Stable cerium isotopes as a tracer of oxidation reactions

Abstract

Redox conditions in past oceans have attracted significant interest and many proxies have been used to probe redox changes through time. For example, the redox dependent behaviour of Ce, resulting in negative or positive elemental Ce anomalies, has been widely used. More recently, mass dependent Ce isotopic variations have been proposed as a powerful tool to study Ce oxidation in natural environments. In this study, we demonstrate, for the first time, that Ce isotopes are fractionated during oxidation reaction, confirming the utility of Ce isotopes to study redox reactions. This result suggests that seawater Ce isotopic composition should be fractionated toward heavy values relative to the continental crust. Measured natural rock samples (carbonates, banded iron formations and Mn nodules) have variable Ce isotopic compositions, ranging from −0.055 ± 0.045 ‰ to +0.280 ± 0.045 ‰. The relation between Ce elemental anomalies and Ce isotopic composition in carbonate rocks suggest that mass dependent Ce isotopes can be used to distinguish elemental anomalies produced by oxidation reactions from those produced by other processes. Coupled with La-Ce chronology, mass dependent Ce isotope analysis is a very powerful tool to study redox reactions in past oceans.