C4 expansion of Central Asia in the middle Miocene linked to the strengthening Indian monsoon

The water vapor source in Central Asia (CA) during the middle Miocene Climatic Optimum (MMCO) is debated. We investigated the Duolebulejin (DLB) section in the Junggar Basin, dated to 18.8–13.2 million years ago (Ma), to reconstruct the history of the vegetation using analyses of multiple indices including pollen, soil organic matter carbon isotopes (δ13CSOM) and magnetic susceptibility (MS). Results reveal a significant regional C4 vegetation expansion during 15.5–14.0 Ma, which was linked to a shift in the configuration of precipitation, manifested by higher seasonality and more summer precipitation under a climatic context conducive to the growth of C4 plants. The water vapor trajectory revealed by paleobotanical data from Eurasia, together with other paleoclimate records, suggests that the synchronous intensification of the Indian summer monsoon (ISM) may have been responsible for this shift. Combining with climate simulations and other evidence, we argue that the summer precipitation pattern in CA during the warming middle Miocene was modulated by the ISM via the monsoon-westerly interactions at high and low latitudes.