31. The last and the penultimate interglacial as recorded by speleothems from a climatically sensitive high-elevation cave site in the alps

Abstract

Within the greater Alpine region, absolutely dated climate records of the penultimate and the last interglacial are exceptionally rare. Speleothems offer an important and still underutilized source of information about the timing and duration of warm periods during the Middle and Upper Quaternary. The focal point of intense research is Spannagel Cave, a large high-altitude (ca. 2200 to 2500 m a.s.l.) cave system in the Zillertal Alps of Austria. The presently low (1.4 to 2.5°C) cave temperature provides a natural threshold for speleothem growth, i.e. the cave acts as a climatically sensitive archive. U-series dating of calcite speleothems, facilitated by exceptionally high U content in combination with high-resolution stable isotope analyses allow identifying warm climate periods. Calcite growth at 236 to 229kyr, 211 to 206kyr and 199 to 192kyr is in good accordance with U-series dated sea-level records and marine sediments whose chronology was tuned to orbital parameters. Oxygen isotope data show that the climate in the Alps was consistently cooler during the penultimate interglacial than during the last interglacial. Carbon isotope data also show a major difference between the two interglacials: while alpine soil and vegetation was apparently well developed during the last interglacial (and similar as today), high C isotope values testify the lack of pedogenic C input during the penultimate warm periods. Accordingly, the area above the cave was either barren or — more likely — covered by a warm-based glacier. Previously regarded as evidence of ice-free conditions early during the penultimate deglaciation speleothem deposition at 136kyr is now seen as an indication of a major change of the glacier's thermal state most likely as a result of the collapse of ice-stream network at the end of the penultimate glacial maximum. Following a return to stadial conditions not conducive to speleothem formation and marked by a hiatus in speleothem growth, fully interglacial conditions did not commence until 130kyr and prevailed until 119 kyr.