GEOTRACES Peru-Tahiti section: Measurement of 7Be as a Tracer of Upper Ocean Processes

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In the ocean, trace elements function variously as nutrients, as contaminants from human activity, and even as convenient tracers of current and past oceanographic processes. Their biogeochemical cycling has major impacts on the ocean carbon cycle and ecosystem dynamics as well as on global climate change. Many processes influencing the distribution and behavior of trace elements in the ocean cannot be directly observed, so consequently a variety of geochemical tracers are used to provide important constraints on their rates and pathways. Beryllium-7 (Be7) is a cosmogenic (i.e., produced in the upper atmosphere) radionuclide that, because of its half-life (53.3d), can be used in the study of environmental processes occurring over seasonal timescales. This timescale is important to studies of biological production, nutrient regeneration, and atmospheric deposition, to name a few. In this project, a research team from the University of Miami will make measurements of water column Be7 along the U.S.GEOTRACES transect from Peru to Tahiti in 2013. The mission of the International GEOTRACES Program (of which the US program is a component) is "to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions." As part of the multi-institutional U.S.GEOTRACES Pacific campaign, the Miami team will address key tasks formulated within the GEOTRACES Science Plan. Specifically, they will: (1) provide realistic estimates of the underlying transport processes influencing measured trace elements of interest (TEI) distributions; they will use water column measurements of Be7 as a tracer of physical processes, such as mixing and upwelling, which redistribute biologically active species; and (2) develop and apply new Be7 methods for quantifying the atmospheric deposition of TEIs measured by other research teams participating in the campaign. Measurements of Be7 in the surface waters and in the lower atmosphere along the cruise track will provide estimates of the atmospheric input of relevant TEIs. The atmospheric input into the global ocean is an important budgetary component of numerous chemical species, yet is very difficult to constrain. The data generated in this work will be available to allow ground-truthing of aerosol deposition models and the atmospheric input of trace elements. Broader impacts: The proposed work is in support of the GEOTRACES program, and as such contributes to many of the broad intellectual objectives of that program. The ability to provide estimates of the atmospheric input of relevant TEIs is an important example, and one that will contribute widely to the field in the future beyond GEOTRACES, including understanding anthropogenic impacts on the oceans. Education and training of graduate students will be essential components of this research.