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

沿海和陆架环境中的地球化学过程影响海洋中的铀浓度，并在过去的冰期/间冰期周期中影响海洋的铀同位素组成。这项研究的总体目的是增进对影响铀从沿海边缘流向公海的循环过程的了解。这项研究特别侧重于密西西比河及其邻近的大陆架，以实现以下目标。(1)确定铀在河流/海洋混合过程中的行为特征；以及确定支配铀在河口行为的过程。(2)查明影响陆架沉积物作为铀的净源或汇的作用的条件和过程；以及评价以河流为主的陆架在整个海洋铀化学中所起的一般作用。水柱和海床样本将从密西西比河附近的大陆架收集。利用过滤和超滤技术，水样将被分为溶解部分、胶体部分和颗粒部分。将检查所有馏分中的铀浓度和同位素比率，以评估河流/海洋混合过程中铀的去向。将从陆架岩心中提取间隙水，检查成岩作用对海底沉积物中铀的吸收或释放，并将利用河水和海水之间的混合实验来研究河口混合对铀的影响。

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.