Multi-phase ecological change on Indian subcontinent from the late Miocene to Pleistocene recorded in the Nicobar Fan

Abstract Modern grasslands on the Indian subcontinent, North and South America, and East Africa expanded widely during the late Miocene – earliest Pleistocene, likely in response to increasing aridity. Grasses utilizing the C4 photosynthetic pathway are more tolerant of high temperatures and dry conditions, and because they induce less C isotope fractionation than plants using the C3 pathway, the expansion of C4 grasslands can be traced through the δ13C of organic matter in soils and terrigenous marine sediments. We present a high-resolution record of the elemental and isotopic composition of bulk organic matter in the Nicobar Fan sediments from IODP Site U1480, off western Sumatra, to elucidate the timing and pace of the C3–C4 plant transition within the ∼1.5 × 106 km2 catchments of the Ganges/Brahmaputra river system, which continue to supply voluminous Himalaya-derived sediments to the Bay of Bengal. Using a multi-proxy approach to correct for the effects of marine organic matter and account for major sources of uncertainty, we recognize two phases of C4 expansion starting at ∼7.1 Ma, and at ∼3.5 Ma, with a stepwise transition at ∼2.5 Ma. These intervals appear to coincide with periods of Indian Ocean and East Asian monsoon intensification, as well as the expansion of Northern Hemisphere glaciation starting at ∼2.7 Ma. Our data from the deep sea for a multi-phased C4 expansion on the Indian subcontinent are in agreement with terrestrial data from the Indian Siwaliks.