Reconstructing Holocene precipitation in the tropical Andes
by
Paul A. Baker
Duke University, Earth and Ocean Sciences
Coauthors: Sherilyn C. Fritz (University of Nebraska, USA), Catherine A. Rigsby (East Carolina University, USA), Stephen J. Burns (University of Massachusetts, USA) and Kirstin H. Coley (University of London, UK)

For the past several years we have studied lacustrine sediments in the tropical Andes in order to reconstruct the history of late Quaternary climate, in particular, effective moisture. On long (orbital) timescales, regional lake levels vary greatly due to mechanisms related to global-scale forcing (e.g. wet-season insolation varying on precessional timescales; global temperature with highest amplitude variability on eccentricity timescales). On shorter (millennial to decadal) timescales regional lake levels appear to be coherent with each other and with tropical/North Atlantic sea-surface temperature (SST) variability, suggesting that the latter is a control on regional precipitation, although there is as yet neither community consensus about this observation nor about the nature of the mechanism. In the instrumental period, regional precipitation variability on interannual timescales is clearly forced by Pacific SST variability—e.g. most ENSO events produce dry and warm conditions in most of tropical South America including the Altiplano. On longer timescales the existence of a Pacific-SST control on Altiplano lake levels (a la Bradley et al. 2003) is less clear.

One of the most important, and most contentious, subjects in Andean paleoclimatology is the interpretation of stable isotopic records, particularly in ice core records such as those recovered from Nevadas Quelccaya, Huascaran, Sajama, and Illimani. We (and others, Pierrehumbert, Hoffman) argue that d18O of precipitation results from regionally-integrated variations in precipitation amount (Thompson and co-workers prefer a paleotemperature interpretation). Taking advantage of the relationship between precipitation amount and its isotopic composition, allows us to infer paleo-precipitation amount from stable isotopic records of ice, sediments, trees, and caves. We will present such a record determined on mid- and late-Holocene carbonate sediments from Lago Umayo, Peru. The reconstructed precipitation, determined at sub-decadal timescales for most of the record, underwent significant wet/dry alternations lasting a few hundred years. The typical amplitude of these alternations is 30 to 40% of the total precipitation. The alternations subjectively appear to be in phase with the Holocene Bond cycles, as we have previously posited. On the basis of the precipitation dependency of modern agricultural yields on the Altiplano, we further speculate that the long-term droughts would have seriously stressed early inhabitants of the region.

Date received: November 12, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caod-15.