GEOTRACES Pacific Section: 227Ac Distribution in Deep Waters as a Constraint on Mixing and Mass Transport of Solutes

由于其作为海洋混合和运输的示踪剂的潜力，227Ac（22年半衰期）已被确定为国际GEOTRACES计划的一个感兴趣的参数，该计划是一个多国努力，旨在绘制和研究全球海洋中一系列微量元素和同位素的地球化学行为。对深海生物地球化学动力学进行量化是了解海洋碳和其他物质循环的必要条件。定义这些动力学需要混合和平流输送速率的知识，这些知识可以用于解释生物地球化学重要溶质的浓度场。尽管227Ac的半衰期非常适合研究深海中的垂直和横向输送，但在深海中很少进行227Ac的测量。在这个项目中，南加州大学的一个研究小组将对2013年美国GEOTRACES太平洋样带（秘鲁到塔希提岛的一段）上的227个ac水柱剖面进行测量，这是美国对国际GEOTRACES项目的几项贡献之一。该小组还将对存档的沉积物进行分析，以限制该同位素的区域底栖生物通量，并为解释其水柱分布提供边界条件。仪器仪表的最新进展促进了这种分析，GEOTRACES计划提供的协同作用将为获得额外数据提供理想的机会，这将满足两个目的。首先，227Ac将在确定南太平洋深处混合率的空间变异性方面具有价值，特别是当与228Ra（6年半衰期）的估计相结合时，228Ra是GEOTRACES合作提议寻求确定的示踪剂领域。其次，由这对示踪剂定义的混合和输运速率将有助于推断GEOTRACES在深水中测量的其他溶质的输运和动力学。溶质运移是由于混合和平流。通过使用具有不同半衰期的多种示踪剂，可以区分这些过程的不同作用，从而更全面地了解溶质行为的动力学。更广泛的影响：作为这项工作的一部分，至少有一名本科生和一名高中生将有机会参与研究经验，深入了解该领域职业发展的可能性，并学习科学研究所采用的方法。

Because of its potential as a tracer of ocean mixing and transport, 227Ac (22 year half-life) has been identified as a parameter of interest for the International GEOTRACES Program, a multi-national effort to map and study the geochemical behavior of a suite of trace elements and isotopes in the global ocean. Quantifying the biogeochemical dynamics of the deep sea is necessary for understanding the oceanic cycling of carbon and other materials. Defining these dynamics requires knowledge of mixing and advective transport rates that can be applied to interpret concentration fields of biogeochemically important solutes. Despite having a half-life that is well suited for the study of both vertical and lateral transport in the deep ocean, few 227Ac measurements have been made in the deep sea. In this project, a research team at the University of Southern California will make measurements of 227Ac water column profiles on the 2013 US GEOTRACES Pacific transect, a section from Peru to Tahiti, one of several American contributions to the International GEOTRACES effort. The team will also do analyses of archived sediments to constrain the regional benthic flux of this isotope and provide a boundary condition for interpreting its water column distribution. Recent advances in instrumentation facilitate this analysis, and the synergy provided by the GEOTRACES program will provide an ideal opportunity to obtain additional data that will serve two purposes. First, 227Ac will be of value in defining the spatial variability of mixing rates in the deep South Pacific, particularly when combined with estimates based on 228Ra (6 year half-life), a tracer field that a collaborating GEOTRACES proposal seeks to determine. Second, the mixing and transport rates defined by this tracer pair will be useful for deducing the transport and the dynamics of other solutes measured by GEOTRACES in deep waters. Solute transport is due to both mixing and advection. By using multiple tracers with different half-lives, the separate roles of these processes may be distinguished, allowing a more complete understanding of the dynamics of solute behaviors. Broader Impacts: As part of this effort, at least one undergraduate and one high school student will have the opportunity to participate in the research experience, gaining insight into the possibilities of careers in this area and learning the approaches taken in scientific research.