Strengthening of the East Asian summer monsoon revealed by a shift in seasonal patterns in diet and climate after 2–3 Ma in northwest China

To examine climate variability in northwest China in the late Cenozoic and to test hypotheses regarding the development of C4 ecosystems and the dynamics of the Asian monsoons, the carbon and oxygen isotopic compositions of 32 bulk and 368 serial tooth enamel samples from herbivores in the Linxia Basin (Gansu Province), ranging in age from 25 Ma to the present, were determined. The results corroborate and improve the record previously obtained from the area, showing that all mammals in the Linxia Basin lived in habitats consisting primarily of C3 vegetation prior to 2–3 Ma and that C4 grasses did not become a significant component of local ecosystems until the Quaternary. The data also show that shifts in climate to drier and/or warmer conditions after about 14, 9.5, 7, and 2.5 Ma, as indicated by positive δ18O excursions in the bulk enamel-δ18O record, were accompanied by increased seasonality; whereas negative δ18O shifts in the bulk data after about 11, 6, and 1.2 Ma, which indicate shifts to wetter and/or cooler climate, were associated with decreases in seasonality. Intra-tooth δ13C and δ18O profiles reveal significant changes in the seasonal patterns of diet and climate after ~ 2–3 Ma. Prior to ~ 2–3 Ma, there was little or no seasonal variation in herbivores' diets and all herbivores fed on C3 vegetation year around. After that time, the data show a significant seasonal variation in the diets of horses and bovids, ranging from a pure C3 to a mixed C3/C4 diet (with C4 plants accounting for up to ~ 60% of the diet). An inverse relationship (or negative correlation) between δ13C and δ18O values within individual teeth — a pattern characteristic of the summer monsoon regime — is observed in younger (< 2–3 Ma) horses and bovids but not in older fossils. These changes in intra-tooth isotopic patterns provide strong evidence for an enhanced monsoon climate since about 2–3 Ma.