Development of the East Asian summer monsoon: Evidence from the sediment record in the South China Sea since 8.5 Ma

128 samples from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea were analyzed for grain size, clay minerals, biogenic opal content and quartz in order to reconstruct changes in East Asian monsoon climate since 8.5 Ma. An abrupt change of terrigenous mass accumulation rate (MAR), clay mineral assemblage, median grain size and biogenic opal MAR about 5.2 Ma suggests that between 8.5-5.2 Ma the source of terrigenous sediment was mainly in the region of surface uplift and basaltic volcanism in southern Vietnam. A simple model of East Asian summer monsoon evolution was based on the clay/feldspar ratio, kaolinite/chlorite ratio and biogenic opal MAR. The summer monsoon has two periods of maximum strength at 8.5-7.6 Ma and 7.1-6.2 Ma. Subsequently, there was a relatively stable period at 6.2-3.5 Ma, continued intensification about 3.5-2.5 Ma, and gradually weakening after 2.5 Ma. Since 1 Ma the monsoon has intensified, with remarkable high-frequency and amplitude variability. Simultaneous increase in sedimentation rates at ODP Sites 1143, 1146 and 1148, as well as in MAR of terrigenous materials, quartz, feldspar and clay minerals at ODP Site 1143 at 3.5-2.5 Ma, may be the erosional response to both global climatic deterioration and the strengthening of the East Asian summer monsoon after about 3-4 Ma.     

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.