Collaborative Research: Evaluating the use of Se/S in marine barite as a tracer of ocean nutrients

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Nitrate is a key limiting nutrient in many oceanic environments, and so a proxy for nitrate concentration would be invaluable for understanding mechanisms driving C export fluxes in the past. Selenate (SeO4=), one of the dissolved chemical species of the trace element selenium, shows an excellent correlation with nitrate, having surface water depletion and deep-water enrichment. In this respect, the biogeochemical cycle of selenium is analogous to that of nitrogen. Furthermore, selenate is almost chemically identical to sulfate and as such is incorporated into barite, a diagenetically stable mineral that is formed inorganically in the water column. Investigators from Old Dominion University and Stanford University will test the suitability of the Se/S ratio in marine authigenic barite as a proxy for upper ocean nitrate concentrations. To achieve this goal, they will evaluate and quantify the processes leading to the incorporation of selenate into marine barite, and develop an empirical core top calibration so that the application of this proxy is as accurate as possible. Their laboratory based approach will include: (1) inorganic/abiotic barite precipitations in the laboratory under varying environmental conditions and selenate/sulfate ratios, and measurements of the Se/S in the resulting barite; (2) barite precipitation in the presence of decaying marine particles (phytoplankton) and varying selenate/sulfate to simulate actual water column processes; (3) isolation of barite and determination of Se/S from available sediment trap samples taken at 2-3 depths in the upper 1000 m; and (4) determinations of Se/S in barites isolated from core-top samples from throughout the world's oceans where there are corresponding selenate and nutrient data. In terms of broader impacts, the proposed research will directly involve a graduate research assistant as part of a Ph.D. dissertation. There will also be opportunities for underrepresented minority students to participate and contribute to the proposed research through Old Dominion University's (ODU) Minority Undergraduate Scholarship and Training REU program, and the joint ODU-Hampton University-Virginia Institute of Marine Sciences Hall- Bonner Program for Minority Doctoral Scholars in Ocean Sciences. The PIs will also maintain their university web sites featuring current research endeavors that are designed for public viewing rather than just internal scientific communication.