MULTIPLE PALEOCENE AND EARLY EOCENE HYPERTHERMALS RECORDED IN THE TERRESTRIAL WILLWOOD FORMATION, NORTHERN BIGHORN BASIN, WYOMING

Paleocene and Eocene hyperthermals are transient greenhouse warming events that are widely viewed as partial analogues of current climate change. Carbon isotope excursions indicate addition of isotopically-light carbon into the atmosphere-ocean exogenic carbon pool during these events caused substantial environmental and biotic change documented in the associated sedimentological and paleontological record. The Paleocene Eocene Thermal Maximum (PETM) is well documented globally, but records of the subsequent smaller events are rare, especially in the terrestrial realm.

Here we present new carbon isotope excursions for two smaller hyperthermal events, I1 and I2, from pedogenic carbonate in the terrestrial Willwood Formation in the northern Bighorn Basin, Wyoming, USA. A total of five (PETM, ETM2, H2, I1, and I2) early Eocene hyperthermals have now been recovered in the Bighorn Basin. The carbon isotope excursions of the I1 and I2 hyperthermal events have magnitudes of ~2.4 and 1.6 per mille, respectively. ETM2, H2, I1, and I2 are now known from a single section, the 210-meter long West Branch section in the McCullough Peaks, where their stratigraphic spacing is proportional to Walvis Ridge records in the Atlantic Ocean. We reconstruct significantly warmer and slightly wetter conditions in the Bighorn Basin during the smaller post-PETM hyperthermals. We construct astronomical age models for these hyperthermal events using sedimentary cyclicity related to orbital climate forcing. This age control is used for detailed correlation and comparison with records from the marine realm.