Emissions of Halomethanes from Bioweathered Lopingian Kupferschiefer Black Shale (Fore-Sudetic Monocline, SW Poland)

Halogenated methane derivatives (halomethanes, HMs) are significant atmospheric sources of halogens, contributing to ozone depletion and radiative forcing. Although both anthropogenic and natural sources are known, the natural production of HMs appears to be underestimated. Studies suggest the existence of a major unidentified natural source. This study investigates whether highly mineralized fossil organic matter (FOM) deposits can be a source of HM emission. Kerogen, the main component of FOM, represents the largest organic carbon reservoir on the Earth. The bioweathered Lopingian Kupferschiefer black shale (BS) in southwestern Poland, rich in both FOM and Fe-bearing minerals, serves as a research material. Using gas chromatography with a mass and electron capture detector, we detected emissions of mono-, di-, tri-, and tetrahalomethanes containing both bromine and chlorine. Based on geochemical properties of the bioweathered BS, we propose a hypothetical model of the abiotic formation of HMs catalyzed by Fe³⁺, involving: (1) organic precursors derived from bioweathered FOM; (2) Fe³⁺ from oxidized minerals; and (3) halides from mine waters and secondary minerals. Our investigation reveals a previously unrecognized natural source of halomethanes in the deep subsurface and, for the first time, indicates the potential emission of tetrachloromethane in a natural setting.