Particulate Organic Carbon Transport From the Himalaya to the Ganga-Brahmaputra Delta

HR: 17:45h
AN: H24B-08 [Abstracts]
TI: Particulate Organic Carbon Transport From the Himalaya to the Ganga-Brahmaputra Delta
AU: Galy, V
EM: vgaly@crpg.cnrs-nancy.fr
AF: Centre de Recherches PＵrographiques et GＰchimiques, BP 20, Vandoeuvre, 54501 France
AU: * France-Lanord, C
EM: cfl@crpg.cnrs-nancy.fr
AF: Centre de Recherches PＵrographiques et GＰchimiques, BP 20, Vandoeuvre, 54501 France
AB: The erosion of the Himalaya generates the world largest influx of sediments to the oceans. Carried by the Ganga-Brahmaputra fluvial system, 1 to 2 billions tons of sediments are discharged to the Bay of Bengal every year. While the particulate organic carbon (POC) concentration is relatively low between 0.6 and 1 % (Saffiulah et al., 1985) this translates into a flux that represents 5 to 25 % of the global continental organic carbon flux to the oceans. Assessing the POC content of river sediments is however difficult as there is a strong dependence to the mineralogical charge. In order to better understand the behaviour of organic carbon during the transport and the role of the transport, suspended and bed sediments have been sampled in the Ganga-Brahmaputra system from the mountain range to the delta. Suspended loads were sampled at different depth in the river channel to take into account sediment variability. Grain size and chemical composition as well as organic carbon contents and isotopic compositions have been measured. This allows 1) to determine the relationship between sediments characteristics and the organic carbon content, 2) to trace organic carbon sources in the rivers sediments and 3) to re-evaluate the mean organic carbon content in the exported sediments. In the bedload sediments, the total organic carbon content (TOC) is always very low, ranging from 0.02 to 0.25 %. The suspended sediments generally have higher TOC although it remains lower than 0.7 %. From surface to deep water of the river, suspended sediments have very contrasted mineralogical composition and grain size characteristics. Surface suspended sediments are very fine grained and clay rich whereas near the bed, suspended sediments are sandy and quartz rich. TOC is dependent of this sorting process with higher concentrations associated to finer surface sediments. All sediments define a good positive correlation between TOC and Al2O3/SiO2 ratio. Throughout all the system, the organic carbon content is therefore determined by the proportion of clay minerals. The organic carbon isotopic compositions (δ¹³C_org) range between -26.3 and -20.6 ‰. In the Himalayan rivers, sediments have stable low δ¹³C_org around -24 ‰. In the plain, Ganga and Brahmaputra suspended sediments present contrasted δ¹³C_org as high as -20.6 ‰ for Ganga. The low δ¹³C_org observed in the Himalayan river sediments characterise an organic carbon derived from C3 plants. In the Gangetic plain, a significant contribution of C4 plants is observed with δ¹³C_org as high as -20.6 ‰. The ubiquity of the relation between TOC and sediment chemistry combined with the evolution δ¹³C_org from the Himalaya to the delta indicate a partial recycling of the organic carbon pool during the deposition-erosion cycles in the plain. Previous studies, based on surface suspended sediments, propose a mean organic carbon content in the suspended sediments of the system ranging between 0.6 and 1.0 %. Because surface suspended sediments always have the higher TOC, our systematic sampling approach lead to a significant decrease of this range. The sediment heterogeneity in the river depth is generated by current velocity gradient from the surface to the bed. Budget of Himalayan erosion flux must therefore integrate sediments characteristics and current velocities repartitions in the river.
DE: 0428 Carbon cycling (4806)
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 1815 Erosion
DE: 1862 Sediment transport (4558)
DE: 9320 Asia
SC: Hydrology [H]
MN: Fall Meeting 2005