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被用来支持这样一种概念，即在海洋边缘加强了对颗粒活性物质的“边界清除”(即优先从海水中清除它们)。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)对沉积231Pa230th的影响可能与边缘增强的清扫作用一样大，甚至更多。如果是真的，那么优先清除海洋边缘的颗粒活性溶质可能没有之前认为的那么重要。还需要进一步研究这两个过程，哥伦比亚大学拉蒙特·多尔蒂地球天文台的研究人员拟议进行的研究将通过分析在北太平洋次北冰洋某一地点采集的样本中溶解的230、232Th和231Pa的浓度和分布，以及分析它们在表层沉积物中的浓度，来解决这一问题和相关目标。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、近底梯度以及检验以下假设，即在沉积物-水界面附近加强清扫作用会在远洋水柱底部产生一个显著的汇。此外，还将结合颗粒相组成在控制化学分馏中的作用的研究，评估沉淀不稳定的生物成因颗粒、成岩相溶解和/或胶体动员的潜在再生和通量。这些结果将用于旨在审查上述各种进程的重要性的建模工作。拟议的工作将增进对加强对海洋边缘颗粒活性物质的化学清除和清除的了解。除了化学海洋学，特别是古海洋学之外，这项研究还将有益于各种领域的研究人员。这一结果将为关于沉积物231Pa230th比值作为深海环流指标的适用性以及从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。这项研究将提供有关活性物质的沉积物-水交换方面的进一步信息。这一结果也可能为今后作为地球物理研究方案的一部分在太平洋开展的研究提供参考。广泛影响：研究的科学方面侧重于提高对边界边缘清理重要性的认识。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.