A 25-MA RECORD OF PALEODIET AND ENVIRONMENTAL CHANGE FROM CARBON AND OXYGEN ISOTOPES IN MAMMALIAN TOOTH ENAMEL AND PALEOSOLS FROM THE NE MARGIN OF THE TIBETAN PLATEAU

Cenozoic sedimentary deposits in the Linxia Basin at the northeastern margin of the Tibetan Plateau contain abundant and well-preserved mammalian fossils. Stable isotope analyses of fossil tooth enamel from 116 individual teeth indicate a diet dominated by C3 plants for all herbivores from ~25 Ma to ~2.5 Ma.  These data suggest that C4 grasses were either absent or insignificant in the Linxia Basin prior to ~2.5 Ma.  This is in striking contrast to what was observed in Pakistan, Nepal, Africa and the Americas where C4 plants expanded rapidly at about 7 to 5 Ma as indicated by a positive d¹³C shift in mammalian tooth enamel and paleosols.  Our data from the Linxia Basin do not show a positive d¹³C shift in the late Miocene or early Pliocene.  In contrast, many herbivores show slightly more depleted d¹³C values after ~ 7Ma, which may indicate a slight increase in rainfall and/or a change in atmospheric conditions.  The d¹⁸O results from the same herbivore species show several significant shifts in climate in the late Cenozoic.  A positive d¹⁸O shift at ~7 Ma indicates a shift to warmer and/or drier conditions and is comparable in timing and direction to the d¹⁸O shift observed in paleosol carbonates in Pakistan and Nepal. The late Miocene climate change observed in the Indian sub-continent and in the Linxia Basin, however, seems to be a regional manifestation of a global climate change.  The negative d¹⁸O shift after ~6 Ma is consistent with the marine d¹⁸O and Mg/Ca records, reflecting the late Cenozoic global cooling trend. The lack of evidence for C4 plants in the Linxia basin prior to ~2.5 Ma suggests that the East Asian summer monsoon, which brings precipitation into northern China during the summer and creates optimal conditions for the growth of C4 grasses, was probably not strong enough to affect this part of China throughout much of the Neogene.  This implies that the Himalaya and the Tibetan Plateau may not have reached the present-day elevation and extent to induce a strong monsoon circulation before ~ 2.5 Ma and regional climatic conditions played an important role in controlling the expansion of C4 plants.