Ancient Microbial Activity In Deep Hydraulically Conductive Fracture Zones Within The Forsmark Target Area For Geological Nuclear Waste Disposal, Sweden

Recent studies reveal that organisms from all three domains of lifeArchaea, Bacteria, and even Eukaryacan thrive under energy-poor, dark, and anoxic conditions at large depths in the fractured crystalline continental crust. There is a need for an increased understanding of the processes and lifeforms in this vast realm, for example, regarding the spatiotemporal extent and variability of the different processes in the crust. Here, we present a study that set out to detect signs of ancient microbial life in the Forsmark areathe target area for deep geological nuclear waste disposal in Sweden. Stable isotope compositions were determined with high spatial resolution analyses within mineral coatings, and mineralized remains of putative microorganisms were studied in several deep water-conducting fracture zones (down to 663 m depth), from which hydrochemical and gas data exist. Large isotopic variabilities of C-13(calcite) (-36.2 to +20.2 parts per thousand V-PDB) and S-34(pyrite) (-11.7 to +37.8 parts per thousand V-CDT) disclose discrete periods of methanogenesis, and potentially, anaerobic oxidation of methane and related microbial sulfate reduction at several depth intervals. Dominant calcite-water disequilibrium of O-18 and Sr-87/Sr-86 precludes abundant recent precipitation. Instead, the mineral coatings largely reflect an ancient archive of episodic microbial processes in the fracture system, which, according to our microscale Rb-Sr dating of co-genetic adularia and calcite, date back to the mid-Paleozoic. Potential Quaternary precipitation exists mainly at 400 m depth in one of the boreholes, where mineral-water compositions corresponded.