Assessing the carbon dioxide content of ancient atmospheres using palaeocalcretes: theoretical and empirical contraints

Recent suggestions that the δ¹³C composition of calcretes can be used to determine the P(CO₂) of ancient atmospheres is critically evaluated. The problem of differentiating pedogenic calcretes from groundwater ones (which could not be used for P(CO₂) determinations) is considered a critical problem. A basic tenet of the use of calcretes for assessing P(CO₂) is that plants using C₄ or cras-sulacean acid metabolism photosynthetic pathways were unimportant until the Miocene. This view is questioned, and evidence is presented from Carboniferous calcified root mats that non-C₃ plants were probably present at this time, confirming speculations by palaeobotanists that crassulacean acid metabolism or C₄ photosynthesis may have arisen numerous times during the evolution of higher plants.