Cerium isotopic behavior during gabbro weathering in a tropical climate

Abstract

Cerium (Ce) is a redox-sensitive element, making it an effective tracer for continental weathering processes. To elucidate mechanisms of Ce isotopic fractionation during chemical weathering processes, we analyzed Ce isotopic compositions across a well-developed latosol profile derived from gabbros in the Leizhou Peninsula, Zhanjiang, Guangdong Province, Southern China. This weathering profile exhibits significant Ce depletion and redistribution, as evidenced by large variations in Ce concentrations (22.5–209.7 μg/g) and τ Th, Ce values ranging from −36.26 to 170.86%. Latosol samples show a small but detectable δ 142/140 Ce variation from −0.061 to 0.053‰ (mean = 0.003 ± 0.053‰, 2S.D., n = 38), which is indistinguishable from that of fresh gabbro ( δ 142/140 Ce = 0.015 ± 0.046‰, 2S.D.) within analytical uncertainty. The δ 142/140 Ce values lack systematic correlations with TOC, aeolian dust, clay minerals, and Fe-Mn oxides, suggesting limited isotopic fractionation associated with these phases. In contrast, a positive correlation with pH (R 2 = 0.49) identifies pH as a likely key control on Ce isotopic composition in this system. Our analysis shows that Ce 3+ released via mineral dissolution is rapidly oxidized in-situ and immobilized as CeO 2 nanoparticles, while pH governs Ce mobility by regulating the assembly and dissociation of Fe oxide-OM-Ce ternary composites. This pH-dependent oxidation-adsorption coupling effectively suppresses stable Ce isotopic fractionation throughout the weathering profile. These findings clarify the geochemical behavior of Ce in intensely weathered laterites and provide novel constraints on the mechanisms controlling terrestrial Ce fluxes to the oceans.