Redox speciation of chromium in the oceanic water column of the Lesser Antilles and offshore Otago Peninsula, New Zealand

Abstract

Chromium redox speciation analysis was carried out onboard ship in several water column profiles off the islands of Dominica, St Lucia and Grenada (Lesser Antilles), and at two stations offshore from the Otago Peninsula (New Zealand). The catalytic adsorptive stripping voltammetry with diethylenetriaminepentaacetic acid as complexing reagent and a hanging mercury drop as a working electrode were used. In the samples from the Lesser Antilles, Cr(VI) concentrations ranged around a relatively constant background of 1.5–2.5 nM, except for one higher value of 4.3 nM, whereas Cr(III) was highly variable and reached maximum concentrations of 12 nM. Variable environmental influences, such as hydrothermal activity and atmospheric input, are probable sources for reduced chromium species. In contrast, the Cr(III) contribution was less than 50% of total Cr in subantarctic non-hydrothermal water offshore from the Otago Peninsula and the presence of Cr(III) was restricted to the upper 100 m of the water column. In these depth profiles, Cr (VI) (up to 3.7 nM) was clearly the dominating species.

Onboard ship redox speciation was shown to be an effective means to avoid storing artefacts in species determination. We conclude that seafloor hydrothermal activity contributes significantly to the reduced Cr species chemistry of the surrounding seawater.