T288 Cenozoic Terrestrial Environmental Change in North America: From Paleosols to Paleobotany

Wednesday, 3 November 2010
1:30 PM - 5:30 PM
Room 607

BIOTIC AND SEDIMENTARY CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM IN WESTERN NORTH AMERICA

Paper number: T288-3
Program: Technical Sessions
Time: 2:00 PM – 2:20 PM

Authors

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) is an ~200 ky period of rapid warming at the onset of the Eocene that had global biotic and climatic effects. Here we describe changes in plants, vertebrates, paleosols and carbon isotopes that occurred across the PETM in the southeastern Bighorn Basin (paleo-lat 49 N). The uppermost 15 m of Fort Union Fm. beneath the PETM has typical latest Paleocene biotas and carbon isotopic values, though there are yellow and red mudstone paleosols that are unusual compared with the earlier Paleocene. The base of the CIE (negative shift of 3-5‰) occurs in the first laterally persistent mudstone paleosol containing CaCO3 nodules, which also preserves the Wa0 vertebrate fauna characteristic of the PETM. The overlying 5-8 m has 1-2 variegated paleosols, Wa0 vertebrates, and a unique flora dominated by legumes and other taxa not present in the Paleocene or post-PETM. The ~20 m of section above these lowest red paleosols has abundant cut-and-fill deposits and poorly developed paleosols with large CaCO3 nodules; carbon isotopes indicate this is the plateau phase of the carbon isotope excursion coincident with the PETM. Wa0 mammals are abundant, and floras of unusual composition are documented from six sites. In the succeeding ~20 m of section carbon isotope values become more positive again, paleosols are thick, purple-red, and lack CaCO3 nodules, and both fauna and flora take on a more standard Eocene composition.

We interpret PETM changes as follows. Floodplains became better drained with the onset of the event, or possibly just before it, because of seasonal precipitation and/or higher temperatures. Biotic change was rapid, occurring in <5 m of section (<20 ky). The middle PETM had highly unstable channels, reflecting increased rates of erosion caused by seasonal precipitation, and biotas were dry tropical. The PETM concluded with a phase of wetter climate, more stable floodplains, and warm temperate biotas. Coincident changes in fauna, flora and paleosols across the PETM suggest that all three were being forced by global environmental change. Drying in our field area is not predicted by climate models of the PETM and raises the possibility that anthropogenic warming may have unexpected effects on precipitation amount and seasonality in the interior of North America.