Palaeoclimatology: Archaean palaeosols and Archaean air

Ferrous carbonate, as the mineral siderite, occurs in Archaean palaeosols (ancient soils). Ohmoto et al. contend that siderite was not in equilibrium with the oxygen in Archaean air and that its presence in palaeosols provides little constraint on the partial pressure of carbon dioxide in Archaean air. But their argument is invalid because it fails to distinguish the different behaviours of the trivial component oxygen and the significant component carbon dioxide in the partly closed system of soil waters. The presence or absence of siderite in ancient soils is a valid constraint on the carbon dioxide partial pressure (pCO2) in ancient atmospheres.