The significance of Himalayan rivers for silicate weathering rates: evidence from the Bhote Kosi tributary

The significance of weathering by Himalayan runoff for both the Sr-isotope marine record and the removal of atmospheric CO2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the Bhote Kosi, a tributary of the Ganges that rises in Tibet from Tethyan sediment bedrock and traverses the major Himalayan lithologies of eastern Nepal before debouching onto the Gangetic plains. Throughout the section, the cation geochemistry of water samples is dominated by Ca and Mg ions, suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the Lesser Himalayas. As the river transects the metasedimentary and granitic lithologies of the High Himalayas the Sr-isotope ratio of the bedload rises rapidly, closely reflecting the isotope geochemistry of the bedrock. In contrast the 87Sr/86Sr ratio of the dissolved load remains roughly constant (0.719–0.723). Downstream of the Main Central Thrust, where the river transects the carbonate-bearing lithologies of the Lesser Himalayas, the 87Sr/86Sr ratio of the dissolved load rises sharply (>0.768). The relative contributions of silicate and carbonate weathering from each of the main Himalayan units has been estimated from major cation, Sr concentration and Sr isotope mass-balance equations. These calculations suggest that the high Sr and high 87Sr/86Sr characteristics of riverine analyses arise initially from a component dissolved from the Tibetan Sedimentary Series which is substantially enhanced by input weathering fluxes, particularly as the river traverses the Lesser Himalayas. Whilst mechanical erosion is maximised within the High Himalayan Crystalline Series, as confirmed by 143Nd/144Nd ratios from the bedload, at least 63% of the dissolved load is acquired by chemical weathering of bedrock lithologies and/or of transported particulates within the Lesser Himalayas, enhanced by higher ambient temperatures and slower discharge rates. This may involve continued dissolution of the High Himalayan Crystalline Series particulates in addition to Lesser Himalayan lithologies. Although Himalayan rivers collectively have a major influence on the Sr-isotope marine record, the high 87Sr/86Sr ratios of their dissolved load results from the mixing of a small component (<10%) of silicate-derived material with an unusually high 87Sr/86Sr ratio (0.75–1.0) and a large component (>90%) of carbonate-derived material some of which is characterised by a high 87Sr/86Sr ratio (up to 0.8). Elevated 87Sr/86Sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates.