Intensification of monsoon, microclimate and asynchronous C 4 appearance: Isotopic evidence from the Indian Siwalik sediments

Abstract

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 C 3 type to a mix of C 3–C 4 type during this time span. Isotopic analysis of samples collected from different locations of the Himalayan Siwalik shows that the late Miocene C 4 appearance was locally asynchronous (by ~ 3 Ma), similar to what is seen on a global scale. The lack of synchronous emergence of C 4 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 C 4 plants during the late Miocene–Pliocene time could be entirely due to monsoon intensification that favoured C 4 respiration in a warm and seasonally variable rainfall regime.