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, changes 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 were 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 a full understanding of the causes of hyperthermals and their
application to future climate scenarios. The primary purpose of this study
was to identify early Eocene hyperthermals in the Fifteenmile Creek area of
the south-central Bighorn Basin, Wyoming, USA. This area preserves a
sequence of fluvial floodplain sedimentary rocks containing paleosol
carbonates and an extensive record of fossil mammals. Previous analysis of
faunal assemblages in this area revealed two pulses of mammal turnover and
changes in diversity interpreted to correlate with the ETM2 and H2
hyperthermals that follow the PETM. This was, however, based on long-distance correlation of the fossil record in this area with chemostratigraphic
records from elsewhere in the basin. We present new carbon isotope stratigraphies using micrite δ13C
values from paleosol carbonate nodules preserved in and between richly
fossiliferous mammal localities at Fifteenmile Creek to identify the
stratigraphic positions of ETM2 and H2. 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. The CIEs for these hyperthermals are
also somewhat 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.
