142Ce minus 146Nd value as a redox indicator in Earth's surface environments

Stable Ce isotope ratios are considered potential indicators for tracing the redox state of Earth's surface; however, their quantitative relationship remains undefined. Here, we investigated the influences of both redox and nonredox processes on stable Ce isotope fractionation in a terrestrial environment by analyzing the weathering products, selective soil chemical extracts, and parent materials of a typical basaltic profile formed in Hainan Island, South China. During redox processes, the preferential enrichment of the heavier 142Ce in the tetravalent Ce and/or the inheritance of the heavier isotope signature from the soluble led to the crystalline Fe-Mn (hydro) oxide phases having the highest delta 142/140Ce values (up to +0.184 +/- 0.040 parts per thousand). This phenomenon is likely governed by the oxidative adsorption of Mn oxides, while Fe -oxides appear to be less important. In addition, distinct Mn concentrations are important in the tau Th,Ce and delta 142/140Ce values but did not correlate with Ce anomalies, indicating that the Mn (hydro)oxides delta 142/140Ce values may be more reliable than Ce/Ce* in quantitatively tracing redox conditions. Regarding non-redox processes, we discussed the impact mechanisms, including external input, pH, organic matter complexation, adsorption/co-precipitation of the poorly crystalline Fe-Mn oxide, and the formation of clay minerals on stable Ce isotopic fractionation. Results indicated that Fe-Mn-Al oxides preferentially enriched the heavier 142Ce, while the formation of kaolinite preferentially incorporated the lighter 140Ce. However, the impact of external input, pH, organic matter complexation, and clay mineral adsorption on stable Ce isotopes appeared to be limited. Furthermore, delta 142/140Ce values displayed a positive correlation with delta 146/144Nd in both the 0.5 mol L-1 HCl leachates and residual phases, suggesting that stable Ce and Nd isotopes behave similarly during non-redox processes. To isolate the signal of stable Ce isotopes in response to redox conditions, we proposed that delta 142/140Ce minus delta 146/144Nd could serve as a quantitative proxy for tracing redox fluctuations in terrestrial environments.