Selenium redox chemistry at GEOSECS I re-occupation

The distribution in profile of the two oxidation states of selenium. Sesup(IV) and Sesup(VI) has been determined at GEOSECS I. The ratio Sesup(IV)/Sesup(VI) increases from 0.1 in the surface layer to a maximum of 0.6 at 1300 m, the deepwater value being 0.55. The ranges are 50-800 pmol/kg for Sesup(IV) and 500-1500 pmol/kg for Sesup(VI). The element is unique in the presence of both oxidation states at comparable levels throughout the water column. Model calculations suggest that this is due to either incorporation of Sesup(IV), produced by reduction of Sesup(VI) in the mixed layer, into refractory carrier phase resulting in a high IV/VI ratio in the deep water or an unique redox cycle involving oxidation of Sesup(IV) in the mixed layer and reduction of Sesup(VI) in the deep water. (orig.)