CARBON-DIOXIDE IN THE ATMOSPHERE - EVIDENCE FROM CENOZOIC AND MESOZOIC PALEOSOLS

A diffusion-reaction model for the isotopic composition of soil CO2 and soil carbonate is evaluated. It shows which variables are important under different conditions and shows that under certain circumstances the carbon isotopic composition of soil carbonate can be used to estimate P(CO2) of the atmosphere from late Paleozoic to the present. The isotopic compositon of soil carbonate produced under uniform conditions is essentially constant below about 20 cm in most soils. Today, the isotopic composition of soil organic matter, which is most important factor in determining the carbon isotopic composition of soil carbonate. However, prior to the advent of C4 plants making up a significant fraction of the biomass, probably in the Tertiary, the concentration of CO2 in the atmosphere can be estimated because of mixing of atmospheric CO2 with CO2 produced in the soil. The diffusion-reaction model also suggests that before the advent of higher land plants most soil carbonate should have delta C-13 values that are strongly influenced by the atmosphere due to the shallow rooting depth of non-vascular plants. Preliminary results reported here suggest that P(CO2) was less than about 700 ppmV during the Eocene and since late Miocene because the limiting carbon isotopic composition of soil carbonate was between -11.5 to -12.5 permil relative to PDB. However, early Cretaceous and early Jurassic paleosols have delta-C-13 values between -6.5 and -8.5 permil suggesting that P(CO2) was significantly higher than today, probably on the order of 1500 to 3000 ppmV.