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1040-.6182(93)E0013-G

Quaternary international, Vol. 21, pp. 163-169, 1994. Copyright O 1994 INQUA/Elsevier Science Ltd.

P~ted in Great Britain. All rights tesaved. 1040--6182/94 $26.00

A CLIMATIC MODEL FOR SOUTHWESTERN AMAZONIA IN LAST GLACIAL TIMES

Edgardo M. Latrubesse* and Carlos G. RamoneU1" *Departamento de Geografia, Universidade do Amazonas, C.P.885, 69011, Manaus, Amazonas, Brazil

t Facultad de lngenier~a y Cs. HMricas, Universidad Nacional del Litoral, C.C. 495, (3000) Santa Fd, Argentina

Muitidisciplinary data indicate that southwestern Amazonia was a savanna environment during Last Glacial times. Interpretation of biogeographical and geological records, as compared with the present climatic conditions, permit us to postulate a dry season more pronounced and prolonged than the present one; city northerly winds (vinodatog] with S O U ~ I~,misphcre trade winds) dominated during the dry season, and the southerly cold air masses (locally termed 'friagem' or 'surazo') would have been more frequent and intensive than today.

INTRODUCTION

The large Amazon rainforest, occupying 5,000,000 lan 2, has been a center of attention of the natural sciences in the last centuries. Although the Amazon rainforest extends into various countries, 80% of the total area is located in Brazil.

The Amazon ecosystem houses more living species than any other single system on Earth. Today, for example, some 80,000 vascular plant species could be identified, and as many as thirty million animal species may exist (Colinvaux, 1989).

In the last two decades, the most frequent explanation for this biodiversity has been that of the forest refuges (Haffer, 1969, 1982; Prance, 1982). According to this theory, during periods of glacial activity Amazonia had a much drier climate, resulting in a rainforest reduction and in an increase in the savanna areas. No refuges were available in the southwestern Amazonia; however, some important biogeographical changes occurred. Two important papers give a review about the geomorphological and paleo- botanical changes in Amazonia during the Quaternary, with emphasis on the Last Glacial Maximum (van tier Hammen, 1991; Clapperton, 1993). In this work, we include both a review and new data, explaining a probable circulation model, specific for the southwestern Amazonia; most data are included in Latmbesse's Doctoral Thesis (1992).

PRESENT CLIMATE IN AMAZONIA

receives most rainfall. The 1TCZ shifts northwards, reaching its extreme northern position from July to August, and is then located over Venezuela and Colombia. The total average rainfall for these three months (winter in the southern hemisphere) decreases to 100--140 mm. According to the facts given above, the southwestern Amazonia has a definite 'dry' season during winter (Fig. la, b). The winter dry season is characterized by dry northerly winds, originated in the anticyclonic circulation of the southern hemisphere tradewinds.

PRESENT CLIMATE IN THE PAMPEAN AND CHACO PLAINS

Conversely to the Amazonia region, where Atlantic trade winds govern wind and rain patterns, in the southern portion of the continent (at 30 ° S), weather patterns produced in the South Atlantic and South Pacific anticyclones prevail. The winds from the South Pacific Anticyclone (SPA) lose their humidity on the west side of the Andes, coming into the Argentine plains cold and dry from the southwest and south during winter.

The winds of the South Atlantic Anticyclone (SAA) are warmer and more humid, coming into the plain from the northeast. On the Pampa, rains result from the interaction between the SAA and SPA air masses. However, in the Chaco region, rains are convective, originating in the SAA air masses.

In the Amazonia rainforest a humid tropical climate prevails. Rainfall, averaging 2000 mm in the whole basin, increases in the northwest to 5000-7000 mm (in Ecuador) and to 10,000 ram in Choco (Colombia), outside the basin. The area south from the centerline of the Solimoes--Amazon River, known as southwestern Amazonia, shows a pronounced dry season with high- and low-peaks of rainfall, which does not hold true for the area north of the Solimoes River. The shifting of the Intertropical Convergence Zone (ITCZ) influences this region.

During summer, in the southern hemisphere, the ITCZ is located from 10 ° to 15 ° S. In this period, southern Amazonia

PRESENT EFFECTS OF THE SPA IN SOUTHWESTERN AMAZONIA

Cold fronts, coming in winter from the cold air masses produced in the SPA, move into southwestern Amazonia. These masses cross the Argentine plains, reaching Brazil. In southwestern Amazonia, they produce the phenomenon locally called 'friagem' or 'surazo'.

