Paleoclimate Proxies and Dynamic Processes in Continental Margin Sediments

Abstract

The primary objective of this dissertation is to explore the dynamic processes, their sensitivities and responses to global climate change within complex ocean margin environments. The first study is based on sediment from the equatorial African continental margin, and the second study is focused on the Antarctic margin. Both studies further our insight into how climate signals are recorded in the sediment record from continental margins. The studies differ in terms of the time scales, oceanographic processes at work, and analytical techniques used. The African margin study area is located off the shelf oceanward of the Niger River Delta. Here we assess the applicability of using the 230Th profiling technique as a Niger River discharge proxy to reconstruct changes in West African Monsoon (WAM) intensity. Our results suggest that West African Monsoon intensity is a function of the migration of the oscillating Intertropical Convergence Zone’s (ITCZ) mean position which, in turn, responds to high-latitude climate events during the last deglaciation and Holocene. Cold stadial events (i.e., Heinrich Stadial 1, Younger Dryas) are characterized by low discharge and high productivity (ITCZ shifts southwards, WAM intensity decreases, and upwelling strengthens due to intensified trade winds), whereas warm interstadial events (i.e., Bølling Allerød and the African Humid Period) are associated with high discharge and low productivity (ITCZ shifts northwards, WAM intensity increases leading to higher Niger River discharge). The Antarctic study site is on the edge of the Ross Sea continental shelf at International Ocean Discovery Program (IODP) Site U1523. Here we used principal component and hierarchical cluster analysis of high-resolution core scanning X-ray fluorescence (XRF) records, together with major and trace element composition, carbonate content, physical property data, lithology, and grain size to assess Antarctic Slope Current strength and intrusions of warm Circumpolar Deep Water (CDW) onto the continental shelf from the Pliocene to present. Intrusions of CDW deliver excess heat onto the shelf, where it transfers across the shelf, triggering basal melting and ice mass loss, which ultimately leads to glacial retreat and ice sheet instability. The results show three different regimes through time. During the Pliocene, the Polar Front was shifted southward, resulting in reduced ice extent and open water conditions over Site U1523. Cooling associated with the Pliocene/Pleistocene boundary resulted in a northward shift of the Antarctic frontal systems, intensifying glacials and limiting the presence of subantarctic diatoms that were common in the Pliocene. The final transition occurred during the mid-Pleistocene transition (MPT), with larger amplitude glacial-interglacial cyclicity, advance of the ice sheet to near the shelf edge, and occasional presence of diatoms suggesting increased polynya development. Both studies use multiple geochemical proxies in conjunction with physical property data to understand and elucidate how the ocean, atmosphere, and cryosphere responded to climate change and how their signals are recorded within ocean margin sediments.