Natural kaolins commonly contain (AI.Fe}-oxides/oxihydroxides and Ti-oxides (Muller and Calas.1993).

Recent developments in spectroscopic methods allow to discuss in details the location of these oxidesin soifs and sediments (Malengreau et al. 1994.199S).

On account of its high sensitivity.second derivative Diffuse Renectance Spectroscopy (ORS) allow theirprecise determination and their distribution (coatings or inciusions}.Its provide c1ues about theevolution of physico-ehemical conditions of clay mineraIs during the formation of sediments and soils.

The objective of this paper is to demonstrate that ORS investigation of kaolinites and associated Fe­oxides provide a basis for an interpretation of the formation of tropical soils (Iatosols) at the expense ofsediments.

A latosol differentiated at the expense of cretaceousltertiary kaolins from the Amazon basin (Pedro andMelfi. 1983: Lucas et al.. 1986: Chauvel et al.. 1987) has been selected for that demonstration.

Samples were collected along a transect cross cutting the sediments and the latosol system.

Ali kaolins were bleached using dithionite-eitrate-bicarbonate (DCB) method of Mehra and Jackson(1960).

Diffuse renectance spectra were recorded at room temperature in the UV-visible range using a CARY2300 spectrophotometer.

Data reduction consists on application of the Kubelka-Munk (KM) formalism to model the absorptionof the scattered light by tightly packed Fe-oxides (Barron and Torrent.l986: Jepson. 1988) under theform of a remission function •. which talœs into account Iight scattering as weil as absorption process(Wendlandt and Hecht. 1966) .. Noise reduction of the experimental spectra was performed by fittingeach spectrum using a. cubic spline smoothing function.Second derivative functions were thencalculated using a numerical. method(Kosmas et al. 1984:Malengreau et aI.l994}.A key feature of thesecond derivative is its ability to show the location of iII-defined absorption bands and to resolve bandsthat are tao close ta be resolved in experimental spectra .

ln the visible spectral range.the position of the absorption bands is indicated by minima on the secondderivative curves (Huguenin and Jones. 1986}.In the UV domain. the absorption edge position due techarge transfer phenomena was determined by the zero of the second derivative curve (Malengreau etal .• 199S). The separation between the minimum and the maximum of each band measured in thesecond-derivative patterns were used ta estimate the amount of Fe and Ti-phases associated withkaolinite.

Second derivative diffuse renectance spectroscopy data of amazonian kaolins:

(i) allowed determination of the Fe and Ti-phases associated with kaolinite: Fe-gels. goethite. anatasetrapped within kaolinites and goethite-hematite-anatase coatings.and.

'1:' 3+(ii) yielded information on substituted( re ) in the crystallattice

The estimate amount of the Fe and Ti-phases showed the following trends :

(i) in raw kaolins. anatase. goethite and hematite content increase From the sediment to the top of thelatosol.

(i) in contrast.in bleached kaolins structural iron.together with trapped Fe-gels and goethite contentdecrease.

146

The obtained spectroscopie data allow to distinguish different generations of kaolinites particles(kaolinite + trapped oxides) along the sediment-Iatosol sequence. There are interpreted in terms ofsuccessive dissolution-recrystallisation of kaolinites.

This process conducts to a progressive leaching of iron in kaolinites and in a parallel direction to anaccumulation of Fe-oxides forming coatings.

REFERENCES:

Barron and Torrent. (1986). j.50iI.Sci.327.499-510

Chauvel.A.. lucas.Y.• Boulet.R.(1987). Experientia,43.137-147.

Huguenin an<t jones.(1986). j.Geophys.Res.91.9.585-9.598.

Jepson.W.B. (1988): ln Iron in Soil and Clay Minerais. J.W.Stucki.B.A.Goodman andU. Schwertmann.eds. Reidel.Dordrecht. 467-536.

Kosmas et al.(1984). Soil Scî.Soc.Amer.j. 48. 401-405.

Lucas.Y.•Chauvel.A..AmbrosiJ.P.(1986):Jn IntSymp.Proc.on Geoch.of the Earth Surface. Granada. 289­299..

Malengreau. N. Muller. jP.. Calas. G. (1994). Clays & Clay Minerais. 42.2.137-147.

Malengreau.N .• MullerJP.• Calas.G.(1995).Clays & Clay Minerals.(in press).

Mehra and Jackson (1960): ln Proc.7th Natl.Conf.aays Clay Miner.• Pergamon press.New york.317­327.

