112 Paleontology: Reconstructing Ecosystems

Monday, 5 November 2012
1:30 PM - 5:30 PM
217A

VEGETATION RECONSTRUCTION OF THE MIDDLE MIOCENE IN SOUTHWEST MONTANA USING PHYTOLITH AND STABLE ISOTOPE ANALYSES

Paper number: 112-15
Program: Technical Sessions
Time: 5:00 PM – 5:15 PM

Authors

Abstract

The warmest climatic event of the last 34 million years occurred during the middle Miocene, culminating in the Middle Miocene Climatic Optimum, (MMCO; 17-15 Ma) which was a period of rapid warming. The climate change likely affected plant community composition as grasses spread across North America, but because plant macrofossils from the MMCO in North America are rare, the response of vegetation to this warm interval is poorly understood. We combine analysis of plant silica (phytolith) assemblages and study of stable carbon isotopes from the same paleosols to document vegetation changes during the MMCO in southwestern Montana. Phytolith assemblage analysis provides information about vegetation structure, as well as the composition of grass communities, whereas isotopic data allows determination of the proportion C3 and C4 plants. The site near Drummond, Montana, has previously produced Barstovian-aged mammals and is productive for phytoliths.

Initial results show that grass silica short cells, diagnostic of grasses, in particular C3 pooid grasses, dominate assemblages (up to 75%). The remainder of the assemblages is made up of phytoliths typical of forest indicators, including palms, woody and herbaceous dicotyledons, and conifers. This suggests a fairly open habitat, with grass communities dominated by C3 grasses. Preliminary stable carbon isotope data are consistent with the phytolith results, indicating dominantly C3 plants (90% or above) in the samples. More samples will be collected both laterally from single paleosols, as well as from other contemporaneous nearby sites to increase the spatial and temporal resolution of the study, and to look for evidence of ecosystem patchiness and regional-scale variability.