Chemical weathering in the Upper Huang He (Yellow River) draining the eastern Qinghai-Tibet Plateau

Abstract

We examined the fluvial geochemistry of the Huang He (Yellow River) in its headwaters to determine natural chemical weathering rates on the northeastern Qinghai-Tibet Plateau, where anthropogenic impact is considered small. Qualitative treatment of the major element composition demonstrates the dominance of carbonate and evaporite dissolution. Most samples are supersaturated with respect to calcite, dolomite, and atmospheric CO 2 with moderate (0.710–0.715) 87Sr/ 86Sr ratios, while six out of 21 total samples have especially high concentrations of Na, Ca, Mg, Cl, and SO 4 from weathering of evaporites. We used inversion model calculations to apportion the total dissolved cations to rain-, evaporite-, carbonate-, and silicate-origin. The samples are either carbonate- or evaporite-dominated, but the relative contributions of the four sources vary widely among samples. Net CO 2 consumption rates by silicate weathering (6–120 × 10 3 mol/km 2/yr) are low and have a relative uncertainty of ∼40%. We extended the inversion model calculation to literature data for rivers draining orogenic zones worldwide. The Ganges-Brahmaputra draining the Himalayan front has higher CO 2 consumption rates (110–570 × 10 3 mol/km 2/yr) and more radiogenic 87Sr/ 86Sr (0.715–1.24) than the Upper Huang He, but the rivers at higher latitudes are similar to or lower than the Upper Huang He in CO 2 uptake by silicate weathering. In these orogenic zones, silicate weathering rates are only weakly coupled with temperature and become independent of runoff above ∼800 mm/yr.