Muller J.P.. and Calas G. (1993) : ln kaolin Genesis and Utilization. H.H. Murray. W.M.Bundy andC.C. Harvey. eds. The aay Minerais Society of America. Boulder. Colorado. 261-289.

Pedro and Melfi.( 1983) : ln Il IntSem.Lat.Proc.• Melfi and Carvalho Ed..Sao-Paulo.3-13.

Wendlandt and Hecht(1966).Interscience publishers. John Wiley & Sons. New York. 298 pp.

147

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'Waie4~ i,u JaieJUtic d.od:L

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1.R MONTORD/

UR 24, Déportement "Eoux (ontinenfoles~ ORSTOM, Bondy

149

INTRODUOION

The Djiguinoum valley is located in an area arrected by West Arrica drought since 1970. The lowlandsdegraded by acidification and salinization are protected against seawater intrusion by an anti-salt dam.Considerable amount or runorr water generated within the catchment during the rainy season iscollected and stored ror rice culture rehabilitation and then ror evaporation in the dry season(MONTOROI. 1994).

The decrease or rice production in the valley makes more errective the pressure or rarmers on theuplands. Forest ecosystem becomes more and more degraded by human domestic activities. Crops.such as groundnut. millet and sorghum. increase regularly and soils become sensible to runorrprocesses.

This paper reports an experiment which was condueted ta evaluate the susceptibility or these soils tarunoff and their contribution ta the water supply or the valley.

MATERIALS AND METHODS

The experiments were carried out in 1990. in the southern part of the watershed., The two rollowingsoils were studied :, a ferrallitic soit (upslope) and a ferruginous one (midslope).

Three parameters were measured :

• Infiltration rate in saturated conditions with a double ring infiltrometer :, the internai ring was 55 cmacross and the external one twice as high. A water amount of 300 mm was applied with a, constantcharge of 3 cm during 2 hours ror ferrallitic soil and less than 1 h for rerruginous soil. For the last one.two tests were carried out

• Infiltration rate in natural conditions. during the rainy season :, soil water profiles were monitored witha neutron probe (Nardeux HumisoQ ta a. maximum of 300 cm.

• Runoff data were collected for each soil, with, a rainfall simulator used at three locations characterizedby cultivated. natural and rallow surfaces. The field experiment was conducted under controlledconditions on a 1 m2 plot (1 m x 1 m) : the rain pattern was adapted ta the regional conditions(CASENAVE and VALENTIN. 1989 : ALBERGEL and al.. 1991).

A water balance was calculated for each soil taking into account the rainrall. runoff and water storageamounts.

RESULTS AND DISCUSSION

Soil water infiltration

Figure 1 shows the maximum water profile during the rainy season and the total porosity profile foreach soil. The saturation conditions are never reached. For the rerrallitic soil. infiltration is regular withan average rate of 90 mm j-1. The maximum saturation rate ranges from 75 to 80 %. For thererruginous soil. temporarly saturated layers related ta the decrease of total porosity can occur with thestrongest rains. The average infiltration rate is 67 mm j-I and the maximum saturation rate ranges fro":\40 ta 60 %.

ln saturated conditions. the permeability is higher for the ferruginous soil (350 to 600 mm h-I) thanthe ferrallitic soil (100 mm h-I). The increase or total porosity in the upper horizons explains thehighest values of permeability.

RunoffISO

Runoff coefficients are higher for the ferruginous soil whatever the type of soil surface. When theferrallitic soil is already wet. runoff can rapidly appear (34 %).

Wafer balance

Table 1 presents the different parameters of water balance carried out for a soil depth of 265 cm.Drainage fluxes are higher for the ferrallitic soil and supply ground water 5 months after the beginningof the rainy season due to the water table depth (about 25 m). For the ferruginous soil, verticaldrainage water reaches ground water 3 months after the first rainfalls.

