Precambrian paleosols and atmospheric CO2 levels

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 23 ÷ 3 × 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. Applying this new approach to other Precambrian paleosols a Proterozoic pCO2 curve is generated, which indicates consistently high pCO2 from 2.5 to 1.8 Ga ago, and a substantial drop in atmospheric pCO2 at some time between 1.8 and 1.1 Ga ago.