Arsenic (As)

Distribution in ocean
The total inorganic arsenic concentration profile is nutrient like, with a slight depletion at the surface and a small enhancement in the deep Pacific, relative to the Atlantic (data). Arsenate ion is the primary component of the inorganic arsenic pool in the Pacific and the Atlantic.  Arsenite concentrations are highest in the euphotic zone, where they may account for about 3 to 10% of the total arsenic.  Methylated arsenic compounds may account for an additional 5% of the arsenic in the euphotic zone.

Speciation
Arsenic exists primarily in the +V oxidation state as the arsenate anion HAsO₄^2-.  Significant amounts of the +III oxidation state arsenite (HAsO₃^2-) are found in near surface waters where it is produced by phytoplankton.  Organic arsenic compounds, including dimethylarsinate, (CH₃)₂AsO(OH), and methylarsonate, CH₃AsO(OH), are also produced in the euphotic zone by algae.

Residence time
The arsenic 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
Arsenic is determined by hydride generation followed by gas chromatography.

References