Processes Affecting the Riverine Flux of Uranium Isotopes from Coastal Margins to the Ocean: A Study of the Mississippi River and Adjacent Shelf

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Geochemical processes in coastal and shelf environments affect uranium concentrations in the ocean, and have influenced the uranium isotope composition of the ocean during past glacial/interglacial cycles. The overall aim of this research is to improve understanding of the cycling processes that affect the flux of uranium from coastal margins to the open ocean. This study focuses specifically on the Mississippi River and the adjacent continental shelf to achieve the following objectives. (1) To characterize the behavior of uranium during river/ocean mixing; and, to identify the processes that govern the estuarine behavior of uranium. (2) To identify the conditions and processes that influence the role of shelf sediments as net sources or sinks for uranium; and, to evaluate the general role that river dominated shelves play in the overall ocean chemistry of uranium. Water-column and Seabed samples will be collected from the continental shelf adjacent to the Mississippi River. Water samples will be partitioned into dissolved, colloidal and particulate fractions using filtration and ultrafiltration techniques. Uranium concentrations and isotope ratios will be examined in all fractions to evaluate the fate of U during river/ocean mixing. Interstitial waters will be extracted from shelf cores and examined for diagenetic uptake or release of uranium from bottom sediments, and mixing experiments between river water and seawater will be used to study effects of estuarine mixing on uranium.