Vertical redox stratification in a Barremian critical-zone weathering profile revealed by extreme Mo isotope fractionation

Redox transformations within continental weathering profiles exert a first-order control on the redistribution and export of redox-sensitive elements, yet the mechanisms governing element retention versus release in semi-arid critical-zone settings remain poorly quantified. Here we examine a calcrete-capped weathering profile developed on Early Cretaceous andesitic lavas (∼125–122 Ma) in the Chaoyang Basin, North China, to investigate how vertically structured redox conditions influence molybdenum (Mo) behavior during weathering. The ~16 m-thick profile displays a pronounced redox stratification, comprising an oxidized, carbonate-cemented calcrete cap overlying progressively reduced, Fe-rich saprolite above fresh bedrock. Integrated iron speciation, bulk geochemistry, and mineral-scale observations reveal the presence of a shallow Fe-reduction front and a deeper ferruginous zone that acts as a transient sink for trace metals. Molybdenum concentrations increase markedly below the calcrete–saprolite boundary, while bulk-rock and pyrite δ⁹⁸Mo values as low as −3.6‰ indicate sequestration of an isotopically light Mo pool at depth. Mass-balance considerations suggest that enhanced Mo mobilization from the upper oxic zone slightly outweighs retention in the reduced saprolite, resulting in a modest net Mo export from the profile. These observations support a vertically organized, two-stage redox filtering system in which climatic wet–dry cycling promotes Mo release near the surface, whereas deeper ferruginous conditions temporarily retain isotopically light Mo. By modulating both the magnitude and isotopic composition of riverine Mo fluxes, such continental redox architectures provide an important upstream control on marine δ⁹⁸Mo signals used to reconstruct past ocean redox conditions. Our results highlight how redox heterogeneity in the terrestrial critical zone shapes trace-element cycling from land to ocean.