Onboard-ship redox speciation of chromium in diffuse hydrothermal fluids from the North Fiji Basin

The chromium species distribution in low-temperature hydrothermal fluids and the overlying water column was determined onboard ship in the North Fiji Basin. In order to differentiate between total dissolved, particulate, total Cr(III), reactive Cr(III) and Cr(VI) the adsorptive stripping voltammetric method using DTPA as complexing reagent in nitrate solution was applied. The results obtained indicate that total chromium (up to 48 nM), mostly present as Cr(III), is enriched in the samples taken directly from the emanation sites. In contrast, Cr concentrations in oxic seawater have been reported to be mostly <10 nM, mainly as chromate. Hydrothermal hydrogen sulfide appears to be the most important redox partner to reduce seawater Cr(VI) in the mixture with the hydrothermal fluid. This hydrothermal Cr(III) signal is transported into the overlying water column for several 100 m above the seafloor and seems to be stabilized by organic complexation of Cr(III) and slow oxidation kinetics of Cr(III)→Cr(VI). Significant amounts of particulate-bound Cr were detected close to the seafloor and in a neutrally buoyant plume about 200 m above the seafloor. As hydrothermalism is a widespread phenomenon in certain regions of the oceans, we assume that it is contributing significantly to the concentrations and speciations of Cr in the deep sea.