DELTA-C-13 OF ORGANIC-CARBON IN THE BENGAL FAN - SOURCE EVOLUTION ANDTRANSPORT OF C3 AND C4 PLANT CARBON TO MARINE-SEDIMENTS

Carbon isotopic measurements on organic carbon (oc) in sediment cores from the Bengal Fan (ODP Leg 116) show a dramatic 10 parts per thousan d increase beginning ca. 7 Ma ago, and a rapid decrease after 0.9 Ma. These shifts reflect changes in the mixing ratio of terrigenous carbon derived from C3 and C4 plants. The rapid increase in delta(13)C Of Be ngal Fan OC at 7 Ma shows that Late Miocene expansion of C4 plants alr eady documented in the Siwaliks was widespread over all the Himalayan foreland. After 7 Ma, relations of delta(13)C With sedimentological pa rameters show that C4 plants dominate in the foreland whereas C3 plant s remain abundant in the mountain range. Variations in the source of t he sediments and of the OC appear to be sensitive to climate-hydrologi c conditions in the basin. Major changes in the isotopic composition o f the carbon flux in one of the worlds largest river systems modify th e isotopic budgets of both marine dissolved carbon and the sedimentary carbon mass.