Two century-scale time series in northern Bolivia constrain the ages of abrupt changes in the physical, geochemical, and biological characteristics of sediments obtained from lakes that formed during deglaciation from the late Pleistocene glacial maximum. The watersheds of Laguna Viscachani (16°12′S, 68°07′W, 3780 m) and Lago Taypi Chaka Kkota (16°13′S, 68°21′W, 4300 m), located on the eastern and western slopes of the Cordillera Real, respectively, contain small cirque glaciers. A high-resolution chronology of the lake sediments is provided by 23 AMS 14C dates of discrete macrofossils. Late Pleistocene glaciers retreated rapidly, exposing the lake basins between 10,700 and 9700 14C yr B.P. The sedimentary facies suggest that after 8900 14C yr B.P. glaciers were absent from the watersheds and remained so during the middle Holocene. An increase in the precipitation-evaporation balance is indicated above unconformities dated to ∼2300 14C yr B.P. in both Lago Taypi Chaka Kkota and Laguna Viscachani. An abrupt increase in sediment accumulation rates after 1400 14C yr B.P. signals the onset of Neoglaciation. A possible link exists between the observed millennial-scale shifts in the regional precipitation-evaporation balance and seasonal shifts in tropical insolation.
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We thank Herbert Wright, Alexander Wolfe, and Ramon Aravena for providing critical comments on an earlier version of the manuscript. The Familia Ando of La Paz, The Servicio Geologico de Bolivia, and ORSTOM-Bolivia all provided valuable logistical assistance. We also thank Chalmers Clapperton and Eric Leonard for their reviews of this manuscript. The work was supported by a graduate fellowship and grants of the NSF-RTG at the University of Minnesota and the Limnological Research Center and field grants from Geological Society of America, Sigma Xi, and the Department of Geology and Geophysics, University of Minnesota.
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