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

9618533麦基摘要要充分利用铀-Th，需要对海洋环境中的铀地球化学有明确的了解。核素作为放射化学工具。海洋的铀同位素特征在很大程度上受到河流主导的陆架环境输入的影响，可以对海洋中的铀浓度产生重大影响，并可能在过去的冰川/间冰期旋回中极大地影响海洋的铀同位素组成。之所以可能这样做，是因为：(1)流向大洋边缘的河流铀通量的80%以上是以颗粒形式存在的，有可能转化为更易溶的相；(2)河流直接向浅层(200米)陆架环境排放颗粒铀，在此过程中，动态过程(沉积、成岩作用、混合)可改变颗粒物质进入海洋；(3)陆架环境对海洋地球化学的可能影响在不同冰川和间冰期(暴露和淹没)之间存在很大差异；在过去100万年中，这一点对维持海洋中稳定的铀同位素浓度具有重要意义。项目目标是：(1)更好地量化与以河流为主的陆架沉积物有关的铀净通量；(2)确定控制以河流为主的陆架沉积物中间隙铀浓度的浓缩/贫化的过程；(3)界定以河流为主的陆架沉积物中存在的活跃微生物，并深入了解生物与化学在这些环境中观察到的锰(四)快速还原及相关铀浓缩过程中的重要性；(4)确定控制陆架沉积物中铀的净通量的物理参数(例如颗粒铀载体相、颗粒沉积和埋藏速度、沉积物混合深度和频率)。为解决目标一，将对主要河流为主地区的河流和陆架沉积物(总计占世界沉积物排放量的35%)进行铀同位素分析，以确定颗粒铀转化为溶解相的程度。为了解决目标二和目标三，建议使用亚马逊和密西西比大陆架的沉积物进行一系列的孔隙水和孵化研究。为了达到最终目标，将在上述每个货架上选定的地点，按照符合河流颗粒供应和沉积物混合的特征模式的时间尺度，对详细的间隙化学(包括铀同位素)进行审查。

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.