Boundary Sources and Sinks of 230Th, 232Th and 231Pa in the NW Pacific

钍（Th）和钚（Pa）同位素的空间分布以前曾被用来支持这样一种观点，即在海洋边缘，粒子反应物质的“边界清除”作用增强（即优先从海水中清除）。然而，最近的证据表明，Th和Pa之间的化学分馏（即溶解相和颗粒相之间的差异分配）的空间变异可能会影响沉积231 Pa/230 Th比值一样多，甚至超过，增强清除边缘。如果是真的，那么在海洋边缘的颗粒反应性溶质的优先去除可能不像以前认为的那么重要。需要进一步的研究来检验这两个过程，哥伦比亚大学拉蒙多尔蒂地球观测站的研究人员提出的研究将通过分析在北太平洋亚北极一个地点收集的样品，分析溶解的230 Th、232 Th和231 Pa的浓度和分布，以及分析它们在表层沉积物中的浓度，来解决这个问题和相关的目标。样本已经收集好了。 拟议工作的其他目标包括评估每种同位素的边界源和汇、近底梯度，并检验以下假设：沉积物-水界面附近的清除作用增强，在水层水柱底部形成一个重要的汇。此外，潜在的再生和通量从沉降不稳定的生物颗粒，从溶解的成岩相和/或胶体的动员将评估结合检查的颗粒相组合物在控制化学分馏的作用。研究结果将用于旨在研究上述各种过程重要性的建模工作。拟议的工作将增进对加强海洋边缘的化学清除和清除颗粒反应物质的了解。这项研究将有利于化学海洋学，特别是古海洋学等各个领域的研究人员。这些结果将为关于沉积231 Pa/230 Th比值作为深海环流代理的适用性以及先前研究得出的各种结论的有效性和重要性的辩论提供信息。这项研究将提供关于活性物质的沉积物-水交换方面的进一步资料。 这些结果也可能为将来作为GEOTRACES计划的一部分在太平洋进行的研究提供信息。更广泛的影响：研究的科学方面侧重于提高对边界边缘清除重要性的认识。这些知识对于理解海洋中许多元素的地球化学是必要的，而不仅仅是本研究的重点同位素。该提案的教育方面涉及研究生教育和相关活动。

The spatial distributions of the isotopes of thorium (Th) and protactinium (Pa) have previously been used to support the notion that there is enhanced "boundary scavenging" of particle reactive substances (i.e. their preferential removal from seawater) at ocean margins. However, recent evidence suggests that spatial variability in the chemical fractionation between Th and Pa (i.e. differential partitioning between the dissolved and particulate phases) may influence sedimentary 231Pa/230Th ratios as much as, or even more than, enhanced scavenging at margins. If true, then preferential removal of particle-reactive solutes at ocean margins may be less significant than previously believed. Further research is needed to examine these two processes, and the proposed study by researchers at the Lamont Doherty Earth Observatory at Columbia University will address this problem and related objectives by analyzing samples collected at a location in the Subarctic North Pacific Ocean for the concentration and distribution of dissolved 230Th, 232Th and 231Pa together with analysis of their concentration in surface sediments. The samples have already been collected. Additional objectives of the proposed work include assessing the boundary sources and sinks for each isotope, the near bottom gradients and testing the hypothesis that enhanced scavenging near the sediment-water interface creates a significant sink at the base of the pelagic water column. Additionally, the potential regeneration and flux from settling labile biogenic particles, from the dissolution of lithogenic phases and/or mobilization of colloids will be evaluated in conjunction with an examination of the role of particulate phase composition in controlling chemical fractionation. The results will be used in modeling efforts aimed at examining the importance of the various processes outlined above. The proposed work will improve understanding of the enhancement of chemical scavenging and removal of particle-reactive substances at ocean margins. The research will be beneficial to investigators in a variety of fields besides chemical oceanography, notably paleoceanography. The results will inform the debate concerning the applicability of sedimentary 231Pa/230Th ratios as a proxy for deep ocean circulation, and the validity and importance of various conclusions derived from prior research. The research will provide further information on aspects of sediment-water exchange of reactive substances. The results may also inform the research that will be conducted in the Pacific Ocean in the future as part of the GEOTRACES Program.Broader Impacts: The scientific aspects of the research focus on improving the understanding the importance of boundary margin scavenging. Such knowledge is necessary for understanding the ocean biogeochemistry of many elements, not just the isotopes that are the focus of this study. The educational aspects of the proposal relate to graduate student education and related activities.