Oxygen minimum zones in oceanic waters have become increasingly important to the marine environment and society. Low oxygen waters affect not only the distribution and abundance of marine organisms, but also impact the solubility and transport of trace elements that are of biological importance, with the chemical speciation and solubility depending on the actual redox poise of the waters. One redox sensitive trace element of interest is selenium, which can be both toxic and essential for

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... ms, depending on its chemical speciation. In 2013, the US GEOTRACES program completed the GP16 transect from Peru to Tahiti, going through the oxygen minimum zone off Peru. Dissolved selenate, selenite, and organic selenide, as well as particulate elemental selenium, were determined in water column samples. Nitrate and nitrite data were used to determine where denitrification was occurring and thus approximately where dissimilatory reduction of selenium should be occurring. Deficits in dissolved selenite+selenate showed that dissimilatory reduction of selenium (bacterial utilization of an oxidized metal as the terminal electron acceptor during respiration) occurred within the oxygen minimum zone at slightly deeper depths than where denitrification was found. The observed selenium deficits can be the result of dissimilatory reduction occurring in situ or can be laterally advected from anoxic coastal sediments. However, using a combination of advection/diffusion modeling, particulate Se(0) data, and 234Th-derived flux rates, the dissimilatory reduction of selenium was shown to mainly occur in situ, rather than via advection/diffusion from coastal sediments.