Intensification of monsoon, microclimate and asynchronous C4 appearance: Isotopic evidence from the Indian Siwalik sediments

Analysis of the D/H ratio of pedogenic clay and the 18O/16O ratio of carbonate nodules collected from Siwalik sediments in India indicates three episodes of monsoon intensification at ~ 11 Ma, 6 Ma and 3 Ma. These episodes coincide with major pulses of tectonic uplift in the Himalayas suggesting causal link between monsoon intensity and uplift. In addition, variation in the carbon isotope ratio of carbonate nodules and residual organic matter indicates a change in vegetation regime from pure C3 type to a mix of C3–C4 type during this time span. Isotopic analysis of samples collected from different locations of the Himalayan Siwalik shows that the late Miocene C4 appearance was locally asynchronous (by ~ 3 Ma), similar to what is seen on a global scale. The lack of synchronous emergence of C4 plants in various parts of Siwalik probably indicates a combined effect of microclimate (e.g. varying monsoon intensity) and habitat disturbance (e.g. forest fire). In contrast, subsequent expansion of C4 plants during the late Miocene–Pliocene time could be entirely due to monsoon intensification that favoured C4 respiration in a warm and seasonally variable rainfall regime.