Low pCO2 in the pore water, not in the Archean air

Low pCO2 in the pore water, not in the Archean air

Nature 474, 7349 (2011). doi:10.1038/nature09960

Authors: N. Dauphas & J. F. Kasting

Arising from M. T. Rosing, D. K. Bird, N. H. Sleep & C. J. Bjerrum Nature464, 744–747 (2010)The solar luminosity during the Archean (3.8 to 2.5 billion years ago) was 20–25% lower than at present, and was probably compensated, at least in part, by a stronger greenhouse effect. Rosing et al. estimate the Archean partial pressure of carbon dioxide to have been about ≈ 10−3 bar, on the basis of the simultaneous occurrence of magnetite (Fe3O4) and siderite (FeCO3) in banded iron formations (BIFs, a type of chemical sediment). Here, we question a central assumption by Rosing et al. that the mineralogy of BIFs reflects near-thermodynamic equilibrium with the atmosphere–ocean system; just as the presence of authigenic pyrite in modern sediments does not imply that the modern atmosphere is anoxic, the mineralogy of BIFs cannot be used to argue for a low- Archean atmosphere. Carbon dioxide is therefore still a viable greenhouse gas candidate with which to explain the warm Archean climate.