Abrupt shifts in the Indian monsoon during the Pliocene marked by high-resolution terrestrial records from the Yuanmou Basin in southwest China

A 650-m-thick sequence of fluvio-lacustrine sediments from the Yuanmou Basin in southwest China was analyzed at 20-cm intervals for grain-size distribution to provide a high-resolution terrestrial record of Indian summer monsoon variations during the Pliocene. The concentrations of the clay and clay-plus-fine-silt fractions are inferred to reflect the water-level status of the lake basin related to the intensity of the Indian summer monsoon and high concentrations reflect high lake levels resulting from the intensified summer monsoon. The frequency of individual lacustrine mud beds is considered to reveal the frequency of the lakes developed in the basin associated with the variability of the Indian summer monsoon and an increased frequency of the lakes reveals an increased variability of the summer monsoon. The proxy data indicate that the Indian summer monsoon experienced two major shifts at 3.57 and 2.78 Ma and two secondary shifts at 3.09 and 2.39 Ma during the Pliocene. The summer monsoon displayed a general trend of gradual intensification during the period of 3.57–2.78 Ma, coeval with an accelerated uplift of the Tibetan Plateau, implying a close link between the monsoon intensification and the plateau uplift. At 2.78 Ma, the summer monsoon was markedly weakened, synchronous with the formation of extensive Northern Hemisphere ice sheets, denoting a quick response of the monsoon regime to the Northern Hemisphere glaciation. The variability of the summer monsoon decreased at 3.09 Ma and increased at 2.39 Ma, presumably suggesting that variations of the Indian monsoon would be modulated by the initiation and periodic fluctuations of ice-sheet covers in Northern Hemisphere high latitudes.