Estimating latest cretaceous and tertiary atmospheric carbon dioxide concentration from stomatal indices

The role of atmospheric CO₂ in determining climate is important for understanding patterns in the geologic record and predicting future climate. Because of the recent anthropogenic rise in temperatures, the role of CO ₂ in globally warm periods is particularly important for any predictions of future climate change. Here, I reconstruct the concentration of atmospheric CO₂ using patterns of stomatal index in the plant cuticles of Ginkgo and Metasequoia for two globally warm intervals, the latest Cretaceous to early Eocene (66–53 Ma) and the middle Miocene (17–15 Ma).; Most vascular C₃ plants show an inverse relationship between the stomatal index in their leaves and the level of atmospheric CO₂. Stomatal index (SI) has been shown experimentally and in the field to be largely independent of every environmental factor (e.g., water stress) except CO ₂. There is a potential, then, to use patterns in SI as a proxy for ancient levels of atmospheric CO₂. One drawback to SI-CO₂ relationships is that they are generally species-specific. Modern Ginkgo and Metasequoia are unusual because both have morphologically identical forms back to the Early and Late Cretaceous, respectively. In addition, at least in the case Ginkgo, its sedimentological and floral associations remain stable throughout the Cenozoic. Thus, both Ginkgo and Metasequoia represent excellent taxa for applying this CO₂ proxy back into the Cretaceous.; Based upon dated herbaria sheets, which document the anthropogenic rise in CO₂, and saplings grown in greenhouses at ambient and elevated CO₂ (350–800 ppmV), the SI-CO₂ relationships for Ginkgo and Metasequoia were determined. These relationships were then applied to fossil cuticles of Ginkgo and Metasequoia from 30 sites in order to reconstruct CO₂. The resulting reconstruction indicates near present-day levels of CO ₂ (300–450 ppmV) for both the early Paleogene and middle Miocene. This suggests that other radiative forcings were more important during these intervals than they are today, and that these intervals may not serve as good analogs for understanding future climate change.