Extended Data Fig. 2: Representative background CO₂ evolutionary scenarios adopted in this study.

These scenarios are selected from the evolutionary tracks of CO₂ derived in Hu et al. (2015)¹⁶ and they are consistent with the present-day pressure and carbon isotopic composition. Scenarios 1 - 3 assume that the photochemical loss rate of carbon depends on the Sun’s Lyman continuum flux to the power of 2. Scenario 1 assumes that carbonate deposition of 40 mbar occurred throughout the Noachian and Hesperian (that is, till 3.0 Ga) in shallow subsurface aquifers. This represents the lower bound of the initial CO₂ partial pressure. Scenarios 2 and 3 assume that carbonate deposition of 290 and 600 mbar occurred in the Noachian and early Hesperian (that is, till 3.5 Ga) in open-water systems. Scenario 3 has an initial CO₂ partial pressure of 1 bar and is the default scenario adopted in this study. Scenario 4 assumes a power-law index of 3, and that carbonate deposition of 1400 mbar occurred in the Noachian and early Hesperian (that is, till 3.5 Ga) in open-water systems. This represents the upper bound of the initial CO₂ partial pressure. Scenario 5 is an endmember scenario where the CO₂ atmosphere is assumed to be collapsed at all times and the pressure constant at 7 mbar.