From May to July, the temperature decreases more than 15°C over 3 or 5 days (Molion, 1987). The average temperature is 26°C between October and April, decreasing 3-4°C in the dry season (Rancy, 1991), the rainfall being

163

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FIG. lb. Normal rainfall in JaliLla/-y (isoyets in ram). The shaded areas are 500 meters above sea level (from Salati e t a l . , 1978).

reduced by 120 mm between July and August. Both formation and movement of these cold air masses were recorded by satellite and field observations (Brinkman et al., 1971; Parmenter, 1976). Therborg (1983) described a sharp temperature decrease, in July 1975, when (at the latitude of Iquitos, at 3 ° S) 8°C were recorded for three consecutive nights. According to the author, the normal average temperature during the 'surazo' would he 14--16°C. In the Bolivian plains, during the 'friagem', the minimum temperature reaches 0-5°C. The 'surazo' has an important biological effect, producing economic losses in agriculture (Ronchail, I992).

Today, southwestern Amazonia is affected by air masses coming from the SPA in winter, which produce a temperature decrease. The more frequent winter winds move

towards the north and northwest, warm and dry. Figure 2 schematically shows this condition.

SOUTHWESTERN AMAZONIA ,DURING Tl tg LAST GLACIAL TIMES: IgNV~ONMBNTAL C ~ E S

FROM MULTIDIT~EI~INARY DATA

Palynology The more specific palynological data on southwestern

Amar~nia come from Rondonia, indicating that savannas replaced rainforest during different periods in the Late Pleistocene (Absy and van der I-Imma~n, 1976). Dry periods were registered at 4I and 18.5 ka BP (van der Hammen, pets. commun.; in Absy, 1993).

Similar results were found in the plateau in the southern

Southwestern Amazonia in Last Glacial Times 165

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HG. 2. Winter atmospheric circulation associated with the 'friagem' or 'surazo' phenoma in Amazonia.

range of Carajas, where dry periods (with replacement of rainforest by savanna) at 60, 40 and 23-11 ka BP were recorded (Absy et al., 1991).

Paleontology of Vertebrates Rancy (1991) found large mammals of Lujanense

Mammal Age in southwestern Amazonia and proposed a savanna environment during the Last Glaciation (Table 1).

The fossils studied by Rancy were found mainly in the upper Jurua River basin. The author also describes f'mdings in the Purus and Madeira Rivers. Previous studies have mentioned the presence of large Late Pleistocene mammals in the Napo and Ucayali rivers. Figure 3 shows the areal distribution of these paleontological sites.

Geology :" Quaternary geological research m southwestern

Amazonia is scarce, the few studies available being mainly made along river banks.

The rivers of southwestern Amazonia can be classified into two groups:

(a) Fluvial systems with headwaters in highlands (Ucayaii, Marafion, Madre de Dios).

(b) Fluvial systems with headwaters in lowlands (Purus, Jurtla, Javari).

Alluvial sediments in the Madre de Dios River were deposited in Middle Pleniglacial times, during the Late Pleistocene (Riisiinen, 1991; Riis~men and Linna, 1992).

In the Ucayali River, Dumont et al. (1991) found that alluvial gravels (up to 10 times coarser than the present sandy bed load) were deposited between 32 and more than 40 ka BP. For these authors, this change is indicative of the irregular fluvial regime during glacial times.

Moreover, when analysing the morphology of ancient channels of the Ucayali, Durnont et al. (1991) concluded that at 13 ka BP, the river discharge was seven to 10 times smaller than the present values.

TABLE 1. Pleistocene megamammals (more than 1 kg) from southwestern Amazonia (from Rancy, 1991)

Genera Habitat Diet

Edentata-Pilosa Eremotherium Forest edge/savanna Grass/browse Ocnopus Forest edge/savanna Grass/browse Glossotherium Savanna Ca'ass/b~wse Lestodon Savanna Grass/browse Scelidotherium Savanna Grass/browse Mylodon Savanna C_a'ass/bfowse Megalonyx Savanna Grass/browse Edentata-Cingulata Propraopus Forest edge/savanna Omnivo~ Dasypus Forest/savanna Insectivore Euphractus Savanna Omnivore Pampatherium Savanna Grass Hoplophortts Savanna Grass Neuryurus Savanna Grass Panocthus Savanna Grass Glyptodon Savanna Grass Notoungulata Toxodon Savanna Grass/low browse Mixotoxodon Savanna Grass/low browse Proboscidea Cuvieronius Savanna Grass/browse)fruit Haplomastodon Savanna Grass/browse/fruit Perissodactyla Tapirus Forest/savanna Browse/fruit Attiodactyla Vicugna Savanna Grass/low browse Palaeolama Savanna Grass/low browse Tayassu Forest/savanna OnmivoreHrugivore Carnivora Eira Forest Carnivore

166 E.M. Latrubesse and C.G. Ramonell

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(adapted from Rancy, 1991).

