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Iron colloids dominate sedimentary supply to the ocean interior

Iron colloids dominate sedimentary supply to the ocean interior
Iron colloids dominate sedimentary supply to the ocean interior
Dissolution of marine sediment is a key source of dissolved iron (Fe) that regulates the ocean carbon cycle. Currently, our prevailing understanding, encapsulated in ocean models, focuses on low-oxygen reductive supply mechanisms and neglects the emerging evidence from iron isotopes in seawater and sediment porewaters for additional nonreductive dissolution processes. Here, we combine measurements of Fe colloids and dissolved δ56Fe in shallow porewaters spanning the full depth of the South Atlantic Ocean to demonstrate that it is lithogenic colloid production that fuels sedimentary iron supply away from low-oxygen systems. Iron colloids are ubiquitous in these oxic ocean sediment porewaters and account for the lithogenic isotope signature of dissolved Fe (δ56Fe = +0.07 ± 0.07‰) within and between ocean basins. Isotope model experiments demonstrate that only lithogenic weathering in both oxic and nitrogenous zones, rather than precipitation or ligand complexation of reduced Fe species, can account for the production of these porewater Fe colloids. The broader covariance between colloidal Fe and organic carbon (OC) abundance suggests that sorption of OC may control the nanoscale stability of Fe minerals by inhibiting the loss of Fe(oxyhydr)oxides to more crystalline minerals in the sediment. Oxic ocean sediments can therefore generate a large exchangeable reservoir of organo-mineral Fe colloids at the sediment water interface (a “rusty source”) that dominates the benthic supply of dissolved Fe to the ocean interior, alongside reductive supply pathways from shallower continental margins.
Iron colloid, Iron isotopes, Ocean sediment, Organo-mineral, Porewater
0027-8424
e2016078118
Homoky, William B.
8e0aa2e9-a88a-4ee5-bfca-1e014cb150ee
Conway, Tim M.
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John, Seth G.
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König, Daniela
80c13d55-6874-468c-a24a-6196bbcddf81
Deng, Feifei
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Tagliabue, Alessandro
23ecb1dd-3cf4-46eb-b059-637a04f2439b
Mills, Rachel A.
a664f299-1a34-4b63-9988-1e599b756706
Homoky, William B.
8e0aa2e9-a88a-4ee5-bfca-1e014cb150ee
Conway, Tim M.
f96668e9-8e9a-4721-bd4e-e9fd7a2a0669
John, Seth G.
1046f19e-4e4e-4850-84e1-6a786236da79
König, Daniela
80c13d55-6874-468c-a24a-6196bbcddf81
Deng, Feifei
4cffab76-0fb3-43d1-a3a9-359b6c023cb8
Tagliabue, Alessandro
23ecb1dd-3cf4-46eb-b059-637a04f2439b
Mills, Rachel A.
a664f299-1a34-4b63-9988-1e599b756706

Homoky, William B., Conway, Tim M., John, Seth G., König, Daniela, Deng, Feifei, Tagliabue, Alessandro and Mills, Rachel A. (2021) Iron colloids dominate sedimentary supply to the ocean interior. Proceedings of the National Academy of Sciences, 118 (13), e2016078118, [2016078118]. (doi:10.1073/pnas.2016078118).

Record type: Article

Abstract

Dissolution of marine sediment is a key source of dissolved iron (Fe) that regulates the ocean carbon cycle. Currently, our prevailing understanding, encapsulated in ocean models, focuses on low-oxygen reductive supply mechanisms and neglects the emerging evidence from iron isotopes in seawater and sediment porewaters for additional nonreductive dissolution processes. Here, we combine measurements of Fe colloids and dissolved δ56Fe in shallow porewaters spanning the full depth of the South Atlantic Ocean to demonstrate that it is lithogenic colloid production that fuels sedimentary iron supply away from low-oxygen systems. Iron colloids are ubiquitous in these oxic ocean sediment porewaters and account for the lithogenic isotope signature of dissolved Fe (δ56Fe = +0.07 ± 0.07‰) within and between ocean basins. Isotope model experiments demonstrate that only lithogenic weathering in both oxic and nitrogenous zones, rather than precipitation or ligand complexation of reduced Fe species, can account for the production of these porewater Fe colloids. The broader covariance between colloidal Fe and organic carbon (OC) abundance suggests that sorption of OC may control the nanoscale stability of Fe minerals by inhibiting the loss of Fe(oxyhydr)oxides to more crystalline minerals in the sediment. Oxic ocean sediments can therefore generate a large exchangeable reservoir of organo-mineral Fe colloids at the sediment water interface (a “rusty source”) that dominates the benthic supply of dissolved Fe to the ocean interior, alongside reductive supply pathways from shallower continental margins.

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Homoky etal_PNAS_Author Accepted MS - Accepted Manuscript
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Accepted/In Press date: 25 January 2021
Published date: 30 March 2021
Additional Information: Funding Information: Edinburgh) for supporting opal analyses by F.D. R.A.M. supported W.B.H. in the collection of sediments and porewater and analyses of oxygen, metals, and carbon through UK Natural Environment Research Council (NERC) Grants NE/F017197/1 and NE/H004394/1. W.B.H. was independently supported by an NERC fellowship at the University of Oxford (NE/K009532/1) to prepare samples for Fe isotope analyses and perform model experiments, and by a University Academic Fellowship at the University of Leeds to write the manuscript. Fe isotope analyses by T.M.C. and S.G.J. were supported by the University of South Carolina. D.K. and A.T. were funded by the European Research Council under the European Union’s Horizon 2020 research and innovation program (Grant Agreement 724289). Publisher Copyright: © 2021 National Academy of Sciences. All rights reserved. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.
Keywords: Iron colloid, Iron isotopes, Ocean sediment, Organo-mineral, Porewater

Identifiers

Local EPrints ID: 448882
URI: http://eprints.soton.ac.uk/id/eprint/448882
ISSN: 0027-8424
PURE UUID: 0234dfe4-d3a6-4f10-a6e2-97fede302246
ORCID for Rachel A. Mills: ORCID iD orcid.org/0000-0002-9811-246X

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Date deposited: 07 May 2021 16:34
Last modified: 23 Jul 2022 05:07

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Contributors

Author: William B. Homoky
Author: Tim M. Conway
Author: Seth G. John
Author: Daniela König
Author: Feifei Deng
Author: Alessandro Tagliabue
Author: Rachel A. Mills ORCID iD

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