INCREASED ATMOSPHERIC CARBON DIOXIDE SURROUNDING THE PALEOCENE-EOCENE THERMAL MAXIMUM FROM STOMATAL INDEX OF GINKGO ADIANTOIDES, BIGHORN BASIN, WYOMING, USA

The Paleocene-Eocene Thermal Maximum (PETM) was a brief interval of intense global warming 56 mya. The interval is marked by a 5-8 °C temperature increase within 20ka, a ~4-6‰ negative carbon isotope excursion (CIE), and widespread marine carbonate dissolution. Together, these are strong evidence for a massive addition of isotopically light carbon into the exogenic carbon cycle. Despite agreement that the PETM reflects a large carbon cycle perturbation, atmospheric CO₂ concentrations (pCO₂) are poorly constrained. In this study we reconstructed Paleocene-Eocene pCO₂ using the stomatal index proxy on leaf fossils of Ginkgo adiantoides from the Bighorn Basin, Wyoming.

Multiple studies have examined the relationship between pCO₂ and stomatal index for extant Ginkgo biloba. Though widely used, these previous calibrations had sparse data or were improperly constructed. We collected new data on stomatal index from herbarium specimens and living trees of Ginkgo biloba that grew under a range of pCO₂ from 290-429 ppm. All specimens were macerated using Cr(VI)O₃ and the internal view of the lower epidermis was imaged by environmental SEM. The new data show a significant decline in stomatal index with increasing pCO₂, but with a shallower slope than previously reported. Applying the new calibration curve to fossils of the nearly identical Ginkgo adiantoides, we reconstruct pCO₂ between 300-350 ppm during a 500ka period of the late Paleocene. However, in the 150ka preceding the CIE, pCO₂ rose quickly to as much as 750 ppm, remaining at least 660 ppm immediately after the recovery from the CIE. Inferred pCO₂ returned to ~300 ppm during the earliest post-PETM (biozone Wa1), but rose again in the early Eocene. No Ginkgo fossils were found in the PETM.

The doubling of pCO₂ in the latest Paleocene coincides with a ~5°C temperature increase inferred from δ¹⁸O in mammalian tooth enamel and fossil leaves; all data sets were generated from fossils in the Bighorn Basin. Our pCO₂ data derived from Ginkgo is consistent with previous estimates of full Earth-system temperature sensitivity to pCO₂ doubling. Relatively enriched CO₂ from submarine volcanism could have warmed latest Paleocene climate without changing the isotopic composition of the exogenic carbon pool, triggering the release of depleted carbon reservoirs at the onset of the PETM.