Sources and temporal variability of lipid biomarkers in Lake Chala (East Africa), with implications for their sedimentary record

Abstract

This thesis emphasizes the importance of extensive studies of the modern-day lake system prior to interpretation of climate-proxy records based on the occurrence and distribution of lipid biomarkers in its sediments. The biomarkers in the modern lake system show a much more complicated picture than usually is expected, and this thesis highlights in various ways the importance of studying modern systems for paleoclimate research. Firstly, certain biomarkers are shown to have a different origin than generally presumed (such as the C28 fatty acid, and des-A-arborenes). Secondly, this thesis describes significant steps in tying established and new biomarkers (such as the C19:1 n-alkene, and long-chain n-alk-1-enes) to their possible microbial sources. Thirdly, the 13C-depleted aquatic biomarkers found in Lake Chala and its sediments can be explained by physical fractionation processes associated with high pH, a common condition in East African lakes. These findings are not only important for the quality and impact of paleoenvironmental reconstructions based on biomarker δ13C values in the sediment record of Lake Chala, but for all (East African) lakes. Finally, this thesis highlights that monitoring programs need to run for many more years (>5) to fully characterize the relation between environmental conditions and the temporal patterns of the studied biomarkers, as well as to reveal the drivers and underlying patterns forcing the lake’s microbial community. Although this is practically challenging and costly to implement, it is crucial in order to improve our understanding of paleoenvironmental reconstructions based on lake sediments.