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In order to better predict future climate change, we need knowledge on past oceanic and atmospheric responses to climate perturbations comparable to the current anthropogenic release of greenhouse gases. Accordingly, we rely on so-called climate archives of which tree rings, ice cores and marine and lacustrine sediments are typical examples. Especially, marine sediments are frequently used, in which we can measure climate proxies which are preserved entities providing indirect information on the climate when the sediments were deposited. This research has focused on organic geochemical proxies based on long-chain diols, which are organic molecules produced by micro-organisms in the ocean, and preserved in the sediment. It was previously observed that ratios of 1,13- and 1,15-diols show a strong relationship with surface seawater temperature, so that measuring these ratios in sediment cores might enable the reconstruction of past seawater temperature variations. In addition, 1,14-diols produced by diatoms typically thriving in upwelling regions, regions of high primary productivity induced by nutrient input from deep waters, may be applied as tracers of variations in past upwelling intensity. This thesis focuses on testing and validating the applicability of these proxies, and on the significance of long-chain diols as environmental indicators. We provide several new insights in the environmental controls determining the long-chain diol distributions we observe in marine sediments and show that long-chain diols can serve as valuable climate proxies for surface seawater temperature, upwelling and riverine input in coastal systems, though several constraints have to be considered before application.

In order to better predict future climate change, we need knowledge on past oceanic and atmospheric responses to climate perturbations comparable to the current anthropogenic release of greenhouse gases. Accordingly, we rely on so-called climate archives of which tree rings, ice cores and marine and lacustrine sediments are typical examples. Especially, marine sediments are frequently used, in which we can measure climate proxies which are preserved entities providing indirect information on the climate when the sediments were deposited. This research has focused on organic geochemical proxies based on long-chain diols, which are organic molecules produced by micro-organisms in the ocean, and preserved in the sediment. It was previously observed that ratios of 1,13- and 1,15-diols show a strong relationship with surface seawater temperature, so that measuring these ratios in sediment cores might enable the reconstruction of past seawater temperature variations. In addition, 1,14-diols produced by diatoms typically thriving in upwelling regions, regions of high primary productivity induced by nutrient input from deep waters, may be applied as tracers of variations in past upwelling intensity. This thesis focuses on testing and validating the applicability of these proxies, and on the significance of long-chain diols as environmental indicators. We provide several new insights in the environmental controls determining the long-chain diol distributions we observe in marine sediments and show that long-chain diols can serve as valuable climate proxies for surface seawater temperature, upwelling and riverine input in coastal systems, though several constraints have to be considered before application.

Understanding past climate variability is of key importance to better predict future climatic evolution. As instrumental records are limited in time, geological archives such as sedimentary records are used. So-called proxies, obtained by the analyses of these sedimentary records, are used to reconstruct past environmental settings and climate change. One type of proxies are biomarkers and this thesis focus on one class and their potential as proxy: the long-chain diols. These compounds are resistant to degradation and can be found in sediments up to 65 million years old. These long-chain diols are useful to reconstruct sea temperature, though this reconstruction is hampered by riverine long-chain diols which bias the original signal. However, the riverine diols can be used to trace past riverine input in coastal margins. In lakes and rivers, diols are produced seasonally by unicellular algae in low-flow areas of river systems. The results described in this thesis allowed for the development of a new diol-based proxy for river input, which, in combination with other proxies, enables a much better understanding of riverine production and transport, and of soil erosion processes. New paleo-reconstructions in both tropical and polar regions have been generated, enabling improved reconstructions of past climatic changes.

Diatoms are one of the major groups of algae which originated relatively recently and evolved in the Late Jurassic/Cretaceous. This thesis presents the results of a comprehensive study of diatom lipids in cultures and in the environment and their applications in the age determination of petroleum and in palaeoenvironmental studies. Diatom DNA sequences were analyzed in order to relate the phylogenetic positions of diatoms to the lipid chemotaxonomy. Forty four different sterols were identified in diatoms, with 24-methylcholesta-5,24(28)-dien-3-beta-ol being most common. 24-Methylcholesta-5,22E-dien-3-beta-ol, previously described as a diatom biomarker, was only the fifth most common sterol and absent in some major diatom groups. All identified sterols have been reported in other algae, but within the diatoms, some sterols and sterol compositions seem to be specific for specific phylogenetic clusters. Sterol compositions confirmed the separate phylogenetic position of the genus Attheya, as also indicated by molecular phylogeny and microscopy. 23-Methyl and 23,24-dimethyl sterols, often associated with dinoflagellate algae, were present in a substantial number of diatoms, suggesting that diatoms may also be a major source. Their phylogenetic position suggests that these diatoms originated from a single common ancestor which evolved in the late Jurassic. In addition to 23-methyl and 23,24-dimethyl sterols, the unusual sterol gorgosterol was found in two diatom cultures of the genus Delphineis. 24-Norsterols were found in the diatom species Thalassiosira aff. antarctica and in dinoflagellate cultures. The evolutionary history of dinoflagellates and diatoms explains the stepwise increases of 24-norsterane concentrations, diagenetic products of 24-norsterols, in petroleum. Long-chain 1,14-diols and 12-hydroxy methyl alkanoates were detected in Proboscia diatoms and may be used as indicators for high-nutrient conditions and upwelling. Their distributions varied between different Proboscia species and culture experiments showed increasing chain-lengths and a decreasing degree of unsaturation for 1,14-diols with increasing growth temperature. Lipid analyses from surface sediments from the eastern South Atlantic suggested a significant relationship between long-chain 1,14-diol chain-length and sea surface temperature, but the degree of unsaturation for 1,14-diols seems also determined by other factors. Sediment trap data from the Arabian Sea confirmed that Proboscia lipids can be used as proxies for upwelling conditions and also showed that long-chain 1,15-diols are not limited to upwelling conditions. A sediment core taken from the Somali continental slope showed strong fluctuations of long-chain 1,14- and 1,15-diols with time; 1,14-diols were relatively high during the Holocene when upwelling occurred and much lower during the Late Glacial Maximum and the last Glacial when upwelling was suppressed. Elevated 1,14-diol concentrations during the first half of Marine Isotope Stage 3 and at the end of Marine Isotope Stage 5.1 suggest intensified glacial upwelling during those periods. Analyses of long-chain diols in a sediment core from the North Western Antarctic Peninsula suggest that Proboscia diatom productivity in this area is associated with upwelling of Upper Circumpolar Deep Water at the shelf break. Comparison of the diol record with melt events in Siple Dome ice core indicates that this upwelling is driven by the same climatic processes that are responsible for changes in regional climate