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Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone

Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone
Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone
Constraints on the variability of chromium (Cr) isotopic compositions in the modern ocean are required to validate the use of Cr isotopic signatures in ancient authigenic marine sediments for reconstructing past levels of atmospheric and ocean oxygenation. This study presents dissolved Cr concentrations (CrT, where CrT = Cr(VI) + Cr(III)) and Cr isotope data (δ53Cr) for shelf, slope and open ocean waters within the oxygen minimum zone (OMZ) of the eastern sub-tropical Atlantic Ocean. Although dissolved oxygen concentrations were as low as 44-90 μmol kg-1 in the core of the OMZ, there was no evidence for removal of Cr(VI). Nonetheless, there was significant variability in seawater δ53Cr, with values ranging from 1.08 to 1.72‰. Shelf CrT concentrations were slightly lower (2.21 ± 0.07 nmol kg-1) than in open ocean waters at the same water depth (between 0 and 160m, 2.48 ± 0.07 nmol kg-1). The shelf waters also had higher δ53Cr values (1.41 ± 0.14‰ compared to 1.18 ± 0.05‰ for open ocean waters shallower than 160m). This is consistent with partial reduction of Cr(VI) to Cr(III), with subsequent removal of isotopically light Cr(III) onto biogenic particles. We also provide evidence for input of relatively isotopically heavy Cr from sediments on the shelf. Intermediate and deep water masses (AAIW and NADW) show a rather limited range of δ53Cr values (1.19 ± 0.09‰) and inputs of Cr from remineralisation of organic material or re-oxidation of Cr(III) appear to be minimal. Authigenic marine precipitates deposited in deep water in the open ocean therefore have the potential to faithfully record seawater δ53Cr, whereas archives of seawater δ53Cr derived from shelf sediments must be interpreted with caution.
0016-7037
Goring-Harford, Heather
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Klar, Jessica
27637ace-8fd9-4859-adb3-a308b58a827e
Pearce, Christopher
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Connelly, D.P.
7120e580-6c32-4711-ae32-a2473c39011c
Achterberg, Eric
685ce961-8c45-4503-9f03-50f6561202b9
James, Rachael
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Goring-Harford, Heather
a0d7f7d0-4593-4bd9-9482-b2d783c0ac97
Klar, Jessica
27637ace-8fd9-4859-adb3-a308b58a827e
Pearce, Christopher
d1521f61-7b4b-4131-9654-51ed5057a026
Connelly, D.P.
7120e580-6c32-4711-ae32-a2473c39011c
Achterberg, Eric
685ce961-8c45-4503-9f03-50f6561202b9
James, Rachael
79aa1d5c-675d-4ba3-85be-fb20798c02f4

Goring-Harford, Heather, Klar, Jessica, Pearce, Christopher, Connelly, D.P., Achterberg, Eric and James, Rachael (2018) Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone. Geochimica et Cosmochimica Acta. (doi:10.1016/j.gca.2018.03.004).

Record type: Article

Abstract

Constraints on the variability of chromium (Cr) isotopic compositions in the modern ocean are required to validate the use of Cr isotopic signatures in ancient authigenic marine sediments for reconstructing past levels of atmospheric and ocean oxygenation. This study presents dissolved Cr concentrations (CrT, where CrT = Cr(VI) + Cr(III)) and Cr isotope data (δ53Cr) for shelf, slope and open ocean waters within the oxygen minimum zone (OMZ) of the eastern sub-tropical Atlantic Ocean. Although dissolved oxygen concentrations were as low as 44-90 μmol kg-1 in the core of the OMZ, there was no evidence for removal of Cr(VI). Nonetheless, there was significant variability in seawater δ53Cr, with values ranging from 1.08 to 1.72‰. Shelf CrT concentrations were slightly lower (2.21 ± 0.07 nmol kg-1) than in open ocean waters at the same water depth (between 0 and 160m, 2.48 ± 0.07 nmol kg-1). The shelf waters also had higher δ53Cr values (1.41 ± 0.14‰ compared to 1.18 ± 0.05‰ for open ocean waters shallower than 160m). This is consistent with partial reduction of Cr(VI) to Cr(III), with subsequent removal of isotopically light Cr(III) onto biogenic particles. We also provide evidence for input of relatively isotopically heavy Cr from sediments on the shelf. Intermediate and deep water masses (AAIW and NADW) show a rather limited range of δ53Cr values (1.19 ± 0.09‰) and inputs of Cr from remineralisation of organic material or re-oxidation of Cr(III) appear to be minimal. Authigenic marine precipitates deposited in deep water in the open ocean therefore have the potential to faithfully record seawater δ53Cr, whereas archives of seawater δ53Cr derived from shelf sediments must be interpreted with caution.

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Goring Harford et al GCA 2018 for eprints - Accepted Manuscript
Available under License Creative Commons Attribution.
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Accepted/In Press date: 2 March 2018
e-pub ahead of print date: 10 March 2018

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Local EPrints ID: 418689
URI: http://eprints.soton.ac.uk/id/eprint/418689
ISSN: 0016-7037
PURE UUID: d7b85684-9522-4aa8-80a3-03fe103b7c59

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Date deposited: 19 Mar 2018 17:30
Last modified: 16 Dec 2019 18:23

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Contributors

Author: Jessica Klar
Author: Christopher Pearce
Author: D.P. Connelly
Author: Eric Achterberg
Author: Rachael James

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