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Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column
Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column
Shelf sediments underlying temperate and oxic waters of the Celtic Sea (NW European Shelf) were found to have shallow oxygen penetrations depths from late spring to late summer (2.2 to 5.8 mm below seafloor) with the shallowest during/after the spring-bloom (mid-April to mid-May) when the organic carbon content was highest. Sediment porewater dissolved iron (dFe, < 0.15 μm) mainly (> 85 %) consisted of reduced Fe(II) and gradually increased from 0.4 to 15 μM at the sediment surface to ~100 to 170 μM at about 6 cm depth. During the late spring this Fe(II) was found to be mainly present as soluble Fe(II) (> 85 % sFe, < 0.02 μm). Sub-surface dFe(II) maxima were enriched in light isotopes (δ56Fe of -2.0 to -1.5 ‰), which is attributed to dissimilatory iron reduction (DIR) during the bacterial decomposition of organic matter. As porewater Fe(II) was oxidised to insoluble Fe(III) in the surface sediment layer, residual Fe(II) was further enriched in lighter isotopes (down to -3.0 ‰). Ferrozine-reactive Fe(II) was found in surface porewaters and in overlying core top waters, and was highest in the late spring period. Shipboard experiments showed that depletion of bottom water oxygen in late spring can lead to a substantial release of Fe(II) Reoxygenation of bottom water caused this Fe(II) to be rapidly lost from solution, but residual dFe(II) and dFe(III) remained (12 and 33 nM) after > 7 hours. Iron(II) oxidation experiments in core top and bottom waters also showed removal from solution but at rates up to 5-times slower than predicted from theoretical reaction kinetics. These data imply the presence of ligands capable of complexing Fe(II) and supressing oxidation rates. The lower oxidation rate allows more time for the diffusion of Fe(II) from the sediments into the overlying water column. Modelling indicates significant diffusive fluxes of Fe(II) (on the order of 23-31 μmol m-2 d-1) are possible during late spring when oxygen penetration depths are shallow, and pore water Fe(II) concentrations are highest. In the water column this stabilised Fe(II) will gradually be oxidised and become part of the dFe(III) pool. Thus oxic continental shelves can supply dFe to the water column, which is enhanced during a small period of the year after phytoplankton bloom events when organic matter is transferred to the seafloor. This input is based on conservative assumptions for solute exchange (diffusionreaction), whereas (bio)physical advection and resuspension events are likely to accelerate these solute exchanges in shelf-seas.
Benthic iron flux, shelf sediment, oxic shelf, porewaters, iron isotopes, redox
0168-2563
49-67
Klar, J.K.
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Homoky, W.B.
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Statham, P.J.
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Birchill, A.J.
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Harris, E.L.
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Woodward, E.M.S
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Silburn, B.
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Cooper, M.J.
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James, R.H.
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Connelly, D.P.
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Chever, F.
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Lichtschlag, A.
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Graves, C.
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Klar, J.K.
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Homoky, W.B.
66fe68ac-26b0-4da7-8e72-75f2c7e3ea46
Statham, P.J.
51458f15-d6e2-4231-8bba-d0567f9e440c
Birchill, A.J.
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Harris, E.L.
b428de4b-83d6-456f-9e44-017f80cdca3c
Woodward, E.M.S
6e57d5d0-c1bd-49e8-b9eb-a1816edaf676
Silburn, B.
05d4e51f-1bbb-494b-990c-2427086e8f50
Cooper, M.J.
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James, R.H.
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Connelly, D.P.
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Chever, F.
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Lichtschlag, A.
f465c42f-b2fd-45a8-ae5b-8b7381d62c13
Graves, C.
9f1c821a-dc07-413b-b577-6e6b55839853

Klar, J.K., Homoky, W.B., Statham, P.J., Birchill, A.J., Harris, E.L., Woodward, E.M.S, Silburn, B., Cooper, M.J., James, R.H., Connelly, D.P., Chever, F., Lichtschlag, A. and Graves, C. (2017) Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column. Biogeochemistry, 135 (1-2), 49-67. (doi:10.1007/s10533-017-0309-x).

