NOAA/WDS Paleoclimatology - Bengal Fan Miocene to Present Plant-Wax Biomarker and Isotope Data

Modern tropical and subtropical C4 grasslands and savannas were established during the late-Miocene and Pliocene, over 20 Myr after evolutionary originations of the C4 photosynthetic pathway. This lag suggests environmental factors first limited and then favored C4 plants. Here, we examine the timing and drivers for the establishment of C4 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 C4 ecosystems in the Ganges-Brahmaputra catchment first emerged at 7.4 Ma and subsequently expanded between 6.9 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 C4 grasslands with further drying at the onset of C4 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 C4 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 datasets. Collectively the fossil, biomarker, and isotopic evidence reveal the development of modern C4 ecosystems on the Indian Subcontinent followed a series of ecosystem transformations driven by drying and fire feedbacks, and possibly declining atmospheric pCO2, beginning at 10.2 Ma and strengthening through the late Miocene.