SPATIAL AND TEMPORAL CONSTRAINTS ON THE EXTENT OF GLACIATION IN THE EUROPEAN ALPS DURING ISOTOPE STAGE 3 TO 6 BY 230TH-DATED SPELEOTHEMS

It well established since the pioneering work of Penck and Brückner (1901/09) that the European Alps have been repeatedly buried beneath an extensive ice sheet during the Pleistocene. Recent studies have shown convincingly that the maximum ice extent during the last glacial cycle (Würm), with valley glaciers advancing well beyond the geographical borders of the Alps, was only reached for a short period of time between c. 25 ka and 20 ka. While this glacial maximum is fairly well constrained by radiocarbon dates, little is known about older ice advances up to and possibly beyond the borders of the Alps, the main reasons being the scarcity of intramontane records and the difficulty of dating them. Carbonate deposits in subsurface cavities and fractures (speleothems) can provide critically needed anchor points for reconstructing past ice sheets, because they (i) require ice-free conditions at the surface and (ii) can be precisely dated using U-series techniques. Ongoing work in the Eastern Alps revealed a number of important observations from cave deposits which are clearly inconsistent with the widely held view of speleothem deposition during interglacial times only. A high-alpine cave site in the Central Alps of Austria (Spannagel Cave) was ice-free subsequent to 135 ± 1.5 ka, providing the first isotopically dated time constraints on the duration of the penultimate glaciation (Riss?) in this part of the Alps. Calcite speleothems present as fracture-lining flowstones show that the central Inn Valley near Innsbruck, which hosted one of the largest valley glaciers in the Eastern Alps during the Pleistocene, was ice-free at 73 ± 1 ka, i.e. during the final stage 5a. A similar age was obtained from a flowstone overlying fossiliferous clastic deposits in cave in the central part of the Northern Calcareous Alps and, more importantly, also in Spannagel Cave (72-78 ka). A fairly large number of dates clustering around 47-59 ka provide evidence of a generally mild climate and limited ice extent during the peak of stage 3 in the Alps, corroborating previously presented evidence of abundant cave bear populations in some of these caves presently situated above the timberline.