The oxygen and carbon isotopic signatures of biogenic carbonates in Gerzensee, Switzerland, during the rapid warming around 14,685 years BP and the following interstadial

摘要

The stable isotope signature of ostracods,molluscs, and charophyte remains fromthe late glacial section of a shallow core from lake Gerzensee, Switzerland, is analyzed along with the bulk carbonate isotope composition in a multi-proxy study aiming to document the biotic responses to the first strong warming ca. 14.6 ka ago after full glacial conditions. The main goal of our contribution is to understand the climatic significance of the oxygenisotope variations in and between the different carbonate species and ideally provide a quantitative estimate of the oxygen isotopic composition of meteoric precipitation, which then could be translated to mean temperature estimates. Corrected for the respective vital offsets, the different carbonates show almost identical oxygenisotope ratios for the time preceding and after the rapid transition from Greenland climate stages GS2 to GI1, indicating low and seasonally constant water temperatures at the sediment-water interface for this period. In the following the difference between cold season and warm-season carbonates increases gradually, pointing to a summer-winter temperature difference of roughly 10 K at the end of GI1. We conclude that this gradual watertemperature increase is independent of climate and is mainly due to sedimentation, shallowing the sediment- water interface, eventually accentuated by a gradual decrease of Gerzensee's water level during GI1. Corrected for the isotope fractionation induced by the long termtrend of suchwater temperature change, the higher resolved δ~(18)O record fromthe bulk carbonates allows calculation of the presumed oxygen-isotope ratio of former lakewater (δ~(18)O_L), which shows striking similarity to the record fromAmmersee. Introducing a tentative hydrological correction close to the present day offset between δ~(18)O_L and the oxygen-isotopes in meteoric precipitation (δ~(18)O_P), we propose a quantitative δ~(18)O_P and mean air temperature record for the Gerzensee region.