Monsoonal intensity dominated the Eocene evolution of paleovegetation and paleoclimate in northern South China Sea

Abstract

The East Asian climate system experienced profound transformation throughout the Eocene epoch, characterized by spatial differences in paleoclimate evolution. However, continuous records of Eocene climate remain scarce, and the driving mechanisms are poorly understood, particularly in low-latitude regions. The Weixinan Sag in the northern South China Sea documents vegetation dynamics and weathering regimes through its continuous clastic sedimentary sequences. Based on palynological records and geochemical proxies from the Eocene Liushagang Formation in the Weixinan Sag, this study suggests a triphasic climatic evolution in the northern South China Sea. During the early Eocene, the Weixinan Sag was dominated by floodplain deciduous broad-leaved forests and montane coniferous forests under moderate weathering conditions, signaling a subtropical monsoon climate. By the middle Eocene, enhanced rainfall triggered peak humidity with intense chemical weathering, alongside the development of evergreen-deciduous broadleaved forests in lowlands and mixed forests in highlands. During the late Eocene, aridification resulted in fern expansion and diminished weathering, yet residual rainfall rhythms persisted. Paleovegetation landscapes and chemical weathering intensity, marked by seasonal precipitation fluctuations, confirm operational monsoonal circulation since the early Eocene and demonstrate the dominance of monsoonal circulation in the Eocene climate of the northern South China Sea. Weak coastal proto-monsoonal cells initiated in early Eocene low-latitude coastal areas through greenhouse-enhanced land-sea thermal gradients, matured into a continental-scale system during middle Eocene paleogeographic restructuring, and then attenuated in the late Eocene due to the effects of global cooling on thermal gradients overriding that of paleogeographic changes. These findings not only reveal the Eocene climatic evolution and driving mechanisms in low-latitude regions, but also recalibrate the developmental chronology of the Eocene East Asian summer monsoon.