The impact of adsorption–desorption reactions on the chemistry of Himalayan 
rivers and the quantification of silicate weathering rates

Knight, Alasdair C. G.; Stevenson, Emily; Bridgestock, Luke; Jotautas Baronas, J.; Knapp, William J.; Adhikari, Basanta Raj; Andermann, C.; Tipper, Edward T.

doi:10.1016/j.epsl.2024.118814

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The impact of adsorption–desorption reactions on the chemistry of Himalayan rivers and the quantification of silicate weathering rates

Authors

Knight, Alasdair C. G.
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Stevenson, Emily
3.3 Earth Surface Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

Bridgestock, Luke
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Jotautas Baronas, J.
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Knapp, William J.
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Adhikari, Basanta Raj
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/persons/resource/alpha01

Andermann, C.
4.6 Geomorphology, 4.0 Geosystems, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

Tipper, Edward T.
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Citation

Knight, A. C. G., Stevenson, E., Bridgestock, L., Jotautas Baronas, J., Knapp, W. J., Adhikari, B. R., Andermann, C., Tipper, E. T. (2024): The impact of adsorption–desorption reactions on the chemistry of Himalayan rivers and the quantification of silicate weathering rates. - Earth and Planetary Science Letters, 641, 118814.
https://doi.org/10.1016/j.epsl.2024.118814

Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5026507

Abstract

Common environmental adsorbents (clay minerals, metal-oxides, metal-oxyhydroxides and organic matter) can significantly impact the chemistry of aqueous fluids via adsorption–desorption reactions. The dissolved chemistry of rivers have routinely been used to quantify silicate mineral dissolution rates, which is a key process for removing carbon dioxide (Image 1) from the atmosphere over geological timescales. The sensitivity of silicate weathering rates to climate is disproportionately weighted towards regions with high erosion rates. This study quantifies the impact of adsorption-desorption reactions on the chemistry of three large Himalayan rivers over a period of two years, utilising both the adsorbed and dissolved phases. The concentration of riverine adsorbed cations are found to vary principally as a function of the concentration and cation exchange capacity (CEC) of the suspended sediment. Over the study period, the adsorbed phase is responsible for transporting ∼70% of the mobile (adsorbed and dissolved) barium and ∼10% of the mobile calcium and strontium. The relative partitioning of cations between the adsorbed and dissolved phases follows a systematic order in both the monsoon and the dry-season (preferentially adsorbed: Ba > Sr & Ca > Mg & K > Na). Excess mobile sodium (Image 2=Na-Cl) to silicon (Si) riverine ratios are found to vary systematically during an annual hydrological cycle due to the mixing of low temperature and geothermal waters. The desorption of sodium from uplifted marine sediments is one key process that may increase the Na*/Si ratios. Accounting for the desorption of sodium reduces silicate weathering rate estimates by up to 83% in the catchments. This study highlights that surficial weathering processes alone are unable to explain the chemistry of the rivers studied due to the influence of hydrothermal reactions, which may play an important role in limiting the efficiency of silicate weathering and hence modulating atmospheric Image 1 concentrations over geological time.