Desert grassland dynamics estimated from carbon isotopes in grass phytoliths and soil organic matter

We document the potential for using carbon isotopes in both soil organic ma tter (SOM) and grass phytoliths in soil to increase the temporal and taxono mic resolutions of long term vegetation dynamics. Carbon isotope values fro m both SOM and phytoliths are expected to describe both the age of material through C-14 dating, and the photosynthetic pathway of the source plant ma terial through ratios of C-12/C-13. Taxonomic resolution is increased becau se the phytoliths examined are specific to grasses, whereas the SOM reflect s the contribution of all the vegetation. Temporal resolution is increased because phytoliths are less mobile in the soil profile than SOM, and can th erefore provide older dates from the same soil depth. Our results, from a d esert grassland site in southwestern North America, largely confirm these e xpectations, and show that C-4 Species have dominated the grass composition for the last 8000 yr, C-3 non-grass vegetation increased about 100-350 yrB P, and no significant C-3 grass or non-grass vegetation existed between 350 -2000 yr BP.