Reconstruction of changes in the Southern Ocean Overturning Circulation using radiogenic Pb and Nd isotopes in marine sediments and Fe Mn crust for the past 19 Ma

Gerhard.Kuhn [ at ]


Radiogenic lead (Pb) and neodymium (Nd) isotopes are sensitive paleoceanographic proxies for the reconstruction of ocean circulation changes in the past. The goal of this dissertation is to develop improved approaches to recover past seawater Pb and Nd isotope signals from marine sediments and ferromanganese (Fe-Mn) crust, and apply it for tracing water mass sourcing changes in the Southern Ocean at (sub-)millennial resolution. Chemical reductive leaching of hydrogenetic Fe-Mn oxyhydroxides from bulk marine sediments offers an efficient and easily accessible way to recover past seawater Pb and Nd isotope signatures. However, the leached seawater derived Pb and Nd isotope signal could be compromised if substantial quantities of Pb and Nd were released from non-hydrogenetic sediment fractions during leaching. In Chapter3, a fast and reliable vortexing leaching method is presented for extracting porewater Pb and Nd isotope signals from sediment samples in the Atlantic sector of the Southern Ocean. The effect of a previously introduced MgCl2 prewash is investigated, as well as the role of chelate ligands and length of leaching time. In order to validate the vortexing leaching method, Pb and Nd isotope signatures were analysed in actual seawater and underlying porewater and sediment leachates from three stations in front of the Filchner-Rønne Ice Shelf, Antarctica. The well-matching results between water sample and leachates corroborate the robustness of this method. Moreover, using the newly developed vortexing leaching method, I generated authigenic Pb and Nd isotopic maps from 70 Southern Ocean coretop sediment samples to help select suitable sites for paleoceanographic and paleoclimatic reconstructions. In Chapter 4, seawater-derived downcore sedimentary Nd and Pb isotope records are presented from three Southern Ocean sites inside and outside the Weddell Sea to resolve changes in Southern Ocean overturning circulation for the past two glacial terminations. The data provide strong evidence for the absence of Weddell Sea AABW outside the Weddell Sea during the last and penultimate glacial maximum. The successive southward displacement of the SO overturning cell following glacial maxima is recorded by increasing contributions of Weddell Sea derived Pb admixtures to regions outside the Weddell Sea during both glacial terminations. The export of Weddell Sea AABW resumed late during glacial terminations, coinciding with the last major atmospheric CO2rise in the transition to the Holocene and the Eemian. While Holocene AABW formation and export out of the Weddell Sea took place without major perturbations, our new records lend strong support for a previously inferred overturning stagnation event during the peak Eemian interglacial. In Chapter 5, an unprecedented continuous high-resolution Pb isotope record was recovered from a Fe-Mn crust grown on the Marie Byrd Seamount in the Pacific sector of the Southern Ocean using laser ablation coupled MC-ICPMS. The Pb isotope data document the ACC history for the past 19 million years and indicate that the final establishment of clockwise (eastward) circulating Antarctic Circumpolar Currents took place at 14 Ma. The Pb isotopic evolution also suggest five large-scale ocean circulation reorganization events which are likely related to changes in Antarctic Bottom Water formation and tectonic events in ocean gateway regions, i.e. Panama Seaway and Drake Passage. Overall, the results presented in this dissertation show deep sea Pb and Nd isotope records are able to resolve the changes in the glacial/interglacial Southern Ocean overturning circulation front shifting and the Antarctic Bottom Water evolution. The Pb isotope system also shows great potential in tracking Antarctic ice dynamics and ocean circulation changes on Cenozoic timescales. Applying these approaches employed in this dissertation to different key locations and into different time period will help resolving more ocean circulation mysteries.

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Huang, H. (2019): Reconstruction of changes in the Southern Ocean Overturning Circulation using radiogenic Pb and Nd isotopes in marine sediments and Fe Mn crust for the past 19 Ma , PhD thesis, Geowissenschaften Mathematisch-Naturwissenschaftliche Fakultät.

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