Transition of predominant regional tectonics to global climate forcing for Asian summer monsoons at ~7.2 Ma

Much has been known regarding late Neogene East Asian summer monsoon (EASM) history and potential forcing mechanisms due to the existence of late Neogene loess deposits on the Chinese Loess Plateau. No continuous loess deposits exist in South Asian monsoon region, and many studies infer South Asian summer monsoon (SASM) variations mainly from low-elevation marine or fluvio-lacustrine sediments. However, inconsistent viewpoints emerge regarding the evolution of the SASM from the late Miocene to the Pleistocene and the potential forcing mechanisms based on these sediments. Here we present an SASM record derived from environmental magnetic parameter in late Neogene fluvio-lacustrine strata from the high-elevation Zhada Basin in southern Tibet. The results show the SASM experienced four stages of variations over the late Miocene to the early Pleistocene, with stronger monsoon precipitation corresponding to warmer climate after ~7.2 Ma. However, SASM intensification prior to ~7.2 Ma was contrary to effects of global climate variations, which we attributed to thermal and mechanical effects of the northeastward growth of the Tibetan Plateau on the SASM. A comparison with the EASM records suggests that the EASM also experienced a transition of dominant driving forcing from the Tibetan Plateau uplift to global climate at ~7.2 Ma, with the exception of the interval 4.5–2.7 Ma when the EASM intensification was controlled by regional tectonics (possibly closure of the Panama Seaway). This study clarifies evolution history of the Asian summer monsoons and recognizes their main forcing mechanisms since 10 Ma.