Sedimentary lipid biomarkers in the magnesium rich and highly alkaline Lake Salda (south-western Anatolia)

Lake Salda located in south-western Anatolia is characterized by the presence of living stromatolites and by a low diversity of both phytoplankton and zooplankton due to high pH and magnesium concentration. The most abundant, free sedimentary lipids of the uppermost centimetres of the lake sediments were studied as potential environmental biomarkers, and proxies based on glycerol dialkyl glycerol tetraethers (GDGT) were tested in this extreme environment. Dinosterol and tetrahymanol are potentially relevant biomarkers for the dinoflagellate Peridinium cinctum and ciliates, respectively. C_20:1 and C_25:2 highly branched isoprenoid (HBI) alkenes, and n-C₁₇ alkane and n-C_17:1 alkene are considered as representing, respectively, diatoms and Cyanobacteria involved in the formation of the stromatolites. Isoprenoid GDGT-0 is assumed to be derived mainly from Euryarchaeota (methanogens), and crenarchaeol from Thaumarchaeota. Allochthonous organic material is represented by long-chain n-alkanes and n-alkanols derived from land plant leaf waxes, as well as branched GDGTs produced by soil bacteria. While pH and temperature proxies based on branched GDGTs are likely not applicable in Lake Salda, TEX₈₆ (tetraether index of tetraethers consisting of 86 carbons), a proxy based on isoprenoid GDGTs, potentially allows estimating mean annual lake surface temperature. Interestingly, C₂₃ and C₂₅ 1,2 diols, which have a yet unknown origin, were found for the first time in lake sediments. This study represents the first investigation of sedimentary lipid distribution in an alkaline and magnesium-rich lake in Anatolia, and provides a basis for future biomarker-based paleoenvironmental reconstruction of Lake Salda.