FOSSIL ATMOSPHERES: AN ELEVATED CO2 EXPERIMENT UTILIZING OPEN-TOP CHAMBERS TO EVALUATE THE GINKGO PALEO-CO2 BAROMETER

The Earth’s climate during the Mesozoic and early Cenozoic was much warmer than today, and was often punctuated by hyperthermal events. The background warmth and the rapid hyperthermals are often attributed to increased atmospheric carbon dioxide (pCO₂), yet paleo-pCO₂ proxy estimates for this time interval disagree widely. Given the wide range of pCO₂ estimates for the same intervals, it is clear that not all of the proxy estimates can be correct. Over the past decade, many of the marine and terrestrial proxies for pCO₂ have been revised by evaluating the underlying assumptions for each proxy. We are evaluating the assumptions that govern the Ginkgo paleo-pCO₂ proxy that depends upon stomatal index, a measure of the proportion of epidermal cells on leaves that are stomatal pores. Historical collections of Ginkgo biloba demonstrate that the stomatal index proxy for paleo-pCO₂ is strongly correlated with pCO₂ over the range of 290-430 ppm. However, despite wide application of the Ginkgo paleo-pCO₂ barometer, our understanding of pCO₂ in the fossil record has been hindered because the morphological and physiological changes in G. biloba stomata under pCO₂ above 400 ppm have been poorly constrained.

To investigate the relationship of Ginkgo to elevated pCO₂ conditions, we are conducting an elevated pCO₂ experiment we call ‘Fossil Atmospheres’. The experiment is designed to quantify the response of Ginkgo to elevated pCO₂ by growing 15 mature Ginkgo biloba trees in open-topped chambers in natural field conditions, under atmospheres with ambient (400), 600, 800, and 1000 ppm of CO₂. Each tree is regularly monitored for changes in stomatal index, and rates of photosynthesis and transpiration to constrain parameters used in gas exchange models of paleo-pCO₂. We have also engaged citizen scientists to help collect stomatal index measurements on the Zooniverse platform, using the interaction to educate citizens about modern climate change from the less-menacing viewpoint of deep-time climate change events. Our experimental results will be used to infer paleo-pCO₂ from stomatal features of Late Cretaceous-Paleogene fossils of the nearly identical species, G. wyomingensis, allowing for paleo-pCO₂ estimates from these terrestrial fossils to be compared with records from other paleo-pCO₂ proxies.