Cerium behavior during the transformation of birnessite to todorokite: Implications for the origin of Ce anomalies in hydro-diagenetic ferromanganese nodules

Abstract Cerium (Ce) anomalies in ferromanganese nodules serve as a crucial indicator of redox conditions during their formation, due to the differential solubility between Ce(III) and Ce(IV). Nodules of different origins always exhibit distinct Ce anomalies, with hydrogenetic nodules displaying positive Ce anomalies, while diagenetic ones typically show negative or slightly positive anomalies. However, the mechanisms behind the variation of Ce anomaly remain unclear. In this study, we investigate a hydro-diagenetic mixed ferromanganese nodule from the northwestern Pacific Ocean, which contains two distinct growth layers: Type 1 and Type 2. Type 1 has low Mn/Fe ratios (<3) and is enriched in Co and Ti, indicating a hydrogenetic origin, whereas Type 2 has higher Mn/Fe ratios (>10) and is enriched in Ni, Cu, and Mg, reflecting a diagenetic process. The Ce anomaly shifts from positive in Type 1 to slightly positive or negative in Type 2, corresponding to differences in mineralogy. To investigate Ce behavior, we further synthesized birnessite with varying Ce concentrations and induced its transformation to todorokite under refluxing conditions. X-ray diffraction and Fourier-transform infrared spectroscopy analyses revealed that Ce inhibits the birnessite-to-todorokite transformation and is partially released into solution during this process. This mineral transformation results in Ce depletion in the solid phase, leading to a shift in the Ce anomalies of natural nodules. Through integrated natural sample analysis and synthesized mineral simulation, our findings demonstrate Ce mobilization and redistribution between hydrogenetic and diagenetic layers in a specific ferromanganese nodule and underscore the importance of considering Mn mineral transformation processes when using Ce anomalies as a redox proxy.