THE USE OF PHYTOLITHS IN CREATING A HIGH-RESOLUTION RECORD OF DEEP-TIME VEGETATION DYNAMICS: A CASE STUDY FROM THE EOCENE OF MONTANA

The Eocene is characterized by a “thermal maximum” in the early part, and a shift to “icehouse” conditions by the end of the epoch, with periods of active volcanism in Western North America throughout. Consequently, this is an interesting time to look at vegetation dynamics and understanding plant responses to environmental change, especially as refinement of global climate models is needed if we are to understand past and future climate change impacts. Macrofossil floras provide important data on vegetation types, but are limited in terms of geographic and temporal coverage. Other paleobotanical evidence, such as phytoliths (plant silica bodies), offers an opportunity to study vegetation composition and dynamics in the absence of macrofossils on a variety of spatial and temporal scales. We examined vegetation patterns based on phytoliths from a section of the Renova Formation, Timberhills region, Montana dated to 39.2 ± 3 Ma. The section is composed of Alfisols, Entisols, Inceptisols, and composite paleosols superimposed onto floodplain sediments that were deposited under generally low-energy conditions. Phytoliths from 27 paleosol horizons were extracted to reconstruct a high-resolution vegetation history for this section. Phytolith morphotypes are predominantly from forest plants, confirming the presence of forests in Montana ~40 Ma. Tropical elements such as palms (Arecaceae) and spiral gingers (Costaceae) are present throughout the section, suggesting this was a paratropical forest. The high-resolution sampling demonstrates that vegetation shifts between three main dominant plant types: woody forest plants, Costaceae, and grasses. The heterogeneity is likely due to succession and vegetation patchiness. High proportions of grasses are correlated with low numbers of aquatic biosilica (diatoms, sponge spicules, chrysophytes) that suggests that these grasses were tolerant of relatively drier conditions, while Costaceae today inhabit forest gaps and margins so represent a specific microhabitat associated with the forest plants. This is the first study that has used phytoliths for a high-resolution record of vegetation dynamics from deep time, and demonstrates that this will a helpful tool for reconstructing Paleogene and Neogene vegetation changes of North America.