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LETTER 613C of Organic Carbon in the Bengal Geochimica et Cosmochimica Acta, Vol. 58, No. 21, pp. 4809-4814, 1994 Copyright 0 1994 ElsevierScience Ltd Pergamon Printed in the USA. All rightsreserved 0016-7037/94 $6.00 + .OO 0016-7037(94)00234-7 LETTER 613C of organic carbon in the Bengal Fan: Source evolution and transport of C3 and C4 plant carbon to marine sediments CHRISTIAN FRANCE-LANORD’ and LOUIS A. DERRY’ ‘Centre de Recherches Pktrographiques et G&ochimiques, CNRS, BP20 54501, Vandoeuvre-les-Nancy, France *Cornell University, Department of Geological Sciences, Ithaca, NY 14853, USA (Received July 8, 1994; accepted in revisedform August 29, 1994) Abstract-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 613C 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 613C with 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. INTRODUCHON 1988) that deep sea sediments in the Indian Ocean are an important sink for OC derived from terrestrial plants. Data THE BENGAL FAN IS one of the largest sedimentary units in from ODP Legs 116 and 117 support this hypothesis. Gas the world, containing ca. 3 X lOI kg of sediment, primarily chromatograph and pyrolysis data show that OC in the Bengal derived from the erosion of the Himalayan orogen (CURRAY, Fan is dominated by organic matter derived from tenigenous 1991; CURRAY and MOORE, 1971; REA, 1992). The rate of sources (BERTRAND et al., 1991; MEYERS and DICKENS, 1992; sediment delivery appears to have increased markedly in early COCHRAN et al., 1989; POYNTER and EGLINTON, 1990). Car- Miocene time (ca. 21 Ma; COCHRAN, 1990; LINDSAY et al., bon isotopic analyses (S13C) of paleosol carbonates, organic 199 1 ), and again at lo- 11 Ma (REA, 1992), implying an av- matter, and fossil teeth (QUADE et al., 1989, 1992; CERLING erage sediment delivery rate of cu. 1.5 X 10” kg/y during the et al., 1993; QUADE and CERLING, 1994) show evidence of late Neogene. This mean rate is similar to estimates of the a major shift in terrestrial vegetation patterns in the Hima- modem rate (MILLIMAN and SYVITSKI, 1992). Measurements layan foreland in late Miocene time. A terrestrial plant com- obtained from drill core of early Miocene to Quaternary age munity dominated by C4 plants appears to replace a C3- in the distal portion of the fan (ODP Leg 116) have an average dominated community between 7 and 6 Ma. The stable iso- total organic carbon (TOC) content of 0.85 wt% (n = 155) topic results raise several questions. Was the expansion of (COCHRAN et al., 1989, and this study). A smaller number C4 plants confined to a relatively small area in the foreland, of analyses (n = 12) from Quaternary sediments in the sub- or was it of genera1 importance in the northern Indian sub- marine portion of the proximal Bengal delta are similar (X continent? What caused the shift in the dominant photosyn- = 0.90 wt%; BOUQUILLON, 1987). If these results are repre- thetic pathway? What effects did the expansion of C4 biomass sentative of the fan as a whole, the mean burial rate of OC have on the budget of OC exported to the ocean and buried in the Bengal Fan since early the Miocene has been = 1.1 in marine sediments? Since the bulk of the OC preserved in X 10” mol/y. Recent published estimates for global OC burial the Bengal Fan is derived from the Ganga and Brahmaputra are from 4 to 10 X 10” mol/y (LASAGA et al., 1985; SMITH river basins, 613C analyses of sedimentary OC provide insight and HOLLIBAUGH, 1993); thus, the Bengal Fan may account into this question. for 10 to 25% of the total global OC burial flux during the Neogene. If the Indus Fan, Indo-Gangetic Plain, and Ganga- THE BENGAL FAN Yamuna Delta are included, the estimate of OC burial in Neogene sediments derived from the Himalaya increases by Sedimentation history of the fan can be illustrated by vari- a factor of roughly 1.5. It is evident that the global OC burial ations in rate of deposition, clay mineralogy, grain size, and budget may have been significantly perturbed by rapid erosion TOC (Fig. I). Bengal Fan sediments older than 7 Ma or of the Himalayan region. younger than 0.9 Ma are dominantly silts with an illite-rich The fraction of biologically labile particulate organic carbon clay fraction, TOC content