lithium isotopes as a probe of weathering processes: orinoco river

Lithium isotopes as a probe of weathering processes:Orinoco River

Youngsook Huh a;*, Lui-Heung Chan b, John M. Edmond c

a Department of Geological Sciences, Northwestern University, 1847 Sheridan Road, Evanston, IL 60208-2150, USAb Department of Geology and Geophysics, Louisiana State University, E229 Howe-Russell Complex, Baton Rouge, LA 70803, USAc Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 42 Carleton Street, Cambridge,

MA 02139, USA

Received 22 December 2000; accepted 27 September 2001

Abstract

Lithium isotopes have the potential to be effective tracers of weathering processes due to their large relative massdifference and therefore fractionation. In this study an attempt is made to fill a major gap in the knowledge of Liisotope fractionation during continental weathering and of the mechanisms involved. Finally the relationship betweenthe suspended and dissolved material is made on a basin-wide scale. The Orinoco basin provides a clear contrast inreaction-limited and transport-limited weathering regimes that has already been documented by a comprehensive studyon its fluvial geochemistry (Edmond et al., Geochim. Cosmochim. Acta 60 (1996) 2949^2976; Edmond et al., Geochim.Cosmochim. Acta 59 (1995) 3301^3325). Conspicuous in our new results is the difference in N6Li of the dissolved loadbetween the Andean (330 to 322x) and Shield (322 to 37x) tributaries, while the N6Li of the suspended load issimilar between the two. To a first approximation, during superficial weathering in high-relief, tectonically activeterrains the dissolved load is high in Li and isotopically heavy (more negative N6Li), whereas in stable Shield regions theconcentrations are low and isotopically light in proportion to the increasing degree of weathering. ß 2001 ElsevierScience B.V. All rights reserved.

Keywords: lithium; stable isotopes; weathering; Orinoco River

1. Introduction

The Earth's average atmospheric temperature,as a measure of its climate, has deteriorated byabout 15³C over the last 50 million yr [3]. Thedecrease of the atmospheric greenhouse gas CO2

has almost certainly been responsible for this.

However, the governing mechanisms of the car-bon cycle over geological time scales are not wellunderstood. Assuming the source term, volcanicemissions from the mantle, varies relatively con-stantly with time, the ideal case would be to ¢ndan explicit, integrative, sedimentary signal of theresponse of the sink term, i.e. silicate weathering.The validity of this `proxy' would have to be¢rmly anchored on examination of its behaviorin the present environment.

It is becoming increasingly clear that no singleproxy will tell the whole story of past silicate

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 2 3 - 4

* Corresponding author. Tel. : +1-847-491-7539;Fax: +1-847-491-8060.

E-mail address: [email protected] (Y. Huh).

EPSL 6025 17-12-01

Earth and Planetary Science Letters 194 (2001) 189^199

www.elsevier.com/locate/epsl

weathering intensity. The marine strontium iso-topes record extreme radiogenic events as occurin the Himalayas where radiogenic Sr is remobi-lized during underthrusting and subsequent un-roo¢ng associated with plate collision and recordminima due to a combination of carbonate weath-ering and hydrothermal input, but not `average'silicate weathering [4^8]. Osmium isotopes are toa large extent indicative of exposures of blackshales [9] but are not necessarily dominated byHimalayan-type events or silicate weathering ingeneral [10^12].

Whereas Sr and Os isotopes do not fractionateduring weathering, it might be expected, becauseof the large relative mass di¡erence between thetwo stable isotopes, 6Li and 7Li, that there could

be large fractionations for Li. The magnitude ofthe fractionation would be indicative of whetherthe reaction is allowed to proceed to completionor not (equilibrium versus kinetic isotope e¡ects)and the Rayleigh-type depletion of the reservoir.Therefore, Li isotopes potentially can be used todi¡erentiate between the reaction-limited weather-ing currently in operation in high-relief, tectoni-cally active regions [1] and on Arctic cratons [13]and the transport-limited, laterite-generating typeas occurs in the Tropical cratons [2].