Ferrallitic sailcm3lcm3

0,1 0,2 0,3 0,4 0,5

Ferruginous sailcm3Icm3

0,1 0,2 0,3 0,4 0,5

300 300 "'-_"'-......1----1_--1_....1z(cm) z(cm)

100 I-----lf-----l~-+--+-..

- Total porosity -- Maxinum Hv - - - - - Hv at the endof rainy season

Figure 1- Moisture profiles for. two soils of the Djiguinoum watershed during the 1990 rainy season(Hv soil water content)

Period RainfaIJ Runoff Waœr Storage n ETR+ Drainagestorage variation

(mm) Ci) (mm) (mm j-I)

Femdlitie soU16 June 490.8

1081.5 19.0 214.9 127 847.6 6.721 October 705.5

Fe'~nous soit16 June 299.1

1068.3 28.8 237.5 127 802.0 6.3210ct0ber 536.6

n number of days for the considered period ETR actual evapotranspiration

Table 1- Water balance for two soils of the Djiguinoum watershed during the 1990 rainy season

151

CONCLUSION

For the two studied soils. water movements are very different and can be explained by their morphological andphysical characteristics. In the watershed. water and solute nuxes lead to soil transformations along the catena(CHAUVEL. (977).

Runoff processes cause colluvial deposits in the lowlands. Geochemical processes in valley soils depend on thesoluble and solid matter supplied by the upper part of the watershed. In lower Casamance. even many soil erosionfeatures are visible in landscapes. the soil loss is not so high as to worry the farmers in the short term.

However. in this c1imatic zone. erosion risks exist for the cropping areas or are potential for the forest surfaces. Itseems that the human activities related to the environmental evolution are the main factor likely to accelerate furthererosion development. Conservation measures (smooth clearing, higher fallow duration, organic matter supply) in theuplands will be recommended to prevent an irreversible transformation of landscapes and to sustain the currentproduction system (ROOSE and SARRAILH. 1989-90).

LrrERATURE CITED.

ALBERGEL O,), BERNARD (A.). BRUNET (O.). MONTOROI O.P.), 1991. Projet pilote -Casamance-. Bas-fond deDjiguinoum. Rapport de synthèse: morpho-pédologie. Multigr.• ORSTOM/ISRAlR3S/IRAT, Dakar, 27 p.

CASENAVE (A.). VALENTIN (C.), 1989. Les états de surface de la zone sahélienne. Innuence sur finfiltration.Didactiques. ORSTOM•. Paris, 230 p.

CHAUVEL (A.) •. 1977. Recherches sur la transformation des sols ferrallitiques dans la zone tropicale. ~ saisonscontrastées. Trav.. et Doc.• ORSTOM, Paris. 62, 532 p.

MONTOROI. O.P.). 1994. La. dynamique de l'eau et la. géochimie des sels du bassin versant de Djiguinoum(Casamance. Sénégal). Conséquences sur la gestion durable de récosystème de mangrove en période de sécheresse.Thèse Doct.. Univ.. Henri Poincaré. Nancy 1. multigr., 349 p.

ROOSE (E.). SARRAILH O.M.). 1989·90. Erodibilité de quelques sols tropicaux. Vingt années de mesure en parcellesd'érosion sous pluies naturelles.. Cah. ORSTOM, sér. Pédol.. XXV( 1-2) : 7-30

152

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Université Catholique cie Louvain3, Place Louis Pasteur, 8-1348 Louvain la Neuve -Belgique

153

Retaining values above a given raw cut off grade. to fix zones to be concerned by a follow up programis a method still used in sorne geochemieal exploration campains.

However. in a methodological study in the Muyinga gold prospect. (North East of Burundi. Fig 1 andPhoto 1 to 4). with samples from bed rock. saprolite. red and dark nodular horizons (Photo 5 to 7).analysed for both gold and multielements in fine. medium and coarse fractions sizes. binary diagramspoint out that sorne values retained by such method may appear less interesting than sorne othersbelow the cut off used. Gold contents are iIIustrated in figures 2 to S.

For trace elements. sorne of them often used as gord pathfinders. graphie processing data. as iIIustratedin the Cr/Fe. V/Fe and As/Fe binary diagrams hereunder. appears as an easy and rapid method allowinggeochemist explorers to distinguish values simply related to supergene enrichment process from thoseassociated with the mineralized bed rock. In the Ct/Fe diagram. ail points values are gathered in a-single- correlation where is weil iIIustrated the supergene enrichment process: low. intermediate andhigh values are respectively associated with bed rock. saprolite and superficial horizons (Fig.6A). Valuesretained by a raw 200 ppm Cr eut off are related to medium and coarse sizes of red and dark nodularhorizon (Fig. 1B).

ln the V/Fe diagram. a raw ISO ppm V cut off subdivides points values in -two groups-: a lowerdomain where is concentrated data related to bed rock or saprolite. and an upper one where we findnodular horizons related data (Fig.7A). In the later•. low and high vanadium values are associatedrespectively with fine and coarse sile. (Fig.7B).

