Isotopic and speciation study on cerium during its solid–water distribution with implication for Ce stable isotope as a paleo-redox proxy

Cerium (Ce) has anomalously high or low concentrations relative to its neighboring elements, lanthanum (La) and praseodymium (Pr), because of its chemical properties; this phenomenon is known as the Ce anomaly. This redox-sensitive property of Ce allows the estimation of the redox state of paleo-ocean environments and the evolution of the atmosphere. However, a consideration of only the relative abundance of Ce may lead to an incomplete understanding of its oxidation process. In the current study, three important geochemical parameters, namely, abundance, stable isotope ratio, and chemical speciation, were obtained for Ce to derive more information from the Ce anomaly. In our adsorption experiments, the distribution pattern of rare earth elements (REEs) suggests the oxidative scavenging of Ce by δ-MnO2. This finding is further supported by the presence of Ce(IV), as detected by the X-ray absorption near-edge structure (XANES) spectra, which is in agreement with previous studies. However, the REE distribution pattern combined with the XANES spectra of the Ce adsorbed on ferrihydrite indicated that Ce may not have been oxidized by ferrihydrite in the Ce/ferrihydrite system during our experimental period. Assuming equilibrium fractionation, the mean isotopic fractionation factors between the liquid and solid phases (αLq–So) of (i) Ce adsorbed on ferrihydrite, (ii) spontaneous precipitation of Ce, and (iii) Ce adsorbed on δ-MnO2 were 1.000145 (±0.000022), 1.000196 (±0.000031), and 1.000411 (±0.000079), respectively. These results indicate that the degree of isotopic fractionation of Ce between the liquid and solid phases becomes larger as the redox condition becomes more oxic in the following order: adsorption without oxidation < spontaneous precipitation < oxidative adsorption. Previously, the appearance of the Ce anomaly and/or XANES analysis constituted the only tool available for exploring the redox state. This study, however, suggests that the stable isotope ratio of Ce can be used to clearly distinguish spontaneous precipitation from oxidative adsorption on δ-MnO2, that occur under more oxic conditions than the Ce(III)/Ce(IV) boundary. Although similar experiments need to be done in a system more similar to natural systems, our results suggest that the combination of the stable isotope ratio and chemical state of Ce can be used to classify the redox condition into three stages based on Ce geochemistry, thereby offering a powerful tool for exploring redox conditions in paleo-ocean environments.