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

Abstract

Carbon isotope ratios (δ¹³C values) of herbivores reflect the δ¹³C values of dietary plants, and the δ¹³C 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 δ¹³C of fossil grazers may serve as a proxy for reconstructing paleoclimates and paleovegetation patterns. However, the accuracy of environmental reconstructions based on herbivore δ¹³C values is often uncertain, because the relationship between the δ¹³C of many animals and the abundance of C₄ and C₃ grasses has not been precisely quantified. We analyzed the δ¹³C 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 δ¹³C 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 δ¹³C values did not differ among habitats and was not correlated with the abundance of C₄ grasses. These results demonstrate that analyses of the δ¹³C 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 δ¹³C of fossil bison and other large herbivores in North America.