Cr isotopic composition of modern carbonates and seawater

Bonnand, P.; Parkinson, I. J.; James, R. H.; Fehr, M. and Connolly, D. P. (2010). Cr isotopic composition of modern carbonates and seawater. In: American Geophysical Union, Fall Meeting 2019., 13-17 Dec 2010, San Francisco.



Recent development in MC-ICP-MS instrumentation, coupled with double-spike techniques has led to the improvement of stable Cr isotopes measurements and allows the determination of Cr isotopes in low concentration samples such as carbonates and seawater. Cr is a redox sensitive element and its isotopes are fractionated during the reduction of Cr(VI) to Cr(III). Chromium isotopic variations in BIFs have been linked to the redox conditions of ancient oceans. However, in order to understand Cr isotopic fractionation in the past it is important to constrain the Cr isotopic composition of modern seawater. Chromium concentrations in seawater are between 2 and 5nM, and therefore the measurement of stable Cr isotopes in seawater is an analytical challenge. We have developed a new technique to measure Cr isotopes in seawater based on the Cr co-precipitation with Fe, the chemical purification of Cr using an anion exchange chromatography and analyses using the double-spike technique with a ThermoFisher Neptune MC-ICP-MS. Using this method, seawater samples from the Argentinean Basin and from Southampton Water (UK) have been analysed, which have Cr concentrations of ~6nM of Cr. Chromium isotopic composition of our seawater samples is consistently heavier than continental crust and mantle values (δ53Cr -0.18‰)with δ53Cr values of ~+0.5‰. We have also measured Cr isotopic compositions in ooids from the Bahamas Banks, which represent chemical precipitates from modern seawater. These also record consistently heavy δ53Cr values (0.6-0.8‰), which overlap the range of modern seawater. We conclude that heavy δ53Cr in seawater reflect either redox cycling of Cr in the oceans or fractionation during the weathering of the continental crust. Moreover, Cr isotopes in modern carbonates are not significantly offset from seawater and therefore, these carbonates reflect the Cr composition of seawater. Thus, Cr isotopes in carbonates can be used to reconstruct the Cr isotopic composition variations in past oceans.

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