Oxidative uptake of Ce by oceanic ferromanganese crusts and implications for paleoredox estimates using Ce isotopes

The oxidative enrichment and isotopic fractionation of cerium (Ce) in contact with vernadite (δ-MnO2) serve as a proxy for past redox conditions in both terrestrial and marine environments. However, the molecular processes that govern the scavenging of Ce from the dissolved 3+ to the insoluble 4+ oxidation states remain obscure. Adsorption experiments on synthetic δ-MnO2 suggest that aqueous Ce(III) precipitates as ceric hydroxide (Ce(OH)4), an unknown mineral. Here, the atomic-scale structure of Ce in natural vernadite from ferromanganese crusts collected across the Pacific, Atlantic, and Indian Oceans was examined using advanced high-energy-resolution extended X-ray absorption fine structure spectroscopy. The findings provide direct evidence for the uptake of Ce as mononuclear Ce(IV) complexes at the layer-edge sites (DES complex) and Mn(IV) vacancy sites of vernadite. Density functional theory-based Gibbs free-energy calculations indicate that hydrolysis of the DES complex promotes the oxidation of Ce(III) to Ce(IV). Quantum mechanical calculations predict that the equilibrium 136Ce/140Ce isotope fractionation factor between Ce(III) dissolved in seawater and the Ce(IV) complexes can reach 1.2-1.3 ‰ at 25 °C, indicating that the 136Ce/140Ce ratio has high potential as a new paleoredox proxy. Seawater cerium (III) forms mononuclear complexes in vernadite from global ferromanganese crusts and is oxidized to cerium (IV) during hydrolysis, as shown by atomic-scale analysis and quantum calculations. This leads to strong Ce isotope fractionation.