Paleohydrology and human driven paleoproductivity during the Late Holocene from Schliersee, Bavaria

Understanding Holocene hydroclimatic variability in the European Alps is challenging due to spatial and temporal disparities between the northern and southern Alps. In addition, interpreting lake sediment records in terms of paleohydrology is complicated by human presence during Roman and Medieval settlements, which increased soil erosion and lake eutrophication. Here, we present a ~4440-year long sediment record from Schliersee, Bavaria, where we applied compound-specific δ2H on leaf waxes, geochemical and diatom analyses to reconstruct hydrology and lake productivity. The terrestrial δ2Hn-C31 records the isotopic composition of precipitation and is similar to leaf wax δ2H from Lake Ghirla, southern Alps, and δ18O from Spannagel cave in Austria. This provides evidence that, on millennial time scales, changes in moisture sources associated with shifts in the position of the Westerlies are one potential explanation regarding the isotope signals across the region. However, doubts remain whether the North Atlantic Oscillation as a winter signal can explain variations in summer-sensitive biomarker δ2H records. The aquatic δ2Hn-C25 records the isotopic composition of lake water and its isotopic offset to δ2Hn-C31 (Δaq–terr) is applied as a proxy for lake evaporation. We find increased evaporation during the Medieval Climate Anomaly in line with a drought reported from tree-ring studies, whereas lower evaporation prevailed during the Little Ice Age, likely due to solar forcing. Lake productivity was higher during the Roman period and Middle Ages, concomitant with land use resulting in higher nutrient inputs into the lake. The intensified use of industrial fertilizers and the drainage of untreated wastewater after the Second World War caused eutrophication during the 1950s. Despite its paleoclimatic significance, this study emphasizes that multi-proxy approaches combining assemblages of geochemical and biological proxies allow robust reconstructions of climate–landscape interactions and human impact.