The University of Southampton
University of Southampton Institutional Repository

The chromium isotopic composition of seawater and marine carbonates

The chromium isotopic composition of seawater and marine carbonates
The chromium isotopic composition of seawater and marine carbonates
Chromium isotopes are fractionated during redox reactions and have the potential to provide a record of changes in the oxygenation levels of the oceans in the geological past. However, Cr is a trace metal in seawater and its low concentrations make isotopic measurements challenging. Here we report the first determinations of View the MathML source for seawater from open ocean (Argentine Basin) and coastal (Southampton Water) settings, using a double-spike technique. The total chromium concentration in seawater from Southampton Water is 1.85 nM, whereas the Cr content of Argentine Basin samples is 5.8–6.6 nM. The View the MathML source value of seawater from the Argentine Basin is 0.491–0.556‰ in intermediate and deep waters, and varies between 0.412 and 0.664‰ in surface waters (<150 m). The View the MathML source value of Southampton Water seawater is 1.505‰, which may reflect in situ reduction of Cr(VI) to Cr(III). All of our seawater samples have higher View the MathML source than crustal and mantle silicates, and mass balance modelling demonstrates that river water must also be enriched in heavy Cr isotopes, indicating that Cr isotopes are fractionated during weathering and/or during transport to the oceans.

We also show that the Cr isotopic composition of modern non-skeletal marine carbonates (0.640– 0.745‰) encompasses the range that we measure for Argentine Basin seawater. Thus, fractionation of Cr isotopes during precipitation of these marine carbonates is likely to be small (<0.2‰), and they have the potential to provide a record of the Cr isotopic composition of ancient seawater. Phanerozoic carbonates are also characterised by heavy View the MathML source and a correlation between View the MathML source and Ce/Ce? suggests that the Cr and Ce cycles in the ocean are linked.
chromium isotopes, seawater, marine carbonates, weathering
0012-821X
10-20
Bonnand, P.
c86a4fef-3857-4b77-8f6f-938275970353
James, R.H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Parkinson, I.J.
55b92c9a-6530-4c57-9804-c9a3eb41937f
Connelly, D.P.
d49131bb-af38-4768-9953-7ae0b43e33c8
Fairchild, I.J.
2ad1db6c-0c1b-45ef-8edb-6839cd1c624d
Bonnand, P.
c86a4fef-3857-4b77-8f6f-938275970353
James, R.H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Parkinson, I.J.
55b92c9a-6530-4c57-9804-c9a3eb41937f
Connelly, D.P.
d49131bb-af38-4768-9953-7ae0b43e33c8
Fairchild, I.J.
2ad1db6c-0c1b-45ef-8edb-6839cd1c624d

Bonnand, P., James, R.H., Parkinson, I.J., Connelly, D.P. and Fairchild, I.J. (2013) The chromium isotopic composition of seawater and marine carbonates. Earth and Planetary Science Letters, 382, 10-20. (doi:10.1016/j.epsl.2013.09.001).

Record type: Article

Abstract

Chromium isotopes are fractionated during redox reactions and have the potential to provide a record of changes in the oxygenation levels of the oceans in the geological past. However, Cr is a trace metal in seawater and its low concentrations make isotopic measurements challenging. Here we report the first determinations of View the MathML source for seawater from open ocean (Argentine Basin) and coastal (Southampton Water) settings, using a double-spike technique. The total chromium concentration in seawater from Southampton Water is 1.85 nM, whereas the Cr content of Argentine Basin samples is 5.8–6.6 nM. The View the MathML source value of seawater from the Argentine Basin is 0.491–0.556‰ in intermediate and deep waters, and varies between 0.412 and 0.664‰ in surface waters (<150 m). The View the MathML source value of Southampton Water seawater is 1.505‰, which may reflect in situ reduction of Cr(VI) to Cr(III). All of our seawater samples have higher View the MathML source than crustal and mantle silicates, and mass balance modelling demonstrates that river water must also be enriched in heavy Cr isotopes, indicating that Cr isotopes are fractionated during weathering and/or during transport to the oceans.

We also show that the Cr isotopic composition of modern non-skeletal marine carbonates (0.640– 0.745‰) encompasses the range that we measure for Argentine Basin seawater. Thus, fractionation of Cr isotopes during precipitation of these marine carbonates is likely to be small (<0.2‰), and they have the potential to provide a record of the Cr isotopic composition of ancient seawater. Phanerozoic carbonates are also characterised by heavy View the MathML source and a correlation between View the MathML source and Ce/Ce? suggests that the Cr and Ce cycles in the ocean are linked.

PDF
Bonnand et al 2013.pdf - Version of Record
Available under License Other.
Download (458kB)

More information

Published date: 15 November 2013
Keywords: chromium isotopes, seawater, marine carbonates, weathering
Organisations: Marine Geoscience

Identifiers

Local EPrints ID: 358976
URI: https://eprints.soton.ac.uk/id/eprint/358976
ISSN: 0012-821X
PURE UUID: b364cd1c-71aa-42db-901e-21e7c63c68b1

Catalogue record

Date deposited: 17 Oct 2013 08:49
Last modified: 18 Jul 2017 03:25

Export record

Altmetrics

Contributors

Author: P. Bonnand
Author: R.H. James
Author: I.J. Parkinson
Author: D.P. Connelly
Author: I.J. Fairchild

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of https://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×