Archaean palaeosols and Archaean air (reply)

Abstract

Ohmoto et al. reply - The idea of a methane-rich Archaean atmosphere has become popular since Rye et al. assumed in their calculation¹ that siderite was absent in pre-2.2-Gyr palaeosols. We have concluded that the absence of siderite in some Archaean palaeosols does not constrain the atmospheric pCO₂, but the presence of much siderite in sedimentary rocks does². Sleep's recognition³ that siderite occurs in Archaean palaeosols substantiates our arguments²: although siderite should be absent in well aerated soils of all geological ages, it may form in waterlogged soils where pO₂ became less than about 10⁻⁶⁰ atm owing to the abundant anaerobic production of H₂. In fact, we have reported this in a 2.6-Gyr soil profile at Schagen, South Africa⁴: abundant ferric-rich minerals formed while the soil was exposed to air, but ferrous-rich carbonate formed while it was apparently submerged under an anoxic pond.