Dissolved inorganic and organic selenium in the Orca Basin

Abstract

The vertical distributions of Se (IV), Se (VI) and dissolved organic Se have been determined in the oxic and non-sulfide-bearing anoxic zones of the Orca Basin. In the oxic waters, the concentration of Se (IV) increases with depth gradually from 0.25 nmole/kg at the surface to a maximum of 0.46 nmole/kg at 750 m and then decreases with depth to a relatively constant concentration of 0.39 nmole/ kg below 1,230 m. The concentration of Se (VI) is rather uniform in the top 250 m at about 0.24 nmole/ kg. Below 250 m it increases with depth to 0.50 nmole/kg at 1.230 m, and it stays relatively constant below this depth. The concentration of organic Se increases from 0.50 nmole/kg at the surface to 1.39 nmole/kg at 78 m. A pronounced broad maximum of organic Se exists between 78 and 250 m. The concentration decreases with depth below 250 m, dropping sharply between 250 and 380 m and more gradually at greater depths. It becomes undetectable at 1,230 m. Organic Se is the dominant species above 250 m. Se (IV) is the most abundant between 250 and 1,000 m while Se (VI) becomes the dominant species below 1,000 m. The distributions of these three species can be explained by the biological uptake of Se in the surface waters and the multi-step regeneration of Se from biogenic particles at greater depths. In suboxic waters at the oxic-anoxic interface, the concentration of Se (IV) increases while that of Se (VI) decreases reflecting a change in redox conditions in the environment. In the anoxic brine, the concentration of Se (IV) is around 0.25 nmole/kg while Se (VI) is undetectable. The concentration of organic Se increases sharply in the suboxic waters and reaches 2.6 nmole/kg in the anoxic brines probably as a result of the decomposition of organic matter and/or a diffusive flux from the underlying sediment.