PALEOGENE PALYNOLOGICAL RECORD OF VEGETATION RESPONSE TO CLIMATIC WARMING IN THE BIGHORN BASIN, WYOMING

The Paleogene palynological record in the Bighorn Basin (Wyoming, USA) allows investigation of vegetation response to climatic warming in a setting different from the Quaternary, where individualistic change seems to dominate. In addition, it can provide data to evaluate long-term effects of the Paleocene-Eocene carbon isotope excursion. I have analyzed samples from lithofacies representing oxbow pondfills and clastic swamps from a 650 m section representing about three million years commencing just below the Paleocene-Eocene boundary. The clastic swamps record local vegetation on the floodplain, whereas the oxbows record local vegetation surrounding the ponds and regional palynomorphs transported by floods. Worldwide data on Late Paleocene to Early Eocene climate as well as leaf physiognomy from the Bighorn Basin itself indicate that mean annual temperatures increased by about 3^o C during deposition of this section. The swamp is dominated by pollen of taxodiaceous conifers, Alnus, and Sparganiaceaepollenites, while the pond is dominated by Coryluspollenites, bisaccate conifer pollen, and Pandaniidites.

The dominant taxa of the swamp and pond assemblages at the base of the interval remain dominant through the section. Only Platycarya pollen makes a significant first appearance within the section, becoming more abundant at the top. The only important taxon lost is Sparganiaceaepollenites. Principal components analysis of each lithofacies's assemblages shows no trend through the section over most of the variance in the data, so the swamp and pond palynological assemblages retain their basic characteristics.

For this Paleogene case of relatively slow climate change in an equable system, the assemblages remained stable over time. Depositional environment is a more potent control on vegetation during this interval than climate. Although there is no preservation of plant fossils within the interval of the Paleocene-Eocene carbon isotope excursion, this record does demonstrate that the long-term effects of this event were slight. These early Paleogene plant communities have a coherence that withstands a significant change in climate, and contrasts with Quaternary studies that suggest a highly individualistic response of plants to climatic change.