PRECAMBRIAN PALEOSOLS AND ATMOSPHERIC CO₂

Precambrian atmospheric pCO₂ levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols. In addition to theoretical flaws with that model, recalculation of pCO₂ levels using more recent thermodynamic data gives significantly lower values. A new model based on the mass balance of weathering in paleosols gives consistent results from three separate ~2.2 Ga old paleosols. The calculated pCO₂ value of 24 ×/÷ 3 times present atmospheric levels is insufficient to overcome the “faint young Sun paradox,” suggesting that another greenhouse gas such as CH₄ was present at elevated levels relative to the present atmosphere to account for the discrepancy. Model results also suggest that recent atmospheric pCO₂ estimates based on massive siderite beds may be overly high unless Precambrian paleosols formed exceptionally rapidly. The mass balance approach is then extended to other Precambrian paleosols to compile a simple curve of atmospheric pCO₂ through time.