USING PALEOSOL CARBONATE TO LOCATE THE EARLY EOCENE H2 HYPERTHERMAL IN A MCCULLOUGH PEAKS FOSSIL LOCALITY, BIGHORN BASIN, WYOMING

Evidence for extreme global warming events (hyperthermals) are recorded in the fossiliferous early Paleogene sedimentary rocks of the Bighorn Basin in Wyoming. The PETM (~56 mya) was the most extreme event, followed ~2 million years later by ETM2 and H2. These hyperthermals have been located within the Willwood Formation of the McCullough Peaks region of the basin by identifying carbon isotope excursions (CIEs) recorded in paleosol carbonate nodules. Previous research on mammal fossils associated with the PETM and ETM2 has suggested that there is a positive relationship between the magnitude of these CIEs and the percentage of body size decrease in early equids. The 2.75‰ H2 CIE has been identified via paleosol carbonates in the Bighorn Basin, although not in locations ideal for collecting fossils. This study reports on the stratigraphic location of H2 so that future research may verify the apparent empirical relationship between the magnitude of warming events and mammal “dwarfing”.

In the Willwood Formation of the McCullough Peaks, H2 is located approximately 20 meters higher stratigraphically than ETM2. A distinct marker bed stratigraphically-contained within the ETM2 hyperthermal was traced approximately 8 km from a previously-established section to a productive fossil-collecting locality containing over 20 meters of strata above the ETM2 marker bed. Paleosol carbonate nodules that range in size from 3 mm to25 mm were collected at this new location along a 20-meter section at a resolution of ~40-100 cm. Micrite from the centers of 90 nodules was analyzed for d¹³C. An analysis of the d¹³C of paleosol carbonate nodules will be reported and used to constrain the location of H2.