Geochemical characteristics, speciation and size-fractionation of iron (Fe) in two marine shallow-water hydrothermal systems, Dominica, Lesser Antilles

Marine shallow-water hydrothermal systems have so far largely been neglected with respect to trace metal fluxes and possible stabilizing complexation processes, even though they emit their fluids directly into the photic zone. The impact of stabilized dissolved metal input by shallow vents into surface waters as well as the effect on the transport and bioavailability of bioactive trace metals within the area of highest primary production rates in the world oceans is therefore, at present, mostly unknown. In this study, we investigated the concentration, size fraction distribution (colloidal and soluble) as well as redox speciation and labile concentrations of the limiting micronutrient iron (Fe) at two marine shallow-water hydrothermal systems (Champagne Hot Springs and Soufriere) off the coast of Dominica, Lesser Antilles Island Arc, Caribbean. Geochemical characterization of the two different vent sites showed that both are affected by meteoric and sea-water influence, with a strongermeteoric influence at Soufriere than at Champagne Hot Springs. Measurements of soluble and labile Fe were performed using a modified competitive ligand exchange -adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime (SA) as the artificial ligand. Our results show that focused fluids discharging at the seafloor, as well as hydrothermal pore fluids are, despite a calculated theoretical oxidation half-life of only 6.4 min, highly enriched in Fe(II), indicating a strong complexation of Fe(II), strong enough to prevent Fe(II) from oxidation and precipitation. Since these fluids show enriched dissolved organic carbon (DOC) concentrations, and very low fractions of chemically labile Fe, complexation may occur by organic carbon, whichwas recently suggested to also be a factor in stabilizing particulate Fe(II) in deep-sea hydrothermal non-buoyant plumes. Our results indicate that shallow-water hydrothermalism off the coast of Dominica releases high concentrations of stabilized, bioavailable Fe(II) into the photic zone, which influences the biogeochemical cycle of Fe in surface waters. Considering the abundance of marine shallow-water hydrothermal systems in many regions, such processes may even play a role in the global oceanic dFe cycle. (C) 2017 Elsevier B.V. All rights reserved.