Decoupling cerium isotope fractionation from cerium anomalies in marine sediments

Abstract

Cerium (Ce) stable isotopes, combined with Ce anomalies, serve as potential proxies for reconstructing Earth’s past redox conditions. However, the behavior of Ce isotopes and Ce anomalies across different oceanic redox environments remains underexplored. This study presents a Ce anomaly and Ce isotope dataset of marine sediments from a range of modern oceanographic regimes, including anoxic continental margins and oxic Equatorial Pacific environments. By integrating bulk sediment geochemistry, sequential extraction, and X-ray absorption spectroscopy data, we reveal the decoupling between Ce stable isotope values (δ¹⁴²Ce) and Ce anomalies (Ce/Ce*). Margin sediments exhibit negative Ce anomalies (0.48 to 0.96) that display a negative correlation with δ¹⁴²Ce (-0.05 to 0.12 ‰), which are close to or higher than the upper continental crust value (-0.03±0.06 ‰). This relationship results from the influence of seawater-derived REEs through authigenic phosphates and organic matter, as well as clastic inputs. In contrast, Equatorial Pacific sediments exhibit positive Ce anomalies (1.05 to 1.23) that are positively correlated to their δ¹⁴²Ce values (0.08 to 0.16 ‰), which exceed the upper crust baseline. This pattern is attributed to Mn(IV)-oxide-driven oxidative adsorption of Ce. Here, we purpose a first-order estimate for the δ¹⁴²Ce value in bottom seawater of the Equatorial Pacific (about 0.3 to 0.4 ‰). These findings enhance our understanding of marine Ce geochemistry and underscore the importance of integrating Ce anomalies with Ce isotope signatures for paleoceanographic redox reconstructions.