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.

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