LAKE HISTORY OF THE SOUTHERN BOLIVIAN ALTIPLANO (18-22°S) OVER THE LAST 120 KA

Recent discussions on impacts of El Niño/Southern Oscillation (ENSO) highlight the importance of the tropics in forcing rapid climate changes over distant parts of the Earth. The Southern Bolivian Altiplano (18-22°S) is situated at the western edge of the Amazon rainfall belt, and is subject to ENSO-related precipitation anomalies. Paleoclimate records from the Altiplano can therefore serve as tests of the relationships between global climate, seasonal insolation, and ENSO.

Dramatic low-latitude climate shifts are shown by extensive tufa-encrusted shorelines and lacustrine sediments laid down by enormous paleolakes that occupied the high altitude Poopo/Coipasa/Uyuni basins in Southern Bolivia. A new and robust chronology for lake events over the past 120 ka in these now dry or oligosaline basins dates two major and several minor lacustrine episodes. Extensive U-Th and 14C dating of shoreline deposits track the rise and fall of the well-documented megalake Tauca, and lead to an improved resolution of its maximum extent between 16 and 14.1 ka. We also recognize a new 115-95 ka megalake, which we designate as Lake Ouki. Deposits from megalake Ouki previously yielded spuriously young radiocarbon dates in the range 28-45 ka, and this lake phase had been described as “Lake Minchin”. In light of our findings we recommend abandonment of the term Lake Minchin. A very minor lake episode occurs at ~46 ka, and oscillations are also found in association with megalake Tauca.

Lake development is out-of-phase with local summer insolation maxima, arguing against the hypothesis that these maxima result in intensification of the South American Summer Monsoon. Currently, wet years on the Altiplano are associated with La Niña-like conditions in the central Pacific, while during dry years El Niño conditions tend to prevent advection of moisture from the Amazon basin to the east. Only one minor rise in lake level occurs in the cold interval between 80 and 20 ka. This relative aridity may be due to persistent El Niño conditions, or simply to a dryer Amazon basin during the 80-20 ka cold interval. The terminal Pleistocene megalake Tauca does coincide with probable intense and persistent La Niña-like conditions in the central Pacific, as established by boron isotopic studies on deep-sea cores by Palmer and Pearson (Science, 2003).