VEGETATION RESPONSE TO THE LATE OLIGOCENE WARMING EVENT IN SOUTHWESTERN MONTANA BASED ON A COMBINED PHYTOLITH-CARBON ISOTOPE RECORD

Paleosol studies on degree of chemical weathering and depth to calcic horizon in southwestern Montana have indicated that the region was characterized by a warm, semiarid climate immediately following the Late Oligocene Warming Event (MAP = 402 mm yr^-1, MAT = 12° C). However, little is known about how the local flora was affected by this climatic change and prior studies have produced conflicting interpretations. Previous sparse plant silica (phytolith) data have suggested that grass-dominated habitats did not emerge in southwestern Montana until the early Miocene whereas paleosol and macrofossil studies indicate the presence of these habitats in the late Oligocene. To resolve this discrepancy, we collected and processed paleosol samples for phytoliths and stable carbon isotopes from a site (Everson Creek) in southwestern Montana dated to 25.3 – 24.5 Ma (Chattian). The isotope analyses yield an average value of δ¹³C = -24.31 ‰, indicative of a C₃-dominated habitat. Preliminary phytolith analysis indicates abundant C₃ pooid open-habitat grasses, as well as the prevalence of bambusoid grasses (C₃) and palms, pointing to the presence of open, grassy patches mixed with more closed vegetation. Bambusoids and palms are today typically associated with wet and relatively warm climates, suggesting comparatively humid conditions in the aftermath of the late Oligocene Warming Event. If these preliminary results hold, this research will provide an earlier date for the presence of grass-dominated habitats in this region than implied by previous phytolith research, and more similar to the age of the spread of grassland ecosystems in the Central Great Plains.