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Coupled, one-dimensional transport equations for CO2 and O2 in pre-Middle Ordovician soils have been developed. The value of the ratio ΔPO2 /ΔPCO2 , where ΔPO2 is the difference between the O2 pressure in the atmosphere and at any level in a particular soil profile, and where ΔPCO2 is the difference between the CO2 pressure in the atmosphere and at the same level in the same soil profile, depends on the relative importance of O2 and CO2 transport by diffusion and transport by advection in soils.

The transport equations were solved in closed form for a particularly simple soil model. Numerical solutions were obtained for more complex soil profiles, and the sensitivity of the PO2 profile in soils to changes in a variety of parameters was explored. The results indicate that free oxygen has been present in the atmosphere at least since early Proterozoic time. The range of atmospheric oxygen pressure permitted by the available paleosol data is large. Our currently preferred range of the oxygen pressure in the atmosphere between ca. 2.5 and 1.8 Ga is between 5 × 10−4 and 1 × 10−3 atm.

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