CLIMATIC AND PALEOBOTANICAL ANALYSIS OF THE EOCENE THERMAL MAXIMUM 2 IN THE BIGHORN BASIN, WYOMING, USA

Eocene Thermal Maximum 2 (ETM2) was a significant hyperthermal event that occurred approximately 53.7 million years ago. Both the carbon isotope excursion and global warming during ETM2 are about half as large as during the Paleocene-Eocene Thermal Maximum (PETM, 56 million years ago). With less forcing of the carbon cycle and climate, we expect floristic change during ETM2 to be less severe than the nearly complete turnover seen during the PETM. Quantification of floral change during this smaller hyperthermal will help reveal how floral change in mid-latitude North America scaled with warming of different magnitudes.

The new site is in the Willwood Fm. (Bighorn Basin, Wyoming), at the 910 m level in the Gilmore Hill stratigraphic section of D’Ambrosia et. al. (2017), which is in the upper part of the carbon isotope excursion associated with ETM2. The fossiliferous mudstone was deposited within an abandoned channel. We examined and tallied over 500 leaves from one quarry and recognized 25 leaf morphotypes, with the two most abundant being Platanites raynoldsii, representing 36% of the flora, and an unnamed fabaceous morphotype, representing 13% of the leaves. During the body of the PETM, Fabaceae are diverse and abundant, whereas taxa like Platanites raynoldsii, which are common in the Western Interior during the Paleocene and Eocene, are absent. Leaf margin analysis yields a mean annual temperature estimate of 13.4 °C, which is warmer than the early Eocene cool period that immediately precedes ETM2 (~11°C), but cooler than the PETM (~20°C) or the late Early Eocene Climatic Optimum.

Comparing the ETM2 site with other early Eocene floral sites in the Bighorn Basin, we find it contains a mixture of stratigraphically long-ranging leaf types as well as a number of leaf types more typical of the PETM or late early Eocene warm period. Less forcing of carbon cycle and climate during ETM2 than the PETM appears to have produced a lower level of floral turnover.