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

Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes
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 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.
Iron isotopes, Hydrothermal plume, East Scotia Ridge, Isotope fractionation
0016-7037
1-20
Lough, A.J.M.
64be5e9d-9ca2-438c-b566-cfee88ff1a37
Klar, Jessica
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Homoky, W.B.
66fe68ac-26b0-4da7-8e72-75f2c7e3ea46
Corner Warner, S.A.
9d702d6c-729a-4c3c-9ad4-89fcc665e57a
Milton, J.A.
9e183221-d0d4-4ddb-aeba-0fdde9d31230
Connelly, D.P.
d49131bb-af38-4768-9953-7ae0b43e33c8
James, R.H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Mills, R.A.
a664f299-1a34-4b63-9988-1e599b756706
Lough, A.J.M.
64be5e9d-9ca2-438c-b566-cfee88ff1a37
Klar, Jessica
817e4fe3-d923-4e9c-84da-a511f602b4f8
Homoky, W.B.
66fe68ac-26b0-4da7-8e72-75f2c7e3ea46
Corner Warner, S.A.
9d702d6c-729a-4c3c-9ad4-89fcc665e57a
Milton, J.A.
9e183221-d0d4-4ddb-aeba-0fdde9d31230
Connelly, D.P.
d49131bb-af38-4768-9953-7ae0b43e33c8
James, R.H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Mills, R.A.
a664f299-1a34-4b63-9988-1e599b756706

Lough, A.J.M., Klar, Jessica and Homoky, W.B. et al. (2017) Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes. Geochimica et Cosmochimica Acta, 202, 1-20. (doi:10.1016/j.gca.2016.12.022).

Record type: Article

Abstract

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.

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More information

Accepted/In Press date: 13 December 2016
e-pub ahead of print date: 5 January 2017
Published date: 1 April 2017
Keywords: Iron isotopes, Hydrothermal plume, East Scotia Ridge, Isotope fractionation
Organisations: Ocean and Earth Science, Geochemistry, Marine Geoscience, National Oceanography Centre

Identifiers

Local EPrints ID: 404479
URI: http://eprints.soton.ac.uk/id/eprint/404479
ISSN: 0016-7037
PURE UUID: 5ed2fa93-444b-4fcb-a76e-9ea44c57c7ba
ORCID for J.A. Milton: ORCID iD orcid.org/0000-0003-4245-5532
ORCID for R.H. James: ORCID iD orcid.org/0000-0001-7402-2315
ORCID for R.A. Mills: ORCID iD orcid.org/0000-0002-9811-246X

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Date deposited: 11 Jan 2017 09:53
Last modified: 05 Apr 2022 01:34

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Contributors

Author: A.J.M. Lough
Author: Jessica Klar
Author: W.B. Homoky
Author: S.A. Corner Warner
Author: J.A. Milton ORCID iD
Author: D.P. Connelly
Author: R.H. James ORCID iD
Author: R.A. Mills ORCID iD

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