Captions
Table 1 : Methane isotope data for synthesized abiotic methane. Bulk 13C/12C and D/H are reported alongside Δ13CH3D and Δ12CH2D2 . Analytical uncertainties are below 0.1‰ (95% confidence intervals) for δ13C and δD. Propagated uncertainties are listed for Δ13CH3D and Δ12CH2D2 in 1σ. They are highly variable and directly reflect counting statistics, i.e., CH4 quantity.
Figure 1: Quantities of synthesized abiotic methane versus temperature. Data display a negative trend with experiment temperature. See text for details. Note: there are nine samples here, but only seven for isotope measurements. The two highest temperatures did not yield enough methane for mass spectrometry.
Figure 2: δ13C (panel a) and δD (panel b) of synthesized abiotic methane versus temperature and observed methane concentrations. δ13C data display a weak negative trend with experiment temperature. See text for details.
Figure 3: Relationships between the two mass-18 isotopologues of methane in experimental samples and the peak experimental temperatures. Data of other methane synthesized abiotically from Young et al., (2017) are also shown. Data and uncertainties are from Table 1.
Figure 4: The two mass-18 isotopologues of methane for relevant experimental and natural data. The FTT data are from this work. The Sabatier experiments are from Young et al., (2017). Hydrothermal gases are from the Rainbow, Von Damm and Lucky Strike sites from Labidi et al., (2020). The Kidd Creek data are only from the 2.9 km level in the mine, from Young et al., (2017). The Oman data are from various vents in the ophioloite, and the data can be found in Nothaft et al., (2021). Other natural data from various geological settings are from Ash et al., (2019), Giunta et al., (2019, 2021), Gonzalez et al., (2019)., Lin et al., (2023), Warr et al., (2021), Young et al., (2017) and Young (2019).