As on the one hand there is no vanadium nor chromium m·inerals in those nodular horizons. mainlycomposed of iron {hydr)oxydes. and on the other hand those later are more abondant in coarse than infine size. we attribute to those Cr and V high values a relative importance related more to the relativeabondance of their phase carreer than to the mineralized bed rock.

ln the As/Fe diagram (Fig SA. B). points values are on the contrary arranged in such way that. on theone hand. a use of a raw 125 ppm As cut off will retain together samples From both nodular horizons.saprolite and bed rock. On the other hand. bed rock and saprolite related points are grouped over whennodular horizons related points are concentrated under a oblique line. independently of their fractionsize. We consider those points values gathering as controlled by two different factor: an anormalouscaraeter related ta the mineralized bed rock for the former and a supergene enrichment process for thelater.

154

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Pascal PODWOJEWSK/lEmmanuel BOURDON2

1ORSTOM, (entre de Géochimie de la Surface (CNRS), " rue Blessig, 67084 Strasbourg2Centre OR5TOM, BP AS, Nouméa, Nouvelle Calédonie

155

A sheet of peridotite. mainly dunite and harburgite. covers 30% of the surface of the tropical island ofNew Caledonia (166° to 168°E. 20 to 2rS). Since the Miocene. intensive tropical weathering hasaffected the peridotite massir. On the peridotite formations. the yearly average rainfall ranges from 2000to over 4000 mm.

Under rapid conditions of drainage. and a humid c1imate. MgO and Si02 (which represent respectively45 and 40% of the peridotite chemical composition) are wholly exported in solution from landscape.and the weathering residues are essentially Fe203 (> 60%). AI203 (5-10%). Cr203 (1-3%). Theexportation of 90 % of the initial rock component creates some"karstic" features. and dolines arefrequent

These weathering residues form very thick oxidic soil profiles (over 20 m) where AI substituted goethite(10-12 % A1203) is the main component of the upper part of the soil profile. These soils are neverindurated when located on slopes. with good drainage conditions. However. the top of most of theperidotitic mounds. fiat peneplains stepped on the sides of isolated perodotite massifs. and the southemplain are composed of strongly indurated hydroxydes forming alto 2 m thick hard horizon. Thishorizon always occurs on the upperpart of the weathering zone. This induration process has beenobserved in a large swampy depression located in the peneplain of the Great Southern Massif. A rapiddecrease in the water level (caused by a tectonic movement) induced the precipitation of a very low AIsubstituted goethite associated in places with lepidocrocite around pores or around plant roots. In thesereduced conditions. goethite is associated with manganese oxydes and with siderite. Goethitepseudomorphs after vegetal cells forms a reticular network. which is composed of vermiform orscoriaceous hard accumulations.. This type of hardening is fast. recent « 120000 yrs). massive andgeneralized to the whole fiat platform that has emerged from the swampy depression. The hardeningmay occur in ail dolines or swampy depression filled with oxidic materials in reduced conditions. whenthe watertable level drops rapidly. This hardening process may also he extended to the formation ofmost. ironpans occurring on New Caledonian peridotites. The vermiform and scoriaceousaccumulations resembles formation of plinthite. but not ferricrete because there are no hematiticconcretion and/or pisolitic accumulation. These hard accumulations do not form in kaolinicenvironments with contrasted c1imatic conditions. but in oxidic environments with humid c1imaticconditions.

156

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L. TROMB/NO' &M. CRfMASCH/2

1Università deg/i Studi di Mi/ana, Dipartimento di Sàenze della Terra, Selione Ge%gida ePa/eont%gia2(onsig/io Nazionale delle Ricerche, (entro di Studio per la Geodinamico, A/pina eQuaternaria

Milano157

GENERAL OUTLINE

The Tadrart Acacus Massif is located in the south-western portion of the Lybia (Fezzan Region): it isFormed by a monoclinal aligned North-South (from 24D30'N to 26DN latitude anf from IODE toIOD3Q4E 10ngitude).The monoclinal is weakly dipping towards East. while the upsfope West scarp isstrongly steepy : due to its asymetric profile. it looks like a cuesta relief. From the geological point ofview. the Tadrart Acacus Massif is constitued of marine and continental sandstones and subordinatelymarine shales. ranging from Cambrian ta Devonian age.

The hydrographie network of the massif is composed of two different units : the ofdest one drain taEast. with a very complex dendritic pattern and seme orders of fluvial terraces : the younger networkdrain ta Westn producing the inversion of the drainage. At the foot of the West scarp of the massifthere are seme siope deposits. Le. glacis and alluvial fans.

