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

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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.