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-iso tope marine record and the removal of atmospheric CO2 through silicate dissolution has been examined by systematic sampling of dissolved loa ds and bedloads from the Bhote Kosi, a tributary of the Ganges that ri ses in Tibet from Tethyan sediment bedrock and traverses the major Him alayan lithologies of eastern Nepal before debouching onto the Gangeti c plains. Throughout the section, the cation geochemistry of water sam ples is dominated by Ca and Mg ions, suggesting that carbonates are th e predominant lithology undergoing dissolution particularly within the Lesser Himalayas. As the river transects the metasedimentary and gran itic lithologies of the High Himalayas the Sr-isotope ratio of the bed load rises rapidly, closely reflecting the isotope geochemistry of the bedrock, In contrast the Sr-87/Sr-86 ratio of the dissolved load rema ins roughly constant (0.719-0.723). Downstream of the Main Central Thr ust, where the river transects the carbonate-bearing lithologies of th e Lesser Himalayas the Sr-87/Sr-86 ratio of the dissolved load rises s harply (>0.768), The relative contributions of silicate and carbonate weathering from each of the main Himalayan units has been estimated fr om major cation, Sr concentration and Sr isotope mass-balance equation s. These calculations suggest that the high Sr and high Sr-87/Sr-86 ch aracteristics of riverine analyses arise initially from a component di ssolved from the Tibetan Sedimentary Series which is substantially enh anced by input weathering fluxes, particularly as the river traverses the Lesser Himalayas. Whilst mechanical erosion is maximised within th e High Himalayan Crystalline Series, as confirmed by Nd-143/Nd-144 rat ios from the bedload, at least 63% of the dissolved load is acquired b y chemical weathering of bedrock lithologies and/or of transported par ticulates within the Lesser Himalayas. enhanced by higher ambient temp eratures and slower discharge rates. This may involve continued dissol ution of the High Himalayan Crystalline Series particulates in additio n to Lesser Himalayan lithologies. Although Himalayan rivers collectiv ely have a major influence on the Sr-isotope marine record, the high S r-87/Sr-86 ratios of their dissolved load results from the mixing of a small component (<10%) of silicate-derived material with an unusually high Sr-87/Sr-86 ratio (0.75-1.0) and a large component (>90%) of car bonate-derived material some of which is characterised by a high Sr-87 /Sr-86 ratio (up to 0.8). Elevated Sr-87/Sr-86 ratios in rivers are th erefore not necessarily indicative of anomalously high dissolution rat es of silicates. (C) 1995 Elsevier Science B.V.