CHEMICAL WEATHERING AND HOT SPRINGS IN THE BHUTAN HIMALAYA

The chemical budgets of rivers draining the Himalayan range have been used as a means of assessing the impact of weathering of silicate minerals in the range on the long-term carbon cycle. Such weathering followed by subsequent marine carbonate precipitation is the fundamental sink for atmospheric CO₂ in the geologic carbon cycle. Weathering reactions produce alkalinity in rivers and thus dissolved load 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. Hot springs are abundant along the Himalayan front and have been shown to significantly impact solute and carbon fluxes as well. While weathering studies are abundant for the drainage basins of the Nepal and Indian Himalaya, they are lacking for Bhutan.

Here we present new major element and isotopic data for four hot spring systems from eastern Bhutan and major element and carbon isotope data for 35 rivers and streams across the Himalayan region of Bhutan. Within Bhutan, watersheds are largely underlain by the gneisses and metasediments of the High Himalayan Crystalline Series (HHC), with only the high reaches of the major streams flowing over the Tethyan Sedimentary Sequence (TSS) carbonates. The rivers of Bhutan generally flow north to south through deeply incised gorges and are major tributaries to the Brahmaphutra. Water samples were collected from hot springs as well as streams at the end of the 2010 monsoon season (late August–September). Because of the strong seasonality of precipitation in Bhutan, these late-monsoon river samples are taken to be reasonable first-order proxies for calculating annual dissolved load fluxes. The rivers are characterized by high calcium, with bicarbonate as the dominant anion, typical of carbonate weathering regimes while hot springs are more variable, with high Na⁺, Cl^- and HCO₃^-. In those basins containing hot springs, the impact is clear with increases in downstream TDS. Hot spring alkalinity is dominantly silicate in origin, typically up to 100%, similar to springs across the range while silicate alkalinity makes up on average 35% of the total alkalinity budget for the rivers of Bhutan, with a minimum of 5% and a maximum of 85%. This despite significant HHC dominated drainages.