A Cr Isotope Proxy For Ocean Deoxygenation

Abstract:

The concentration and distribution of Cr in the oceans is strongly controlled by its oxidation state. Cr(VI) (as soluble chromate) is the dominant oxidation state in oxygenated seawater. Chromate is typically greater than 70% of total dissolved Cr in the open ocean, consistent with thermodynamic predictions. However, lower than average chromate concentrations in coastal seas and oxygen minimum zones suggest that chromate is being removed in these settings by reduction to Cr(III), which favours particle reactive species. Cr is an element whose isotopes are fractionated by redox changes. Reduction of Cr(VI) causes light isotopes of Cr to be enriched in the product Cr(III). Accordingly, any local-scale increase in reductive Cr removal fluxes will cause the seawater Cr concentration to decrease and the δ⁵³Cr value to increase. A recent study of Cr isotopes in the oceans¹ supports this prediction. Cr isotopes show a range of δ⁵³Cr values correlating inversely with Cr concentration. The fractionation factor deduced from this correlation is –0.80 ±0.03 ‰ (2s) on a global scale. The difference in solubility of oxidized and reduced Cr in seawater, and the isotopic fractionation between them, is the basis on which the Cr isotope proxy may be used for tracing ocean deoxygenation events in the geological past. More specifically, changes in the size of the chromate inventory of seawater, both locally and globally, should be traceable from reconstructions of seawater-derived Cr isotope variations in marine sedimentary successions. Geological records of Cr isotope changes in the oceans during past deoxygenation events may be used to gauge the impact of global warming on future deoxygenation of the oceans, particularly if proxy records of temperature and ocean pH are also reconstructed. However, study of the modern ocean Cr cycle is still in its early stages, and important knowledge gaps need to be filled going forward. In this talk, we present results of our seawater Cr isotope investigations in the modern ocean, and address the implications for the Cr isotope proxy.

¹Scheiderich et al. (2015) EPSL 423, 87-97.