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

The stable isotope signature of ostracods, molluscs, and charophyte remains from the 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 oxygen-isotope 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 oxygen-isotope 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 water-temperature increase is independent of climate and is mainly due to sedimentation, shallowing the sediment–water interface, eventually accentuated by a gradual decrease of Gerzenseeʹs water level during GI1. Corrected for the isotope fractionation induced by the long term trend of such water temperature change, the higher resolved δ18O record from the bulk carbonates allows calculation of the presumed oxygen–isotope ratio of former lake water (δ18OL), which shows striking similarity to the record from Ammersee. Introducing a tentative hydrological correction close to the present day offset between δ18OL and the oxygen–isotopes in meteoric precipitation (δ18OP), we propose a quantitative δ18OP and mean air temperature record for the Gerzensee region.