| Summary: | This thesis describes the structure and occurrence of branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids, their potential biological origin and shows their application in reconstructions of past environmental conditions. Based on the stereo-configuration of the glycerol backbone, it was concluded that these membrane lipids are synthesised by Bacteria rather than Archaea. As they are abundant in the anoxic part of peat bogs and in world-wide soils, they are expected to be synthesised by anaerobic soil bacteria. Branched GDGTs are fluvially transported to the marine environment where they become part of the marine sedimentary archive. It is shown that their abundance in marine sediments relative to crenarchaeol (an isoprenoid GDGT derived from marine Crenarchaeota), expressed in the Branched vs. Isoprenoid Tetraether (BIT) index, can be used as a proxy for the relative fluvial input of terrestrial organic matter. This proxy was applied in a sediment core from the Bay of Biscay. During the last glacial, when the North Sea and Channel were emerged as result of the sea-level low-stand, this core location was situated in front of the Channel River, which drained all major river catchments of north-western Europe. The BIT index revealed an early reactivation of this river at the onset of the deglaciation. Because of the increased freshwater discharge, a 'lid' of fresh cold water was created on the Bay of Biscay which hindered water evaporation. Consequently, climate on land shifted back to a drier state which was, subsequently, recorded as an abrupt fall in the BIT index in this core. The relative abundance of individual branched GDGTs varied from place to place. Analysis of 134 globally distributed soil samples revealed that the degree of cyclisation, expressed in the Cyclisation ratio of Branched Tetraehters (CBT), is related to soil pH and the degree of methylation, expressed in the Methylation index of Branched Tetraethers (MBT), is related to both soil pH and annual mean air temperature (MAT). This observation could be explained by the fact that micro-organisms adapt the molecular composition of their cell membrane in response to ambient conditions in order to keep their membrane functioning. Using these relations, records of annual MAT and soil pH changes for central Africa were obtained by analysing the branched GDGT distribution in a marine sediment core close to the Congo River outflow. For the annual MAT, this revealed a gradual increase of ~4C since the last glacial. An advantage of reconstructing continental temperatures in a marine core is the possibility to directly compare continental and sea surface temperature changes. This comparison revealed that the temperature difference between land and sea in equatorial Africa exerted a strong control, through the thermal pressure gradient, on African precipitation patterns during the deglaciation. A similar application in a core from the Arctic Ocean revealed a temperature increase of 8C on the Arctic continents during the Palaeocene-Eocene thermal maximum. During this time interval of increased atmospheric greenhouse gas concentrations, subtropical values of 25C were reached which are indicative of a strongly reduced latitudinal thermal gradient.
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