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Sedimentary biogeochemistry and palaeoceanography of the South China Sea during the late Pleistocene Kienast, Markus

Abstract

The South China Sea (SCS) is the largest marginal basin off the Asian continent and its surface hydrography as well as sedimentation within the basin are strongly influenced by the SE Asian monsoon and eustatic sea-level changes. The rapidly accumulating sediments in this basin are therefore a unique and sensitive monitor of past variations in monsoonal climate, ocean-continent linkages, deglacial sea-level, and marine biogeochemical processes and their relationship to climate change. This thesis contributes to our understanding of these key aspects of glacial-interglacial palaeoceanography by presenting multi-proxy organic and inorganic sediment geochemical records from a large number of gravity cores and surface sediment samples from throughout the SCS. Records of deglacial sea surface temperature (SST) and summer monsoon variability point to a close coupling with the climate of the circum- North Atlantic realm, whereas sedimentological changes associated with variations in winter monsoonal intensity suggest a concordant deglacial development with SST changes in the open equatorial Pacific. Together, these records demonstrate a complex interaction of Northern and Southern Hemisphere influences on the climate of this region. The biogeochemical cycles of nitrogen and carbon in the open SCS as reflected in their time-varying isotopic composition are not significantly affected by monsoonal climate or the specific sedimentological and geographical setting of this marginal basin but appear instead to reflect the regional marine biogeochemistry. On the other hand, variations in basin configuration related to eustatic sea level changes leave a distinct inprint in the rate and geochemical composition of terrigenous sediment supply to the SCS, which are used to constrain the deglacial history of sea level rise and its impact on near-shore sedimentation. A comparison of various geochemical and micropalaeontological methods to estimate palaeo-sea surface temperatures (SSTs) demonstrates a quantitative agreement between alkenone (U[sup K']₃₇), foraminiferal Mg/Ca, and foraminiferal tranfer function FP- 12E SST estimates in recording an annual average cooling of 2-2.5 °C of the tropical southern SCS during the last glacial period. In contrast, the foraminiferal transfer functions RAM and SIMMAX show an annual average glacial cooling of only

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