Record type: Article

Abstract

Shelf sediments underlying temperate and oxic waters of the Celtic Sea (NW European Shelf) were found to have shallow oxygen penetrations depths from late spring to late summer (2.2 to 5.8 mm below seafloor) with the shallowest during/after the spring-bloom (mid-April to mid-May) when the organic carbon content was highest. Sediment porewater dissolved iron (dFe, < 0.15 μm) mainly (> 85 %) consisted of reduced Fe(II) and gradually increased from 0.4 to 15 μM at the sediment surface to ~100 to 170 μM at about 6 cm depth. During the late spring this Fe(II) was found to be mainly present as soluble Fe(II) (> 85 % sFe, < 0.02 μm). Sub-surface dFe(II) maxima were enriched in light isotopes (δ56Fe of -2.0 to -1.5 ‰), which is attributed to dissimilatory iron reduction (DIR) during the bacterial decomposition of organic matter. As porewater Fe(II) was oxidised to insoluble Fe(III) in the surface sediment layer, residual Fe(II) was further enriched in lighter isotopes (down to -3.0 ‰). Ferrozine-reactive Fe(II) was found in surface porewaters and in overlying core top waters, and was highest in the late spring period. Shipboard experiments showed that depletion of bottom water oxygen in late spring can lead to a substantial release of Fe(II) Reoxygenation of bottom water caused this Fe(II) to be rapidly lost from solution, but residual dFe(II) and dFe(III) remained (12 and 33 nM) after > 7 hours. Iron(II) oxidation experiments in core top and bottom waters also showed removal from solution but at rates up to 5-times slower than predicted from theoretical reaction kinetics. These data imply the presence of ligands capable of complexing Fe(II) and supressing oxidation rates. The lower oxidation rate allows more time for the diffusion of Fe(II) from the sediments into the overlying water column. Modelling indicates significant diffusive fluxes of Fe(II) (on the order of 23-31 μmol m-2 d-1) are possible during late spring when oxygen penetration depths are shallow, and pore water Fe(II) concentrations are highest. In the water column this stabilised Fe(II) will gradually be oxidised and become part of the dFe(III) pool. Thus oxic continental shelves can supply dFe to the water column, which is enhanced during a small period of the year after phytoplankton bloom events when organic matter is transferred to the seafloor. This input is based on conservative assumptions for solute exchange (diffusionreaction), whereas (bio)physical advection and resuspension events are likely to accelerate these solute exchanges in shelf-seas.

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Accepted/In Press date: 6 February 2017
e-pub ahead of print date: 27 February 2017
Published date: 1 September 2017
Keywords: Benthic iron flux, shelf sediment, oxic shelf, porewaters, iron isotopes, redox
Organisations: Geochemistry, Ocean and Earth Science, Marine Biogeochemistry, Marine Geoscience, National Oceanography Centre

Identifiers

Local EPrints ID: 406248
URI: https://eprints.soton.ac.uk/id/eprint/406248
ISSN: 0168-2563
PURE UUID: 62fdac4c-4454-42b1-b408-fbcc21ae6aa5
ORCID for M.J. Cooper: ORCID iD orcid.org/0000-0002-2130-2759

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Date deposited: 10 Mar 2017 10:43
Last modified: 03 Dec 2019 01:53

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Contributors

Author: J.K. Klar
Author: W.B. Homoky
Author: P.J. Statham
Author: A.J. Birchill
Author: E.L. Harris
Author: E.M.S Woodward
Author: B. Silburn
Author: M.J. Cooper ORCID iD
Author: R.H. James
Author: D.P. Connelly
Author: F. Chever
Author: A. Lichtschlag
Author: C. Graves

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