Chromium isotope fractionation during black shale weathering and its environmental implications
Science of The Total Environment(2021)
Abstract
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.
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Key words
Black shales,Cr isotope,Weathering profiles,Redox,Enshi Prefecture
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