Towards quantifying Younger Dryas cooling in Europe using lipid biomarkers archived in lake sediments

The Younger Dryas (YD, 12,900-11,700 cal. yr BP) was a major abrupt cooling event caused by a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), which sharply reduced heat transport in the Northern Hemisphere. However, the magnitude, temporal pattern and seasonality expression of the YD cooling across Europe remain difficult to constrain due to the lack of quantitative proxies and confounding factors on proxy responses. Here, we aim to reconstruct YD mean annual temperatures of months above freezing (MAF) using temperature-sensitive bacterial membrane lipids, so-called branched glycerol dialkyl glycerol tetraethers (brGDGTs), stored in lake sediments from Retournemer (eastern France) and Steisslingen (southern Germany). In both lakes, brGDGT-derived MAFs show only a minor cooling (~1-2C) during the YD, whereas more established GDGT-based proxies indicate a deeper oxic layer and shifts in lake microbial communities consistent with colder and windier conditions across Europe. As such, our brGDGT records confirm that most of the cooling was expressed during winters, in line with previously suggested seasonality patterns. Subsequent examination of the much less explored branched glycerol monoalkyl glycerol tetraethers (brGMGTs) that are characterized by an additional carbon–carbon bond between their alkyl chains reveals a stronger response during the YD in both lakes. However, translation to absolute temperature is hampered by their distinct composition from that in East African lakes on which the only currently existing transfer function is based. Regardless, our results show that brGMGTs have potential as indicators of YD cooling in future studies.