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 modem feral equids and their environment has not been thoroughly studied. We analyzed the carbon and oxygen isotope values (delta(13)C and delta(18)O) of tooth enamel carbonate and the delta(13)C values of fecal samples from modern feral horses. We compared those values with the delta(13)C values of local vegetation and the delta(18)O values of local waters. Herds were studied in two contrasting localities: eastern Oregon, where grasslands consisted of 100% C-3 species, and New Mexico, where >95% of the grasses were C-4 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 delta(13)C values are consistent with a diet containing 100% C-3 plants. Microhistological. analyses of fecal samples from New Mexico indicate a diet averaging 75% grass, while enamel delta(13)C values suggest that diets averaged 85% C-4 plants (range = 72-97%). Thus, reconstructions of the C-3/C-4 ratio of grasses in ancient grasslands that are based on the delta(13)C values of fossil equid teeth may underestimate the abundance of C-4 grasses. The mean delta(18)O values of tooth enamel paralleled the trends observed in the mean delta(18)O values of precipitation. However, the mean delta(18)O values of enamel carbonate from Oregon and New Mexico differed by only 3.3parts per thousand, which is less than the difference in the mean delta(18)O values of precipitation (6.5parts per thousand). In addition, the range of delta(18)O values within New Mexico enamel samples (6.5parts per thousand) was greater than the difference between mean enamel delta(18)O values at each site. Calculated values for the delta(18)O of water ingested by, horses are 2-3parts per thousand more positive than mean delta(18)O values for corresponding precipitation, suggesting that horses consumed waters that were enriched in delta(18)O due to evaporation. While our results confirm that local climatic and hydrological conditions can influence the delta(18)O values of equid enamel, they also show that the delta(18)O values of equid teeth are not always a direct proxy for the isotope ratios of precipitation. (C) 2003 Elsevier B.V. All rights reserved.