A previous reconnaissance study of the dis-solved load of major world rivers showed a largerange in N6Li from 36 to 332x, encompassingthe mantle as inferred from MORB (34x) andseawater values (332x) [14]. The discharge-

Fig. 1. Sample locations (solid circles) in the Orinoco drainage basin in South America. The stippled area indicates the GuayanaShield. The base map is from [1].

EPSL 6025 17-12-01

Y. Huh et al. / Earth and Planetary Science Letters 194 (2001) 189^199190

weighted mean was 323x. It is this range, com-parable to that of C, S and B, that requires ex-planation. Since the chemistry of Li is predomi-nantly inorganic like B, and unlike C and S, themechanism must lie in the weathering cycle and insubsequent diagenetic reactions both terrestrialand marine. The very `well-behaved' oceanic sys-tem, where the fractionation factor associatedwith low-temperature alteration of basalt by sea-water could be determined directly [15], led to theexpectation that the much more complicatedwater^rock interactions that occur during conti-nental weathering can respond on relatively shorttime scales to climatic changes depending on thelithologic exposure mechanism. In the presentstudy, we have analyzed both the suspended anddissolved loads of the major tributaries of theOrinoco basin in order to deduce the behaviorof Li isotopes during weathering.

2. Samples

The Orinoco is the third largest in the world inwater and 10th in sediment discharge; its drainagearea is about 106 km2. It is perhaps the best char-acterized among the large pristine rivers in termsof the suspended and dissolved £uxes [1,2,16,17].The river £ows along the boundary between twomarkedly di¡erent terrains (Fig. 1). The rightbank tributaries drain the deeply eroded crystal-line bedrock of the Precambrian Guayana Shield.The material in the left bank tributaries is gener-ated in the Andes and is transported across thewide area of Tertiary alluvial plains, the `Llanos'.Fluxes of suspended and dissolved loads areasymmetric between the left and the right banktributaries, with 90^95% of the suspended sedi-ment and s 80% of the dissolved load suppliedby the three largest tributaries of the Andes (theGuaviare, Meta and Apure) and only a minorquantity derived from those of the GuayanaShield [17] (Fig. 2a,b). The dichotomy is notice-able in other chemical elements as well. The rightbank rivers, relative to the left, are very dilute indissolved cations, with negligible Cl and SO4, andalkalinities commonly negative due to the pres-ence of signi¢cant concentrations of organic acids

dissolved from their soil [2]. The Shield rivers alsosupply V40% of the K, as large areas are com-posed of K-rich granitic rocks, e.g. the ParguazaBatholith. Sources of water and dissolved K andalso silica are almost equally divided between An-dean and Shield tributaries, the latter re£ectingthe intense weathering of the basement rocks tokaolinite and gibbsite [2] (Fig. 2c,d). The sus-pended sediment samples of rivers that drain themountain belts are rich in unstable and cation-rich 2:1 clays, including illite, vermiculite, smec-tite [16,18]. After passing through the Llanos, thesediments undergo further weathering and aremostly kaolinite with minor illite [16,18]. In theShield tributaries the suspended sediments are cat-ion-depleted, consisting of kaolinite and minorgibbsite [16,18]. Areal weathering rates (mol/km2/yr) estimated from exposure ages [19] and£uvial £uxes [1] are at least two orders of magni-tude greater in the Andes than on the Shield [20].The Andes shows the weathering-limited featuresof a tectonically active zone whereas on theGuayana Shield weathering is slow but goes tocompletion [1,2,21].

The seasonal contrasts in water discharge (fac-tor of 8^54) and river depth (10^12 m di¡erence)are large [22] with the maxima in August. Andeansamples and those from the main stem of the low-er Orinoco used in this study were collected atfalling stage. Of the Shield tributaries, the Cauraand Parguaza were sampled at falling stage, theCaroni at rising stage, and the Orinoco above theVentuari and the Ventuari itself at low £ow. Forthis reason and because of the inherent uncer-tainty in discharge and suspended £ux estimates,a strict mass balance calculation of Li and itsisotopes cannot be carried out with the presentdata set.

