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. Paleogene hyperthermals, including the Paleocene–Eocene ThermalMaximum (PETM) and several other smaller events, represent globalperturbations to Earth's climate system and are characterized by warmertemperatures, changes in floral and faunal communities, and hydrologicchanges. These events are identified in the geologic record globally bynegative carbon isotope excursions (CIEs), resulting from the input ofisotopically light carbon into Earth's atmosphere. Much about the causes andeffects of hyperthermals remains uncertain, including whether allhyperthermals were caused by the same underlying processes, how bioticeffects scale with the magnitude of hyperthermals, and why CIEs are largerin paleosol carbonates relative to marine records. Resolving these questionsis crucial for a full understanding of the causes of hyperthermals and theirapplication to future climate scenarios. The primary purpose of this studywas to identify early Eocene hyperthermals in the Fifteenmile Creek area ofthe south-central Bighorn Basin, Wyoming, USA. This area preserves asequence of fluvial floodplain sedimentary rocks containing paleosolcarbonates and an extensive record of fossil mammals. Previous analysis offaunal assemblages in this area revealed two pulses of mammal turnover andchanges in diversity interpreted to correlate with the ETM2 and H2hyperthermals that follow the PETM. This was, however, based on long-distance correlation of the fossil record in this area with chemostratigraphicrecords from elsewhere in the basin. We present new carbon isotope stratigraphies using micrite δ13Cvalues from paleosol carbonate nodules preserved in and between richlyfossiliferous mammal localities at Fifteenmile Creek to identify thestratigraphic positions of ETM2 and H2. Carbon isotope results show that theETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, arerecorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previouslyrecognized pulses of mammal turnover. The CIEs for these hyperthermals arealso somewhat smaller in magnitude than in more northerly Bighorn Basinrecords. We suggest that basin-wide differences in soil moisture and/orvegetation could contribute to variable CIE amplitudes in this and otherterrestrial records.

Terrestrial carbon isotope stratigraphy and mammal turnover during post-PETM hyperthermals in the Bighorn Basin, Wyoming, USA