BROAD SYNCHRONY OF A LATE-GLACIAL GLACIER ADVANCE AND THE HIGHSTAND OF PALAEOLAKE TAUCA IN THE BOLIVIAN ALTIPLANO

The morphology and sedimentology of glacially influenced fan-deltas on massifs at the margin of the southern Altiplano, Bolivia, suggest a b roadly synchronous expansion oi glaciers and palaeolake Tauca during t he late-glacial interval. This is shown by sedimentary successions of glacigenic, glacifluvial and glacideltaic facies linking palaeoglacier s with palaeolake Tauca on the flanks of Cerro Azanaques and Cerro Tun upa at altitudes of 3770-3720 m. Radiocarbon dates from peat overlain by glacial diamict and glacifluvial outwash indicate that glaciers in this area reached their last glacial maximum extent after ca. 13 300 C -14 yr BP. Glacifluvial fan-deltas contiguous with the moraines confir m that the advance coincided with a highstand of palaeolake Tauca radi ocarbon dated to the interval ca. 13 500-11 500 yr BP. Modeling of cli matic controls on this glacier advance suggests the primary forcing wa s increased summer (wet season) moisture, possibly amounting to 600 mm above the modern values of 200-400 mm. Greater cloud cover probably d epressed local temperatures and reduced the evaporation rate. The cons equent rise in effective annual moisture (P-E) comfortably accommodate s a palaeolake 48-50 x 10(3) km(2) in area and up to 100 m deep in the southern Altiplano. Because the palaeoglacier equilibrium-line altitu des rose toward the south and west, like the gradient of modern precip itation totals, we conclude that the increased late-glacial moisture w as brought by weather systems similar to those of the present, but tha t atmospheric conditions were cloudier and cooler. (C) 1997 by John Wi ley & Sons, Ltd.