All major tributaries, Apure, Meta, and Gua-viare, draining the Andes and the Upper Orinoco,Ventuari, Caura and Caroni draining the Shield,were analyzed (Table 1). They account for 60% ofthe water discharge and 90% of suspended sedi-ment discharge and s 90% of the dissolved andsuspended lithium. One Andean river rich in con-tinental evaporites, the Cojedes, and one ex-tremely radiogenic Shield river, the Parguazawhich drains a rapakivi terrain and has 87Sr/86Sr

EPSL 6025 17-12-01

Y. Huh et al. / Earth and Planetary Science Letters 194 (2001) 189^199 191

ratios s 0.9, are also included to provide some ofthe extremes [14]. In addition to the Orinoco trib-utaries, two rivers draining ma¢c terrains and thathave the distinctively unradiogenic Sr isotope ra-

tios of such substrates (V0.704; Table 1), wereanalyzed since such terrain was not represented inthe previous reconnaissance work [14]. One is theMayn, a tributary of the Anadyr in far-eastern

Fig. 2. The Orinoco drainage system is represented in simpli¢ed bar graphs with head of the river at the bottom and the mouthat the top with Shield tributaries on the right and Andean tributaries on the left. Size of bars indicates size of £uxes regardlessof direction. (a) Mean suspended sediment discharge [16]; (b) dissolved load transport [1]; (c) mean water discharge [16]; (d) dis-solved silica £ux [1]; (e) suspended Li £ux; (f) dissolved lithium £ux. We would like to emphasize that there are 30^50% uncer-tainties associated with the water and sediment discharge estimates [16], which further propagates to £uxes of dissolved load, Siand Li even though the precision of chemical measurements are þ 5%.

EPSL 6025 17-12-01


Tab

le1

Lit

hium

conc

entr

atio

nsan

dis

otop

icra

tios

indi

ssol

ved

and

susp

ende

dlo

ads

Riv

erna

me

Sam

ple

num

ber

Dat

eG

eolo

gyb

Dis

solv

edlo

adSu

spen

ded

load

87Sr

/86

Sre

Li

(nm

ol/k

g)N6

Li

Met

hodc

Li

(Wg/

g)N6

Lid

Met

hodc

Ori

noco

basi

nC

ojed

es@

Sucr

eO

R75

003

/13/

86A

ndes

(N.

Mer

ida)

,E

0.70

9872

33

22.1

ICP

MS

Apu

re@

El

Per

roO

R31

711

/29/

82A

ndes

(Mer

ida)

,C

0.71

2483

.13

32.0

ICP

MS

62.3

36.

4F

ESA

,S

Met

aO

R30

711

/20/

82A

ndes

(Ori

enta

l)0.

7164

127

336

.1F

ESA

71.3

32.

3F

ESA

,S

Gua

viar

eab

ove

Inir

ida

OR

302

11/1

7/82

And

es(O

rien

tal)

0.71

7138

.93

30.1

IDT

IMS

38.3

30.

8F

ESA

,S

Ori

noco

abov

eV

entu

ari

OR

470

01/0

1/83

SWSh

ield

0.74

1573

.63

18.8

IDT

IMS

7.2

+0.

9ID

,F

Ven

tuar

iO

R47

103

/01/

83SW

Shie

ld0.

7415

77.4

313

.3F

ESA

5.2

35.

4ID

,F

Par

guaz

aO

R30

911

/22/

82Sh

ield

,ev

olve

d0.

8535

70.7

36.

6ID

TIM

SC

aura

blT

iqui

reO

R32

212

/02/

82Sh

ield

0.73

2356

.63

22.3

IDT

IMS

19.1

33.

3ID

,F

Ori

noco

@B

oliv

ara

OR

451

03/0

1/83

mai

nch

anne

l@

mou

th0.

