Organic Matter Occurrence and Alteration in the Uranium-Rich Alum Shale Formation: Implications for Microbial Activity

Summary The Early Palaeozoic Alum Shale Formation in northwestern Europe is rich in uranium up to several hundreds of ppm. Despite the absence of higher land plant precursors, the primary Type II kerogen in the Alum Shale has an abnormally strong aromatic character even at low thermal maturities, probably due to α-particle bombardment. FT-ICR-MS data reveal that the macromolecules in the uranium-rich Alum Shale samples are less alkylated than less irradiated counterparts, and that oxygen containing-compounds are enriched in the uranium-rich samples but are not easily degradable into low-molecular-weight products due to irradiation-induced crosslinking. Despite strong organic matter alteration, CO2 and CH4 are formed using U-rich samples in incubation experiments, and DNA-based high-throughput sequencing allows a detailed phylogenetic characterization of the indigenous microbial communities with implications of an uranium-adapted biosphere. The alteration of macromolecular organic matter in the Alum Shale Formation is suggested as an Earth analogue of the Martian surface, in response to α-particle irradiation. Especially a comparison of potential habitats on the Martian surface with Kolm nodules of the Alum Shale in terms of irradiation dosage and organic geochemical characteristics reveals that they have received similar levels of irradiation.