Hydrologic Changes Drove The Late Miocene Expansion Of C-4 Grasslands On The Northern Indian Subcontinent

Modern tropical and subtropical C-4 grasslands and savannas were established during the late-Miocene and Pliocene, over 20 Myr after evolutionary originations of the C-4 photosynthetic pathway. This lag suggests environmental factors first limited and then favored C-4 plants. Here, we examine the timing and drivers for the establishment of C-4 grasslands on the Indian Subcontinent using carbon and hydrogen isotope signatures of plant-wax n-alkanes recovered from turbidites in the Bengal Fan. Like prior studies, we find C-4 ecosystems in the Ganges-Brahmaputra catchment first emerged at 7.4 Ma and subsequently expanded between 6.9 to similar to 6.0 Ma. Hydrogen isotope values varied from 10.2 to 7.4 Ma and then increased after 7.4, which suggests intermittent drying began before the establishment of C-4 grasslands with further drying at the onset of C-4 expansion. Synthesis of published plant fossil data from the Siwalik Group of the Himalayan foreland basin documents an ecosystem trajectory from evergreen tropical forests to seasonally deciduous forests, and then expansive C-4 grasslands. This trajectory coincided with a seasonally uneven drying trend due to both increased evaporation of plant leaf and soil waters and reduced rainfall, as identified in soil carbonate and tooth enamel data sets. Collectively the fossil, biomarker, and isotopic evidence reveal the development of modern C-4 ecosystems on the Indian Subcontinent followed a series of ecosystem transformations driven by drying and fire feedbacks, and possibly declining atmospheric pCO(2), beginning at 10.2 Ma and strengthening through the late Miocene.