Late Holocene hydroclimate dynamics in the northern Alps based on compound-specific δ2H from Schliersee, Germany

A comprehensive understanding of Holocene hydroclimate variability in the European Alps remains challenging because of the great spatial and temporal disparities between the northern and southern Alps, mainly caused by changes in atmospheric circulation patterns and different climate settings. Most of the hydroclimate studies are based on lake level and high-resolution flood reconstructions that can be potentially biased by catchment-specific effects and anthropogenic impacts. Moreover, floods are only single events and just one important aspect of paleohydrology. Phases of enhanced evaporation, transpiration and droughts are equally important ecologically and can occur between flooding events. Stable isotopes (δ¹⁸O) in speleothems and lake carbonates were applied to track past changes in atmospheric circulation and hydrology, but in the northern Alps, such studies mainly focus on the Late Glacial and Early Holocene.

We present the first compound-specific δ²H record based on terrestrial (n-C₃₁) and aquatic (n-C₂₅) n-alkanes from a sediment core collected from Schliersee, a pre-alpine lake located in Bavaria (Germany), and covering the Late Holocene (past ~4.3 ka). Based on previous calibration studies and new data, we use the δ²H record of n-C₃₁ as a proxy for the isotopic composition of precipitation. We find that δ²H_n-C31 from Schliersee shows depleted values between ~1200 and ~500 cal. yr BP and enriched values before (2500 – 1200 cal. yr BP) and thereafter (500 cal. yr BP until today). This pattern is in good agreement with speleothem δ¹⁸O from Spannagel cave, Austria, and compound-specific δ²H from Lake Ghirla, southern Alps and was previously interpreted to reflect changes in moisture source. Therefore, our results support the concept that northern hemispheric cooling and changes in the North Atlantic Oscillation cause changes in moisture source related to shifts in the position of the Westerlies. Based on our results we conclude that this mechanism seem to have affected the isotopic composition of precipitation in both northern and southern Alps.

Moreover, aquatic δ²H_n-C25 is enriched by several tens of permille compared to terrestrial δ²H_n-C31, because of evaporative enrichment of lake water (Grafenstein & Labuhn, 2021 in: Ramstein et al., Springer Cham). Thus, we use their isotopic difference, expressed by Δ_aq–terr, as a proxy for evaporative enrichment. Our Δ_aq–terr shows a striking coincidence with tree-ring based drought reconstructions for Europe since the Medieval. This highlights that a “warm and dry” hydroclimate occurred during the Medieval (~1000 cal. yr BP), whereas “cool and wet” conditions prevailed during the Little Ice Age (~600 cal. yr BP). Furthermore, minima in Δ_aq–terr during the Little Ice Age seem to correspond to minima in solar forcing. High evaporative enrichment coincides with the observed anthropogenic warming during the last 250 years.

Our δ²H-record from Schliersee is consistent with other regional reconstructions and provides additional insights into the paleohydrology of the northern Alps. This highlights the potential of compound-specific δ²H analyses as a powerful tool for paleohydrological reconstructions and helps to better understand the hydroclimate dynamics across the Alps.