Carbon isotope stratigraphy and mammal turnover during post-PETM hyperthermals

Abstract. Paleogene hyperthermals, including the Paleocene-Eocene Thermal Maximum (PETM) and several other smaller events, represent global perturbations to Earth's climate system and are characterized by warmer temperatures, shifts in floral and faunal communities, and hydrologic changes. These events are identified in the geologic record globally by negative carbon isotope excursions (CIEs), resulting from the input of isotopically light carbon into Earth's atmosphere. Much about the causes and effects of hyperthermals remains uncertain, including whether all hyperthermals are caused by the same underlying processes, how biotic effects scale with the magnitude of hyperthermals, and why CIEs are larger in paleosol carbonates relative to marine records. Resolving these questions is crucial for their full interpretation and application to future climate scenarios. The Fifteenmile Creek area of the central Bighorn Basin, Wyoming U.S.A., exposes an early Eocene floodplain sedimentary sequence that preserves paleosol carbonates and an extensive fossil mammal collection. Previous analysis of faunal assemblages revealed two pulses of mammal turnover and changes in diversity interpreted to correlate with the ETM2 and H2 hyperthermals that immediately follow the PETM. This was, however, based on long distance correlation of chemostratigraphic records. We present new carbon isotope stratigraphy using micrite δ13C values from paleosol carbonate nodules preserved in and between richly fossiliferous localities at Fifteenmile Creek to identify the stratigraphic positions of ETM2 and H2. Additionally, we used differential GPS elevations to establish a new stratigraphic framework that assists in correlation and is independent from the biostratigraphy and previous composite lithostratigraphic sections from the area. Carbon isotope results show that the ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previously recognized pulses of mammal turnover. Comparisons between the new chemostratigraphy and fossil record suggest that the recorded amplitude of these faunal changes may be 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. We suggest that basin-wide differences in soil moisture and/or vegetation could contribute to variable CIE amplitudes in this and other terrestrial records.