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

摘要

To examine climate variability in northwest China in the late Cenozoic and to test hypotheses regarding the development of C_4 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 25Ma 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 C_3 vegetation prior to 2-3Ma and that C_4 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 2.5Ma, as indicated by positive δ~(18)O excursions in the bulk enamel-δ~(18)O record, were accompanied by increased seasonality; whereas negative δ~(18)O shifts in the bulk data after about 11, 6, and 1.2Ma, which indicate shifts to wetter and/or cooler climate, were associated with decreases in seasonality. Intra-tooth δ~(13)C and δ~(18)O profiles reveal significant changes in the seasonal patterns of diet and climate after ~2-3Ma. Prior to ~2-3Ma, there was little or no seasonal variation in herbivores' diets and all herbivores fed on C_3 vegetation year around. After that time, the data show a significant seasonal variation in the diets of horses and bovids, ranging from a pure C_3 to a mixed C_3/C_4 diet δ. An inverse relationship (or negative correlation) between δ~(13)C and δ~(18)O values within individual teeth - a pattern characteristic of the summer monsoon regime - is observed in younger (<2-3Ma) 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 2-3Ma.