The c1imate of the Tadrart Acacus Massif area is hyperarid : annual rainfall ranges from 0 to 10 mmand the mean annual temperature is 30·C (minimum 4.5DC. maximum 45DC). with a mean annualrelative humidity of 32 %.

ARID AREAS PEDOGENESIS

Due ta its c1imatic conditions. we can ragards Tadrart Acacus Massif area as a really arid (hyperarid)region. In facto from the pedologieal point of view. we can found seme of the typical desert soilfeatures :

• soirs c1eveloped on c1astic materials (mechanieal weathering of the bedrock). on poorly sorted fluvialsediments or on aeolian dunes :

• silt and clay sizedhorizons connected with extemal sources of allochthonous materials. for instanceatmospheric fa II-out of aeolian dust and hydromorphic phenomena (water table fluctuations) :

• Water-seluble salts accumulation:

• Clay fraction characterised by juvenile weathering minerais.

On the other hand. in the desert areas is often possible identify some traces of quite differentpedogenetie phases : ancient and relict soils. soil crusts (paleosols in a broad sense). that are notcorresponding ta the pedoenvironmental condition of the present day. Ali these evidences are usuallyinterpreted as paleoclimatic indicatars of seme "humid" phases that look place in the past

This poster presentation is focused on micromorphological approach concerning some of these palee­evidences located precisely in the Tadrart Acacus Massif area. We must specify that the present workconstitute a section of a more complex study spanning From the geomorphology to thearchaeopedology : at this precise moment we are exploiting the paleopedological approach. carrying outfurther analyses (micromorphometry and image analysis. particle-size. XRF. XRD. Feoxl Feo) on ailprofiles of which we are discussing in this presentation.

PALEOSOLS ON TERRACES AND PEDIMENTS IN THE TADRART ACAruS MASSIF

Two main evidences were identified in the study area : the first one is located on the fluvial terracessystem of the eastern side of the Massif. the second one pertains to the pediments and glacis. footslopeof the West fringe of the trench.

158

A first group of some profiles (LEO. T 18. T19 and TAO) are located on the terraces of the eastern slopeof the Tadrart Acacus . They are Iying on a sandstone substratum and are characterised by thepresence of a weathered red (2.5 YR from 4/6 to 5/6) B horizons. up to 1 m thick. that showsubangular blocky structure and a great amount of silty and c1eyey fractions in the grain-size : theblocks are hard or very hard. pores are common or dominant ; in each profile were identified someCaC03 concretions but they can show the maximum expression at different deep. At the top of these Bhorizons there are usually few decimetres of yellow sand with very dominant pebbles of sandstoneblack coated ; at the bottom of the profiles there are less weathered horizon (C horizon). up to 60 cmthick. often with carbonate concretions. which overlie the sandstone parent rock. Some archeologicalartefacts. referred to Acheulean age. were found at the top of the profiles and they can suggest theMiddle Pleistocene as the minimum age for these soils.

A sequence of pediments and glacis of different ages is located footslope of the West fringe of theTadrart Acacus Massif. At their top they show relict paleosols that are developed bath in erosionalsurfaces and in the gravel of the glacis. The surface of the glacis is generally cevered by graved andpebblezs of standstone. coated by the black deset varnish. The described profiles (TM. TA6. TAS-SER)are composed of a red B2 horizon (sub-horizon sequence B21. Bn. B23) showing a colour rangingfrom 2.5 YR 5/6 to 5 YR 5/4. silty-Ioamy grain-size and subangular blocky structure: an allochthonouscontribution of aeolian sand was found within the top levels of these horizons. The thickness of thehorizon is comprised hetween. 25 and 105 cm. The B2 horizons owerlie a sandy/sandylloamy reddish­brown/brown C horizons showing weathered graver and sometimes saline crusts in the topcentimetres.. The desceihed paleosols can be ordered in a postincisive chronosequence in whichthickness. complexity. rubification and fine fractions content of the B horizons increase in the oldestmemœrs.

MICROMORPHOLOGICAL APPROACH

The micromorphological approach was carried out following the terminology of the Handbook of ThinSoil Sections Description (Bullock et al.• 1985). We propose a short qualitative description of mainfeatures of maximum weathering horizons (B) for each profile.

