Decreasing summer monsoon precipitation during the Mid-Pleistocene transition revealed by a pollen record from lacustrine deposits of the Northeast Plain of China

The Mid-Pleistocene transition (MPT) was characterized not only by the transition of ice volume periodicities from 40 kyr to 100 kyr, but also by major changes in high latitude ice sheets, global sea level and CO2 / ocean carbon storage after ∼900 ka, which had a major influence on the evolution of the East Asian summer monsoon (EASM). However, there are different views regarding the trend of East Asian summer monsoon precipitation during the MPT and its driving mechanism. Together with published paleomagnetic and 26Al/10Be isotopic chronologies, we used pollen analysis of the lacustrine sediments of a drill core to reconstruct the vegetation history of the Northeast Plain of China. The results demonstrate a pattern of vegetation succession from coniferous and broadleaved mixed forest dominated by Pinus to woodland-steppe during the MPT, indicating a change from a semi-humid to a semi-arid climate. Palynological records from the Liaodong Peninsula, North China Plain, and the Tibetan Plateau also indicate a drying climate during the MPT, which was related to the decreasing trend of EASM precipitation. We suggest that falling global sea level caused by the expansion of high-latitude ice sheets, and the resulting the exposure of continental shelves and the increased transport distance of oceanic water vapor to the continental interior, was the main cause of the reduction in EASM precipitation. The weakening of the Atlantic meridional overturning circulation and the southward movement of the Intertropical Convergence Zone in the tropical Pacific may have been additional causes of the decrease in EASM precipitation.