Northward expansion of Cenozoic Asian humid climate recorded by sporopollen

The distribution pattern of arid-humid climate in the modern northwest-southeast confrontation in Asia evolved from the zonal distribution pattern of the Paleogene. However, this evolutionary process remains poorly constrained by geological evidence. In this paper, we analyze the characteristics of sporopollen assemblages recorded in Late Eocene strata of the Jianchuan Basin in southwest China. We show that vegetation patterns exhibited a remarkable shift from tropical-subtropical sparse forest to subtropical-temperate mixed broadleaved-coniferous forest at -40.6 Ma; this shift is further indicated by the rapid decrease in drought-tolerant plants, mostly Ephedra and Chenopodiaceae, evidence that the previous hot-dry climate was replaced by a warm-humid climate at this time. This evidence suggests that the Asian monsoon began to affect the study area at -40.6 Ma. We then comprehensively integrated sporopollen results from several basins across East Asia, revealing that the frontal edge of the humid monsoon climate moved intermittently to the northwest after the Late Eocene, and may have advanced to the Lunpola-Lanzhou line by the early Late Oligocene, this being similar to the position of the leading edge of the modern monsoon. Moreover, the frontal edge of the humid monsoon climate may have extended as far as the Qaidam Basin during the Middle Miocene, but then gradually retreated to the position of the modern monsoon front. The uplift of the Tibetan Plateau and global climate change may have played dominant roles in this evolution.