Filamentous and shrub-like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatollites of the ca. 1200 Ma Society Cliffs Formation, Baffin and Bylot Islands, Arctic Canada, are 400 m.y. older than previously reported examples. In vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms (CCMs) and is a direct ecophysiollogical link to atmospheric CO 2 concentration. CCMs are induced in present-day cyanobacteria under experimental conditions when pCO(2) is below similar to 0.36% (similar to 10 times present atmospheric level, PAL). Society Cliffs calcified cyanobacteria consequently imply pCO(2) levels of <0.36% at ca. 1200 Ma. This inference is consistent with marine carbon isotope modeling that suggests pCO(2) of 7-10 PAL in the late Mesoproterozoic. Combined, petrographic, experimental, and modeling results therefore suggest that Mesoproterozoic pCO(2) concentrations were not substantially different from Phanerozoic values and were significantly less than previous estimates of up to 200 PAL. Assuming 10% lower solar luminosity in the late Mesoproterozoic, pCO(2) levels of 10 PAL or less require the presence of additional greenhouse gases for maintenance of an ice-free Earth. At 10 PAL pCO(2), methane concentrations of 100-200 ppm would have been sufficient to sustain warm Earth surface conditions. The low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of Earth's Proterozoic biosphere.