Tectonic and orbital forcing of the South Asian monsoon in central Tibet during the late Oligocene.

The modern pattern of the Asian monsoon is thought to have formed around the Oligocene/Miocene transition and is generally attributed to Himalaya-Tibetan Plateau (H-TP) uplift. However, the timing of the ancient Asian monsoon over the TP and its response to astronomical forcing and TP uplift remains poorly known because of the paucity of well-dated high-resolution geological records from the TP interior. Here, we present a precession-scale cyclostratigraphic sedimentary section of 27.32 to 23.24 million years ago (Ma) during the late Oligocene epoch from the Nima Basin to show that the South Asian monsoon (SAM) had already advanced to the central TP (32°N) at least by 27.3 Ma, which is indicated by cyclic arid-humid fluctuations based on environmental magnetism proxies. A shift of lithology and astronomically orbital periods and amplified amplitude of proxy measurements as well as a hydroclimate transition around 25.8 Ma suggest that the SAM intensified at ~25.8 Ma and that the TP reached a paleoelevation threshold for enhancing the coupling between the uplifted plateau and the SAM. Orbital short eccentricity-paced precipitation variability is argued to be mainly driven by orbital eccentricity-modulated low-latitude summer insolation rather than glacial-interglacial Antarctic ice sheet fluctuations. The monsoon data from the TP interior provide key evidence to link the greatly enhanced tropical SAM at 25.8 Ma with TP uplift rather than global climate change and suggest that SAM's northward expansion to the boreal subtropics was dominated by a combination of tectonic and astronomical forcing at multiple timescales in the late Oligocene epoch.