40Ar/39Ar evidence for early deglaciation of the central Chilean Andes

Abstract

Spatial variability of deglaciation and warming since the last glacial maximum is critical for relating hemispheric or regional climate-forcing mechanisms to global orbitally-induced insolation changes. Radiocarbon ages of moraines, ice cores, and deep sea sediments from the southern hemisphere suggest that initial warming and glacial recession began before 23, 000 years ago, but that in latitudinal belts of concentrated precipitation, glaciers readvanced 18, 000 and 13, 000 years ago. 40Ar/39Ar dating of glaciated and unglaciated lava flows 2200 m in elevation at Laguna del Maule, central Chilean Andes, implies that at 36° S, the southern Andean ice cap retreated between 25, 600±1, 200 and 23, 300±600 years ago and did not expand afterwards. These ages are reconciled if increased insolation ablated sea-ice cover and mountain glaciers in the southern hemisphere 5, 000 years before the northern, and if subsequently Laguna del Maule lay north of the westerly winds delivering moisture to more southerly (39-52° S) portions of the ice cap.