Chromium isotope fractionation during black shale weathering and its environmental implications

Black shale contains abundant pyrite and organic matter that are susceptible to weathering when exposed to the air. In the process of weathering, acid mine drainage can be produced, and a range of toxic trace elements including Cr can be released and transported into rivers, groundwater and soils, potentially leading to severe environmental pollution. In order to study Cr migration and Cr isotopic fractionation during black shale weathering, we sampled metalliferous black shales and cherts from two weathering profiles at Shadi and Yutangba from the Permian Maokou Formation in Enshi Prefecture. The unweathered samples in Shadi and Yutangba have high Cr contents (1562 μg/g and 643 μg/g, respectively), and highly fractionated Cr isotopic compositions (2.04 ± 0.11‰ and 1.91 ± 0.09‰, respectively, expressed as δ⁵³Cr). The narrow range of authigenic δ⁵³Cr in these unweathered shales suggests that the δ⁵³Cr value of the seawater was relatively stable during the period of deposition. Strongly weathered shales in Shadi and Yutangba both display significant Cr losses compared to the unweathered counterparts. Their average δ⁵³Cr values are 1.75 ± 0.12‰ and 1.85 ± 0.39‰ for Shadi and Yutangba, respectively, which are isotopically lighter than fresh samples. This indicates that heavier Cr isotopes are preferentially leached into fluids, leaving the residues enriched in lighter isotopes during black shale weathering. However, the δ⁵³Cr values of the samples close to the water table are higher than those of the unaltered ones, which can be explained by adsorption or quantitative reduction of Cr(VI) near the water table. The fact that Cr isotopes are fractionated during black shale weathering may complicate the application of δ⁵³Cr in polluted samples to identify the Cr sources in areas with exposed black shales. The δ⁵³Cr of seepage water can be measured and treated as a more realistic source signal.