Isotopic analysis of tooth enamel carbonate from modern North American feral horses: Implications for paleoenvironmental reconstructions

The accuracy of paleoenvironmental reconstructions based on isotope analyses of equid teeth is currently uncertain because the exact relationship between the isotope composition of modern feral equids and their environment has not been thoroughly studied. We analyzed the carbon and oxygen isotope values (δ¹³C and δ¹⁸O) of tooth enamel carbonate and the δ¹³C values of fecal samples from modern feral horses. We compared those values with the δ¹³C values of local vegetation and the δ¹⁸O values of local waters. Herds were studied in two contrasting localities: eastern Oregon, where grasslands consisted of 100% C₃ species, and New Mexico, where >95% of the grasses were C₄ species. Carbon isotope analyses of fecal material and tooth enamel suggest that horses consumed primarily grass, but some New Mexico horses also consumed significant amounts of shrubs and/or forbs. Microhistological analyses of fecal samples show that Oregon horses consumed 95% grass, and Oregon enamel δ¹³C values are consistent with a diet containing 100% C₃ plants. Microhistological analyses of fecal samples from New Mexico indicate a diet averaging 75% grass, while enamel δ¹³C values suggest that diets averaged 85% C₄ plants (range=72-97%). Thus, reconstructions of the C₃/C₄ ratio of grasses in ancient grasslands that are based on the δ ¹³C values of fossil equid teeth may underestimate the abundance of C₄ grasses. The mean δ¹⁸O values of tooth enamel paralleled the trends observed in the mean δ¹⁸O values of precipitation. However, the mean δ¹⁸O values of enamel carbonate from Oregon and New Mexico differed by only 3.3‰, which is less than the difference in the mean δ¹⁸O values of precipitation (6.5‰). In addition, the range of δ¹⁸O values within New Mexico enamel samples (6.5‰) was greater than the difference between mean enamel δ¹⁸O values at each site. Calculated values for the δ¹⁸O of water ingested by horses are 2-3‰ more positive than mean δ¹⁸O values for corresponding precipitation, suggesting that horses consumed waters that were enriched in ¹⁸O due to evaporation. While our results confirm that local climatic and hydrological conditions can influence the δ ¹⁸O values of equid enamel, they also show that the δ ¹⁸O values of equid teeth are not always a direct proxy for the isotope ratios of precipitation.