Uranium-series radionuclide records of paleoceanographic and sedimentary changes in the Arctic Ocean
Hoffmann, Sharon S.
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The radionuclides 231Pa and 230Th, produced in the water column and removed from the ocean by particle scavenging and burial in sediments, offer a means for paleoceanographers to examine past dynamics of both water column and sedimentary processes. I show for the first time that a state of balance exists between 230Th production and burial in the Central Arctic basins, based on measured sedimentary 230Thxs inventories in box cores, establishing this nuclide’s utility as a paleoceanographic indicator of sedimentary processes and as a normalization tool. I present the first 230Th-normalized particle fluxes calculated for the central Arctic: vertical particle fluxes were extremely low during the late glacial, rose during the deglaciation due to particle inputs from shelf inundation, increased productivity and ice-rafted debris, and fell again following the establishment of interglacial conditions. A major event of lateral sediment redistribution, inferred from surplus 230Thxs inventories, occurred in the Makarov Basin during the deglaciation and may have been due to destabilization of slope and shelf sediments as sea level rose. I present the first high-resolution, radiocarbon-dated downcore records of sedimentary 231Pa/230Th from the Arctic Ocean. Low ratios indicate that 231Pa was exported from all sites during the late glacial period, with export decreasing during the deglaciation and Holocene. 231Pa/230Th measurements in cores from three continental slope sites show no evidence for a 231Pa sink related to boundary scavenging on the continental slopes. Holocene 231Pa/230Th ratios show a very significant variation by depth, with strong export of 231Pa at deep sites but little or no export at shallow sites, a result which echoes findings for the South Atlantic and the Pacific. The Arctic thus appears fundamentally similar to other ocean basins in its 231Pa and 230Th dynamics, despite its peculiar qualities of sea ice cover, low particle flux, and relatively isolated deep waters.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2009.
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