MAJOR ELEMENT GEOCHEMISTRY AND STRONTIUM ISOTOPE RATIOS OF THE UPPER REACH OF THE HUANG HE

We examined the geochemistry of the upper reach of the Huang He (Yellow River) on the Qinghai-Tibet Plateau to determine whether lithology or climate controls the weathering fluxes in the basin. The results show that carbonate and evaporite weathering are dominant, contributing 71 to 96% of the dissolved cations in the river waters. Weathering of silicates contributes a minor proportion (4 to 28%) of the total cations. The two main tributaries, the Huang Shui and Tao He, have different major ion composition but yield similar amounts of dissolved load. Given the similar lithology of sandstone, limestone and evaporite-bearing red beds but different climate in the two basins, this finding indicates climate (particularly, precipitation and evaporation) does affect the major element concentrations in river waters but has little effect on the weathering fluxes. Net CO₂ consumption rates (20 -100 x 10³ mol/km²/yr), calculated from silicate weathering fluxes, in the study basin are low compared to other rivers draining high mountains such as the Amazon and Orinoco draining the Andes and the Fraser draining the Rockies, but higher than the Mackenzie draining the Rockies and the Yukon. We attribute the variation in silicate weathering fluxes in the above basins to the lithology, i.e., type and extent of exposures (silicates vs. carbonates or evaporites). The Amazon and Orinoco drain marine limestones, continental red bed and andesites on the Andes, the Fraser drain metamorphic and granitic rocks and Tertiary volcanics on the Rockies; and the Mackenzie draining the Rockies and the Yukon are underlain with carbonates, sandstones and shales with rare igneous rocks. On the other hand, the total dissolved yields of these rivers are all comparable with each other regardless of their different climate regimes (runoff, temperature).