δ13Cof organic carbon in the Bengal Fan : Source evolution and transport of C3 and C4 plant carbon to marine sediments

Geochimica et Cosmochimica Acta, Vol. 58, No. 21, pp. 4809-4814, 1994

Carbon isotopic measurements on organic carbon (oc) in sediment cores from the Bengal Fan (ODP Leg 116) show a dramatic 10 %Oincrease 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 δ13C of Bengal Fan OC at 7 Ma shows that Late Miocene expansion of C4 plants already documented in the Siwaliks was widespread over all the Himalayan foreland. After 7 Ma, relations of δ13Cwith sedimentological parameters show that C4 plants dominate in the foreland whereas C3 plants remain abundant in the mountain range. Variations in the source of the sediments and of the OC appear to be sensitive to climate-hydrologic conditions in the basin. Major changes in the isotopic composition of the carbon flux in one of the worlds largest river systems modify the isotopic budgets of both marine dissolved carbon and the sedimentary carbon mass.

Figure : 613Cof TOC in Bengal Fan sediments plotted against age in Holes 7I7C and 718B. The increase of 6‰ values at ca. 7 Ma reflects the increase of the C4/C3 plant ratio in the source of the organic matter. The 6‰ increase near 7 Ma closely parallels that observed in paleosol studies in the Himalayan foreland (e.g., CERLING, et al., 1993), but the decrease after 0.9 Ma is not observed in the paleosol data. Variations in 6‰ are correlated with variations in sedimentation rate, grain size and clay mineralogy (Fig. I).

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