Late Miocene weakening of the South Asian Monsoon: Insights from the Siwalik of Nepal

Abstract

The South Asian Monsoon (SAM) is the strong summer monsoon within the Asian Monsoon System, driven by the thermal gradient between the warmer Asian landmass and the cooler Indian Ocean. High-resolution marine proxies indicate a decline in SAM strength after approximately 10 million years ago (Ma); however, terrestrial evidence has been limited. We present high-resolution and well-dated leaf assemblages from the Lower and Middle Siwalik of central Nepal, supported by paleoclimate modelling, which suggest a weakening of the SAM between 10.24 and 8.5 Ma. This decline is linked to a reduction in moisture supply, possibly due to a weakening of the Somali Jet. Our study highlights the potential of leaf physiognomy to reconstruct monsoon strength.