Regional Paleovegetation Records of the Eocene-Oligocene Transition of Montana

Abstract

The climate change that occurred over the Eocene-Oligocene transition (EOT; ~33.7 Ma) is the most significant and dramatic of the Cenozoic, reflecting a change from “hothouse” to “icehouse” conditions. Paleoclimate and paleovegetation studies over the last 50 years have put together a broad picture of vegetation change in southwestern Montana. Although general predictions concerning biome response to climate change are useful, a refined understanding is necessary to forecast the effects of future anthropocentric climate change. This study investigates five sites in southwestern Montana (North Hough Draw, Little Spring Gulch, Big Stonerpipe, Little Pipestone, and Matador Ranch) using geochemical analyses of fossilized soils (paleosols) in combination with plant biosilica (phytoliths) to infer the regional response of vegetation during the EOT. In contrast with the marine record of the EOT, whole rock geochemical analyses indicate little change in mean annual temperature and precipitation. Phytoliths from the five sites are indicative of a forested or woodland landscape with a small component of (likely understory) grasses (~22.1%). The most significant vegetation change occurs in regard to the percentage of open-habitat grasses at each site, with a clear increase in open habitat grasses around 34.5 Ma, followed by a return to a more heavily forested landscape over the EOT. Stable carbon isotope data from organic carbon inclusions in paleosols change in tandem with the abundance of open-habitat grasses, and indicate that open-habitat grasses became more abundant as drier conditions prevailed. These results affirm findings of lower resolution studies of southwestern Montana, suggest a long-term, gradual cooling and drying trend over the EOT, and highlight the importance of local geography as a predictor of response to climate change.