GC23I-06:
Did Hydrological Change Drive the Expansion of C4 Grasslands in the Siwaliks during the Late Miocene?
Tuesday, 16 December 2014: 2:55 PM
Jennifer Lee Pensky, Barnard College, New York, NY, United States, Samuel R Phelps, Columbia University of New York, Palisades, NY, United States, Kevin T Uno, Lamont -Doherty Earth Observatory, Palisades, NY, United States and Pratigya J Polissar, Lamont-Doherty Earth Observato, Nyack, NY, United States
Abstract:
The vegetation change from C3 forests to C4 grasslands in India, Pakistan, and Nepal during the late Miocene has been well documented, with previous records indicating that carbon isotope changes in paleosols from Pakistan and marine sediments from the Bengal Fan correspond with a C4 plant expansion. The uplift of the Tibetan plateau, monsoon variability, late Cenozoic cooling, and hydrological changes have all been suggested as potential causes of the C4 grassland expansion, but the ultimate driver of this shift is still unknown. We investigate whether hydrological change drove the expansion of C4 grasslands in the Siwaliks around 6 Ma. We analyzed leaf wax biomarkers in Bengal Fan deep-sea core sediments deposited between 20 Ma and 0.5 Ma from ODP sites 717 and 718, where terrestrial sediments from the Ganges and Brahmaputra Rivers are deposited on the sea floor. n-Alkane concentrations and carbon and hydrogen isotope ratios were measured to characterize the vegetation shift and to determine changes in rainfall isotopic composition and continental aridity. We found that an increase of carbon isotopic ratios in the long-chain alkanes occurred very abruptly between 6.35 and 5.5 Ma, indicating a change from a C3 woodland to a C4 grass-dominated environment. This ecosystem shift was accompanied by an increase in the δD values of the plant-wax biomarkers, although the δD shift begins almost 1 My prior to the vegetation shift. Source water isotopic values were computed from the plant-wax n-alkane δD values using a landscape apparent fractionation value based upon the n-alkane carbon isotope values and reasonable vegetation end members. The source water isotope shift is larger than the lipid shift and some combination of an increase in δD of rainfall or aridity preceded the vegetation shift, and may have played a role in driving the C4 vegetation expansion. Ongoing work comparing these results to published oxygen isotope values from sedimentary archives in the Siwaliks may help differentiate these two effects.
