Suspended Sediment Variability and Erosion Geochemical Budget of the Ganga Basin

Himalayan erosion generates one of the world largest sediment transport system. Assessing modern fluxes is difficult in such large river system as there is a large seasonal variability and because floodplain sequestration and the bedload transport are potentially important but not measurable. Measurements on rivers in Bangladesh provide data on dissolved and particulate fluxes. Galy and France-Lanord [1] used a geochemical budget to approach these additional fluxes and proposed that they should be approximately equal to that of the suspended load flux (about 400 Mt/yr for the Ganga). However, such budget relies on the representativeness of the chemical composition attributed to the different endmembers. Here we present new data on river sediments of the Ganga in Bangladesh and in the Gangetic plain that allow a much more reliable assessment of the suspended sediment composition. Sampling has been performed during monsoon periods of 2001, 02 and 04. Sampling includes depth profiling in order to take into account the mineral sorting during transport. Depth profiles show systematic trends. For instance, Al2O3/SiO2 ratio evolve from 0.25-0.35 at the surface down to 0.15-0.27 at 10 m depth mainly as a response to the progressive decrease of the (micas+clays)/quartz ratio. Grain size distribution also evolves in parallel from an unimodal distribution around 20 μm at the surface to a bimodal distribution with a second mode around 200 μm at depth. Elements like Na, which is controlled by plagioclase, and Ca controlled by carbonates do no not show any systematic evolution with depth. Along the Ganga course, the suspended sediments are more progressively depleted in SiO2 likely as a response to the deposition of quartz rich sediments in the floodplain. The data show that in Bangladesh, the suspended sediments are clearly enriched in Al and Fe relative to the average Himalayan sources. Using a simple geochemical mass balance and assuming steady state erosion of the Himalayan part of the basin, these data imply that the fluxes of bedload transport and floodplain sequestration do represent 50 to 70 % of the suspended load fluxes. This result confirms our preliminary budget and suggests that the Himalayan range drained by the Ganga delivers an erosion flux above 1100 Mt/yr. Taking into account the mineralogical sorting associated to the transport dynamic of the river is essential for geochemical approaches of erosion budgets. It also allows more accurate characterization of the sediment sources and of the weathering stage of the sediments. [1] Geology 29, 23-26, 2001.