Measurement of Fe(II) in surface water of the equatorial Pacific

The distribution of Fe(II) in euphotic waters of the equatorial Pacific Ocean was examined with novel in situ sampling and analytical methodology. Dissolved Fe(II) was isolated and preconcentrated, at depth, with a ligand-exchange chromatographic technique and then determined spectrophotometrically. The distribution of Fe(II) in the upper 100 m was both temporally and spatially variable but generally exhibited maxima near the surface and often at depths with higher concentrations of Chl a. The concentration of Fe(II) varied from below the detection limit of 0. 1 2 to 0. 5 3 nmol kg-1. Exposure of equatorial Pacific water to simulated sunlight in the laboratory resulted in similar Fe(II) concentrations, suggesting that photochemical reduction may be an important source for Fe(II) in these waters. Field measurements indicate that Fe(II) may comprise a significant fraction of the Fe present in euphotic ocean waters, which typically contain < 1.0 nmol kg-1 of dissolved Fe. The presence of detectable steady state concentrations of Fe(II) is believed to be the result of faster rates of photochemical and(or) biological reduction of Fe(III), relative to rates of Fe(II) oxidation by oxygen, peroxides, and other oxidants.