Biomarkers for understanding paleoclimate and organic matter inputs in the Southern Ocean

The opening of the Tasmanian Gateway around the Eocene/Oligocene boundary is a crucial evolution stage for the formation of the Antarctic Circumpolar Current, and plays a pivotal role in global ocean circulation, regional climate, and terrestrial vegetation variations. Multiple proxies such as microfossils or terrestrial palynomorphs, have been used to study Paleogene tectonic, climatic, oceanographic, and vegetation evolution in the Tasmanian Gateway. However, except for glycerol dialkyl glycerol tetraethers (GDGTs) and alkenones, which contain water temperature information, biomarkers have rarely been studied in ODP Leg 189 samples. Nevertheless, they could provide useful information about organic matter input and depositional environment. This PhD thesis provides the first detailed organic geochemical analysis of hydrocarbons and biomarkers in four cores (1168, 1170, 1171, 1172) from the early Paleogene interval of Ocean Drilling Program (ODP) Leg 189 in the Tasmanian Gateway region to the south of Australia. 114 sediment samples were analysed by gas chromatography-mass spectrometry (GC-MS), including multiple-reaction monitoring (MRM) mode. Results showed that sediments are all immature with significant microbial input. Interesting polyaromatic hydrocarbon features were observed in sediments. Land-derived perylene is only concentrated at the site (1168) closest to Tasmania under dysoxic conditions, while the terrigenous-sourced organic matter was detected at all four sites. This was attributed to the short transportation distance and fast deposition rate of the organic matter, which preserve the precursors of perylene. This study indicates that perylene cannot serve as a palaeoenvironmental marker for anoxia, and sustained reducing conditions are not necessarily essential for the formation of perylene from its precursors. Unusual alkylnaphthalenes and alkylphenanthrenes isomer distributions were detected in Site 1168, with unknown terrigenous or algal-derived sources. Reworking, combustion or preferential biodegradation processes cannot be validated. Microbial interference was assumed to be the most plausible explanation for such specific distributions. These findings also suggest that the application of alkylnaphthalenes and alkylphenanthrenes isomers in maturity or combustion indexes should be done cautiously, as microbial input could bias the results. The Palaeocene-Eocene Thermal Maximum (PETM), marked by a negative carbon isotope excursion, is one of the most prominent hyperthermal events during the Paleogene. Sediments in Site 1172 recorded this event. The variations of hopanoids indicate an intensified bacterial input, while total organic carbon and sulfur contents suggest diluted detrital input across the PETM in the Tasmanian Gateway. Sea level fluctuations could significantly influence the composition and accumulation of organic matter. This study investigates the geochemical features of sediments in the Tasmanian Gateway in the early Paleogene, which shed light on the explanation of regional tectonic, climatic, and oceanographic variations. Also, it deepens our understanding of the application of biomarkers. The Totten Glacier, and the Moscow University Ice Shelf on its eastern flank, form the largest part of the extensive drainage system for basal meltwater to the ocean in East Antarctica, and are particularly sensitive to ongoing climate change. The global sea level could rise 3.85 m based on meltwater flowing through the Totten Glacier alone. This is similar to the entire probable contribution of the West Antarctic Ice Sheet to global sea level rise. Satellite records have shown that the Totten Glacier has the fastest melting rate in East Antarctica. Lipid compounds in 10 surface marine sediments from the continental slope offshore from the Totten Glacier were analysed by GC-MS, including MRM mode and high-performance liquid chromatography (HPLC). Sediments have a low thermal maturity. n-Alkane and steroid distributions suggest abundant terrigenous input into this region related to eroded older sediments and/or dust deposition. Sea ice proxies based on highly branched isoprenoids indicate a highly variable sea-ice extent in the study area, consistent with the satellite records. Both isoprenoidal GDGTs (isoGDGTs) and hydroxylated isoprenoidal GDGTs (OH-GDGTs) are used to reconstruct ocean temperatures. Although all indices provide unrealistic estimates of temperatures, OH-GDGTs seem to be more promising ocean temperature proxies in cold regions, with smaller differences from satellite records. This study provides the first biomarker records in surface marine sediments from the continental slope offshore from the Totten Glacier, Antarctica, which improves our understanding of the behaviour of the Totten Glacier, an area significant for future changes in global sea level. Overall, this thesis provides enhanced information on the application of specific biomarkers and hydrocarbons for palaeoenvironment reconstructions. The biomarker results enlarge the database in the Southern Ocean, which lack published field data, and will be crucial for future modelling studies. Also, this thesis sheds light on the organic matter input, paleoclimate and paleoenvironment of marine sediments in two critical regions in the Southern Ocean, which will greatly improve our understanding of these areas.