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

Lough, A.J.M.; Klar, J.K.; Homoky, W.B.; Comer-Warner, S.A.; Milton, J.A.; Connelly, D.P.; James, R.H.; Mills, R.A.. 2017 Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes. Geochimica et Cosmochimica Acta, 202. 1-20. 10.1016/j.gca.2016.12.022

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© 2017 Elsevier B.V. This is the author’s version of a work that was accepted for publication in Geochimica et Cosmochimica Acta Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was/will be published in Geochimica et Cosmochimica Acta doi:10.1016/j.gca.2016.12.022
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Abstract/Summary

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 cycle if these boundary fluxes have distinct signatures. Seafloor hydrothermal vents emit metal rich fluids from mid-ocean ridges into 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 plume dispersal in the deep ocean are understood. In this study we assess the behaviour of Fe isotopes in a Southern Ocean hydrothermal plume using a sampling program of Total Dissolvable Fe (TDFe), and dissolved Fe (dFe). We demonstrate that ?56Fe values of dFe (?56dFe) within the hydrothermal plume change dramatically during early plume dispersal, 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 plume samples age. The dFe present in the hydrothermal plume includes stabilised dFe species with potential to be transported to the deep ocean. We estimate that stable dFe exported from the plume will have a ?56Fe of -0.28 ± 0.17 ‰. Further, we show that the proportion of authigenic iron-sulfide and iron-oxyhydroxide minerals precipitating in the buoyant plume exert 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 signatures under the authigenic conditions reported from modern vent sites elsewhere, and so ought to be considered during iron isotope reconstructions of past hydrothermalism from ocean sediment records.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1016/j.gca.2016.12.022
ISSN: 00167037
Additional Keywords: Iron isotopes; Hydrothermal plume; East Scotia Ridge; Isotope fractionation
Date made live: 11 Jan 2017 10:44 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/515784

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