A New Record of the ETM2 and H2 Hyperthermals from the Fifteenmile Creek Area (Bighorn Basin, Wyoming): Implications for the Terrestrial Preservation of Hyperthermals and Their Biotic Response

Abstract:

The Paleogene experienced several short-lived global warming events, known as hyperthermals, that include the Paleocene–Eocene Thermal Maximum (PETM) as well as smaller events before and after the PETM. Hyperthermals are identified in the geologic record by a negative carbon isotope excursion (CIE) and their associated biological and environmental shifts offer insight into possible responses to future warming. The geologic records of these events vary considerably, including differing biotic responses and a poorly understood amplification of CIEs in paleosol carbonate relative to marine records. Resolving these questions is crucial for their full interpretation and application to future scenarios. The Fifteenmile Creek (FC) area of the central Bighorn Basin exposes an early Eocene fluvial sedimentary sequence and has produced an extensive fossil vertebrate collection with over 32,000 mammal specimens. Previous paleontological analysis of these assemblages revealed two brief pulses of mammal turnover and changes in diversity interpreted to correlate with smaller hyperthermals immediately following the PETM (ETM2 and H2). This was based, however, on long distance correlation of chemostratigraphic records. Our study establishes a carbon isotope stratigraphy using micrite δ¹³C from paleosol carbonate nodules preserved in richly fossiliferous localities at FC to identify the stratigraphic position of ETM2 and H2. Over 750 isotope samples were collected from five stratigraphic sections and correlated with 18 fossil localities. We used differential GPS elevation to establish a new stratigraphic framework that assists in correlation and is independent from the biostratigraphy and previous composite lithostratigraphic sections from the area. The ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded in FC. ETM2 and H2 overlap with the two pulses of mammal turnover and comparisons between the new chemostratigraphy and fossil record suggest that the recorded amplitude of these faunal changes may have been muted as a result of some stratigraphic averaging of fossils. The CIEs for these hyperthermals are also smaller in magnitude than in more northerly Bighorn Basin records and provide an opportunity to better evaluate the regional and global variation in CIE amplification.