Abnormally heavy cerium stable isotope composition in regolith: Implications for redox tracing

This study investigated the cerium (Ce) geochemical composition in the regolith at Tanakami Mountain to assess the effectiveness of using Ce proxies for redox tracing during regolith formation. Based on the in-profile Ce isotope pattern, we divided this regolith into three different layers: I (0–2.0 m), II (2.0–5.0 m), and III layers (5.0–8.0 m). The regolith exhibits Ce anomalies (Ce/Ce^∗) ranging from 0.2 to 4.2, with Ce primarily in its tetravalent form (85.3 %). Notably, δ¹⁴²Ce values range from 0.060 to 0.307 ‰, predominantly heavier than those in the bedrock (∼ − 0.1 to 0 ‰), which is contrary to previous observations in weathering profiles. Eolian deposition has a minor impact on the δ¹⁴²Ce composition in the regolith, as supported by the ε_Nd pattern. Under weakly acidic conditions (pH: 4.88–6.91), both O₂-driven spontaneous oxidation and oxidative adsorption on MnO₂ cause preferential retention of isotopically lighter Ce while heavier Ce isotopes migrate downward. We suggest that the pre-existing surface layer (E layer), which is enriched in isotopically lighter Ce, has likely been eroded, while the deeper I layer (δ¹⁴²Ce: 0.131–0.307 ‰), enriched in heavier Ce isotopes, has been preserved. In addition, the oxidation–reduction cycling of Mn in the II layer promotes the downward migration of isotopically heavier Ce (δ¹⁴²Ce: 0.139–0.248 ‰) and an increase in δ¹⁴²Ce value with depth. The reprecipitation of Mn oxides and subsequent oxidative adsorption transforms Ce into its tetravalent form (Bir-Ce⁴⁺, Ce⁴⁺ adsorbed on birnessite) in the II layer. The bottom III layer, close to the substrate with subtle weathering signals exhibits limited Ce isotope fractionation (δ¹⁴²Ce: 0.078–0.161 ‰, nearly identical to the bedrock). In summary, the combination of elemental, isotopic, and speciation information of Ce provides a comprehensive picture of terrestrial redox status and fluctuations. Moreover, our findings emphasize the need to consider erosion processes when using redox-sensitive elements, including Ce and other redox-sensitive elements (e.g. S, Fe, Mn, and Mo), for redox tracing in Earth’s surface environment.