The chromium isotopic composition of seawater and marine carbonates

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.

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ORCID for R.H. James: orcid.org/0000-0001-7402-2315

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