Biogeochemical responses of lake Übeschi (Swiss Plateau) during Late-Glacial periods of rapid climate variations 

The present-day rapid degradation of freshwater ecosystems worldwide, due to eutrophication and hypolimnetic anoxia, is primarily driven by human activities and climate change. A comprehensive understanding of these processes and their underlying mechanisms is essential. Traditionally, the study of these phenomena has been challenged by the limited availability of high-resolution sedimentary records and suitable proxies, particularly for past anoxia events. However, recent advancements in Hyperspectral Imaging (HI) techniques have opened up new avenues for examining variability in past lake biogeochemical cycles under changing redox conditions and related chemical feedbacks, providing insight into how freshwater ecosystems have responded to natural and anthropogenic perturbations. We investigate the responses of lake productivity, redox conditions and chemical feedbacks (P, Fe, Mn) during periods of rapid climate shifts (Dansgaard-Oeschger Events) under natural conditions in the past. Furthermore, we seek to reconstruct the lake's history prior to the Bølling warming (14.6 kBP) to gain insights into the lake ecology during initial stages of the lake formation in a cold-dry tundra environment during Heinrich Stadial 1 (HS1, Oldest Dryas). To answer these questions a kettle hole lake Übeschisee in the upper Swiss Plateau was studied. The lake is well suited for analyzing abrupt changes in the lake and in the lake catchment during the Late-Glacial since it formed after the retreat of the Aare glacier >18 kBP and offers a continuous sediment record since then. The biogeochemical composition of lake Übeschi sediments was acquired by X-ray fluorescence core scanning, HI, HPLC measurements of pigment extracts, and ICP-MS measurements of P, Mn and Fe sequential extractions. Bacteriopheophytin a produced by Purple Sulfur Bacteria (PSB) is used as a proxy for hypolimnetic anoxia. Our results show an increase in aquatic primary production already during HS1, i.e. long before the Bølling warming. Cyanobacteria were among the first photosynthetic organisms to colonize the young lake in a deglacial environment. Additionally, hypolimnetic anoxia events indicated by bacteriopheophytin a generally occurred during the cold periods of the Oldest Dryas, the Older Dryas and the Younger Dryas and not during the Bølling warming, as initially hypothesized. Apparently, these anoxia events are attributed to very long ice cover of the lake and a very short open period with strong lake stratification and a chemocline in the photic zone where PSB could grow. Finally, redox-sensitive fractions of Mn and Fe are mostly absent from the sediment during anoxic events, suggesting remobilization of these elements by reductive dissolution. This provides evidence for pre-anthropogenic chemical feedbacks and internal P loads under natural conditions.