Kah and Riding report the finding of petrographic fabrics in 1.2-billion-year-old carbonate strata that suggest the process of cyanobacterial calcification. When ambient atmospheric partial pressure of CO2 (pCO2) concentrations fall below roughly 10 times present atmospheric levels (PAL), cyanobacteria begin to use a combination of dissolved carbonate species (CO32- and HCO3-) in the photosynthetic production of organic matter. The cyanobacteria, however, must perform a series of biochemical gymnastics to utilize HCO3- in this process. As a byproduct of these biochemical changes, the pH of the microbial sheath rises dramatically and induces the precipitation of calcium carbonate minerals. Identification of these calcified sheaths in the geologic record thus place an upper limit on pCO2. Atmospheric concentrations of CO2 <10 times PAL in the Mesoproterozoic are significantly lower than previous estimates, and indicate the requirement of additional greenhouse gasses, such as methane, in the atmosphere to keep the oceans from freezing.-Geological Society of America
