PRESENT DAY CHEMICAL WEATHERING IN THE LARGE RIVERS OF THE TIBETAN PLATEAU AND ADJACENT AREAS

According to the uplift weathering hypothesis (Raymo and Ruddiman, 1992), active tectonic uplift associated with the collision of India and Asia in the Cenozoic caused strong weathering of freshly exposed rock. This increases the riverine transport of dissolved elements to the ocean thereby perturbing their long-term geochemical cycle. Chemical weathering of silicate rocks removes CO₂ from the atmosphere and cools the climate. The rapid rise in seawater ⁸⁷Sr/⁸⁶Sr and ¹⁸⁷Os/¹⁸⁸Os ratios and δ¹⁸O as recorded in marine sediments is attributed to the uplift and weathering of Tibet-Himalayas and resulting climatic deterioration. In this context, it is of some importance to determine the spatial variation of chemical weathering rates and pattern in the Tibet-Himalaya region.

For this we turn to the major river systems draining the Himalayas and the Tibetan Plateau—the Indus, Ganges and its major tributaries (Yamuna, Ghaghara, Gandak, and Sepat Kosi), Brahmaputra, Salween, Mekong, Jinsha (upper Yangtze), Red, and Yellow. The diverse lithologies that the rivers drain are manifested in their major element composition. The rivers are predominantly Ca-HCO₃ type, but there are tributaries that show high Na-Cl-SO₄ content due to proximity to evaporite deposits or hot springs. Headwater tributaries of the Indus and Ganges are comparatively higher in Si, resulting from silicate weathering. The Indus headwaters drain the arc and have unradiogenic Sr isotope values and are magnesian. The Ganges headwaters in the high Himalayas draining high grade metamorphic rocks have radiogenic ⁸⁷Sr/⁸⁶Sr. The Mekong and Jinsha headwaters drain greenschist facies flysch and shales, and their fluvial chemistry is dominated by carbonate and unradiogenic ⁸⁷Sr/⁸⁶Sr. The headwaters of the Yellow drain the arid interior of the plateau and again unradiogenic in terms of ⁸⁷Sr/⁸⁶Sr. Overall, based on our ~350 samples and a similar number of samples reported in the literature, the headwaters of the Ganges and its tributaries are unique in their high Sr isotope ratios and concentrations. Because of the volcanics and the weakly metamorphosed lithology, other major river systems draining the Himalaya-Tibet do not have such anomalous Sr values. Total chemical weathering rates in the eastern Tibet are of similar magnitude as the Central portion.