Evaluating the Flux of Uranium Isotopes from River-Dominated Shelf Sediments

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9618533 MCKEE ABSTRACT A clear understanding of uranium geochemistry in marine environments is required to fully utilize U-Th. nuclides as radiochemical tools. The uranium isotope signature of the ocean is greatly influenced by inputs from river-dominated shelf environments can have a dramatic impact on uranium concentrations in the ocean, and may have greatly influenced the uranium isotopic composition of the ocean during past glacial/interglacial cycles. This is possible because: (1) over 80% of the riverine uranium flux to ocean margins is in a particulate form which can potentially be transformed to a more soluble phase; (2) rivers discharge particulate uranium directly into shallow (200 m) shelf environments where dynamic processes (deposition, diagenesis, mixing) can alter particulate materials before entering the ocean; and, (3) the possible influence of shelf environments on oceanic geochemistry varies greatly between glacial and interglacial periods (exposure and inundation); a point that has important implications for the maintenance of steady-state uranium isotope concentrations in the ocean during the past million years. Project objectives are: (1) To better quantify the net uranium flux associated with river-dominated shelf sediments; (2) To identify the processes controlling enrichment/depletion of interstitial uranium concentrations in river-dominated shelf sediments; (3) To define the active microorganisms present in river-dominated shelf sediments and to gain some insight into the importance of biology vs. chemistry in the rapid Mn (IV) reduction and associated U enrichments observed in these environments and, (4( To identify the physical parameters that control net uranium fluxes from shelf sediments (e.g., particulate uranium carrier phases, rates of particulate deposition and burial, depth and frequency of sediment mixing). To address objective one, riverine and shelf sediments from major river-dominated area (totaling -35% of world sediment discharge) will be analyzed for uranium isotopes to ascertain the extent to which particulate U is transformed to a dissolved phase. To address objectives two and three, a series of porewater and incubation studies are proposed using sediments from the Amazon and Mississippi Shelves. To address the final objective, detailed interstitial chemistry (including U isotopes) will be examined at selected sites on each of the above shelves on time scales compatible to the characteristic patterns of riverine particulate supply and sediment mixing.