Chronology of Dansgaard-Oeschger cycles

The climate during the long glacials of the youngest period of the Earth`s history (the Quaternary) was characterized by large instability. Interstadials started with abrupt warmings, but only lasted for a maximum of ca. 3000 years and lead to a gradual cooling culminating in very cold and dry stadials. This striking asymmetric structure of the ice-age climate - known as Dansgaard-Oeschger cycles - has been puzzled the paleoclimate community for many years and no consensus has been reached as far as the underlying forcing mechanisms are concerned. A major difficulty in studying this paleoclimate phenomenon is the precise chronology of individual Dansgaard- Oeschger cycles; within the last glacial cycle alone there were at least two dozens of them. A new and promising approach to identify and date these short-lived events in sediments is the study of dripstones. Within this project dripstones from caves in the Eastern and Western Alps will be examined as previous studies by our group have demonstrated that alpine caves are excellent paleoclimate archives. The traces of past ice-age climate will detected using the stable isotopes of oxygen in the calcite of these inorganic carbonate deposits, while the precise dating relies on the radioactive decay of the trace elements uranium and thorium. The results of this research will help, among others, to significantly improve the current chronology of the important Greenland ice cores.

In this project stalagmites from selected caves in Austria and Switzerland were studied as archives of ice-age climate variability. Rapid and drastic climate changes during the last glacial period (so-called Dansgaard-Oeschger cycles) were identified based on the isotopic composition of the oxygen atom in the dripstone calcite, analyzed at high resolution along the extension axis. These changes were compared to the same climate signals as recorded in the deep ice of Greenland. The high similarity of the isotope curve from Greenland - meanwhile an icon of climate change - with the curve from the Alpine region underscores the close climatic relationship between these two regions. In this project the age and growth dynamics of stalagmites were determined at high precision and a robust age scale of these glacial climate transitions was established. The precise timing, rate and pattern of these past drastic climate transitions could be determined with high quality. This work was done in collaboration with the word-famous age dating laboratory at the University of Minnesota in Minneapolis, USA. A composite, albeit incomplete, climate curve was established which for the first time and reveals the true dimension of climate fluctuations in the Alps between 118,000 and 64,000 years before present. This curve may serve as valuable reference for a future improved chronology of the Greenland ice cores.