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 broadly synchronous expansion of glaciers and palaeolake Tauca during the late-glacial interval. This is shown by sedimentary successions of glacigenic, glacifluvial and glacideltaic facies linking palaeoglaciers with palaeolake Tauca on the flanks of Cerro Azanaques and Cerro Tunupa 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 ¹⁴ C yr BP. Glacifluvial fan-deltas contiguous with the moraines confirm that the advance coincided with a highstand of palaeolake Tauca radiocarbon dated to the interval ca. 13 500–11 500 yr BP. Modeling of climatic controls on this glacier advance suggests the primary forcing was increased summer (wet season) moisture, possibly amounting to 600 mm above the modern values of 200–400 mm. Greater cloud cover probably depressed local temperatures and reduced the evaporation rate. The consequent rise in effective annual moisture ( P − E ) comfortably accommodates a palaeolake 48–50 × 10 ³ km ² in area and up to 100 m deep in the southern Altiplano. Because the palaeoglacier equilibrium-line altitudes rose toward the south and west, like the gradient of modern precipitation totals, we conclude that the increased late-glacial moisture was brought by weather systems similar to those of the present, but that atmospheric conditions were cloudier and cooler. © 1997 John Wiley & Sons, Ltd.