The stabilisation and transportation of dissolved iron from high temperature hydrothermal vent systems

•Two typical hydrothermal plumes contained high concentrations of labile Fe.•The Fe was bound to weak ligands in a range of sizes and reactivities.•The species are <0.2 µm in size so will not settle and can be transported off axis.•∼7.5% of hydrothermal Fe was stabilised in these complexes.•Hydrothermal vents are an important source of Fe to the deep sea.

Iron (Fe) binding phases in two hydrothermal plumes in the Southern Ocean were studied using a novel voltammetric technique. This approach, reverse titration-competitive ligand exchange-adsorptive cathodic stripping voltammetry, showed that on average 30±21% of dissolved Fe in the hydrothermal plumes was stabilised by chemically labile binding to ligands. The conditional stability constant (log K′FeL) of the observed complexes was 20.61±0.54 (mean±1 SD) for the two vent sites, intermediate between previous measurements of deep ocean ligands (21.4-23; Kondo et al., 2012) and dissolved weak estuarine ligands (<20; Gerringa et al., 2007).Our results indicate that approximately 7.5% of all hydrothermal Fe was stabilised by complexation with ligands. Furthermore, 47±26% of the dissolved Fe in the plume existed in the colloidal size range (0.02-0.2 µm). Our data suggests that a portion (∼7.5%) of hydrothermal Fe is sufficiently stabilised in the dissolved size fraction (<0.2 µm) to make an important impact on deep ocean Fe distributions. Lateral deep ocean currents transport this hydrothermal Fe as lenses of enhanced Fe concentrations away from mid ocean ridge spreading centres and back arc basins.

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