The silicon isotopic composition of the Ganges and its tributaries

The silicon isotopic composition (δSi30) 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 e...

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Bibliographic Details
Published in:Earth and planetary science letters 2013-11, Vol.381, p.21-30
Main Authors: Fontorbe, Guillaume, De La Rocha, Christina L., Chapman, Hazel J., Bickle, Michael J.
Format: Article
Language:English
Subjects:
chemical weathering
Concentration (composition)
Earth Sciences
Freshwater
Ganges
Himalayas
Minerals
Oceanography
Porous media
river chemistry
Rivers
Sciences of the Universe
Silicon
Silicon dioxide
silicon isotopes
Tributaries
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Summary:The silicon isotopic composition (δSi30) 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 δSi30 and DSi that may represent mixing between a low DSi, low δSi30 (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 δSi30 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 δSi30 ranged from +1.03±0.03‰ to +2.46±0.06‰. Highest values of δSi30 occurred in the Gomati and its tributaries. In general, the lower DSi and higher δSi30 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. •We present Si concentrations and isotopic compositions for the Ganges and tributaries.•Controls on DSi and δSi30 are illustrated by a simple 1-D reactive flow model.•DSi and δSi30 exhibit a positive correlation in the mountains.•δSi30 continues to increase as DSi decreases in the alluvial plain.•Weathering in the Ganges system is far-from-equilibrium.
ISSN:0012-821X
1385-013X
DOI:10.1016/j.epsl.2013.08.026