Late Eocene East Asian Monsoon in South of the Yangtze River Revealed by a Major Vegetation Transition from Desert to Forest

The vast humid regions in south of the Yangtze River (SYR) are strongly influenced by the East Asian monsoon (EAM), and are among the most unique landscapes in the subtropical regions of the world. The EAM plays an integral role in human society of SYR; however, the origin and underlying mechanisms driving the EAM in this area remains elusive. Here, we present an Eocene palynological record from the Qingjiang Basin, Jiangxi Province, which reveals how the vegetation landscape changed from an early Eocene desert to late Eocene swamp forests. This change was driven by a climatic shift from arid conditions with mean annual precipitation (MAP) lower than 200 mm to humid monsoonal conditions with MAP of 1479±373 mm, implying that the modern-like EAM has prevailed in SYR since at least the late Eocene, much earlier than previously thought. We conclude that the establishment of the East Asian summer monsoon in SYR by the late Eocene was primarily controlled by paleogeographic changes, not CO2. These results provide a new perspective on the early evolutionary history of the EAM, and challenges the prevailing view that the origin of humid SYR was a Neogene phenomenon.