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

钍（Th）和镤（Pa）同位素的空间分布已经被用来支持这样一种观点，即在海洋边缘，粒子反应性物质的“边界清除”（即它们优先从海水中清除）增强。然而，最近的证据表明，Th和Pa之间化学分馏的空间变异性（即溶解相和颗粒相之间的差异分配）可能影响沉积的231Pa/230Th比率，其影响程度与边缘增强的清除作用相同，甚至更大。如果这是真的，那么海洋边缘颗粒反应性溶质的优先去除可能没有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Lamont Doherty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地区收集的样品，分析溶解的230Th， 232Th和231Pa的浓度和分布，以及分析它们在地表沉积物中的浓度，来解决这一问题和相关目标。样品已经收集好了。拟议工作的其他目标包括评估每种同位素的边界源和汇、近底部梯度和检验这样一种假设，即在沉积物-水界面附近增强的清除作用在上层水柱底部产生了一个重要的汇。此外，将结合颗粒相组成在控制化学分馏中的作用，评估沉淀不稳定的生物颗粒、产岩相溶解和/或胶体动员的潜在再生和通量。结果将用于旨在检查上述各种过程的重要性的建模工作。拟议的工作将增进对加强海洋边缘化学清除和去除颗粒反应性物质的理解。这项研究将对化学海洋学以外的其他领域，特别是古海洋学的研究人员有很大的帮助。这些结果将为关于沉积231Pa/230Th比值作为深海环流代表的适用性的争论以及先前研究得出的各种结论的有效性和重要性提供信息。这项研究将提供有关反应性物质的沉积物-水交换方面的进一步资料。这些结果也可以为将来在太平洋进行的研究提供信息，作为GEOTRACES计划的一部分。更广泛的影响：研究的科学方面侧重于提高对边界清除重要性的理解。这些知识对于理解许多元素的海洋生物地球化学是必要的，而不仅仅是同位素，这是本研究的重点。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.