VEGETATION AND LITHOLOGY RESPONSE TO THE PALEOCENE-EOCENE THERMAL MAXIMUM IN THE HANNA BASIN, WY

The Paleocene-Eocene Thermal Maximum (PETM), a rapid global warming event ~56 Ma caused by a large release of isotopically-light carbon into earth’s atmosphere and oceans, provides geologic parallels to anthropogenic climate change. This study compares lithology and reconstructions of vegetation structure of three ancient fluvial landscapes before, during, and after the PETM in the Hanna Basin, southeastern Wyoming, U.S.A. During the ~200 kyr PETM warming event, temperatures increased by 4-8ºC and atmospheric CO₂ levels likely significantly increased. Studies on vegetation change in the Bighorn Basin of Wyoming show high degree of taxonomic turnover during the PETM; however, the Hanna Basin serves as an important comparison record of the vegetation and fluvial change in a generally humid environment.

This study focuses on vegetation changes within channel and overbank depositional sequences. At each site, vegetation structure was assessed using reconstructed Leaf Area Index (LAI) measurements, which quantifies vegetation structure [LAI = foliage area (m²)/ground area (m²)]. We reconstructed LAI using the morphology of fossil leaf epidermal cells preserved as cuticles dispersed in organic-rich strata. Cuticle assemblages containing cells with higher aspect ratios and more undulatory cell walls on average indicate more densely vegetated forests, while more circular, less undulated cell assemblages represent more open environments. Modern LAI values generally fall between 0 (fully open landscape) to 6 (dense canopied forest). LAI values from predominately mudstone lithologies of Pre-PETM strata ranged from 3.4 to 4.9, with an average of 4.22 (+/- 2.0). During the PETM, LAI values ranged from 1.2 to 5.5, with an average of 3.5 (+/-1.9). Post-PETM, in dominantly coal and shale lithologies, LAI values ranged from 4.1 to 7.5, with an average of 5.5 (+/- 2.0). These results overall demonstrate that dense forested environments existed in the late Paleocene, then became more open during the PETM but fluctuated rapidly. Post-PETM, early Eocene sites show the densest canopies. The variation in LAI values within our dataset demonstrates that the PETM had a significant impact on terrestrial landscapes.