Collaborative Research: Impact of Atmospheric Deposition on the Distribution and Speciation of Trace Elements in the Upper Ocean - Focus on Iron in the Sargasso Sea

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Recent studies in the Sargasso Sea (BATS region) have revealed a large, seasonal-scale change in the dissolved iron (Fe) concentration of the surface waters where concentrations decreased from ~1-2 nM in summer 2003, when aerosol Fe concentrations were high, to ~0.1-0.2 nM in spring 2004, when aerosol Fe concentrations were low. These changes were interpreted as the result of the seasonal dust input into the highly-stratified, oligotrophic surface waters during the summer, followed by the removal of this aeolian iron due to the convective mixing, biological uptake and particle scavenging during the winter-early spring. Researchers from the Bermuda Biological Station for Research, the University of Delaware and Woods Hole Oceanographic Institute have devised a simple model that implies that aeolian input of dissolved Fe into the surface ocean does not scale with total aerosol Fe concentrations. To test this hypothesis, the scientists will use the Sargasso Sea as a natural laboratory to carry out the following objectives: (1) achieve a quantitative understanding of the impact of mineral aerosol deposition (dry and wet) on the concentration and speciation of Fe in surface waters of the Sargasso Sea; (2) test the new model for aerosol Fe solubility that have been developed for the Sargasso Sea, and thus evaluate the potential for application of this model at the global scale; and (3) evaluate the need for an autonomous, buoy-mounted aerosol sampler for quantifying the concentration and solubility of aerosol iron over the open ocean. To attain these goals, both time-series observations and intensive process studies will be done. The work will include shipboard water-column sampling, atmospheric sampling at the Tudor Hill tower on Bermuda, the use of an autonomous aerosol sampler on the Bermuda Testbed Mooring, and subsequent analyses of the concentration and speciation of Fe and other trace metals in seawater, aerosol, and rainwater samples. As regards broader impacts, this study will provide needed data to improve predictive capabilities of biogeochemical-ecosystem models. In addition, this project will document the utility of an autonomous, buoy-mounted aerosol sampling systems linking atmospheric and oceanographic processes for environmental monitoring and research in marine geochemistry and biogeochemistry. The goals of this research are compatible with the focus of the SOLAS and GEOTRACES programs. One graduate student and one undergraduate student (funded as part of the REU program) from the University of Delaware will be trained as part of this project.