Cerium isotopic constraints on oceanic redox conditions: Insights from first-principles calculations

Oceanic redox conditions are important for understanding life explosions, mass extinctions and surface environments. Stable cerium (Ce) isotopes may provide a useful proxy for oceanic redox conditions by combining Ce anomalies in stratigraphic profiles. However, the quantitative response of Ce isotope compositions to redox conditions has yet to be investigated. Here, we estimate equilibrium Ce isotope fractionation factors between marine authigenic minerals to constrain Ce isotope compositional variations under different redox conditions using first-principles calculations. Cerium isotope fractionations (103lnαTotal) between minerals are controlled by mass-dependent (103lnαMass) and mass-independent isotope fractionation factors (103lnαNVE). Our results show that the magnitude of 103lnαNVE is comparable to or even larger than that of 103lnαMass. The 103ln142/140αTotal values between seawater and minerals are +0.40, −0.34, +0.11 and −0.07 ‰ for manganese oxides, phosphates and Ce3+- and Ce4+-doped calcites, respectively. In a locally oxidised ocean, the precipitation of manganese oxides increases the Ce isotope composition (δ142/140Ce) of seawater; in an anaerobic ocean, the precipitation of phosphates decreases the δ142/140Ce value of seawater. Therefore, calcite can faithfully record seawater δ142/140Ce values because of the limited Ce isotope fractionations in those redox conditions.