Estimating paleoatmospheric pCO 2 during the Early Eocene Climatic Optimum from stomatal frequency of Ginkgo, Okanagan Highlands, British Columbia, Canada

Abstract

Estimates of pCO 2 for the early Paleogene vary widely, from near modern-day levels to an order of magnitude greater, based on various proxy measures. Resolving the relationship between climate and pCO 2 during this globally warm period is a key task in understanding climate dynamics in a warmer world. Here, we use the stomatal frequency of fossil Ginkgo adiantoides from the Okanagan Highlands of British Columbia, Canada to estimate pCO 2 during the Early Eocene Climatic Optimum (EECO), the interval of peak warmth in the Cenozoic. We also examine a dataset of modern Ginkgo biloba leaves to critically assess the accuracy and precision of stomatal frequency as a proxy indicator of pCO 2. Early Eocene fossil G. adiantoides has significantly lower stomatal frequency than modern G. biloba, suggesting pCO 2 levels > 2× modern pre-industrial values. This result is in contrast to earlier studies using stomatal frequency of Ginkgo that indicated near modern-day levels of pCO 2 in the early Paleogene, though not including samples from the EECO. We also find that levels of pCO 2 as indicated by stomatal frequency are correlated with trends in climate (mean annual temperature) over time at the Falkland fossil locality, suggesting that climate and pCO 2 were coupled during the EECO hyperthermal.