7182

53.3

332

.2ID

TIM

S58

.2+

1.9

ID,

FO

rino

co@

Bol

ivar

OR

326

12/0

6/82

mai

nch

anne

l@

mou

th54

/5+

0.5

FE

SA,

SC

aron

i@

Ord

azO

R32

712

/06/

82Sh

ield

,bl

ack

0.73

2227

.13

17.0

ICP

MS

9.7

30.

6ID

,F

Ma¢

cR

iver

sM

ayn

(tri

bof

Ana

dyr)

AY

107

09/0

1/97

Okh

otsk

^Chu

kotk

a0.

7049

57.4

328

.7ID

TIM

S27

.33

1.8

ID,F

Bag

hich

a(t

rib

ofIn

dus)

PA

612

/10/

94op

hiol

ites

0.70

4526

23

22.2

IDT

IMS

Num

bers

init

alic

sar

efr

om[1

4].

aSu

spen

ded

load

was

anal

yzed

onO

R32

6(1

2/6/

82)

Ori

noco

@P

.O

rdaz

.b

E,

evap

orit

es;

C,

carb

onat

es.

cM

etho

dus

edto

dete

rmin

eL

ico

ncen

trat

ions

.IC

PM

S,in

duct

ivel

yco

uple

dpl

asm

am

ass

spec

trom

etry

;F

ESA

,£a

me

emis

sion

stan

dard

addi

tion

;ID

,is

otop

edi

lu-

tion

TIM

S;

F,

¢lte

red

;S,

tota

ldi

gest

edse

dim

ents

.dN6

Li

dete

rmin

edby

bora

teis

35.

4fo

rO

R31

7an

d3

2.1

for

OR

307.

eD

ata

from

liter

atur

e[1

,2,2

3]an

d(H

uh,

unpu

blis

hed

data

).D

ata

for

OR

750

and

OR

471

are

for

sam

ples

colle

cted

atdi

¡er

ent

tim

esfr

omth

ose

used

for

Li

anal

y-se

s.

EPSL 6025 17-12-01


Siberia draining the Okhotsk^Chukotka volcanicbelt [23]; the other is the Baghicha, a headwatertributary of the Indus draining the ophiolites ofthe Kohistan Arc in Ladakh (Huh, unpublisheddata).

3. Analytical methods

Samples are from archives held at MIT withcollection dates ranging from 1982 to 1997 (Table1). They were all ¢ltered through 0.4 Wm Nucleo-pore membrane ¢lters in the ¢eld within 24 h ofcollection with the exception of the Baghicha(PA6). Samples for analysis of the dissolvedload were stored in cleaned polyethylene bottles ;the suspended load samples, the material retainedon the ¢lters, were dried at room temperature andstored in plastic petri dishes. The latter ¢lters wereput to total dissolution in 4:1 (v/v) HF^HClO4.The desorbable and carbonate fractions, at leastin marine sediments and probably also in riverineand lacustrine precipitates, contain very little Li,and the Li isotope composition is controlledmainly by the silicates [24,25]. When largeamounts of sediments were available, the ¢lterswere scraped with Te£on spatulas; when amountswere small, whole ¢lters were digested in the dis-solution step. This digestion step was repeated

for sample OR317 and 0.5 ml of H2O2 was add-ed to sample OR307 until there was no residueleft.

For both suspended and dissolved load, Li wasisolated using cation exchange columns andloaded as Li3PO4 [26]. For two Andean samples(Apure and Meta), enough material was availablefor con¢rmation using the borate method usinglarger column sizes and loaded as Li2B4O7 [27].The results agreed within analytical error (Table1). Only a small percentage of the ion exchangecapacity of the resin was used for all samples.Samples were run on Finnigan MAT 262 thermalionization mass spectrometer at LSU [26].

The precision was better than V1x (1c)based on repeat measurements of standards andsamples. Following convention, N6Li is reportedin x units:

N6Li �

6Li7Li

� �sample3

6Li7Li

� �standard

6Li7Li

� �standard

0BBB@1CCCAW1000

where (6Li/7Li)standard = 0.08274 þ 0.00003 for apure Li2CO3 standard, NBS L-SVEC. Unlike oth-er isotopes, the lighter isotope is less abundantthan the heavier one, and more negative valuesare thus heavier.

