ATMOSPHERIC CARBON-DIOXIDE CONCENTRATIONS BEFORE 2.2-BILLION YEARS AGO

THE composition of the Earth's early atmosphere is a subject of contin uing debate(1-4). In particular, it has been suggested that elevated c oncentrations of atmospheric carbon dioxide would have been necessary to maintain normal surface temperatures in the face of lower solar lum inosity in early Earth history(5,6). Fossil weathering profiles, known as palaeosols, have provided semiquantitative constraints on atmosphe ric oxygen partial pressure (pO(2)) before 2.2 Gyr ago(36,37). Here we use the same well studied palaeosols to constrain atmospheric p(CO2) 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 atmospher ic p(CO2) must have been less than 10(-1.4) atm-about 100 times today' s level of 360 p.p.m., and at least five times lower than that require d in one-dimensional climate models to compensate for lower solar lumi nosity at 2.75 Gyr. Our results suggest that either the Earth's early climate was much more sensitive to increases in p(CO2) than has been t hought, or that one or more greenhouse gases other than CO2 contribute d significantly to the atmosphere's radiative balance during the late Archaean and early Proterozoic eons.