For rivers with lowland headwaters, the more illustrative study was carried out in the upper Jurua basin by Simpson and Paula Couto (1981). In this region, Simpson describes the 'bone-bearing conglomerate' or conglomerate facies 'PL-type'.

Quaternary conglomerates are also found in some other rivers (Acre and Madeira Rivers). Our research included fieldwork in the upper and middle Acre, lower Iaco, Purus (near Boca do Acre), Moa and upper Jurua Rivers (field data in Latrubesse, 1992); these rivers today carry 98% of the sediment load as suspended load (Gibbs, 1967); the bed load is very small and sand sized.

The more precise Quaternary data come from the upper Jurua basin. The bone-bearing conglomerate is vertically and laterally discontinuous, and it is facially related to sandy deposits. The prevailing colors range from black to red-brown, due to precipitation of iron oxides; most pebbles are hard concretions, some quartzite also occurs; the pebbles reach 10 cm in diameter, and the matrix is sandy to clayey-sandy.

A rich vertebrate fauna of Lujanean Mammal Age is found in the deposit (Simpson and Paula Couto, 1981; Rancy, 1991). Rancy assigns a Late Quaternary age for the fauna which inhabited the region during the Last Glacial.

Some preliminary conclusions can be obtained from the above mentioned data. Rivers with headwaters in the Andes had a strong dynamics between ca. 56 and 26 ka BP (Dumont et al., 1991; R~is~inen, 1991; Risiinen and Linna, 1992), associated with the Maximum Glacial in the central and

northern Andes (van der Hammen et al., 1981; Clapperton, 1986, 1993). The occurrence of pebbles (10 cm diameter) deposited by lowland rivers clearly indicates the magnitude of changes produced on the hydrological variables.

In short, the southwestern Amazonian rivers had a high energy, produced by strong precipitation in the Andes and climatic deterioration in the lowlands, which reached its maximum during the Last Glacial Maximum.

THE PAMPEAN AND CHACO PLAINS DURING THE LAST GLACIAL MAXIMUM

The Pampean Plain The surface sediments of the Pampean region are aeolian

silts and sands. The aeolian sands compose an extensive sand sea (inactive under the present climatic conditions) over 200,000 km 2, from 38 ° to 33 ° S. Great longitudinal dunes, more than 100 km long, were detected as the first landforms in the sand sea (Iriondo, 1990a; Ramonell and Latrubesse, 1991; Ramonell et al., 1992a). Peripherally to the northern and northeastern areas, on the sand sea's leeward side, there exist loessic sediments up to 30 ° S. The loessic sediments were deposited in a peridesertic environment (Iriondo, 1990a), over the pre-existing fluvial landscape; loess deposits covered the drainage systems in the plains (Iriondo 1991), as well as the upper portions of the San Luis and C6rdoba Pampean Ranges (Ramonell and Latrubesse, 1990; Cantd, 1992).

The ~,eolian sedimentary system was defined and named

Southwestern Amazonin in Last Glacial Times 167

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FIG. 4. The Pampean Aeolian System (modified from Iriondo, 1990a)

Pampean Aeolian System by Iriondo (1990a; Fig. 4). The Pampean Aeolian System was generated during the Last Glacial Maximum (Iriondo, 1990a; Ramonell and Latrubesse, 1991) and became inactive in the Lower Holocene (Tonni, 1992; Ramonell et al., 1992a, hi. The climate was dry and cold, dominated by SPA winds strengthened by Katabatic winds coming from the Ice Field on the Patagonian Andes (lriondo and Garcia, 1993).

The Chaco Plain In western Chaco a dry climate occurred during the Late

Pleistocene, with deposition of aeolian sand and loess transported by northerly winds (Iriondo, 1990b). In the middle section of the loess sequence, '4C dating indicates an age of 16,900 ± 270 BP (Iriondo, 1993).

