Constraints on the Origin of Paleolake Expansions in the Central Andes

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During the late Pleistocene, at least one episode of lake expansions 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 paleolakes, while others have hypothesized that deglaciation contributed to the water source for the expanding lakes. From estimates of the potential meltwater stored in the glaciers during their maximum extents, the authors conclude that insufficient meltwater was available to fill the large paleolakes. However, the meltwater hypothesis remains viable south of the main plateau region where, in five 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 balance and bulk transfer methods. The model indicates that a 10°C 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.