Fig. 3. The N6Li of the major tributaries of the Orinoco draining the Guayana Shield and the Andes. Solid symbols are for sus-pended Li; open symbols are for dissolved Li.

EPSL 6025 17-12-01


4. Results

The Andean tributaries supply almost all thesuspended lithium (Fig. 2e). The dissolved lithiumdischarges are balanced between right and leftbanks (V54% from Andean rivers) (Fig. 2f) asfor silica [1]. This suggests that Li is held up inthe altered material like Si, and not leached intothe dissolved load like Ca or Na. The Meta notonly has the highest suspended sediment dis-charge; it has the highest content of Li in thesuspended load (71 ppm). In the dissolved loadlithium is also disproportionately high in theMeta. The Meta basin is rich in shales and thereare some salt domes and associated salt springs inthe headwater [28].

For all samples the suspended Li is lighter thanthe corresponding dissolved Li and relatively con-stant, in the range 36.4 to +1.9 (Table 1, Fig. 3).On the other hand, there is a very large range inthe dissolved load. The Andean rivers are heavy,330 to 336x, and comparable to seawater, andShield rivers are light, ranging from values closeto the suspended Li to 322x. It follows thatdi¡erences in the Li isotopic composition betweensuspended and dissolved loads are greater (26^34x) for Andean than for Shield (8^20x) riv-ers (Fig. 3). For comparison, fresh MORB is34x [15] and pelagic clays V311x [25,29].

The dissolved Li isotopic composition for theOrinoco basin shows some correlation with otherweathering-related parameters. The ratio of Si tototal cations corrected for the evaporite contribu-tion using Cl (Si/TZ�*), is a rough index of theextent of weathering in the fresh silicate rock-sec-ondary clay^kaolinite^gibbsite sequence. The Srisotope ratio is a broad indicator of the in£uenceof aluminosilicates (s 0.72) versus carbonates orevaporites (0.709). Shield samples are radiogenic,high in Si and light in Li [2]. Andes samples areless radiogenic, have more cations, and are heav-ier in Li [1].

The Mayn and Baghicha draining ma¢c ter-rains had N6Li of 328.7x and 322.2x, respec-tively. The 87Sr/86Sr values are very unradiogenicV0.705, representative of the lithology. The sus-pended Li is again very light, in the range of othersuspended Li of the Orinoco basin.

5. Discussion

It is clear that the Li isotope signatures of thedissolved load are generated during the weather-ing process and are not simply inherited from thelithology or rainwater. Though the rainwater ofthe Orinoco drainage basin has not been analyzedfor Li, its contribution to the dissolved Li is ex-pected to be negligible. Reported Li concentra-tions in snow samples from the Alps are 0.65^0.97 nM [30], less than 3% of that found in theOrinoco River waters. To be a lithologic tracer,the speci¢c rock type has to contain largeamounts of Li, weather fast and have a homoge-neous isotope ratio. Trace amounts of carbonatesand black shales can dominate the Sr and Os iso-topic systems because of the high solubility andcontent and characteristic isotopic ratios. How-ever, carbonates generally have very low Li con-centrations (1^2 ppm in ancient carbonates fromYellowstone, Chan, unpublished data), so theywill only have an in£uence if the weathering ofcarbonates is dominant. Foraminiferal shells andcarbonate sediments from ODP cores have Li/Camolar ratios of 0.5^2W1035 [25,26], whereas theratios in the dissolved load of the Orinoco ares 1.5W1034. Lithium is known to be enriched inshales, but because it is not sensitive to redoxpotential, black shales, which undergo fast oxida-tive weathering compared to ordinary shales, areprobably not a signi¢cant source of Li. Neverthe-less, the sulfuric acid generated during weatheringof reduced minerals, e.g. pyrite, may lead to fur-ther dissolution of Li-rich clay layers embeddedwith the black shales. Even in the latter case, theisotopic composition from weathering of shales isso variable [14] that it cannot be considered as alithologic signature. The occurrence of marineevaporites in the Andes and their absence in theShield cannot explain the dichotomy in the dis-solved N6Li, as the Li/Cl ratios for all tributariesare orders of magnitude higher (Li/Cl molar ra-tio = 92^1400W1035) than the marine ratio (Li/Clmolar ratio = 4.7W1035). The lack of a correlationbetween Li concentrations and N6Li in generaland the range in the N6Li (332 to 38x) fromterrains with radiogenic Sr (87Sr/86Srs 0.715) fur-ther suggest that lithologic e¡ects are only second-

