GEOCHEMISTRY AND CHEMICAL WEATHERING BUDGETS FOR HIMALAYAN RIVERS OF BHUTAN

Weathering of silicate minerals in the Himalaya since the Miocene has been suggested as a significant sink for atmospheric carbon dioxide. Weathering reactions produce alkalinity in rivers and thus river chemistry can be an effective means for assessing the total weathering budget of a watershed as well as for apportioning weathering fluxes between silicate and carbonate mineral sources. While weathering studies using river chemistry are abundant for the drainage basins of the Nepal and Indian Himalaya, they have until now been lacking for Bhutan. Here we present major element data for 35 rivers and streams across the Himalayan region of Bhutan. Most of the rivers in Bhutan flow south to north through deeply incised gorges. Watersheds are largely underlain by the gneisses and metasediments of the Greater Himalayan sequence of rocks. Samples were taken from all the major and most minor rivers of Bhutan at the end of the monsoon season (late August–September 2010). Because of the strong seasonality of precipitation in Bhutan, these late-monsoon samples are likely a reasonable proxy for calculating annual dissolved load fluxes. The rivers are characterized by high calcium, with bicarbonate as the dominant anion. We note that in some cases, there is the strong influence of hot spring fluids in our stream samples, with 100-fold increases in downstream TDS. Initial analyses of our samples show that silicate alkalinity makes up on average 62% of the total alkalinity budget for the rivers of Bhutan, with a minimum of 38% and a maximum of 94%. This is higher than in Nepal, where carbonate weathering typically dominates stream dissolved loads (silicate alkalinity ~40%). This indicates that weathering in Bhutan may be a larger carbon dioxide sink than in neighboring areas along the Himalayan arc.