The demise of the Last Interglacial recorded in isotopically dated speleothems from the Alps

Abstract

The transition from interglacial to glacial climate has attracted much attention in paleoclimatic research, partly because orbital forcing dictates that such a return to a new ice age is inescapable (albeit in the distant future only). The detailed series of steps that leads to glacial inceptions remains poorly understood, however, and studies of past glacial inceptions are often compromised by incomplete records, imprecise chronological control, and/or uncertainties associated with proxy-transfer functions. Here we present results from U–Th-dated speleothems from a cave in the Austrian Alps (Entrische Kirche cave) which grew continuously between ca 127 and 114 ka and intermittently also during subsequent stadials and interstadials. The alpine setting of this cave is prone to periglacial and glacial processes as well as temperature and precipitation changes which are recorded in these cave deposits. We use high-resolution stable isotope data to constrain paleoenvironmental changes associated with the demise of the Last Interglacial, unprecedented in detail for central Europe. Peak interglacial conditions are characterized by high δ 18O values and high growth rates from ca 127 to 124 ka. A major drop in δ 18O by 3‰ occurred at ca 118 ka, which coincided with cold event C26 in the North Atlantic. This isotopic shift, which occurred at or close to isotopic equilibrium, cannot be explained by simple uniform cooling. Modelling suggests that enhanced seasonality, dominated by significant changes in winter precipitation, are required to reconcile the isotope data with the fact that calcite deposition continued across the MIS 5e/d transition (i.e., no cave freezing). This period of enhanced seasonality lasted for several millennia during which the catchment area of the cave remained geomorphologically stable and ice free. Eventually, speleothem growth terminated at ca 114 ka, which is in line with widespread ice-rafting in the North Atlantic (cold event C24) and the onset of fully stadial conditions during MIS 5d.