CHEMICAL WEATHERING, EROSION, AND CO2 CONSUMPTION IN THE SOUTHERN TIBETAN PLATEAU AND EASTERN SYNTAXIS OF THE HIMALAYA: MAJOR ION CHEMISTRY OF THE YARLUNG TSANGPO/BRAHMAPUTRA RIVER

The Yarlung Tsangpo/Brahmaputra river drains a large portion of the eastern Himalaya and southern Tibetan plateau as well as the eastern Himalayan syntaxis, one of the most tectonically active areas of the globe. We measured the solute chemistry of over one hundred tributaries along the Yarlung Tsangpo/Brahmaputra river to identify the controls of stream chemistry in this area, to examine the relationships between tectonic, climatic, and geologic factors and chemical weathering rates, and to quantify chemical weathering fluxes from the eastern syntaxis of the Himalaya. We show that the dissolved stream chemistry of the tributaries of the Yarlung Tsangpo is dominated by the weathering of carbonate, which provides 80 to 90 percent of all dissolved Ca2+ and Mg2+ cations in streams, and silicate weathering reactions which result in the breakdown of plagioclase and production of kaolinite. More importantly, chemical weathering rates throughout this area are strongly influenced by both climatic and tectonic factors. In the eastern syntaxis, high annual precipitation and rapid tectonic uplift generates high dissolved chemical fluxes. Chemical weathering rates in this portion of the basin are several times the Brahmaputra average. These data show that as a result of tectonic and climatic factors, 15 to 20 percent of the total CO2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 3.5 percent of the total land area of the basin.