We use the evolution of river sediment characteristics and sedimentary Corg from the
Himalayan range to the delta to study the transport of Corg 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 Corg content in order to evaluate the controls on Corg loading. Contributions
of C3 and C4 plants are estimated from Corg stable isotopic composition (?13Corg). We use the
evolution of ?13Corg values from the Himalayan range to the delta in order to study the fate of
Corg during floodplain transit.
Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In
spite of 4 to 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 Corg loading. The general correlation between the total Corg content and
Al/Si ratio indicates that Corg 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
Bed and suspended sediments have distinct ?13Corg values. In bed sediments, ?13Corg 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 ?13Corg
values (-24.8 ‰ average) indicating a dominant proportion of C3 plant inputs. In the
Brahmaputra basin, ?13Corg 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
?13Corg values (-22.4 to -20.0‰), 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 Corg coming from the Himalaya is oxidised and replaced by floodplain
Corg. This renewal process likely occurs 40 during successive deposition-erosion cycles and river
course avulsions in the plain.