CLIMATIC SIGNIFICANCE OF LARGE LAKES ON THE BOLIVIAN ALTIPLANO OF SOUTH AMERICA

Numerous paleoclimate studies have documented the growth and demise of large lakes on the Bolivian Altiplano at several times during the late Quaternary. The terminal basin of the Altiplano drainage is the Salar de Uyuni, where large paleolakes characteristic of wetter and/or colder periods (signaled in the Salar by mud deposition) alternated with shallow salt pans characteristic of drier and/or warmer periods (signaled in the Salar by deposition of gypsum and halite). Although there are major discrepancies in published records about the timing of these paleolakes, our previously published work, based on radiocarbon and U/Th isochron dating of sediments from a continuous 220 m drill core, indicates that the only long-lived lakes in the basin occurred during the past 60,000 years. The existence and persistence of these lakes is evidenced respectively by the presence and thickness of lacustrine mud in both the drill core and the natural gamma logging record.

Sr isotopic data, on both salt and lacustrine mud units in the Salar drill core, indicate a major shift from less radiogenic values prior to ca 60 Ka to more radiogenic values afterwards. This shift suggests a change in source waters. A previously published energy balance/water mass balance model of paleolakes on the Salar (Blodgett et al., 1997) failed to take into account the potential large contribution of water inputs to the paleolakes from Lake Titicaca (LT) overflow. Today and in the Holocene (we do not have a Sr isotopic record of MIS3 in LT), the Sr isotopic composition of LT is similar to the radiogenic composition of the last 60,000 years of Salar sedimentation. Thus, it seems likely that LT overflow was a major component of the Salar water budget and was necessary for the maintenance of long-lived lakes in the Salar basin during the past 60,000 years. We test this idea with our own paleohydrologic model that evaluates the contribution of LT overflow.

A more global question is what happened ca 60,000 years ago that caused the initiation of apparently wetter conditions on the Altiplano? As pointed out recently by Larry Peterson (PAGES OSM 2009) other records on other continents also suggest a more widespread wetting of the tropics. We will evaluate the causes for this climatic evolution.