From the micromorphological point of view ail the described B horizons of the identified paleosols arevery similar. They are characterised by iron rich clay material showing microped paleosols are verysimilar. They are characterised by iron rich clay material showing microped patteans rpseudosand- andundifferentiated to weakly speckled b-fabric; the coarse material is composed by quartz grains wealdyweathred. sometimes clay and iron coated. and by fragments of sandstone thzt show evidences ofpreceding weathering and Fe-Me impregnation. and couId he interpreted as -pedorelicts- according toBrever's terrninology : the pedofeatures are constitued by two different groups: few. deeply fragmentedand deformed clay coatings. related to voids and/or grains: few or frequent calcite nodules. typic andimpregnative. with a great dimensional range in diFferent profiles. We would like to precise that weprefer conceive the weathered sandstone as coarse material (i.e -pedorelictl and not as amorphouspseudomorphic pedofeatures. in order to emphasise the diHerent pedogenetic processes thet affectedthese relict paleosols.

CONCLUSIONS

Field and. above ail. micromorphological characteristics indicate that these relict paleosols canredardedas lateritic soils and can he classified as oxic materials. It is c1ear that they are no true laterites sensustricto. but however they show Features that can be related to stong weathering lateritic-like :particularly we would Iike to underline :

• the homogeneity of the groundmass159

• the compact micropeds of the micromass

• The weakly expressed b-fabric (undifferentiated or speckled)

• Coarse fraction of least weatherable elements

• the texturai pedofeatures. few and often fragmented and deformed

After this general pedogenetic characterisation. we can precise the significance of other featuresdescribed in the paleosols: the presence of calcite nodules can be explained by the present daypedogenetic processes that have taken place since the onset of dry conditions : the microstructuresgrain dominated (bridged and pellicular grain) described for two profiles could be related to sandy oxicmaterials. with clay content not sufficiently high (Iess than 15%). but with the othermicromorphologica/ features corresponding to the normal oxic materials. This hypothesis is inconformity with particle size analyses.

As far as the chronosequence soils is concerned. first micromorphological observations are inagreement with fields descriptions (e.g. expression of features. df ratio) •. but an exhaustive study of thechronosequence is necessarily tied to comparisons of ail horizons of who/e profiles. that willdelayed tofurther studies.

Finally. from thez paleoclimatic point of view. we can conclude that the descnbed paleosols are not inequilibrium respect to the hyperarid condition of the present day. but the cou/d be. connected to thestrong c1imatic changes that took place. in the Sahara area. since Late Tertiary. during the Middle andEarly Pleistocene. when less arid conditions were favourab/e to a pedogenetic regime characterised bystrog weathering?

ACKNOWLEDGEMENTS

First of ail we must specify that the whole analytical approach of the present work has been carryingout thank to support by the CNR - Centro di Studio per la Geodinamica Alpina e Quatemaria and wewould Iike to thank the director. Prof. P.M. Rossi for his heJp.

A special thank to ail the members of the Libyannn ltalian Archaeological Joint Missions (1989-1994)and to our colleagues of the Libyan Department of Antiquities.

ESSENl'IAL BIBLIOGRAPHY

ABLlDELGAWAD G.M. & BEN MAHMOUD K (1980) - Avanced Paleoclimate Weathering in Soil ofthe Fezzan Area of Libya • In SALEM j. & BUSREWIL S. (En) - The Ge%gy of Lybia - pp 1803-1825 ­Elsevier Amsterdam

BLlLLOCK P.• FEDOROFF N.. JONGERIUS A.. STOOPS G.• TURSINA T. & BABEL C. (1985) ­Handbook for Soil Thin Section Description - Waine Reseach Publication. Albrighton

BREWER R.( 1964) - Fabric and Mineral Analiysis of Soils - John Wiley & Sons. New york

DOUGLAS L.A. & THOMPSON M. L. (ED) (1985) - Soil Micromorphology and Soil Classification ­SSSA Special Publication Number 15. Madison.

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HAMILTON R. (1964) • Microscopic Studies of Laterite Formations - ln JONGERIUS A (ED) • SoilMieromorphology • pp 269-278 - Elsevier. Amsterdam

MC FARLANE M.J. (1976) • Laterite and Landscape - Academie Press. London

TARDY Y. (1994) - Petrologie des Laterites et des Sols Tropicaux· Masson. Paris.

WATSON A.(1992) - Desert Soils - ln: MARTINII.P.. CHESWORTH W. (ED) - Weathering. Soils andPaleosols - pp 225-260 - Elsevier. Amsterdam.

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