Forms and distribution of Ce in a ferromanganese nodule

We studied the distribution and speciation of Ce in a previously-characterized ferromanganese nodule from the South Pacific Gyre using X-ray microspectroscopy.We find Ce in three types of environment – diffusely in all parts of the nodule except the Fe-rich, Mn-poor “matrix”, fluorapatite grains with a negative Ce anomaly suggestive of an authigenic origin, and localized spots whose composition is consistent with a pseudobinary mixture of sorbed Ce⁴⁺ and a mix of sorbed Ce³⁺ and CeO₂. In contrast with some other elements such as Ti, there is no clear correlation of the Ce concentration in the diffuse areas with either that of Mn, Fe or any linear combination of the two.We argue that the CeO₂ came from oxidation of Ce³⁺ in the water column, while the Ce in the Mn-rich regions may have been initially deposited as Ce⁴⁺ and then partially reduced.