Rapid summer temperature changes during Termination 1a: high-resolution multi-proxy climate reconstructions from Gerzensee (Switzerland)

Quantitative July temperature reconstructions using fossil chironomid and pollen data have been carried out with a high temporal resolution for the Termination la and the late-glacial Interstadial (ca 15 000-13 000 cal years BP) on a sediment core from Gerzensee (Switzerland). The biota-based July temperature reconstructions show a rapid warming of 4-5 degrees C (pollen) and 2-3 degrees C (chironomids) at Termination la (i.e. the onset of the late-glacial Interstadial, ca 14 650 cal. BP). These temperature changes go parallel with a shift of more than 3% in a high-resolution stable oxygen isotope record measured on bulk carbonates from the same core, also indicating a substantial and rapid warming.Pollen-inferred July temperatures follow the shape of the oxygen-isotope record with a generally decreasing trend of ca 2 degrees C throughout the Interstadial, showing even minor cold oscillations of 0.5-1 degrees C in July temperature recorded in many central European sites (e.g. Aegelsee and Gerzensee Oscillations) and the Greenland ice cores (e.g. Gl-ld and GI-1 b). Chironomid-inferred summer temperatures, however, show a gradual increase of ca 2-3 degrees C throughout the Interstadial, interrupted by some cold oscillations (e.g. GI-lb) of 0.5-1 degrees C.The reconstructions based on both proxies are in agreement with comparable studies in the Jura Mountains and the Alps. Given the close correlation between the oxygen isotope record and the polleninferred temperatures, the discrepancies between the two biota-specific July temperature inferences could be explained by the higher sensitivity of vegetation to changes in seasonality and precipitation, whereas aquatic organisms such as chironomids are more responsive to summer season conditions.