Late Eocene onset of the East Asian Monsoon in the Qingjiang Basin of Central Jiangxi Province (Southeast China) revealed by a major vegetation transition from desert to forest

The vast humid regions in southeast China are strongly influenced by the East Asian Monsoon (EAM); however, the origin and underlying mechanisms driving the EAM in this area remains uncertain. Here, we present Eocene palynological records from two sedimentary successions in the Qingjiang Basin of central Jiangxi Province, southeast China, which reveals a major vegetation transition from an early Eocene Ephedra-dominated shrub desert to late Eocene taxodioid Cupressaceae-dominated swamp forests. Using the Coexistence Approach and the Climatic Amplitude method, we quantitatively reconstructed climatic parameters, which indicate a climate shift from arid conditions with mean annual precipitation (MAP) lower than 200 mm to humid conditions with a MAP of 1479 ± 373 mm. We infer that this sharp increase in humidity is linked to the onset of a modern-style EAM, implying that the EAM has prevailed in southeast China since at least the late Eocene, much earlier than previously thought. We conclude that the establishment of the EAM in southeast China by the late Eocene was primarily controlled by paleogeographic changes, not changes in atmospheric CO2 levels. These results provide a new perspective on the early evolutionary history of the EAM, and challenges the prevailing view that the origin of humid southeast China was a Neogene phenomenon.