Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes

Iron is a scarce but essential micronutrient in the oceans that limits primary productivity in many regions of the surface ocean. The mechanisms and rates of Fe supply to the ocean interior are still poorly understood and quantified. Iron isotope ratios of different Fe pools can potentially be used to trace sources and sinks of the global Fe biogeochemical cycleif these boundary fluxes have distinct signatures. Seafloor hydrothermal ventsemit metal rich fluids from mid-ocean ridgesinto the deep ocean. Iron isotope ratios have the potential to be used to trace the input of hydrothermal dissolved iron to the oceans if the local controls on the fractionation of Fe isotopes during plumedispersal in the deep ocean are understood. In this study we assess the behaviour of Fe isotopes in a Southern Ocean hydrothermal plumeusing a sampling program of Total Dissolvable Fe (TDFe), and dissolved Fe (dFe). We demonstrate that ?56Fe values of dFe (?56dFe) within the hydrothermal plumechange dramatically during early plumedispersal, ranging from -2.39 ± 0.05 ‰ to -0.13 ± 0.06 ‰ (2 SD). The isotopic composition of TDFe (?56TDFe) was consistently heavier than dFe values, ranging from -0.31 ± 0.03 ‰ to 0.78 ± 0.05 ‰, consistent with Fe oxyhydroxide precipitation as the plumesamples age. The dFe present in the hydrothermal plumeincludes stabilised dFe species with potential to be transported to the deep ocean. We estimate that stable dFe exported from the plumewill have a ?56Fe of -0.28 ± 0.17 ‰. Further, we show that the proportion of authigenic iron-sulfideand iron-oxyhydroxide minerals precipitating in the buoyant plumeexert opposing controls on the resultant isotope composition of dissolved Fe passed into the neutrally buoyant plume. We show that such controls yield variable dissolved Fe isotope signaturesunder the authigenicconditions reported from modern vent sites elsewhere, and so ought to be considered during iron isotope reconstructions of past hydrothermalism from ocean sediment records.