ESTIMATES OF ATMOSPHERIC CO2 ACROSS THE PETM HYPERTHERMAL USING STOMATAL INDEX VALUES FROM GINKGO ADIANTOIDES AND PLATANITES RAYNOLDSII

The Paleocene-Eocene Thermal Maximum (PETM) was a geologically brief interval 56 million years ago in which global temperature increased rapidly by 5-8 °C, were accompanied by widespread marine carbonate dissolution, and a ~4-6‰ negative carbon isotope excursion (CIE). Though the PETM clearly reflects a massive carbon release, atmospheric concentrations of CO₂ (pCO₂) during the event are loosely constrained.

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.