Improving the Ginkgo CO2 barometer: Implications for the early Cenozoic atmosphere

Stomatal properties of fossil Ginkgo have been used widely to infer the atmospheric concentration of CO2 in the geological past (paleo-pCO(2)). Many of these estimates of paleo-pCO2 have relied on the inverse correlation between pCO(2) and stomatal index (SI the proportion of epidermal cells that are stomata) observed in recent Ginkgo biloba, and therefore depend on the accuracy of this relationship. The SI-pCO(2) relationship in G. biloba has not been well documented, however. Here we present new measurements of SI for leaves of G. biloba that grew under pCO(2) from 290 to 430 ppm. We prepared and imaged all specimens using a consistent procedure and photo-documented each count. As in prior studies, we found a significant inverse relationship between SI and pCO(2), however, the relationship is more linear, has a shallower slope, and a lower correlation coefficient than previously reported. We examined leaves of G. biloba grown under pCO(2) of 1500 ppm, but found they had highly variable SI and a large proportion of malformed stomata. We also measured stomatal dimensions, stomatal density, and the carbon isotope composition of G. biloba leaves in order to test a mechanistic model for inferring pCO(2). This model overestimated observed pCO(2), performing less well than the SI method between 290 and 430 ppm. We used our revised SI-pCO(2) response curve, and new observations of selected fossils, to estimate late Cretaceous and Cenozoic pCO(2) from fossil Ginkgo adiantoides. All but one of the new estimates is below 800 ppm, and together they show little long-term change in pCO(2) or relation to global temperature. The low Paleogene pCO(2) levels indicated by the Ginkgo SI proxy are not consistent with the high pCO(2) inferred by some climate and carbon cycle models. We cannot currently resolve the discrepancy, but greater agreement between proxy data and models may come from a better understanding of the stomatal response of G. biloba to elevated pCO(2), better counts and measurements of fossil Ginkgo, or models that can simulate greenhouse climates at lower pCO(2). (C) 2016 Elsevier B.V. All rights reserved.