| Summary: | The fluids at black schist-rich bedrock in the Fennoscandian shield have been shown to carry extensive methane (Kietäväinen and Purkamo 2015, Kietäväinen et al. 2017). The sources of methane, abiotic, microbial, thermogenic, or their mixtures, are not well understood (Etiope and Sherwood Lollar 2013, Douglas et al. 2017). While previous field and laboratory studies have concentrated on oxic degradation of relatively low metamorphic grade black shales (e.g., Matlakowska et al. 2012, Petsch et al. 2005), our goal was to explore the genetic potential of microbial communities in naturally anoxic, oligotrophic and moderately saline bedrock fluids in contact with high-metamorphic grade organic carbon containing black schist. We tested if the microbial metabolisms could explain the extensive methane detected from the fluids at black schist -rich bedrock in the Fennoscandian shield. We aimed to determine the difference between abiotic and biotic methane formation in Palaeoproteorozoic bedrock using novel methane isotopologue measurements and evaluate the ability of natural microbial communities to use black schists as a carbon source in enrichment cultures and compare these to the previously reported cultures. Two study sites, namely the Outokumpu Deep Scientific Drill Hole at depth of 1470 m and Juuka/Miihkali116 overflowing deep drill hole in Finland, were selected for comprehensive geochemical and microbiological sampling. The sampling campaign involved collecting samples for methane isotopologues, intrinsic microbial community, and fluid for inoculation of laboratory microcosms. Ground and sterilized black shists of two different maturities obtained from Finnish bedrock, 13C-labeled graphite, cellulose, acetate and CO 2 were used as different carbon sources for intrinsic deep subsurface fluids, and these microcosms were incubated for 8-20 months. Subsequently, the gas phase of the microcosms was analyzed for CH 4 , CO 2 , N 2 O, O 2 , and N 2 concentrations, as well as isotopic ratios of carbon in CH 4 and CO 2 . ...
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