Holocene Paleohydrology of the Tropical Andes from Lake Records

southern basin of Lake Titicaca (Lago Wiñaymarka) conTwo century-scale time series in northern Bolivia constrain the tains an incomplete Holocene record because it was almost ages of abrupt changes in the physical, geochemical, and biological completely desiccated prior to about 3500 C yr B.P. (Wirrcharacteristics of sediments obtained from lakes that formed durmann et al., 1990; Wirrmann and Mourguiart, 1995; Abbott, ing deglaciation from the late Pleistocene glacial maximum. The 1995). Also, all of the closed basin lakes in the cordillera watersheds of Laguna Viscachani (16712*S, 68707*W, 3780 m) and are ephemeral lakes lacking a complete Holocene record Lago Taypi Chaka Kkota (16713*S, 68721*W, 4300 m), located on that extends through the proposed mid-Holocene dry phase. the eastern and western slopes of the Cordillera Real, respectively, Therefore, we targeted glacier-fed lakes with a positive water contain small cirque glaciers. A high-resolution chronology of the balance to obtain continuous Holocene paleoclimate selake sediments is provided by 23 AMS C dates of discrete macroquences. Although the glacial lakes in this study maintain fossils. Late Pleistocene glaciers retreated rapidly, exposing the overflowing stages throughout the dry season (May–Nolake basins between 10,700 and 9700 C yr B.P. The sedimentary facies suggest that after 8900 C yr B.P. glaciers were absent from vember), the erosion and deposition histories of Lago Taypi the watersheds and remained so during the middle Holocene. An Chaka Kkota and Laguna Viscachani provide strong eviincrease in the precipitation–evaporation balance is indicated dence for a dry middle Holocene. We suggest that cirque above unconformities dated to Ç2300 C yr B.P. in both Lago glaciers were absent from the watersheds during the middle Taypi Chaka Kkota and Laguna Viscachani. An abrupt increase Holocene, and without ice to provide a meltwater buffer in sediment accumulation rates after 1400 C yr B.P. signals the during the dry season, a lake would have been seasonally onset of Neoglaciation. A possible link exists between the observed lower or become desiccated. The absence of a meltwater millennial-scale shifts in the regional precipitation–evaporation buffer also rendered these high-altitude lake systems more balance and seasonal shifts in tropical insolation. q 1997 University of susceptible to annual to century-scale droughts during the Washington. middle Holocene. Previous water-level studies in Lake Titicaca imply a negative trend in the regional precipitation–evaporation balance INTRODUCTION from the latest Pleistocene to the middle Holocene (Servant Lake-sediment records from both sides of the Cordillera and Fontes, 1978; Wirrmann and De Oliveira Almeida, 1987; Real, Bolivia, were analyzed to investigate the Holocene Mourguiart, 1990). Studies of glacier retreat and higher environmental dynamics of the Bolivian Andes (Fig. 1). snowlines for this period provide further evidence supporting These time series also extend the 3500-yr record of changes a negative water balance in the region (Gouze et al., 1986; in the precipitation–evaporation balance for the region esSeltzer, 1992). tablished by dating water-level fluctuations in Lake Titicaca Here we present new proxy climate data from two sites in tropical South America indicating a middle Holocene dry (Abbott et al., in press; Binford et al., in press). The shallow