Atmospheric carbon dioxide concentrations before 2.2 billion years ago

Abstract

THE composition of the Earth's early atmosphere is a subject of continuing debate^1–4. In particular, it has been suggested that elevated concentrations of atmospheric carbon dioxide would have been necessary to maintain normal surface temperatures in the face of lower solar luminosity in early Earth history^5,6. Fossil weathering profiles, known as palaeosols, have provided semi-quantitative constraints on atmospheric oxygen partial pressure (po₂) before 2.2 Gyr ago^36,37. Here we use the same well studied palaeosols to constrain atmospheric pco₂ between 2.75 and 2.2 Gyr ago. The observation that iron lost from the tops of these profiles was reprecipitated lower down as iron silicate minerals^7–9, rather than as iron carbonate, indicates that atmospheric pco₂ must have been less than 10^–1.4atm—about 100 times today's level of 360 p.p.m., and at least five times lower than that required in one-dimensional climate models to compensate for lower solar luminosity at 2.75 Gyr. Our results suggest that either the Earth's early climate was much more sensitive to increases in pco₂ than has been thought, or that one or more greenhouse gases other than CO₂ contributed significantly to the atmosphere's radiative balance during the late Archaean and early Proterozoic eons.