1. Astrobiology Research Center of the NASA Astrobiology Institute and Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA
2. Oyo Corporation, Miyazaki Branch, Oshima-cho, Miyazaki City, 0995-61, Japan

Correspondence to: Hiroshi Ohmoto¹ Email: ohmoto@geosc.psu.edu

Abstract

It is generally thought that, in order to compensate for lower solar flux and maintain liquid oceans on the early Earth, methane must have been an important greenhouse gas before 2.2 billion years (Gyr) ago^{1, 2, 3, 4, 5}. This is based upon a simple thermodynamic calculation that relates the absence of siderite (FeCO₃) in some pre-2.2-Gyr palaeosols to atmospheric CO₂ concentrations that would have been too low to have provided the necessary greenhouse effect¹. Using multi-dimensional thermodynamic analyses and geological evidence, we show here that the absence of siderite in palaeosols does not constrain atmospheric CO₂ concentrations. Siderite is absent in many palaeosols (both pre- and post-2.2-Gyr in age) because the O₂ concentrations and pH conditions in well-aerated soils have favoured the formation of ferric (Fe³⁺)-rich minerals, such as goethite, rather than siderite. Siderite, however, has formed throughout geological history in subsurface environments, such as euxinic seas, where anaerobic organisms created H₂-rich conditions. The abundance of large, massive siderite-rich beds in pre-1.8-Gyr sedimentary sequences and their carbon isotope ratios indicate that the atmospheric CO₂ concentration was more than 100 times greater than today, causing the rain and ocean waters to be more acidic than today. We therefore conclude that CO₂ alone (without a significant contribution from methane) could have provided the necessary greenhouse effect to maintain liquid oceans on the early Earth.

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