2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 14
Presentation Time: 11:15 AM


SHELDON, Nathan D., Department of Geology, Royal Holloway Univ of London, Egham, Surrey, TW20 0EX, United Kingdom, n.sheldon@gl.rhul.ac.uk

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.