Lacustrine sediments often contain relatively high amounts of organic matter because of limited bottom water oxygenation and relatively high sedimentation rates. The membrane lipids of Crenarchaeota, a major group of the domain Archaea, consist of isoprenoid glycerol dialkyl glycerol tetraether (GDGTs) containing cyclopentane moieties, a characteristic considered an adaptation mechanism to temperature of the membrane. Analyses of core top sediments showed that the distribution of isoprenoid GDGTs varies with sea surface temperature (SST), a relation expressed in the TetraEther indeX of lipids with 86 carbon atoms (TEX86). Branched GDGTs were detected in peat bogs, soils, and coastal marine and lacustrine sediments. Their abundance in marine sediments decreases relative to crenarchaeol (an isoprenoid GDGT) with increasing distance from river mouths, indicating transport of branched GDGTs from land to aquatic environments. This relation was expressed in the Branched versus Isoprenoid Tetraether (BIT) index. Branched GDGTs contain a different number of cyclopentane moieties and also a different degree of methylation, which were shown to be related to air temperature and soil pH via two newly developed proxies: CBT (Cyclisation ratio of Branched Tetraethers) and MBT (Methylation index of Branched Tetraethers). This thesis describes the distribution and biological sources of GDGT membrane lipids in lacustrine environments, their potential use as temperature proxies and their application in reconstructing past continental temperatures. Isoprenoid and branched GDGTs were detected in variable amounts in particulate matter and top 5 cm of the sediment from 47 European lakes. The results show that not only large lakes but also smaller size lakes with high in situ productivity of isoprenoid GDGTs are suited for the use of TEX86. Winter temperatures correlate better with TEX86 indicating that the isoprenoid GDGT producing organisms probably have their peak abundances during this season. In an extended set of lakes we observed that the distribution of branched GDGTs varied substantially among the lake sediments. The MBT/CBT-reconstructed temperatures were consistently below annual mean air temperature of the catchment of the lake. Still, a significant correlation exists between the reconstructed temperature and the temperature measured on site. The results of a seasonal study in Lake Lucerne show that fluxes of GDGTs and concentrations in the water column vary according to a seasonal pattern, with a similar trend in suspended particulate matter and sediment traps. Fluxes and concentrations of isoprenoid GDGTs increase with depth, indicating production of isoprenoid GDGTs by Group 1 Crenarchaeota in the deep, aphotic zone. To reconstruct temperature changes during the Late Glacial Interstadial, the Younger Dryas and the Early Holocene (~14,600 to 10,600 cal. yr BP) the relative distribution of GDGTs in a sediment core from the same Lake Lucerne was determined. Reconstructed lake temperatures indicate a step-wise pattern of climate change, with a warming at the onset of the late-glacial interstadial, followed by an abrupt cooling at the onset of Younger Dryas and a rapid warming from 5.5 to 9 C at the Younger Dryas/Holocene transition within less than 200 years
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