Reconstruction of atmospheric CO2 during the Oligocene based on leaf fossils from the Ningming Formation in Guangxi, China

Recently, several leaf fossils with good cuticles were discovered from the Oligocene deposits in Guangxi, China. The high fossilization potential of these leaves and easily identifiable stomata and epidermal cells make them an excellent source for CO2 reconstruction. In this study, atmospheric CO2 of the Oligocene is reconstructed using the mechanistic-theoretical model based on stomatal data, photosynthesis and gas exchange control and using an alternative method based only on stomatal ratio. Fossil leaves of Buxus ningmingensis, Chuniophoenix slenderifolius and Cephalotaxus ningmingensis, two angiosperms and one gymnosperm, were considered. It was found that some differences were present between the results obtained by the two methods. The SR-based method yields a somewhat narrower CO2 range (from 355 to 578 ppmv) than the model approach (from 255 to 972 ppmv) which encloses the former range. The CO2 results derived from the three species using the two approaches are overlapped to a high degree with CO2 data from other terrestrial plants and phytoplankton material. More data provided by different proxies are desirable to unravel the interactions between atmospheric CO2 and climate change for the Oligocene.