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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 (0.02–0.2µm). Our data suggests that a portion (∼7.5%) of hydrothermal Fe is sufficiently stabilised in the dissolved size fraction (<0.2µm) 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. [ABSTRACT FROM AUTHOR]

Abstract: Iron is mostly bound to poorly characterised organic ligands; thus, organic ligands are paramount in defining Fe biogeochemical cycling and its control on oceanic primary productivity. Since 1994, Fe chemical speciation has been determined by Competitive Ligand Exchange–Adsorptive Cathodic Stripping Voltammetry (CLE–AdCSV) at room temperature. However, chemical speciation is strongly dependent on temperature and some organic ligands can be temperature sensitive. Here, we compare the use of the CLE–AdCSV at room temperature and at 4°C—a temperature closer to that found in the Southern Ocean, one of the largest iron-limited regions. For both temperatures, similar detection limits and total Fe concentrations were found. However, at 4°C the analytical detection window (αFe(TAC) 2 ) was shifted by 1.4-fold towards the detection of weaker ligands, resulting in up to 2-fold lower ligand concentrations as well as a 2- to 5-fold and 10- to70-fold lower conditional stability constants with inorganic Fe (Fe′) and Fe(III), respectively. As a result, the Fe′ concentration at 4°C was 2-fold greater, resulting in direct implication for Fe bioavailability. Results show that difference in Fe chemical speciation at 4°C was not solely explained by temperature effect on thermodynamics with the exchange ligands or the diffusion of the electroactive complex towards the Hg drop. Lowering analytical window during analysis at room temperature is proposed as a first estimate of temperature effect on iron chemical speciation. [Copyright &y& Elsevier]