EPSL 6025 17-12-01


ary [14]. Previous work indicated that riversdraining evaporites (Amazon at Iquitos, Cojedesof the Orinoco, and the Biryuk on the Siberianplatform) range from 320 to 322x and werethe most distinctive lithologic e¡ect seen in rivers.The newly analyzed samples from ma¢c/volcanicterrains also have values that are not distinctive oflithology (Table 1).

The suspended Li is systematically lighter thanthe dissolved load and rather homogeneous be-tween +2 and 36x. Mineralogical study of¢ne-grained sediments (6 0.2 Wm) of the Orinocotributaries showed that those from Shield rivershad cation-depleted weathering products, mostlykaolinite with minor gibbsite, and those drainingthe thrust and fold belt of the Andes had cation-rich products, e.g. vermiculite and smectite in ad-dition to kaolinite [18]. The latter after havingtraversed the foreland basin, the Llanos, is morekaolinite-rich [18]. The secondary clays are en-riched in K and Mg relative to bedrock [18]. Lith-ium behaves like Mg (similar ion size) duringweathering, and therefore as bedrock disintegratesand forms secondary clays, lithium is incorpo-rated in the clays along with Mg and Al whilethe major elements are preferentially lost. A cor-roborating observation is that higher amounts ofLi are found in ¢ner-grained material of coastalsediments [31]. Isotopic fractionation at this stage

is not known but in analogy with low-temperatureweathering of basalt, where Li is taken up fromsolution in the altered minerals with a preferencefor the lighter isotope (K= 1.019) [15,24], 6Li isexpected to be preferentially taken up in altera-tion products. Therefore, the unweathered bed-rock is expected to have been heavier than thesuspended load. Given that the transit time ofwater from the headwaters to the mouth is V40days [1] it is not likely that the dissolved Li under-goes signi¢cant exchange with the suspended Liwithin the river channels, except fast adsorp-tion^desorption. The latter is reversible in the ab-sence of large changes in concentrations of com-peting cations, e.g. NH4� [24]. The partitioningand fractionation of Li probably occur mostly insoils.

The reason for 6Li enrichment in clays has notbeen addressed speci¢cally. Two major processescan be envisioned for isotope exchange betweenminerals and £uids: di¡usion and surface reaction(recrystallization or the formation of a new phase)[32]. The average velocity of the lighter isotope insolution is faster (8% for 6Li), and as Li has high-er solid state di¡usion rates than other alkali cat-ions, at least in albite [33], di¡usion is expected tobe an important mode of transport and of frac-tionation of Li during the weathering process.Bonds containing the lighter isotope have higher

Fig. 4. The relationship between the isotopic composition of the dissolved load and other weathering-related parameters: leftpanel: Si/TZ�* and right panel: 87Sr/86Sr. TZ�* is the total cationic charge corrected for evaporite (halite and gypsum/anhydrite)input assuming that negligible amount of Cl and SO4 are from sea-salts or other lithologies. TZ�* (equiv-alents) = 2[Ca]+2[Mg]+[Na]+[K]3[Cl]32[SO4]. Squares, Shield; triangles, Andes; circles, Orinoco main channel.

