A major environmental shift by the middle Eocene in southern China: Evidence from palynological records

Abstract

The modern environment of southern China is dominated by a humid monsoon climate, and presents a striking contrast to the widespread deserts found at similar latitudes elsewhere. The formation of a monsoon climate marks a major environmental shift in southern China during the Cenozoic; however, the origin and possible driving mechanism of the climate transition remain unclear. Paleogene palynological records from seven basins across southern China offer an exceptional opportunity to address this knowledge gap. The Paleogene palynoflora reveals two completely different vegetation and climate patterns: the Paleocene to early Eocene was characterized by a relatively high abundance of xerophilous taxa, suggesting an arid/semi-arid climate; while the middle Eocene to Oligocene supported subtropical wet evergreen and deciduous broad-leaved mixed forest, characterized by a significant increase in broad-leaved, coniferous taxa and the almost complete disappearance of xerophilous taxa, together indicating the emergence of a warm–humid climate at this time. Combined palynological, sedimentological, paleozoological, and paleobotanical proxy evidence reveal that the major environmental transition occurred roughly around the middle Eocene; this transition may mark the establishment of the monsoon climate. Furthermore, middle Eocene climatic parameters obtained through the Coexistence Approach (CA) were compared with those of modern sites with known climate regimes. The middle Eocene parameters were most similar to those exposed to the modern East Asian Monsoon (EAM), further indicating that the middle Eocene climate of southern China was predominantly influenced by the EAM. The evolution of the Paleogene palynoflora in southern China is largely consistent with global climate change. We infer that the enhancement of the East Asian winter monsoon (EAWM) during the middle Eocene was closely related to long-term global cooling, while the intensified East Asian summer monsoon (EASM) was primarily caused by the northward drift of the Indian Subcontinent and the uplift of the Tibetan Plateau.