POTENTIAL TERRESTRIAL RECORD OF HYPERTHERMAL C24RH7 F, A LOW MAGNITUDE GLOBAL WARMING EVENT, IN THE EARLY EOCENE OF THE BIGHORN BASIN, WYOMING

The early Eocene was punctuated by numerous hyperthermal events, primarily identified as significant negative stable carbon isotope excursions (CIEs). Numerous CIEs are well documented in the marine record, whereas only high amplitude events, such as the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM-2), have been identified in terrestrial sedimentary basins, limiting chronostratigraphic correlation and our understanding of paleoclimate change of various magnitudes. Marine carbon isotope records indicate the presence of low amplitude CIEs, stratigraphically between the PETM and ETM-2 and contemporaneous with the early Wasatchian (Wa) North American Land Mammal Age, an interval of time preserved in the Willwood Formation of the Bighorn Basin. A low-magnitude CIE was previously detected using dispersed organics from the northern McCullough Peaks, constraining a potential hyperthermal to a 4-meter interval. We collected pedogenic carbonate nodules from a 37-meter composite stratigraphic section in the McCullough Peaks with the goal of producing an inorganic δ¹³C record of this event. Pedogenic carbonate nodules are not susceptible to contamination by recalcitrant, lithogenic organic material from the sedimentary provenance, and sampling micrite from these nodules provides a more reliable record of the regional carbon cycle during the Eocene. A statistically-significant, but small, CIE (∆δ¹³C = -0.55 ‰) is present immediately above a mature red paleosol with distinct B sub-horizons and low clay and organic carbon content. This red paleosol is the lowest occurrence of Homogalax protapirinus, an isectolophid perissodactyl, in the study area and represents the Wa-2/Wa-3 faunal boundary, a horizon distinguished by several first appearances, and relatively few last appearances, of fossil mammal taxa. Age models based on long-term Willwood sedimentation rates suggest the low-magnitude CIE has an age of 55.31-55.08 Ma, making it correlative to the 55.165 Ma C24rH7-F hyperthermal event recorded in both the Atlantic and Pacific basins. Regional characterization and correlation of this low magnitude CIE across the Bighorn Basin has the potential to provide more precise numerical age and rate estimates for geologic and biotic events preserved in the Willwood Formation.