Calcium isotope fractionation during weathering of argillaceous carbonates in a humid climate

The continental weathering is a key process that controls calcium (Ca) transportation from the continental crust to the waters. To elucidate the behavior of Ca isotopes during carbonate weathering, the concentrations and δ^44/40Ca (relative to NIST SRM 915a) of bulk saprolites, exchangeable, acid-leachable and residual phases of a weathering profile developed on the marine carbonates, Guangdong province, South China, were investigated. Upwards the profile, δ^44/40Ca values of the bulk saprolites systematically decrease from 0.77 ± 0.12 ‰ to -0.44 ± 0.12 ‰, suggesting that significant Ca isotope fractionation occurred during chemical weathering. The exchangeable fractions have δ^44/40Ca values higher than those of the bulk saprolites with Δ^44/40Ca_{exchangeable-saprolite} varying from -0.01 ‰ to 0.73 ‰, suggesting that heavy isotopes are preferentially adsorbed onto the clays. The acid-leachable phases display a relatively narrow δ^44/40Ca range from 0.52 ‰ to 0.74 ‰ with Ca fractions varying from 7.4 % to 100.3 %, potentially indicating that limited Ca isotopic fractionation occurs during the dissolution of primary carbonates. The residual Ca pool is strongly fractionated with δ^44/40Ca ranging from 0.64 ± 0.08 ‰ to -0.98 ± 0.02 ‰, systematically lower than their bulk saprolites, perhaps indicating light Ca isotopes are preferentially incorporated into the clay lattices. Altogether, it seems that the Ca isotopic fractionation directions are opposite between clay structural incorporation and adsorption. Our study provides important insight of Ca behavior and Ca isotopic fractionation during chemical weathering, which is critical to shape Ca isotopic compositions of the upper continental crust and trace the global biogeochemical cycle of Ca.