Reconstructing grassland vegetation and paleotemperatures using carbon isotope ratios of bison tooth enamel

Abstract

Carbon isotope ratios ( 13 C values) of herbivores reflect the 13 C values of dietary plants, and the 13 C values of grazers (animals that consume 90% grass) reflect the local abundance of C3 versus C4 grasses. Because grassland C3/C4 ratios correlate with climate, the 13 C of fossil grazers may serve as a proxy for reconstructing paleoclimates and paleovegetation patterns. However, the accuracy of environmental reconstructions based on herbivore 13 C values is often uncertain, because the relationship between the 13 C of many animals and the abundance of C4 and C3 grasses has not been precisely quantified. We analyzed the 13 C of tooth enamel carbonate from modern bison (Bison bison bison) from nine localities in the United States. The C4 grass biomass at these sites ranged from 1% to 95% of the total grass biomass. The mean 13 C of enamel for each population correlated well with the local abundance of C4 grasses and with variations in mean annual temperatures. The variability of enamel 13 C values did not differ among habitats and was not correlated with the abundance of C4 grasses. These results demon- strate that analyses of the 13 C values of fossil bison can be used as a quantitative proxy for reconstructing grassland C3/C4 ratios and paleotemperatures, and they will serve as a baseline for interpreting the 13 C of fossil bison and other large herbivores in North