Revisiting the diachronous transition of C₃ to C₄ plants in the Himalayan foreland and other parts of the globe: A sedimentological perspective

The Himalayan foreland basin and other parts of the world had witnessed a dramatic change in the ecological structure during the Late Miocene, as indicated by the increase in abundance of C 4 plants in a C 3 dominated ecosystem. However, the asynchronous expansion of C 4 plants across the latitudes, as well as within the different sub‐basins of the Himalayan foreland basin, tentatively suggests that regional climatic factors and sedimentary architecture vis‐à‐vis geomorphological settings might have an important influence in controlling the abundance of C 4 plants. This study has carried out sedimentological and palaeohydrological analyses of the Siwalik channels at Naladkhad and Ranital regions of the Kangra sub‐basin, north‐west Himalaya, and estimated slope and sinuosity of the modern Himalayan fan rivers ( n > 100), which provides a hydrological analogue for the Siwalik channels. The results suggest that the Siwalik Group in the Kangra sub‐basin was deposited by braided rivers, situated at the proximal part of palaeo‐alluvial fan. In addition to climate‐driven changes, the comparison between sedimentary architecture and published carbon isotope data from the Kangra sub‐basin suggests that channel‐fill dominated fan‐proximal Naladkhad and Ranital regions favoured the growth of C 3 plants until ca 6 Ma, a time when Pakistan Siwaliks were dominated by C 4 plants. In comparison to the Kangra sub‐basin, Pakistan Siwaliks are characterized by a higher abundance of floodplain sediments and possibly represent a distal fan deposit. Data from other parts of the world similarly suggest that, in addition to climate forcing, the dominance of overbank fines favoured the growth of C 4 plants. Therefore, detailed knowledge of the depositional environment and palaeo‐geomorphic setting of the sedimentary archives is essential to understand the influence of sedimentary architecture on the spatio‐temporal variation in the abundance of C 4 vegetation, especially for foreland basin settings, where lateral facies transition rate is high.