Holocene paleohydrology and glacial history of the central Andes using multiproxy lake sediment studies

Here we document at century to millennial scale the regional changes of precipitation-evaporation from the late Pleistocene to present with multiproxy methods on a north-south transect of lake sites across the eastern cordillera of the central Andes. The transect of study sites covers the area from similar to 14degreesS to 20degreesS and includes core studies from seven lakes and modem calibration water samples from twenty-three watersheds analyzed to constrain the down-core interpretations of stable isotopes and diatoms. We selected lakes in different hydrologic settings spanning a range of sensitivity to changes in the moisture balance. These include: (1) lakes directly receiving glacial meltwater, (2) overflowing lakes in glaciated watersheds, (3) overflowing lakes in watersheds without active glaciers, and (4) lakes that become closed basins during the dry season. The results of our current work on multiple lakes in the Bolivian Andes show that while the overall pattern of Holocene environmental change is consistent within the region, conditions were not always stable over centennial to over millennial timescales and considerable decadal- to century-scale climate variability is evident [Abbott et al., Quat. Res. 47 (1997) 70-80, Quat. Res. 47 (1997) 169-180, Quat. Sci. Rev. 19 (2000) 1801-1820; Polissar, Master's thesis, University of Massachusetts (1999)]. Comparison of the paleoclimate record from one well-studied site, Lago Taypi Chaka Kkota (LTCK), with others within the region illustrates a consistent overall pattern of aridity from the late glacial through the middle Holocene. Previous work noted a difference between the timing of water-level rise in Lake Titicaca similar to 5.0-3.5 ka B.P. [Abbott et al., Quat. Res. 47 (1997) 169-180; Cross et al., Holocene 10 (2000) 21-32; Rowe et al., Clim. Change 52 (2002) 175-199] and the onset of wetter conditions at 2.3 ka B.P. in LTCK, a lake that drains into the southern end of Lake Titicaca [Abbott et al., Quat. Res. 47 (1997) 70-80]. Sedimentary and oxygen isotope evidence from Paco Cocha (13degrees54'S) located in the northern reaches of the expansive 57000 km(2) Titicaca watershed, which spans similar to 14degreesS to 17degreesS, indicates that glaciers returned to the watershed around 4.8 ka B.P. In addition, sedimentary and geochemical data from Llacho Kkota (15degrees07'S), located between LTCK (16degrees12'S) and Paco Cocha, indicate wetter conditions around 3.4 ka B.P. This suggests wetter conditions occurred first in the northern reaches of the Titicaca watershed and resulted in rising water levels in Lake Titicaca while the LTCK watershed remained unglaciated and seasonally desiccated. Comparison of the paleoclimate records from Paco Cocha, LTCK, and Potosi with other paleoclimate records from the region including Lake Titicaca and Nevado Sajama illustrates a consistent overall pattern of aridity from the late Pleistocene through the middle Holocene, but wetter conditions occurred in the northern areas first, and the aridity in the north was of shorter duration and less severe. The progressive increase in wet season (summer) insolation across the region during the Holocene likely resulted in an increasingly southward position and intensification of the zone of intense summer convection - known as the Intertropical Convergence Zone over the tropical Atlantic and Pacific. We suggest this is the cause of the overall pattern of increasingly later dates for the onset of wetter conditions moving north to south that we observe across the seven lakes discussed here that span four degrees of latitude. However, this does not explain the decadal- to century-scale variability that demonstrably exists at these same sites. Overall the last 2.3 ka has been the wettest period during the Holocene, but even during this relatively wet phase there are century-scale lowstands in lakes, including Titicaca, Llacho Kkota, Juntutuyo (17degrees33'S), and Potosi (19degrees38'S), and significant changes in the extent of glacial activity in the glaciated watersheds of Paco Cocha, LTCK, and Viscachani (16degrees11'S), indicating the continued sensitivity of the region to shifts in the moisture balance. (C) 2003 Elsevier Science B.V. All rights reserved.