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

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Abstract
Recent suggestions that the δ 13 C composition of calcretes can be used to determine the P (CO 2 ) of ancient atmospheres is critically evaluated. The problem of differentiating pedogenic calcretes from groundwater ones (which could not be used for P (CO 2 ) determinations) is considered a critical problem. A basic tenet of the use of calcretes for assessing P(CO 2 ) is that plants using C 4 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-C 3 plants were probably present at this time, confirming speculations by palaeobotanists that crassulacean acid metabolism or C 4 photosynthesis may have arisen numerous times during the evolution of higher plants.