Sources and cycling of selenium in the western and equatorial Atlantic Ocean

The concentration and chemical speciation of selenium were determined at six vertical profile stations along a 11,000-km-long horizontal transect from 34°S to 8°N in the western Atlantic. The depth profiles of total dissolved selenium, selenite (SeIV), and selenate (VI) all showed surface-water depletion and deep-water enrichment characteristic of the nutrient-like behavior of selenium that has been observed in other ocean basins. In North Atlantic Deep Water, the Se(IV)/Se(VI) ratios were generally similar to those found in the eastern Atlantic and North Pacific (0.7), but waters originating in the higher latitudes of the southern hemisphere, Antarctic Intermediate (AAIW) and Bottom Water, and Upper Circumpolar Deep Water (UCDW), were enriched in selenate and had correspondingly low Se(IV)/Se(VI) ratios (ca. 0.4). In contrast to these inorganic selenium species, organic selenide had maxima in the surface waters of the oligotrophic stations and undetectable concentrations in the mid- and deep waters. One exception to this pattern was found at the southernmost station (33°S) where a secondary organic selenide maximum was found in the AAIW and UCDW (700–1900 m). This observation can be explained by considering the 10-year residence time of organic selenide in the water column and the relatively young age (<3 yr) of these subducted surface waters. In the surface transect, total dissolved selenium showed only minor variations with oceanic regime (0.55–0.83 nM) except in the offshore plume of the Amazon River, where concentrations dropped as low as 0.19 nM. Organic selenide was the predominant form of dissolved selenium in surface waters (50±11%), followed by selenate (36±13%) and then selenite (14±6%). Cross-flow ultra filtration experiments indicated that surface water dissolved organic selenide is in a<1-kD fraction and thus truly dissolved. Selenate had higher concentrations in the southern hemisphere (0.24 nM) than in the north (0.17 nM), but the highest value (0.37 nM) was found in the area of weak equatorial upwelling. In contrast, selenite was quite low in the southern hemisphere (0.06 nM) and had its highest concentrations in the equatorial region and under the Intertropical Convergence Zone (0.11 nM). The higher selenite concentrations corresponded to surface waters, which had elevated aluminum levels due to the input of North African dust. While selenium is not enriched in mineral aerosols themselves, air masses from Europe can be entrained in those leaving North Africa, enriching selenium as a consequence. The estimate of atmospheric deposition of selenium to the equatorial Atlantic is ca. 10×106 mol yr−1, while relatively low selenium concentrations in the Amazon River (0.48 nM) only deliver ca. 2×106 mol yr−1. Atmospheric selenium inputs dominate fluxes to the equatorial Atlantic, but these and Amazonian inputs profoundly affect the distribution and speciation of selenium in this region.