(a) Sulphate and methane concentrations34 in the groundwater. (b) δ13Ccalcite and groundwater δ13CDIC. (c) δ18Ocalcite. Panel c includes a range for hypothetical calcite precipitated from the current groundwater at the same depth where the AOM- or methanogenesis-calcite coatings were collected. Equation 1,000 lnα(Calcite−H20)=18.03(103T−1)−32.42 (ref. 36) is used to calculate the fractionation factor (α) between oxygen in water and calcite at borehole water temperatures of 5–20 °C (hence the range). (d) δ34S values of pyrite in paragenesis with AOM-related crystals, together with pyrite in paragenesis with methanogenesis-related calcite and pyrite without any indicated methane relation31. For the stable isotope analyses, the symbol sizes are larger than the analytical uncertainties. Groundwater data are listed in Supplementary Table 2 and full stable carbon, oxygen and sulphur isotope data for calcite and pyrite in Supplementary Tables 1 and 3.