LATE-GLACIAL AND EARLY HOLOCENE LAKE-SEDIMENTS, GROUNDWATER FORMATION AND CLIMATE IN THE ATACAMA-ALTIPLANO 22-24-DEGREES-S

Precipitation rates in the Atacama Altiplano 22-24 degrees S were 400-500 mm yr(-1) during late glacial and early Holocene times as opposed to 200 mm yr(-1) today. This humid phase (Tauca phase) was likely due to strenghtened tropical (monsoonal) circulation, which brought continental moisture to the Atacama Altiplano. The lake level of Laguna Lejia (23 degrees 30'S, 4350 m) at that time was up to 25 m higher than it is today. Mg/Ca and Sr/Ca data from lake sediments show that, what today is a highly saline lake was a freshwater lake at that time. Seasonally-laminated calcareous sediments were deposited between 13 500 and < 10 400 yr B.P. indicating the maximum of the humid phase. Climatic changes in the past are important for current groundwater resources. C-14 and H-3 data from lake-, ground- and well water suggest that modern groundwater formation (i.e. water < 40 years) in the Altiplano is very limited under current arid conditions. We conclude that significant amounts of the water resources in this area originated during the time of the late-glacial and early Holocene humid climate. Tritium data from snow samples show that the moisture in the Altiplano at 22-24 degrees S is mainly of continental origin, whereas precipitation from the westerlies hardly contributes to the water supply in this area. This precipitation pattern matches the paleodata, and we suggest that current precipitation formation may provide an analogue framework for late-glacial circulation in this area.