The cerium isotope fingerprints of redox fluctuation in the regolith

Abstract

Understanding the fractionation mechanisms of the stable isotopes of redox-active metals in terrestrial records is a prerequisite for refining the application of redox reconstruction. We measured the Ce concentration, oxidation state, and isotope composition of bauxites forming on the Columbia River Basalts (CRBs) to evaluate the impacts of redox and non-redox processes on the Ce isotopic variability on Earth’s surface. Bauxites recovered from drill cores show upward increasing Ce concentration (5.4–88.7 μg/g) and overall depletion of Ce (mass transfer coefficient, τCe,Nb: -0.96 to -0.52, i.e., net depletion as much as 52-96% relative to their parent CRBs). There is a wide range of the Ce anomaly (Ce/Ce*) from 0.4 to 6.3 and 0.6 to 1.4 in the Cowlitz and Columbia bauxites, respectively. The most positive Ce anomalies appear at the shallow regolith (2-5 m depth). Cerium primarily presents in its trivalent form in the dust and CRBs, and the downward increase in the fraction of Ce(IV) in the bauxites corresponds with Mn (III/IV) enrichment. Regolith δ142Ce ranges from -0.16±0.03 to -0.02±0.04‰ and -0.28 ±0.03 to -0.05±0.03‰ in the Cowlitz and Columbia profiles, respectively. There are negative Ce isotope shifts from the basaltic parents (CRBs, δ142Ce from -0.08±0.03 to -0.01±0.03‰) in the deep regolith (5 to 9 m), consistent with the enrichment of high-valence Ce and Mn. Regolith Ce isotope records primarily reflect the redox cycle of Ce and may be masked by dust addition on the top. The vertical difference between Ce/Ce* and δ142Ce may be explained by the oxidation-reduction cycle of Mn, leaving discrete CeO2 in the shallow regolith while transferring isotopically light Ce linked to Mn (III/IV) deposits down to the deep regolith. We conclude the combination of Ce isotopes and anomalies in terrestrial records probably records Ce redox fluctuation.