Abstract
Cerium (Ce) anomalies and oxidation state distributions in marine sediments are important redox indicators in ocean geochemistry, owing to Ce's unique redox behavior compared to other rare earth elements. In oxic seawater, Ce(III) is typically oxidized to insoluble Ce(IV), resulting in characteristic positive Ce anomalies in sediments. This oxidation process has traditionally been attributed to the catalytic action of manganese oxides. Consequently, the potential role of other minerals in Ce redox transformation has been largely overlooked.This study reveals that natural montmorillonite (SWy-3) can oxidize Ce(III) to Ce(IV) in artificial seawater. In batch experiments containing 50 mg SWy-3 and 0.5 uM Ce(III), we observed the oxidation of approximately 30% of the Ce(III) within 24 hours. Analysis using Ce L3-edge XANES and EXAFS spectroscopy confirmed the formation of Ce(IV) species on the mineral surface. In contrast, no oxidation occurred in identical experiments conducted under freshwater conditions.These results indicate that clay minerals, which have traditionally been regarded as redox-inert, may also participate in Ce oxidation under specific geochemical conditions. The input of clay minerals into marine systems through hydrothermal or tectonic activity could contribute to the formation of Ce anomalies, potentially affecting interpretations of past marine environments.
