PHYTOLITH RECORD OF DEEP-TIME VEGETATION DYNAMICS: LATE-MIDDLE MIOCENE OF VASA PARK, WASHINGTON

Vegetation changes during the middle-late Miocene, a period of cooling following the Middle Miocene Climatic Optimum (MMCO) are not well known, because of the relative paucity of appropriately aged paleobotanical data. The middle-late Miocene fossil flora from Vasa Park, Washington State, constitutes an opportunity to reconstruct vegetation and regional climate during this crucial time in the Pacific Northwest. Previous work at the site has focused on macrofossils (leaves and reproductive structures) and palynomorphs (pollen/spores); this study adds information about plant silica (phytolith) assemblages, allowing us to compare different sources of paleobotanical data for a single site. Phytoliths form when silica from ground water is deposited within and around plant cells, resulting in microscopic shapes that are diagnostic of different plant types. When fossilized, soil phytolith assemblages permit reconstruction of vegetation type (grassland vs. forest), as well as aspects of climate. Preliminary phytolith evidence shows predominantly forest forms but also grasses, including members of the warm-adapted PACMAD clade. This compares well to macrofossils and palynomorphs, which indicate a forested habitat with several warm-adapted taxa (e.g., Liquidambar, Taxodium). Our results are consistent with a scenario in which regional temperatures were elevated compared to modern climate.