COORDINATED CHANGE IN CLIMATE, SEDIMENTS AND BIOTAS DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

We traced paleosols and measured sections representing the PETM for 35 km along the southeastern margin of the Bighorn Basin, placing plant and vertebrate fossils, stable isotopic measurements, paleosols and sedimentary features in a unified stratigraphic framework with a precision of a few meters. The uppermost 15 m of Fort Union Fm. beneath the PETM has typical latest Paleocene plants, mammals (zone Cf3), and isotopic values, although locally there are yellow and red mudstone paleosols atypical of lower Fort Union rocks. The base of the CIE (negative shift of 3-5‰) occurs in the first laterally persistent red mudstone paleosol containing CaCO3 nodules, which also contains the characteristic Wa0 fauna and a unique flora dominated by legumes and other taxa not present in the Paleocene. The 5-8 m overlying the lowest PETM paleosol has 1-2 variegated paleosols and also produces Wa0 fossils. The ~20 m of section above these lowest red paleosols is characterized by large CaCO3 nodules (possibly cemented burrow-fills), poorly developed, local paleosols, and abundant cut-and-fill deposits. Wa0 mammals are abundant, as are fossil plants representing the legume-dominated PETM flora. The highest ~20 m of the PETM occurs in a striking sequence of thick, laterally extensive purple-red paleosols lacking CaCO3 nodules. This fauna belongs to the Wa1 zone and the flora contains a mixture of range-through taxa, those unique to the PETM, and a few Eocene indices. The changes in fauna, flora and paleosol features occur prior to the end of the CIE.

Our interpretation of the PETM sequence is that floodplains became better drained with the onset of the event, or possibly shortly prior to it, indicating more seasonal precipitation as well as higher temperatures. Biotic change was rapid, occurring within 5 m of section. The middle PETM was a period of highly unstable channels, reflecting increased rates of erosion typical of more seasonal precipitation. The upper part of the PETM, marked by an absence of CaCO3, and thick, purple-red paleosols, was deposited in a wetter climate with more stable floodplains. The fauna and flora here resemble later early Eocene assemblages even though the CIE had not yet ended. Coincident changes in fauna, flora and paleosols across the PETM suggest that all three were being forced by global environmental change.