Soluble iron conservation and colloidal iron dynamics in a hydrothermal plume

Iron (Fe) limits or co-limits primary productivity and nitrogen fixation in large regions of the worlds oceans, and the supply of Fe from hydrothermal vents to the deep ocean is now known to be extensive. However, the mechanisms that control the amount of hydrothermal Fe that is stabilised in the deep ocean, and thus dictate the impact of hydrothermal Fe sources on surface ocean biogeochemistry, are unclear. To learn more, we have examined the dispersion of total dissolvable Fe (TDFe), dissolved Fe (dFe) and soluble Fe (sFe) in the buoyant and non-buoyant hydrothermal plume above the Beebe vent field, Caribbean Sea. We have also characterised plume particles using electron microscopy and synchrotron based spectromicroscopy. We show that the majority of dFe in the Beebe hydrothermal plume was present as colloidal Fe (dFesFe = cFe). During ascent of the buoyant plume, a significant fraction of particulate Fe (pFe=TDFedFe) was lost to settling and exchange with colloids. Conversely, the opposite was observed in the non-buoyant plume, where pFe concentrations increased during non-buoyant plume dilution, cFe concentrations decreased apparently due to colloid aggregation. Elemental mapping of carbon, oxygen and iron in plume particles reveals their close association and indicates that exchanges of Fe between colloids and particles must include transformations of organic carbon and Fe oxyhydroxide minerals. Notably, sFe is largely conserved during plume dilution, and this is likely to be due to stabilisation by organic ligands, in contrast to the more dynamic exchanges between pFe and cFe. This study highlights that the size of the sFe stabilising ligand pool, and the rate of iron-rich colloid aggregation will control the amount and physico-chemical composition of dFe supplied to the ocean interior from hydrothermal systems. Both the ligand pool, and the rate of cFe aggregation in hydrothermal plumes remain uncertain and determining these are important intermediate goals to more accurately assess the impact of hydrothermalism on the ocean’s carbon cycle.