The Paleocene Eocene Thermal Maximum in the Hanna Basin, WY: Constraints from Organic Carbon Isotopes and Palynological Data

The P-E boundary, approximately 56 Ma, coincides with a global climatic event, the Paleocene-Eocene Thermal Maximum (PETM). The PETM is believed to have resulted from a 2-8 fold increase in atmospheric pCO2 in less than 10,000 years, which resulted in increased temperatures of 5-8°C globally. The PETM is an event of great interest as it is believed to be the best ancient analogue for modern climate change. This study aims to more precisely pinpoint the stratigraphic location of the PETM in the Hanna Formation in the Hanna Basin, WY. Previous studies have identified the late Paleocene at 2350 meters above the base of the section and the early Eocene as 2800 meters above the base of the section. In order to accomplish this, organic carbon isotopes from carbonaceous shales and coal deposits were measured in order to establish the presence of the characteristic CIE associated with the PETM and the P-E. Palynological samples were also extracted from carbonaceous shales and coals in order to determine the presence of the index pollen, which were used as biostratigraphic markers to determine the exact placement of the P-E boundary within the section. Additionally, pollen abundance and occurrence were determined throughout the section in order to see if the palynological record suggests paleoecological changes associated with warming at the PETM. Results show an approximately -2 / shift in organic carbon isotopic signature between approximately 2600-2650 meters above the base of the Hanna Formation. Platycarya platycaryoides pollen first occurs just down section of the observed CIE in organic carbon, first appearing at 2540 meters above the base of the section. The first occurrence of Platycarya platycaryoides near the observed carbon isotope excursion suggests that the onset of the PETM and the P-E boundary in the Hanna Basin are located between 2540 and 2650 meters. Thus, this study succeeded in more precisely locating the PETM with in the Hanna Formation. Moreover, this study shows that the Hanna Basin records the PETM event over a greater thickness of section with higher stratigraphic resolution than adjacent basins with a disjunct first occurrence of P-E indicator palynomorphs and the characteristic negative carbon isotope excursion of the PETM. Thus the Hanna Basin reveals greater biogeochemical complexity than other adjacent basins and suggests that either local factors such as old heavy carbon erosion, early floral immigration or changing environmental circumstances complicated the local record, or that greater stratigraphic resolution indicates that biotic change and carbon cycle shifts were not coincident. If the latter, then rapid climatic warming may have post-dated major biological perturbations, which has implications for modern global warming.