EVIDENCE FOR A PLIO-PLEISTOCENE STRENGTHENING OF THE ASIAN MONSOON AND ITS IMPORTANCE TO THE UNDERSTANDING OF MAMMALIAN EVOLUTION IN NORTHWEST CHINA

The uplift of the Tibetan Plateau during the Cenozoic Era is considered to be the driving force in the development of the Asian monsoons, and the timing of this development is crucial to the understanding of mammalian evolution in China. The physical barriers created by the plateau are also important to the understanding of the migrations of mammals. Linxia Basin, located on the northeastern margin of the Tibetan Plateau, has produced an abundance of well-preserved horse fossils. Bulk and serial stable carbon and oxygen isotopic analyses of tooth enamel carbonate from 15 individuals within the genera Equus and Hipparion, ranging in age from 11.5 to 0.05 Ma, have allowed reconstruction of the ancient climates and ecologies of those individuals. The bulk δ¹³C values of tooth enamel carbonate from all individuals indicate a primary diet of C3 grasses, with the exception of one individual from 1.2 Ma, that incorporated a considerable amount of C4 grasses in its diet. The serial δ¹³C and δ¹⁸O values of tooth enamel from 0.05 and 1.2 million-year-old horses show strong seasonality, while the δ values of all individuals between 2.5 and 11.5 Ma do not exhibit similar evidence of seasonality. This apparent seasonality after 2.5 Ma may be related to the strengthening of the summer monsoon in northwest China, which would suggest that the Tibetan Plateau did not reach its current elevation until at least the Late Pliocene. The development of this high barrier at a time much later than generally accepted may help to explain why the Miocene-aged Hipparion fauna that lived between 500 and 1000 m elevations in China is also found at 4200 m in the high Himalayas.