Geochemistry of the headwaters of the Yangtze River, Tongtian He and Jinsha Jiang: Silicate weathering and CO2 consumption

Water and sediment samples were collected from the headwaters of the Yangtze River, Tongtian He and Jinsha Jiang (upstream of the Yangtze River which flows on the eastern Qinghai-Tibet Plateau). A detailed geochemical study of the river system was carried out to determine: (i) temporal and spatial variations of the major ions and their implications; (ii) contribution of carbonate, silicate and evaporite to the river dissolved load and (iii) CO₂ consumption via silicate weathering. Results show that cations derived from evaporite dissolution account for 44.7–82.8% of the total cations in the headwaters of the Yangtze River and increasing from SE to NW of the drainage basin. The contribution from silicate weathering gradually increases from the headwaters due to exposure of intrusive rocks and volcanic rocks in the Jinsha Jiang suture belt. Proportion of cations derived from silicate weathering to the total cations in river waters reaches a maximum at Panzhihua City, which is consistent with the abundant exposure of Cenozoic granitoids and Precambrian high-grade metamorphic rocks around Panzhihua. The Jinsha Jiang basin has higher silicate weathering rates but lower carbonate weathering rates than the middle and lower reaches of the Yangtze River. The calculated enrichment factors of potentially harmful metals in the river sediments are within the range of 0.33–2.59, indicative of level 1 or 2 contamination. The highest enrichment factor for Co, Cr and V is found in Panzhihua City, indicating that it has been influenced by anthropogenic sources.