Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: Chemical weathering, erosion, and CO2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

Abstract

The Yarlung Tsangpo-Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo-Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO 2 consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo-Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km À2 yr À1 and the silicate cation flux more than 34 tons km À2 yr À1 . In total, chemical weathering in this area consumes 15.2 · 10 5 mol CO 2 km À2 yr À1 , which is twice the Brahmaputra average. These data show that 15-20% of the total CO 2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na + and Cl À fluxes throughout southern Tibet, comprising more than 50% of all Na + in some stream systems. Carbonate weathering generates 80-90% of all dissolved Ca 2+ and Mg 2+ cations in much of the Yarlung Tsangpo catchment.

Hydrochemistry of the middle and upper reaches of the Yarlung Tsangpo River system: weathering processes and CO2 consumption

“…To address the additions of solute cations and Cl -from atmospheric input, the method of multiplying the measured molar concentration of each cation by the corresponding ion/Cl -ratio measured in rainwater is adopted. The correction for rainwater input is performed under the assumption that rainwater has the Cl -content of the drainage with the lowest Cl -concentration in the catchment (Hren et al 2007). The lowest Cl -concentration at site S5 (Cl -= 5.0 and 9.2 lmol/l during two periods, respectively) is taken as being supplied entirely by atmospheric input.…”

“…Therefore, we referred to previous studies of local silicate weathering to determine the ratios. In the following discussion, we assign 0.55 and 0.3 to Ca/Na, Mg/Na (Hren et al 2007). …”

“…Recently, those rivers originating in the southern slopes of the Himalaya have gained great concern. A number of arguments have suggested that the uplift of the Himalaya and heavy monsoonal precipitation promote rapid physical erosion and chemical weathering and thereby enhance CO 2 consumption (Dalai et al 2002;Hren et al 2007;Wu et al 2008a).…”

“…However, there are only a few studies on the hydrochemistry of Yarlung Tsangpo, especially the middle and upper parts, due to the high elevation and oxygen-deficient atmospheres. Previous studies on the Yarlung Tsangpo have focused on water quality (Huang et al 2011) and the data are mainly from the mainstream (Hren et al 2007). No information is available on weathering processes and the associated CO 2 consumption rates about the reaches and tributaries in the middle and upper parts of Yarlung Tsangpo River.…”

Comparison of surface water chemistry and weathering effects of two lake basins in the Changtang Nature Reserve, China

“…In recent decades, research on the surface water chemistry of the Qinghai-Tibet Plateau has mostly focused on areas with convenient access and significant impacts of human activities, such as the Qaidam Basin (Tan et al, 2011), Qinghai Lake (Xiao et al, 2012;Xu et al, 2010), and river source regions (Noh et al, 2009;Qin et al, 2006;Wu et al, 2005Wu et al, , 2008 in the Qinghai Province, China. Investigations and analyses of large lakes and rivers located in central and southern Tibet are equally abundant, such as studies of Lake Nam Co Zhang et al, 2008), Lake Yamzhog Yumco (Sun et al, 2013;Zhang et al, 2012), Lake Mapam Yumco (Wang et al, 2013), and the Yarlung Tsangpo River Basin (Guilmette et al, 2009;Hren et al, 2007). These studies have greatly enhanced the knowledge about the chemical characteristics and evolution of lake and river waters on the Qinghai-Tibet Plateau.…”