Major ion chemistry in the headwaters of the Yamuna river system: chemical weathering, its temperature dependence and CO₂ consumption in the Himalaya

Abstract

The Yamuna river and its tributaries in the Himalaya constitute the Yamuna River System (YRS). The YRS basin has a drainage area and discharge comparable in magnitude to those of the Bhagirathi and the Alaknanda rivers, which merge to form the Ganga at the foothills of the Himalaya. A detailed geochemical study of the YRS was carried out to determine: (i) the relative significance of silicate, carbonate and evaporite weathering in contributing to its major ion composition; (ii) CO₂ consumption via silicate weathering; and (iii) the factors regulating chemical weathering of silicates in the basin. The results show that the YRS waters are mildly alkaline, with a wide range of TDS, ~32 to ~620 mg^-1. In these waters, the abundances of Ca, Mg and alkalinity, which account for most of TDS, are derived mainly from carbonates. Many of the tributaries in the lower reaches of the Yamuna basin are supersaturated with calcite. In addition to carbonic acid, sulphuric acid generated by oxidation of pyrites also seems to be supplying protons for chemical weathering. Silicate weathering in YRS basin contributes, on average, ~25% (molar basis) of total cations on a basin wide scale. Silicate weathering, however, does not seem to be intense in the basin as evident from low Si/(Na+K) in the waters, ~1.2 and low values of chemical index of alteration (CIA) in bed sediments, ~60. CO₂ drawdown resulting from silicate weathering in the YRS basin in the Himalaya during monsoon ranges between (4 to 7) × 10⁵ moles km^-2 y^-1. This is higher than that estimated for the Ganga at Rishikesh for the same season. The CO₂ consumption rates in the Yamuna and the Ganga basins in the Himalaya are higher than the global average value, suggesting enhanced CO₂ drawdown in the southern slopes of the Himalaya. The impact of this enhanced drawdown on the global CO₂ budget may not be pronounced, as the drainage area of the YRS and the Ganga in the Himalaya is small. The CO₂ drawdown by silicates in the YRS basin is marginally higher than the reported values of CO₂ release from oxidation of organic rich sediments, estimated using Re as a proxy. This comparison shows the need to constrain CO₂ sources and sinks better to balance its budget in a regional scale. The results also show that silicate weathering rate in the YRS basin is not, vert, similar 10 mm ky^-1 and on the Ganga basin, it is not, vert, similar 5 mm ky^-1, which are several times lower than the carbonate weathering rates. The significantly higher silicate weathering rate observed in the YRS basin seems to be governed by rapid physical erosion in this region. The apparent activation energy for overall silicate weathering in the YRS basin, derived from Na and Si concentrations and water temperature, ranges from ~50 to 80 kJ mol^-1. These values are comparable to those reported for granitoid weathering in natural watersheds and feldspar weathering in laboratory experiments. This study brings to light the sources contributing to major ions, enhanced chemical weathering rates in the Yamuna River Basin and interdependence of silicate weathering on physical erosion and temperature.