Cadmium isotopic evidence for the redistribution and fate of cadmium during carbonate weathering

Abstract

The weathering of carbonate rocks releases considerable amounts of cadmium (Cd) thus posing potential risks to the ecosystem and human health. The biogeochemical cycling of Cd during weathering processes is poorly understood because of the absence of efficient tracers for Cd biogeochemical processes. In this study, the Cd isotope technique was applied to elucidate the redistribution and fate of Cd during carbonate weathering. The results showed that the weathering of Cd-rich limestone resulted in elevated Cd contents in soils (2.63–22.7 mg/kg) and plants (0.20–1.16 mg/kg). The chemical fractions of Cd in the overlying soils were dominated by Fe and Mn oxides, and organically-bound Cd increased with increasing organic matter content. The heavier Cd isotopic composition of bedrock (δ^114/110Cd: 0.38 ‰) than that of soil (0.23 ‰) demonstrated the loss of Cd during carbonate weathering. Minor isotope fractionation (∆^114/110Cd < 0.20 ‰) observed in the paddy soil profiles coincides with the limited migration and redistribution of Cd during the development of paddy soil. Additionally, the retention of Cd from weathering fluid by Fe/Mn oxides and organic matter contributes to the enrichment of heavy Cd isotopes in soils. Furthermore, the positive correlation between δ^114/110Cd values and soil organic matter contents indicates the addition of Cd from the litter (δ^114/110Cd: 0.30–0.94 ‰). These results shed light on the response of Cd isotope signals to biogeochemical processes and the cycling of Cd during weathering processes.