Earth Interactions x Volume 1 (1997) x Paper No. 1 x Page 1 Copyright q 1997. Paper 1-001, 9,697 Words, 4 Tables, 11 Figures, 1 Mathematica. http://EarthInteractions.org Constraints on the Origin of Paleolake Expansions in the Central Andes Troy A. Blodgett,* John D. Lenters,** and Bryan L. Isacks* *Department of Geological Sciences, Cornell University, Ithaca, New York **Atmospheric Science Program, Cornell University, Ithaca, New York Received 11 March 19; accepted 6 December 1996. ABSTRACT: During the late Pleistocene, at least one episode of lake ex- pansions occurred in the internally draining high plateau region of Bolivia. Some researchers have advocated that a wetter climate associated with a change in atmospheric circulation caused the development of the large paleo- lakes, while others have hypothesized that deglaciation contributed to the wa- ter source for the expanding lakes. From estimates of the potential meltwater stored in the glaciers during their maximum extents, the authors conclude that insuf®cient meltwater was available to ®ll the large paleolakes. However, the meltwater hypothesis remains viable south of the main plateau region where, in ®ve small drainage basins, the volume of available glacial meltwater was 3±16 times greater than the volume of water in the paleolakes. Pollen, dunes, and other eolian features indicate that the region surrounding the Altiplano was much drier during at least one interval of the late Pleistocene. Although the timing of the dry period with respect to the paleolakes is still unknown, a pluvial explanation for the existence of paleolakes seems unlikely. Decreased evaporative loss, however, remains a possible explanation. To understand what factors could have been associated with a decrease in evaporation rates over the drainage basin, an evaporation model is developed based on the energy * Corresponding author address: Troy Blodgett, 2112 Snee Hall, Cornell University, Ithaca, NY 14853. E-mail address: [email protected] Unauthenticated | Downloaded 09/25/21 01:08 PM UTC Earth Interactions x Volume 1 (1997) x Paper No. 1 x Page 2 balance and bulk transfer methods. The model indicates that a 108C drop in air temperature or a doubling in cloud cover could have caused the paleolakes to reach their highest levels. Alternatively, a 50% increase in precipitation rate could have also maintained the paleolakes. KEYWORDS: Paleoclimatology Evapotranspiration Geomorphology Hy- drologic budget Snow and ice 1. Introduction 1.1. Evidence of paleolakes Evidence of former lakes in the central Andes was ®rst documented in the mid- 1800s when explorers observed lacustrine deposits and high shorelines around Lake Titicaca and identi®ed algal-carbonate beach ridges 4 m above the basin ¯oor near Lake Poopo (Figure 1) (Clapperton, 1993). Distinct erosional shore platforms and depositional gravel beach surround most of the salars and lakes between 158 and 238S and throughout much of the internally drained portion of the central Andes. The modern climate encompassing the region of paleolake development is diverse and include both arid regions receiving less than 200 mm of precipitation per year and more humid areas receiving over 800 mm per year. Figure 1. Titicaca±Uyuni±Coipasa±Poopo drainage basin. Within the numerous internally draining basins on the high plateau (Altipla- no), deposits and shorelines corresponding to several episodes of paleolake ex- pansions have been identi®ed. The oldest and highest of the paleolakes identi®ed is Lake Mataro in the basin containing Lake Titicaca. Lake Mataro is tentatively dated as late Pliocene because it lies just above a 2.8 6 0.4 Ma ash bed (Lavenu, 1986). Evidence of two other paleolakes named Lake Cabana and Lake Ballivian have also been found within the Titicaca basin and are roughly mid- to late Pleistocene in age (Clapperton, 1993). During high paleolake phases in the southern Altiplano, the present PoopoÂ, Coipasa, and Uyuni drainage basins were integrated into one basin (Figure 1, Figure 2, Figure 3). Within the Poopo±Coipasa±Uyuni drainage basin, Ahlfeld and Branisa (Ahlfeld and Branisa, 1960) identi®ed conspicuous erosional bench- es cut into the ¯anks of the volcanic stock Cerro Lipillipi at the elevations of 3680, 3685, 3710, and 3735 m. Based on radiocarbon ages of 26,700 to 28,300 yr BP, 25,700 to 26,900 yr BP, and an AMS (accelerator mass spectrometry) date of 30,640 to 31,750 yr BP, the four terraces and a higher depositional shoreline at 3760 m were originally associated with Lake Minchin, while a shoreline consisting of clayey and calcareous diatomites 2±5 m thick at 3720 m was considered evidence of another paleolake named Lake Tauca (Clapperton, 1993; Clayton and Clapperton, 1995; Servant and Fontes, 1978). Since dates acquired by Bills et al. (Bills et al., 1994) show that the maximum level of Lake Minchin (3760 m) was attained by the Tauca lake phase at about 13,790 yr BP, a distinction no longer exists between the maximum elevation of Lake Minchin