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. 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. Fossil weathering profiles, known as palaeosols, have provided semi-quantitative constraints on atmospheric oxygen partial pressure (pO2) before 2.2 Gyr ago. 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, rather than as iron carbonate, indicates that atmospheric pCO2 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 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.

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“…The presence of siderite in the nakhlites and ALH84001 suggests high pCO 2 conditions. This line of argument follows the same geochemical constraint used in determining pCO 2 of the early Earth: for example, pCO 2 in the late Archaean is deduced as <50 mbar based on the absence of siderite and the presence of hydrous iron silicates in paleosols (Rye et al, 1995). Essentially, in the expected presence of dissolved silica, siderite tends to form at pCO 2 >50 − 100 mbar, whereas hydrous iron silicates will form at lower pCO 2 (Catling, 1999).…”

Palaeoclimates: the first two billion years

“…The solar constant at that time was approximately 80% of its present value (Gough 1981). The two dashed curves represent the freezing point of water and an upper limit on P CO 2 derived from palaeosols (Rye et al 1995). If the palaeosol data are accepted as a constraint, solutions must lie in the upper left-hand corner of this diagram.…”

Low HesperianPCO2constrained from in situ mineralogical analysis at Gale Crater, Mars

“…Estimates of P CO2 on the Archean (>2.5 Ga) Earth, derived from paleosols and banded iron formations, also use the thermodynamic stability of clay minerals and magnetite (17)(18)(19), but are criticized because they rely on assumptions of mineral equilibrium and do not account for enhanced delivery of carbon and reducing power to sediments via biological activity (20,21). Metamorphic overprinting of original mineral assemblages in terrestrial rocks this old is ubiquitous and complicates interpretations further (22).…”