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

The concentration and chemical speciation of selenium were determined at si x vertical profile stations along a 11,000-km-long horizontal transect from 34 degreesS to 8 degreesN in the western Atlantic. The depth profiles of t otal dissolved selenium, selenite (SeIV), and selenate (VI) all showed surf ace-water depletion and deepwater 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 (UCD W), were enriched in selenate and had correspondingly low Se(IV)/Se(VI) rat ios (ca. 0.4). In contrast to these inorganic selenium species, organic sel enide had maxima in the surface waters of the oligotrophic stations and und etectable concentrations in the mid- and deep waters. One exception to this pattern was found at the southernmost station (33 degreesS) where a second ary organic selenide maximum was found in the AAIW and UCDW (700-1900 m). T his observation can be explained by considering the 10-year residence time of organic selenide in the water column and the relatively young age (< 3 y r) of these subducted surface waters. In the surface transect, total dissol ved selenium showed only minor variations with oceanic regime (0.55-0.83 nM ) except in the offshore plume of the Amazon River, where concentrations dr opped as low as 0.19 nM. Organic selenide was the predominant form of disso lved 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 fra ction and thus truly dissolved. Selenate had higher concentrations in the s outhern hemisphere (0.24 nM) than in the north (0.17 nM), but the highest v alue (0.37 nM) was found in the area of weak equatorial upwelling. In contr ast, selenite was quite low in the southern hemisphere (0.06 nM) and had it s highest concentrations in the equatorial region and under the Intertropic al Convergence Zone (0.11 nM). The higher selenite concentrations correspon ded to surface waters, which had elevated aluminum levels due to the input of North African dust. While selenium is not enriched in mineral aerosols t hemselves, air masses from Europe can be entrained in those leaving North A frica, enriching selenium as a consequence. The estimate of atmospheric dep osition of selenium to the equatorial Atlantic is ca. 10 x 10(6) mol yr(-1) , while relatively low selenium concentrations in the Amazon River (0.48 nM ) only deliver ca. 2 x 10(6) mol yr(-1). Atmospheric selenium inputs domina te fluxes to the equatorial Atlantic, but these and Amazonian inputs profou ndly affect the distribution and speciation of selenium in this region. (C) 2001 Elsevier Science Ltd. All rights reserved.