Precambrian paleosols and atmospheric CO2 levels

Precambrian atmospheric pCO(2) 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 pCO(2) 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 similar to 2.2 Ga old paleosols. The calculated pCO(2) value of 23(divided by 3)(x3) 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 pCO(2) curve is generated, which indicates consistently high pCO(2) from 2.5 to 1.8 Ga ago, and a substantial drop in atmospheric pCO(2) at some time between 1.8 and 1.1 Ga ago. (c) 2006 Elsevier B.V. All rights reserved.