Chemical weathering in the upstream and midstream reaches of the Yarlung Tsangpo basin, southern Tibetan Plateau

The Tibetan Plateau (TP) is a focus of chemical weathering research worldwide. However, scant studies on the northern flank of the Himalayas and the vast inland region of the TP hinders understanding of the role the TP has played in the global carbon cycle. The upstream and midstream of the Yarlung Tsangpo (UMRYT) is the longest river in Tibet with the highest elevation in the world. Two sampling campaigns were conducted in December 2012 and August 2013 to estimate the intensity of chemical weathering, CO2 consumption rates, and their controlling factors in the UMRYT basin. Results show that the total dissolved cations and anions have a broad range of changes with an average of 199 mg/L (46-637 mg/L) in December and 196 mg/L (43-523 mg/L) in August. This parameter displays small seasonal variations for most sampling sites; these are just slightly diluted in August even though runoff is 6-12 times higher than that in December. The evidence from TZ(+) */HCO3-* (*indicates after deduction from atmospheric inputs) demonstrates the proportion of rock weathering caused by H2SO4 is surprisingly high in the sub-basins to the extent that it is generally above 50% in both seasons. Dogxung Tsangpo, Nianchu River, and the small tributaries at the southern bank are severely affected by rock weathering involving sulfuric acid (above 90%). The chemical budget shows that the average proportion of TZ(+) in river water derives from atmospheric inputs, weathering of silicate, carbonate, and evaporite are 9%, 19%, 50%, and 22%, respectively. The contribution of carbonate dissolution to TZ(+) is 8-16% higher during the monsoon season. In contrast, the proportions of silicate weathering and evaporite dissolution are 4-16% and 1-8% larger in the non-monsoon period, respectively. Chemical weathering intensity shows significant spatial differences. Carbonate weathering rates (CWR) and silicate weathering rates (SWR) in the main tributaries vary from 12.6 +/- 4.8 to 20.0 +/- 5.8 t/km(2)/y and from 2.9 to 5.6 t/km(2)/y, respectively. Lithology exerts a profound influence on the spatial heterogeneity of CWR in basins of the main tributaries of the UMRYT, while the spatial distribution of SWR is mainly governed by runoff. The CWR and SWR in the whole basin are 13.2 +/- 4.0 t/km(2)/y and 3.9 t/km(2)/y, respectively, indicating the chemical weathering intensity and its CO2 sequestration are much lower than most large rivers draining the Tibetan Plateau and other orogenic belts around the world.