EPSL 6025 17-12-01


vibrational energy and during a chemical reaction(e.g. weathering), molecules bearing the lighterisotopes will react more readily, enriching the re-action product in the lighter isotope. It is pre-dicted based on the above kinetic isotope e¡ectsthat 6Li-containing bonds will preferentially breakand di¡use. After the bonds are broken enrich-ment of the light isotope in the altered solid phaserather than the percolating water can be explainedby the hydration e¡ect, which is very strong forLi. Because of the high vibrational frequency ofwater (1600^3900 cm31) compared to commonminerals (6 1000 cm31), water has a tendencyto incorporate the heavy isotope preferentially inorder to lower the free energy of the system [32].Thus, the clay fraction ends up with the lighterisotope. During cation exchange chromatographya similar e¡ect is observed: 6Li preferentially goesinto the resin phase regardless of the type of cat-ion exchanger and the counter ion in the solutionphase (K= 1.00089^1.00171 [34] ; 1.022 [35]). Thisis opposite what is found for other alkali ions, e.g.K and Rb, attributable to the stronger isotopee¡ect associated with hydration than phase ex-change for Li, where the opposite is true for Kand Rb [34].

If indeed the observed isotopic variation in theriver dissolved Li is primarily due to fractionationduring di¡usion and bond breaking during forma-tion of clays, then the clear di¡erence betweenheavier Andean rivers and lighter Shield rivers(Fig. 3) and the correlation with Si/TZ�* and87Sr/86Sr (Fig. 4) can be explained by the `trans-port- versus reaction-limited' argument. In re-gions where, due to tectonics or lithology, theweathering is rapid and super¢cial, kinetic isotopee¡ects are important and 6Li is preferentially in-corporated into alteration phases. In stable Shieldregions where transport is the limiting factor, thereaction is slow but complete, and equilibrium isapproached between water and the alterationclays. That the system achieves close to equilibri-um conditions in the Shield tributaries is also evi-denced by the phase diagrams and the Rb^Sr iso-chron relationship seen in the dissolved load [2].This is a situation of Rayleigh fractionation whereone begins to see a `reservoir e¡ect'. The heavyLi is removed in solution and the alteration

phases become lighter and lighter as reaction pro-ceeds.

The fact that the dissolved N6Li of the Orinocoat the mouth is in the range of the Andean riversis puzzling, considering the fact that the Shieldand Andean rivers supply equal amounts of dis-solved Li. The possibility exists that this is anartifact of using samples from two di¡erent expe-ditions, though every e¡ort was made to use sam-ples collected at similar stages and therefore attimes of similar discharge. Alternatively, it maybe due to exchange processes between dissolvedand suspended load during transport. Furtheranalyses of samples at di¡erent river stages andat di¡erent locations along the main channel arenecessary to address this.

6. Conclusion

All important tributaries of the Orinoco havebeen analyzed for Li and N6Li in the suspendedand dissolved loads and can be interpreted in thecontext of the published data on these rivers. Theresults con¢rm that the Orinoco is indeed a well-constrained system for such a study:

b Suspended sediments for both the Shield andAndes are homogeneously light. The valuesare close to that estimated for fresh MORBbut lighter than pelagic clays.

b Dissolved Li is heavier than suspended, andthere is a clear di¡erence between heavier An-dean rivers (322 to 336x) and lighter Shieldrivers (37 to 322x).

b There is a correlation with a semi-weatheringintensity proxies, Si/TZ�* and 87Sr/86Sr.

Based on the observed data, we hypothesizethat Li, and 6Li in particular, preferentially frac-tionates into the secondary clays during weather-ing. In weathering-limited regions, kinetic isotopefractionation causes the dissolved load to be en-riched in 7Li. In transport-limited basins, Ray-leigh-type extraction of the heavy isotope from areservoir and more intense reaction lead the N6Li

EPSL 6025 17-12-01


values of dissolved load to approach that of thesuspended load. To be a suitable weatheringproxy, this fractionation needs to be better under-stood and documented in other weathering envi-ronments.

Acknowledgements

The help of T. Blanchard in the lab is gratefullyacknowledged. We thank C. France-Lanord andan anonymous reviewer for their careful reviews.This work was supported by NSF grants EAR-9506390 to L.-H.C. and OCE-9616599 toJ.M.E.[EB]

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lithium isotopes as a probe of weathering processes: orinoco river

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