Weathering processes in the Ganges–Brahmaputra basin and the riverine alkalinity budget

• Published in: Chemical geology 1999-07, Vol.159 (1-4), p.31-60
• Main Authors: Galy, Albert, France-Lanord, Christian
• Format: Article
• Language: English
• Subjects: Alkalinity; Atmospheric CO2; Continental interfaces, environment; Himalaya; Metamorphic CO2; Sciences of the Universe; Sulfide; Weathering
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/S0009-2541(99)00033-9

We present river chemistry data for a network of rivers draining the western and central Nepal Himalaya. Our sampling locations cover the system from the sources of rivers in Tibet to the Gangetic plain. Water samples were collected throughout the year, including the monsoon season, for rivers in Nepal and for the Ganges and Brahmaputra in Bangladesh. Rivers draining the North Himalaya are characterized by low discharge under a cold and arid climate. Main stream waters have δ13CDIC near 0‰ and high [SO42−]/([SO42−]+[HCO3−]) ratios (XSO4) with values around 40 Eq% and high [Cl−]. Ca is the dominant cation (Ca2+/∑cations=55 to 75 Eq%, after correction of sodium by chloride). Dissolved sulfate is produced in waters from the Tethian Sedimentary Series whereas chlorine is related to thermal waters. Dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution. δ13CDIC values of up to 3.9‰ show that metamorphic CO2 is an important weathering agent. Rivers of the North Himalayan basins have about 50% of their alkalinity derived from carbonate dissolution, 20% from biogenic activity and 30% from metamorphic CO2. On the south flank of the Himalaya, rivers are more depleted in 13C and have, on average, lower XSO4. Most rivers have δ13CDIC and XSO4 values compatible with a simple mixing between soil CO2 and sulfate derived from sulfide oxidation. During the monsoon, discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced. XSO4 and δ13C decrease while Ca/∑cations increases, implying enhanced dissolution of pedogenic calcite. As a whole, the G–B riverine flux of alkalinity derived from silicate weathering is around 2.7×1011 mol/year, modest at the global scale. Sulphuric acid controls 20–30% of the weathering reaction in the Brahmaputra and 6 to 9% in the Ganges. Na+ and K+ balance 60 to 65% of the silicate-derived alkalinity flux, and the long term CO2 consumption by Ganges and Brahmaputra is near 6.4×1010 mol/year. The flux of metamorphic CO2 converted to alkalinity via weathering reactions is ca. 1×1010 mol/year.