STABLE C AND O ISOTOPES IN LATE CENOZOIC MAMMALIAN TOOTH ENAMEL FROM THE QAIDAM BASIN, CHINA AND THEIR PALEOENVIRONMENTAL AND PALEOECOLOGICAL IMPLICATIONS

The evolution of C4 plants and the uplift of the Tibetan Plateau are two of the hotly debated issues, focusing mainly on the timing and the driving mechanisms. However, these are not directly recorded in the geologic history, and have to be deduced from indirect evidence. Here we report results of stable C and O isotope analyses of a wide variety of late Cenozoic mammalian tooth enamel samples, including deer, giraffe, horse, rhino, and elephant, from the Qaidam Basin in the northern Tibetan Plateau.

The δ¹³C values of these samples generally show only small variations and are mostly less than -8‰ for modern samples and less than -7‰ for fossils, indicating that the local ecosystems were composed of nearly pure C3 vegetation. Interestingly, however, the serial samples from a rhino tooth from Shengou (CD0722, early late Miocene, with a current best estimated age of ~8–10 Ma) show unambiguous C4 signal, with δ¹³C values up to -4.1‰. This could indicate that C4 plants were a significant component in this rhino’s diet (up to ~53%), and that C4 plants were a significant component in the local ecosystem. However, comparison with other herbivores from the area as well as data from the Chinese Loess Plateau and those from the Linxia Basin, both of which are located to the east of the Qaidam Basin, suggests that the rhino from CD0722 was more likely to be a migrant who grew up in places where C4 plants were present and migrated to the Shengou area.

The δ¹⁸O values of these samples did not increase monotonously with time. However, the range of variation seems to be increasing, with modern teeth showing the largest variation (11.5‰ for a modern horse tooth from Barunyawula, BRY-RP4), probably indicating increasingly severe aridification. The mean δ⁸O values of large mammals display significant temporal variations from the early late Miocene to the present, most likely reflecting variations in the δ¹⁸O of meteoric water, which was in turn caused by climatic and/or tectonic changes.