Fig. 2: Gibbs energies, ΔG_r, of the nine methanogenic catabolic pathways considered in this study as a function of sediment depth at the three study sites.

The solid lines represent average ΔG_r values and the shaded envelopes around them refer to the possible range of ΔG_r for that reaction based on a range of substrate concentrations (see Supplementary Table 2). The dashed lines above 50 mbsf indicate methane concentrations were below the detection limit and an activity of 10⁻⁷ was used in the calculations. The vertical dashed black lines refer to ΔG_r = 0. The black blocks indicate a magmatic sill, and the grey blocks represent the sulfate-methane transition zone, SMTZ, at each site. *No ranges for the Gibbs energies of hydrogenotrophic methanogenesis are shown because in situ measurements of CO₂ and H₂ were used rather than a range of substrate concentrations (see methods).