Isotopic signals in fracture-filling calcite showing anaerobic oxidation of methane in a granitic basement

Understanding the long-term redox conditions and related carbon cycle in groundwater is essential for safety assessment because they affect performance of barrier systems radionuclide transport geological disposal. However, it difficult to identify those changes directly. To help understand this, we conducted a paleohydrogeological study on calcite mineralization associated with fracture-controlled flow-paths Toki Granite central Japan, focusing its oxygen stable isotope characteristics. Previous studies revealed four generations fracture-filling Granite. Therefore, isotopic analysis both bulk samples spatially-resolved microsamples discrete within zoned crystals. The δ18OVPDB ranging between −32.7‰ −0.59‰ that precipitated was derived from various origins over history area, including early hydrothermal fluids late-stage cooling granite (less than −17.2‰); freshwater invasion surface following regional uplift (−18.5‰∼ −8.3‰), and; seawater penetrated during periods marine transgression (−8.7‰ ∼ −0.3‰). range δ13CVPDB values (−56.5‰ +6.0‰) wider dissolved inorganic (DIC) originated hydrothermal, meteoric, sources (−25‰ +2‰). Calcite low less −25‰ believed have DIC provided by anaerobic oxidation methane (AOM), whereas higher +2‰ containing 13C-enriched as source methanogenesis. These processes influencing formation are comparable at other crystalline rock sites European countries. AOM methanogenesis transition origin meteoric seawater. these granitic can provide important insights into relevant assessing evolution geoenvironmental systems.