Stable isotopic variations in modern herbivore tooth enamel, plants and water on the Tibetan Plateau: Implications for paleoclimate and paleoelevation reconstructions
Stable isotope analyses of fossil teeth and other authigenic minerals have been used to reconstruct the paleoenvironment and paleoelevation of the Tibetan Plateau. The accuracy of such reconstructions is limited by the lack of a comprehensive modem comparative database from the region. We analyzed the carbon and oxygen isotopic compositions (delta C-13 and delta O-18 values) of tooth enamel from modem herbivores, the delta C-13 values of gasses and the delta O-18 values of water samples collected from various elevations within the Tibetan Plateau to examine their relationships with modem environment/elevation. The delta C-13 values of enamel samples from horses, yaks and goats display a narrow range of variation, with a mean of -10.7 +/- 1.4 parts per thousand (n=301), indicating that these modem herbivores were feeding predominantly on C3 plants, consistent with the current dominance of C3 vegetation in the region. Some of the samples have delta C-13 values between -7.3 and -10 parts per thousand. Although these higher delta C-13 values could suggest consumption of some C4 plants by the animals, the lack of significant seasonal delta C-13 variations within individual teeth indicates that these higher enamel delta C-13 values are due to consumption of C3 plants experiencing water stress and/or some CAM plants rather than C4 plants. Our data show that the conservative "cut-off' delta C-13 value for a pure C3 diet within the Tibetan Plateau should be -8 parts per thousand for modem herbivores and -7 parts per thousand (or even -6.5 parts per thousand) for fossils if the region was as and in the past as today. In contrast to the small intra-tooth delta C-13 variations within individual teeth, serial enamel samples display large intra-tooth delta O-18 variations, reflecting seasonal variations in the delta O-18 of meteoric water. The mean delta O-18 values of tooth enamel from yaks and horses show a strong correlation with water delta O-18 values, confirming that the delta O-18 of tooth enamel from obligate drinker generally tracks the 6180 of meteoric water. Unfortunately, elevation alone cannot explain most of the variance in the delta O-18 of precipitation and tooth enamel, suggesting that quantitative reconstruction of the paleoelevation of the Tibetan Plateau using re-constructed delta O-18 values of paleo-meteoric water from fossil enamel or other oxygen-bearing minerals is not warranted. For a given environment, horses have the lowest enamel-delta O-18 values while goats display the highest enamel-delta O-18 values among the species studied. The large inter-species delta O-18 variations are likely due to differences in physiology and diet/drinking behavior of the animals. This underscores the importance of species-specific studies when interpreting delta O-18 data of fossil mammalian teeth in a stratigraphic sequence as a record of paleoclimate changes. (C) 2008 Elsevier B.V. All rights reserved.