Loading and fate of particulate organic carbon from the Himalaya to the Ganga-Brahmaputra delta

We use the evolution of river sediment characteristics and sedimentary C-org from the Himalayan range to the delta to study the transport of C-org in the Ganga-Brahmaputra, system and especially its fate during floodplain transit. A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and C-org content in order to evaluate the controls on C-org loading. Contributions of C3 and C4 plants are estimated from C-org stable isotopic composition (delta C-13(org)). We use the evolution of delta C-13(org) values from the Himalayan range to the delta in order to study the fate of C-org during floodplain transit. Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar Corg content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on C rg loading. The general correlation between the total C rg content and Al/Si ratio indicates that C-org loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates. Bed and Suspended sediments have distinct d(13)C(org) values. In bed sediments, d(13)C(org) values are compatible with a dominant proportion Of fossil Corg derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low d(13)C(org) values (-24.8 parts per thousand average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, d(13)C(org) values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher d(13)C(org) values (-22.4 parts per thousand to -20.0 parts per thousand), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic C-org coming from the Himalaya is oxidised and replaced by floodplain C-org This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain. (C) 2008 Elsevier Ltd. All rights reserved.