Global Cooling‐Driven Summer Monsoon Weakening in South China Across the Eocene‐Oligocene Transition.

Recent research has suggested that East Asia has experienced a prevailing monsoon climate since the Eocene. However, there is little knowledge about the development of the East Asian monsoon system before the Miocene, particularly in southern China, due to a lack of well‐dated continuous sediment records. Here, we present new magnetic proxy records from International Ocean Discovery Program Site U1501 in the northern South China Sea. We conducted rock magnetic experiments, made scanning electron microscopy observations and performed diffuse reflectance spectrum analysis on the late Eocene‐early Oligocene core sediments. The magnetic signal of the sediments is dominated by detrital magnetite and titanomagnetite formed during the silicate weathering and erosion processes, which were used to infer the evolution of summer monsoon precipitation in southern China. Our results along with geochemical and clay mineral data from Site U1501 strongly indicate that the East Asian summer monsoon generally weakened across the Eocene‐Oligocene transition. This change was linked to coeval global cooling rather than tectonic processes. Prior to the Eocene‐Oligocene transition, the summer monsoon intensified, likely due to the latest Eocene warming event caused by the enhancement of the Atlantic Meridional Overturning Circulation, which may have preconditioned the Earth system for the greenhouse‐to‐icehouse transition. Plain Language Summary: To date, there is little knowledge regarding the long‐term evolution of monsoon climate in South China before the Miocene due to a lack of well‐dated continuous sediment records. The recent International Ocean Discovery Program Expedition 368 retrieved continuous late Eocene‐early Oligocene sediments at Site U1501 within the northern South China Sea, thus providing an opportunity to fill this gap. In this study, we investigate the evolution of the East Asian summer monsoon in South China during the late Eocene‐early Oligocene through a comprehensive analysis of the magnetic properties of the sediments. The results strongly indicate that the summer monsoon generally weakened across the Eocene‐Oligocene transition, which was linked to coeval global cooling. A short‐term strengthening of the summer monsoon prior to the Eocene‐Oligocene transition was also recorded. This change is attributed to the warming caused by the latest Eocene strengthening of the Atlantic Meridional Overturning Circulation, which may have preconditioned the Earth system state before causing a transition into an icehouse Earth. Key Points: The environmental magnetic record in the northern South China Sea indicates monsoon evolution during the late Eocene to early OligoceneThe East Asian summer monsoon weakened across the Eocene‐Oligocene transitionGlobal cooling drove the long‐term weakening of East Asian summer monsoon [ABSTRACT FROM AUTHOR]