Estimates of paleodischarge for the more important rivers of the Chaco Plain in Argentina (Pilcomayo, Bermejo and Salado Rivers) indicate considerably reduced flows during that time, amounting to approximately 20% of present discharges (Iriondo and Garcia, 1993).

WIND CIRCULATION MODEL FOR SOUTHWESTERN AMAZONIA

Data collected and revised demonstrated that southwestern Amazonia supported savanna climates during the Last Glaciation. The more precise data are restricted to periods close to the Last Glacial Maximum, and between 40 and 9 kaBP.

Presently, the winter dry season is characterized by the dominance of dry northerly winds, originated in the anticyclonic circulation of the southern hemisphere trade winds; when the dry northerly winds reach southwestern Amazonia, the ITCZ is placed in its northern position. Considering the Late Pleistocene savanna environment, the more probable scenario was similar to the above-mentioned one, but with a more extended dry season. New geological data in central Amazonia (see Iriondo and Latrubesse, this volume) are in agreement with this assumption.

In addition, the 'friagem' or 'surazo' phenomena should have been more frequent and intensive than today, a

168 E. M, Latrubesse and C. G. Ramonell

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hypothesis supported by the environmental scenario prevailing in the Pampean Plain in Last Glacial times.

Figure 5 shows schematically the wind circulation model (or the more frequent one) for southwestern Amazonia during Last Glacial times.

FINAL REMARKS

(1) Southwestern Amazonia supported a savanna environment during the Last Glacial Maximum, with a dry season more pronounced and prolonged than the present one.

(2) The dry northerly winds, originated in the anticyclonic circulation of southern hemisphere trade winds, dominated during the dry season.

(3) The 'friagem' or 'surazo' should have been more frequent and intensive than today, having great importance as a biD-regulation factor because of the temperature decrease.

ACKNOWLEDGEMENTS

We wish to express our thanks to the Laboratorio de Paleontologia, Universi__de,~__ do Acre, for its professional aid; especially, to A. Rancy, J.

Pereira de Souza Filho, J.C. Bocquentin ViUanueva~ J. dos Santos and R. Negri. Part of the support for the fieldwork came from the Universid___~le- do Acre. Figures drafted by A. Romagnoni are kindly acknowledged.

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Absy, M.L and van der I-lammen, T. (1976). Some pa]eoecological data from Rondonia, southern part of the Amazon Basin. Acre Amaz6nica. 6(3), 293-299.

Absy, M.L., Cleef, A., Foornier, M.. Ma~dn, L.. Servant M., Siffeddine, A.. Ferreira Da Silva, M.) Soubies. F., Suguio, K., Turq, B. and van Der H ~ , T. (1991). Mise en evideace de quM_~- phases d'ouvemu~ de la foret dense clans le sudest de l'Amazonie au cours des 00.000 demieres ana~. Premiere compareimn avec d'autres regions tropic, ales. CR. Acad. Sci. Paris, t. 312, Set. If, 673.-678.

Brinkman, W.LF.. Weinman, J.A. and Goes Ribeiro. M.N. (1971). Air temperatures in cenuai Anmzoaia. Acre Anm~n/ca, 2, 5t-56.

Cant~, M. (1992). Provincia de C6rdobL In: Iriondo, M. (ed.), El Holoceno en/a Argembm, Vol. 1, pp. 1-I6. CADINQUA.

Clapporton. Ch.M. (1986). Glacial leomorphology. Quaternary glacial sequence and l~aeoclimatic iafemaces in the Ecuadm'ian Andes. In: Garcfiner. V. (ed.). International G-eomorphology, Part I], pp. 843-870. Wiley, London.

Clapperton, Ch.M. (1993), Nature of environmental changes in South

Southwestern Amazonia in Last Glacial Times 169

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Gibbs, R. (1967). The geochemistry of the Amazon River System: Part I. The factors that control the salinity and the composition and concentration of suspended solids. Geological Society of America Bulletin, 78, 1203-1232.

Haffer, J. (1969). Speciation in Amazonian birds. Science, 165, 131-137. Haffer, J. (1982). General aspects of the refuge theory. In: Prance, G.T.

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Iriondo, M. (1990a). Map of the South American plains. Its present state. In: Rabassa, J. (ed.), Quaternary of South America and Antarctic Penn., Vol. 6, pp. 297-308. A.A. Balkema, Rotterdam.

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