INTEGRATING PHYTOLITH AND CARBON ISOTOPE ANALYSES TO RECONSTRUCT VEGETATION IN THE NORTHERN ROCKY MOUNTAINS DURING THE MIDDLE MIOCENE

The evidence for Earths most recent global warming event in the middle Miocene has been independently confirmed by δ¹³C and δ¹⁸O isotopes from benthic foraminifera, fossil leaf floras, pollen assemblages, and paleosols. This event, the Mid-Miocene Climatic Optimum (MMCO), is the closest parallel to the global warming event currently underway. Thus, turnovers of fauna and flora during this interval are our best model system for the ongoing climatic crisis. The MMCO was an important period of biotic modernization with accelerated spread of grass-dominated ecosystems. Though studies have tried to track paleovegetation changes, the rarity of well-preserved macrofossils and palynofloras stymies reconstruction of the complete story. Our approach, which integrates phytoliths (microscopic silica bodies formed by many vascular plants) with carbon isotopes from paleosols, is not hindered by a comparable paucity of data. Whole rock samples with carbonaceous rootlets were collected from late Hemingfordian - late Barstovian–aged strata from the Railroad Canyon Sequence (RCS) of Idaho and were processed for both phytolith and carbon isotope analyses. Preliminary results reveal phytolith assemblages dominated by C₃ pooid grass morphotypes and a mean δ¹³C_org value of -24.38 ‰ (V-PDB) during the late Hemingfordian. These results suggest that C₃ grassland communities were already dominating the local vegetation of the northern Rocky Mountains during the early stage of the MMCO. Future work will address younger (late Hemingfordian-late Barstovian) rock of the RCS to examine in detail a continuous record of vegetation response to climate changes through the MMCO in this region.