RAPID GLOBAL WARMING AND FLORAL CHANGE AT THE PALEOCENE-EOCENE BOUNDARY

At the onset of the Eocene the earth warmed by 4-8 °C over a period of 10-20 ky. This sudden global warming, called the Paleocene-Eocene Thermal Maximum (PETM), lasted approximately 150 ky and appears to have been caused by the release of ~4,500 Gt of carbon to the atmosphere-ocean system. Fossil floras from the upper Fort Union and lower Willwood Formations in the Bighorn Basin of Wyoming provide a continental record of this climatic event and demonstrate its effects on terrestrial vegetation. Leaf margin analysis of the floras shows ~5 °C of warming during the PETM, consistent with the magnitude of warming estimated from oxygen isotope change in continental vertebrates and planktonic foraminifera. Leaf size analysis of PETM floras suggests that precipitation decreased at the beginning of the event, then fluctuated, consistent with paleosol features in the same area.

Comparison of PETM plant fossils with latest Paleocene and early Eocene floras in the same area shows a nearly complete turnover in composition over this brief time. Pre- and post-PETM floras are dominated by deciduous plants in Betulaceae, Platanaceae, Cercidiphyllaceae and Taxodiaceae, among others. PETM floras are characterized by a high abundance and/or diversity of Fabaceae and Anacardiaceae among other families, and conifers are absent. Some PETM plant species present in the Bighorn Basin migrated there from farther south, as shown by Paleocene records from the Gulf Coast and southern Rocky Mountains, and several belong to lineages that are common in seasonally dry subtropical habitats today. The plant fossil record of the PETM demonstrates the large effect of global warming on the composition of mid-latitude vegetation, probably resulting from local extinction and continental-scale change in the geographic ranges of plants over a geologically brief time. The similarity of pre- and post-PETM floras demonstrates that rapid climate change of this magnitude did not result in lineage extinction, implying that cooler-adapted plants survived in higher-latitude or higher-altitude refugia.