ESTIMATES OF ATMOSPHERIC CO2 ACROSS THE PETM HYPERTHERMAL USING STOMATAL INDEX VALUES FROM GINKGO ADIANTOIDES AND PLATANITES RAYNOLDSII
Here we describe new estimates of PETM pCO2 made using the stomatal index proxy. We developed a new stomatal index calibration curve from living Ginkgo biloba and applied it to fossils of Ginkgo adiantoides from the late Paleocene and early Eocene of Wyoming, estimating that pCO2 increased from ~330 to ~550 ppm in the 100 ka preceding the CIE. This coincides with an estimated pre-PETM 5°C temperature rise documented in δ18O of mammal tooth enamel and fossil leaves from the Bighorn Basin. Ginkgo adiantoides is absent from the PETM, but returns just after the recovery phase of the CIE, and indicates that pCO2levels returned to ~550 ppm before dropping in the early Eocene.
To make stomatal index-based pCO2 estimates for the PETM, we extracted dispersed cuticle from a plant macrofossil site deposited during the early recovery phase of the CIE. Two morphotypes of dispersed cuticle recovered from the PETM are similar to cuticle removed from late Paleocene macrofossils of Platanites raynoldsii, a member of the sycamore family (Platanaceae). Epidermal features shared by the dispersed cuticle morphotypes and Platanites raynoldsii are typical of modern and fossil Platanaceae. They include a raised oval ledge surrounding a sunken stomatal pore, surface striations oriented perpendicular to the stomata, a relatively uncutinized oval zone around the raised ledge, elongated glandular trichomes on veins, round intercostal trichome bases, and puzzle-piece shaped epidermal cells. The numerous surface striations most resemble those of living Platanus orientalis, a Middle Eastern tree that grows along streams in seasonally dry habitats, conditions similar to those in Wyoming during the PETM. We then compared stomatal index values of the dispersed platanoid cuticle fragments with those of the late Paleocene Platanites raynoldsii to estimate relative change in pCO2 from the latest Paleocene to the early recovery phase of the CIE.