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Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK

Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK
Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK
Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the δ53Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and δ53Cr values
of inorganic Cr (Cr(III)+Cr(VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The δ53Cr values of dissolved inorganic Cr ranged from -0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmol kg-1 and Cr(III) concentrations from 0.11 to 3.21 nmol kg-1. Both Cr(VI) concentrations and δ53Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus δ53Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the δ53Cr signature of Beaulieu River water was modified only by mixing
between the river and seawater endmembers. The calculated average δ53Cr value of the river water endmember (-0.39 ± 0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr(III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low δ53Cr values (-0.11 to -0.75‰).While input of Cr from anoxic weathering
processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the δ53Cr values of estuarine and coastal waters on the local scale. The δ53Cr value of the coastal seawater endmember (1.6 ± 0.4‰) was also higher than the range observed in the deep open ocean, due to in situ biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary δ53Cr records.
anoxic weathering, chromium cycling, chromium isotopes, chromium speciation, estuarine mixing
0012-821X
Goring-Harford, Heather
c1f25875-afe9-44af-b730-fb7b78774081
Klar, Jessica K.
3beaa216-b22d-43f3-8a1b-9683c949939b
Donald, Hannah
574e955f-ec6a-49e1-b86a-8bf7d1ab877c
Pearce, Christopher
d1521f61-7b4b-4131-9654-51ed5057a026
Connelly, Douglas
d49131bb-af38-4768-9953-7ae0b43e33c8
James, Rachael
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Goring-Harford, Heather
c1f25875-afe9-44af-b730-fb7b78774081
Klar, Jessica K.
3beaa216-b22d-43f3-8a1b-9683c949939b
Donald, Hannah
574e955f-ec6a-49e1-b86a-8bf7d1ab877c
Pearce, Christopher
d1521f61-7b4b-4131-9654-51ed5057a026
Connelly, Douglas
d49131bb-af38-4768-9953-7ae0b43e33c8
James, Rachael
79aa1d5c-675d-4ba3-85be-fb20798c02f4

Goring-Harford, Heather, Klar, Jessica K., Donald, Hannah, Pearce, Christopher, Connelly, Douglas and James, Rachael (2020) Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK. Earth and Planetary Science Letters, 536, [116166]. (doi:10.1016/j.epsl.2020.116166).

Record type: Article

Abstract

Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the δ53Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and δ53Cr values
of inorganic Cr (Cr(III)+Cr(VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The δ53Cr values of dissolved inorganic Cr ranged from -0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmol kg-1 and Cr(III) concentrations from 0.11 to 3.21 nmol kg-1. Both Cr(VI) concentrations and δ53Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus δ53Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the δ53Cr signature of Beaulieu River water was modified only by mixing
between the river and seawater endmembers. The calculated average δ53Cr value of the river water endmember (-0.39 ± 0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr(III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low δ53Cr values (-0.11 to -0.75‰).While input of Cr from anoxic weathering
processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the δ53Cr values of estuarine and coastal waters on the local scale. The δ53Cr value of the coastal seawater endmember (1.6 ± 0.4‰) was also higher than the range observed in the deep open ocean, due to in situ biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary δ53Cr records.

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Beaulieu accepted manuscript for PURE - Accepted Manuscript
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Accepted/In Press date: 13 February 2020
e-pub ahead of print date: 21 February 2020
Published date: 15 April 2020
Keywords: anoxic weathering, chromium cycling, chromium isotopes, chromium speciation, estuarine mixing

Identifiers

Local EPrints ID: 438162
URI: http://eprints.soton.ac.uk/id/eprint/438162
ISSN: 0012-821X
PURE UUID: 72d595ad-2d72-4a68-a050-f0f41adbc944
ORCID for Rachael James: ORCID iD orcid.org/0000-0001-7402-2315

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Date deposited: 03 Mar 2020 17:44
Last modified: 28 Apr 2022 05:13

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Contributors

Author: Heather Goring-Harford
Author: Jessica K. Klar
Author: Hannah Donald
Author: Christopher Pearce
Author: Douglas Connelly
Author: Rachael James ORCID iD

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