The cerium isotope fingerprints of redox fluctuation in bauxites

Abstract

We measured the Ce concentration, oxidation state, and isotope composition of bauxites developed 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 of 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 deposited exogenous materials (wind-blown dust from an old, weathered region of the continent) and primary CRBs. There is a downward increase in the fraction of Ce (IV) in the regolith. Bauxite δ142Ce ranges from -0.161±0.034 to -0.018±0.043‰ and -0.277±0.034 to -0.046±0.034‰ in the Cowlitz and Columbia profiles, respectively. The Ce isotope record reflects the redox cycle of Ce and Mn and is masked by dust accretion on the top. The enrichment of Ce (IV) occurs in the transition zone (the bottom of the shallow regolith) with minor Ce isotope shifts from the CRBs (δ142Ce from -0.080±0.034 to -0.04±0.034‰) likely caused by minor isotopic fractionation linked to O2-driven CeO2 precipitation. There are negative Ce isotope shifts from the basaltic parents and the enrichment of oxidized Ce and Mn in the deep regolith (5-9 m depth), where large isotopic fractionation probably induced by Ce oxidative adsorption on MnO2 plays a vital role. The vertical difference between Ce/Ce⁎ and δ142Ce may be attributed to the climatic oxidation-reduction cycle of Mn and Ce, leaving discrete CeO2 grains in the shallow regolith while transferring isotopically light Ce associated to oxidized Mn deposits down to the deep regolith. We conclude that a combination of Ce species isotopes and anomalies potentially records terrestrial redox status and fluctuation.