Selenium (Se)

Atomic number:	34
Atomic weight:	78.96
Average concentration in ocean:	1.9 nmol/kg
Residence time:	26,000 years

Distribution in ocean
Dissolved selenium has a nutrient-like vertical profile. Concentrations are low at the surface and they increase with depth (data).

Speciation
Dissolved selenium exists as the selenate (SeO₄^2-) and selenite (SeO₃^2-). Selenium is in the +VI and +IV oxidation states, respectively. The Atlantic and Pacific vertical distributions show complete depletion of selenite at the surface and only moderate depletions of selenate. In addition, organic selenides (-II oxidation state) are found

Residence time
The selenium residence time is from Tracers in the Sea (Broecker, W. S. and Peng, T.-H., 1982. Eldigo Press, Palisades, NY). It was calculated from the global average river input.

Analysis
Selenium is determined by gas chromatography after forming a volatile organic derivative (5-nitropiaselenol).

References & Notes