and Lake Tauca. Additional AMS and uranium series dates corresponding to several lower lake levels indicate that either subsequent lesser high lake phases Unauthenticated | Downloaded 09/25/21 01:08 PM UTC Earth Interactions x Volume 1 (1997) x Paper No. 1 x Page 3 Figure 1. Titicaca±Uyuni±Coipasa±Poopo drainage basin. developed or the Tauca lake phase persisted until about 9500 yr BP (Clayton and Clapperton, 1995; Servant et al., 1995). To avoid confusion associated with new shoreline dates collected within the Uyuni±Coipasa±Poopo basin, we sim- ply consider the highest shoreline level (3760 m) to be the highest elevation attained by the Tauca lake phase. Lower shoreline elevations are considered lower levels of the Tauca lake phase. Although the age of the highest shoreline at 3825 m near the shores of Lake Titicaca has not been established, for our calculations we consider the 3825-m-level part of Tauca (3760 m), because a Unauthenticated | Downloaded 09/25/21 01:08 PM UTC Earth Interactions x Volume 1 (1997) x Paper No. 1 x Page 4 Figure 2. Paleolake and Pleistocene glacier reconstruction within the Titicaca± Uyuni±Coipasa±Poopo drainage basin. highstand in the Lake Titicaca basin is likely to have been contemporaneous with a highstand in the Titicaca±Uyuni±Poopo±Coipasa basin (Wirrmann and Mourguiart, 1995). Servant and Fontes (Servant and Fontes, 1978) also dated shorelines within a small basin to the south that now contains Laguna Khota (Figure 4) at 11,000 to 12,500 yr BP. This small paleolake's age suggests that the cause of the Tauca phase may also have been felt outside of the Uyuni± Coipasa±Poopo drainage basin. Glacial meltwater was one of the ®rst hypotheses proposed to account for Unauthenticated | Downloaded 09/25/21 01:08 PM UTC Earth Interactions x Volume 1 (1997) x Paper No. 1 x Page 5 Figure 3. Schematic pro®le of modern drainage. Thick black lines represent rela- tive slope length and slope gradients (slopes are vertically exaggerat- ed). the paleolake expansions (Servant and Fontes, 1978). Kessler (Kessler, 1984) proposed as an alternative hypothesis that pluvial conditions accounted for the expanded lakes. He argued that the glaciers melted during a regressive lake phase and thus contributed no meltwater to the expansive lake phase. He further proposed that a southerly shift in the intertropical convergence zone had in- creased the amount of Amazonian moisture reaching the plateau and estimated that at least a 30% increase in precipitation would have been required to account for the Lake Tauca (3720 m) shorelines. A similar model was constructed by Hastenrath and Kutzbach (Hastenrath and Kutzbach, 1985), who also rejected the idea that glacial meltwater could account for the paleolakes. They estimated that the expansion of two of the Lake Tauca phases at 3720 and 3760 m resulted from precipitation increases of greater than 50% and 75%, respectively. A de®nitive test of the hypothesis that melting glaciers ®lled the basins would be to reconstruct the former glacier and lake water volumes to see which is greater. This is undertaken in the present study for the Titicaca±Uyuni±Coipasa± Poopo basin as well as for ®ve small basins in the southern Altiplano (Figure 4). Furthermore, since reduced evaporation remains a potential explanation for the lake highstands, an evaporation model is constructed to investigate the sensitivity of evaporation rates to altered atmospheric conditions. 1.2. Modern and late Pleistocene drainage patterns Because many of the drainage basins on the Altiplano over¯owed and coalesced during the paleolake expansions, the modern series of drainage basins have been grouped into just two basins connected by the Desaguadero River (Figure 1, Figure 3). Evidence of basin coalescence has also been documented in modern Unauthenticated | Downloaded 09/25/21 01:08 PM UTC Earth Interactions x Volume 1 (1997) x Paper No. 1 x Page 6 Figure 4. Map showing extent of former glaciers, paleolakes, and present salars/ lakes in the southern Altiplano. time. During the rainy season in the wet year of 1986, water not only reached Lake Poopo as is typical, but the inundation also ¯ooded the Coipasa and Uyuni salars (Roche et al., 1991). One complication to this simple approach is that presently a small drainage area south of Lake Titicaca drains northward when Lake Titicaca is low but drains southward into the Desaguadero River watershed Unauthenticated | Downloaded 09/25/21 01:08 PM UTC Earth Interactions x Volume 1 (1997) x Paper No. 1 x Page 7 when Lake Titicaca is high. For modeling present conditions, we include this alternating drainage as part of the Titicaca basin (Figure 1). However, since the highstand around Lake Titicaca was probably coincident with the Tauca lake phase, and the alternating drainage region area was likely to have drained directly into the Uyuni±Poopo±Coipasa basin during inundation episodes, the alternating drainage area is considered part of the Uyuni±Coipasa±Poopo basin during the Tauca lake phase.