Sporopollen evidence for Late Miocene stepwise aridification on the Northeastern Tibetan Plateau

Abstract. Holding a climatically and geologically key position both regionally and globally, the northeastern Tibetan Plateau provides a natural laboratory for understanding the interactions between tectonic activity and the evolution of Asian aridification. Determining when and how the Late Miocene climate evolved on the northeastern Tibetan Plateau may help us understand better relations between tectonic uplift, global cooling and ecosystem evolution. Previous paleoenvironmental research has focused on the western Longzhong Basin. Late Miocene aridification data derived from sporopollen now requires corroborative evidence from the eastern Longzhong Basin. Here, we present a Late Miocene sporopollen record from the Tianshui Basin in the eastern Longzhong Basin. Our results show a two-stage stepwise aridification: a temperate forest with a more humid climate developed in the basin between 11.4 and 10.1 Ma, followed by a temperate open forest environment with a less humid climate between 10.1 and 7.4 Ma; and an open temperate forest-steppe environment with a relatively arid climate occupied the basin during 7.4 to 6.4 Ma. The vegetation succession demonstrates that Asian aridification occurred after ~ 7–8 Ma, which is confirmed by other evidence from Asia. Furthermore, this persistent aridification on the northeastern Tibetan Plateau parallels the global cooling of the Late Miocene; the stepwise vegetation succession is consistent with the major uplift of the northeastern Tibetan Plateau during this time. These integrated environmental proxies indicate that global cooling may have been a potential driving force for Asian interior aridification, most likely enhanced by stepwise uplift of the Tibetan Plateau.