Extended Data Fig. 6: Carbon isotopic composition of the Olla samples as a function of CO₂ concentration for data from this study (dark blue circles) and those collected without ³He analysis¹⁰ (unfilled circles).

One standard deviation error bars are within symbol size. Dashed lines show endmember methanogenesis and dissolution fractionation (at pH 7) trajectories. Tick marks represent the total amount of CO₂ trapping within the system, relative to sample O5. The shaded region represents trapping by the combination of both microbial methanogenesis and dissolution. The same combination of dissolution and microbial methanogenesis has been modelled (grey region, cf. Fig. 2). Lines labelled ‘consumption’ at 5%, 13% and 19 % show the proportion of original CO₂ that has been removed by net microbial methanogenesis. One sample from ref. ¹⁰ has a CO₂ concentration greater than our ‘most pristine’ CO₂/³He sample (O5). This illustrates that by using sample O5 as our least altered composition we calculate a conservative (minimum) amount of CO₂ consumption, and that initial CO₂ concentrations were probably greater.

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