usually I 0.5 wt%, show relatively (POC) in the Ganga river is low, which suggests (ITTEKKOT, little chemical weathering, and were deposited at high sedi- 4809 C. France-Lanord and L. A. Derry +Smectite +Illite 6 ;a 8 2 - 10 a% 12 16 18 0 200 400 600 800 0 0.3 0.6 0.9 200 400 600 0 0.5 f 1.5 2 Depth below sea floor (m) (Illite+Chlorite)/CClay Max. quartz grain size (pm) TOC fwt%) FIG. I. Age dependence with (a) sediment thickness (GARTNER, 1990). (b) clay mineralogy (B~UQLJILLONet al., 1990), (c) maximum quartz grain size, and (d) TOC content (COCHRANet al.. 1989) at ODP Sites 717 (0) and 7 18 (0) and data in this study (+). (a) lower slopes correspond to higher sedimentation rates. (b) (Illite + chlorite)/Xclay ratios are measured on <2 pm fractions. They mostly reflect the relative abundance of illite vs. smectite. In the Bengal Fan illite and chlorite are derived from metamo~hic precursors by physical erosion whereas smectite is primarily derived from weathering of the same material (FRANCE-LANORDet al., 1993). %TOC appears to reflect grain size/ specific surface area of sediments (e.g., KEIL et al.. 1994) mentation rates. In contrast, between 7 and 0.9 Ma, sedi- measured for TOC content, and analysed on a VG602D mass spec- mentation rates of mostly smectite-rich muds and fine silts trometer. Reproducibility is better than 0.2% and d”C of NBS-22 was measured at -29.7 i 0.1 %Orelative to PDB. Deposition ages of were lower, the degree of chemicai weathering was much sediment were estimated from biostrati~phic data (GARTNER,f 990) more intense, and TOC 2 1 wt%. However, neodymium, following the chronostratigraphy of BERGGRENet al. (1985). strontium, oxygen. and hydrogen isotopic data show that these variations were not caused by changing sediment 6°C on total organic carbon (TOC) was analyzed for sources, but rather by changing continental alteration histories twenty-eight samples from ODP Holes 7 17C and 7 1SC (Fig. (BOIJQUILLONet al., 1990; FRANCE-LANORD et al., 1993). 2). Together, these two holes provide a continuous 18 Ma Comparable variations in sedimentation rate and weathering record of turbidite sedimentation (STOW et al.. 1990). Prior intensity are also observed in the Siwaiik Himalayan foreland to ea. 7 Ma, 613C values in TQC are -27 to -25%, typical sequence (BURBANK et al., 1993; QUADE and CERLINC, of C3 plants. The Si3C data show a dramatic positive shift of 1994). These variations have been interpreted to reflect a 10% beginning near 7 Ma. 613C values in Pliocene sediments change in the rate of sediment supply and the residence time of the Bengal Fan reach -15 to -17%0. In late Quaternary of sediment in the foreland basin (BURBANK et al., 1993; sediments 6’“C values decrease rapidly to -24%. Limited FRANCE-LANORD et al., 1993). According to this hypothesis, &13C data from OC at DSDP Hole 218 show comparable between 7 and 0.9 Ma sediments were stored longer and values with the exception of one sample (ERDMAN et at., weathered more intensely in the Indo-Gangetic Plain. The 1974). The range of the 613C variations we observe is consid- beginning of this interval follows oceanographic evidence for erably greater than that which could be produced by diagen- the intensification of the Asian monsoon ca. 7-8 Ma (mOON esis (ca. 2%; e.g., MEYERS, 1994); hence, the isotopic shift et al.. 1991; PRELL et al., 1992). must represent a change in the source of OC. Mixing of carbon from terrestrial C3 plants with OC derived from marine pri- METHODS AND RESULTS mary production fb’-‘C N --..20%) cannot account for values as high as - 15Z0, and in any case its relative abundance in Total Organic Carbon has been analyzed on whole-rock sediments. Bengal Fan sediments is low (BERTRAND et al., 199 I; MEYERS Data are available upon request to the authors or in NOAA data and DICKENS, 1992; COCWRAN et al., 1989; POYNTER and base. Samples were first acidified with 2 N HCI at 100°C to eliminate carbonate, rinsed, collected on a silica filter, and dried at 80°C. The EGLINTON, 1990). Thus, it is unlikely that most of the vari- sample on its filter was then oxidized under vacuum at 950°C with ation in SL3C is caused by the presence of variable amounts CuO/CuZO. The CO* released was purified at - 140°C.
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