Geochemical Constraint on Sediment Sorting, Transport and Deposition Throughout the Himalayan River System

The Himalayan river system transfers annually ca. a billion ton of sediment from the Himalayan slopes to the Bay of Bengal. The transport conditions are highly contrasted with a very steep mountainous part and a long and very flat floodplain and deltaic part. The modest slope and the subsidence in the floodplain tend to favor deposition of pebbles in the vicinity of the Himalayan outlet and of sandy sediments downstream. Because deposition preferentially involves coarse and quartz rich sediments it tends to geochemically fractionate the overall sediment load transported by the rivers [1]. This can be tracked using the evolution of major element concentrations in the sediment tacking into account the potential bias due to chemical erosion. Al/Si ratio best describes the partition between coarse quartz rich sand and phyllosilicate rich silty-clays.

We use a set of chemical compositions and granulometric distribution for modern sediment samples of Himalayan rivers and selected locations in the floodplain and the Bangladesh delta. Sampling includes river depth sampling during the flood season to document river variability due to settling processes in the water column, daily sampling on the Narayani-Gandak river to document seasonal variability. We also use data from sediments deposited in the floodplain to document the geochemical effect of floodplain sequestration.

Data show that grain size/mineralogical segregation becomes more pronounced downstream and is dependent on the hydrodynamic conditions [1]. At the Himalayan front, data show that Al/Si ratios vary from 0.29 to 0.20 with limited variation in the water column. Their integrated ratio is between to 0.23 and 0.24. Average pebble composition near the outlet of the Narayani is highly enriched in silica with a ratio of 0.11. Sediments sequestered in the floodplain have an average composition between 0.15 and 0.20. Downstream, Ganga in Bangladesh typically varies from 0.32 at the surface to 0.13 in the bedload with an integrated value of 0.23. This represents quartz proportion varying from less than 30% in the upper suspension to ca. 60% in the bottom sediments. The modest evolution of Al/Si ratios from Himalaya to the Delta implies that deposition in the floodplain is limited around 10% ±10. [1] Lupker et al. 2011: JGR doi:10.1029/2010JF001947