Altitude Effect on the Stable Isotope Chemistry of Tooth Enamel from Modern Herbivores in Tibet

A total of 123 bulk and serial enamel samples were obtained from modern goats, horses and yaks from southern Tibet for C and O isotope analysis. The δ13C and δ18O values of tooth enamel were compared with the δ13C values of local vegetation and the δ18O values of local waters to examine the relationship between the isotopic composition of modern herbivores and their environment. The δ13C values of enamel samples from horses range from -11.2‰ to -13.9‰ with an average δ13C value of -12.7 ± 1.0‰ (n=13). The δ13C values of yak tooth enamel range from -7.3‰ to -14.2‰, averaging -10.1±1.4‰ (n=84). The goat teeth have δ13C values ranging from -7.8‰ to -12.1‰, with a mean of -10.2 ± 1.2‰ (n=26). These enamel δ13C values indicate that these modern herbivores were feeding predominantly on C3 plants, consistent with the current dominance of C3 vegetation in the region. Some of the samples have δ13C values between -7.3 and -9‰. Although these higher δ13C values could suggest consumption of some C4 plants by the animals, the lack of significant seasonal δ13C variations within individual teeth indicates that these higher enamel δ13C values are more likely due to consumption of C3 plants experiencing water stress and/or some CAM plants. This suggests that the "cut-off" δ13C value for a pure C3 diet can be as high as -8‰ in water-stressed environments. The δ13C variations within and between species primarily reflect the variations in the δ13C values of the C3 plant foodstuffs consumed by the animals. The δ13C values of tooth enamel do not show a trend with increasing elevation. Oxygen isotopic compositions of tooth enamel varied widely within and between species. In contrast to the small intra-tooth δ13C variations within individual teeth, serial enamel samples display large intra-tooth δ18O variations, reflecting seasonal variations in the oxygen isotopic composition of meteoric water. The mean δ18O values of tooth enamel from goats showed a correlation with water δ18O values, suggesting that the δ18O of tooth enamel can be used as a proxy for the δ18O of meteoric water. Unfortunately, the oxygen isotopic compositions of water and tooth enamel do not show a clear trend with increasing elevation in the study area, suggesting that quantitative reconstruction of paleoelevation in this region using reconstructed δ18O values of paleo-meteoric water from fossil tooth enamel or other O-bearing minerals may not be warranted. For a given elevation/environment, horses have the lowest enamel δ18O values while goats display the highest enamel δ18O values among the three species studied. The large inter-species δ18O variations are due to differences in physiology and diet/drinking behavior of the animals. This confirms the importance of species-specific studies when interpreting δ18O data of fossil mammalian teeth in a stratigraphic sequence as a record of paleoclimate changes.