The silicon isotopic composition of the Ganges and its tributaries

The silicon isotopic composition ( δ Si 30 ) of the headwaters of the Ganges River, in the Himalaya, ranged from + 0.49 ± 0.01 ‰ to + 2.17 ± 0.04 ‰ at dissolved silicon (DSi) concentrations of 38 to 239 μM. Both the concentration and isotopic composition of DSi in the tributaries increased between the highest elevations to where the Ganges leaves the Himalayas at Rishikesh. The tributaries exhibit a linear correlation between δ Si 30 and DSi that may represent mixing between a low DSi, low δ Si 30 (e.g., 40 μM, + 0.5 ‰ ) component potentially reflecting fractionation during adsorption of a small fraction of silicon onto iron oxides and a high DSi, high δ Si 30 component (e.g., 240 μM, + 1.7 ‰ ) produced during higher intensity weathering with a greater proportional sequestration of weathered silicon into secondary minerals or biogenic silica. On the Ganges alluvial plain, in the Ganges and the Yamuna, Gomati, and their tributaries, DSi ranged from 122 to 218 μM while δ Si 30 ranged from + 1.03 ± 0.03 ‰ to + 2.46 ± 0.06 ‰ . Highest values of δ Si 30 occurred in the Gomati and its tributaries. In general, the lower DSi and higher δ Si 30 of DSi in these rivers suggests control of both by removal of DSi by secondary mineral formation and/or biogenic silica production. A simple 1-dimensional model with flow through a porous medium is introduced and provides a useful framework for understanding these results.