Palynological and geochemical evidence for East Asian monsoon evolution from the northern South China Sea during the middle to late Eocene

Abstract

The evolution of the East Asian monsoon (EAM) profoundly affected the climate and environment of southeast China. This paper presents middle to late Eocene palynological records from the WC36-A borehole in the northern South China Sea (SCS) to improve understanding of EAM evolution and its driving mechanism during this key time interval. Findings reveal that vegetated landscapes transformed from mixed evergreen and deciduous broad-leaved forest in the middle Eocene to the mixed coniferous and broad-leaved forest ecosystem in the late Eocene. Furthermore, palynology-based CREST climate reconstructions suggest the existence of a subtropical monsoon climate and a modern-like EAM in the northern SCS region since the middle Eocene. Coupled with chemical weathering proxies, our analysis reveals the three-stage monsoon climatic evolution from cool-subhumid conditions to warm-humid conditions and then returning to cool-subhumid conditions. Our study suggests that the uplift of the Central-Southern Tibetan Plateau drove the EAM enhancement in the middle to late middle Eocene. The weakened EAM was likely caused by global cooling in the early middle and late Eocene and further promoted by the global sea level fall in the late Eocene. Our study proposes different interaction processes between forcing factors and provides new insight from the northern SCS to reconstruct the EAM evolution during the middle to late Eocene.