Cerium is a rare-earth element (lanthanide) characterized by two different redox states: III and IV. Contrary to other lanthanide elements, which are only trivalent (with the notable exception of Eu), Ce can be oxidized by atmospheric oxygen (O) to Ce under alkaline conditions.
The cerium anomaly relates to the decrease in solubility, which accompanies the oxidation of Ce(III) to Ce(IV). Under reducing conditions, Ce is relatively soluble, while under oxidizing conditions CeO precipitates. Sediments deposited under oxic or anoxic conditions can preserve on the long term the geochemical signature of Ce or Ce upon reserve that no early diagenetic transformation altered it.