PII S0016-7037(02)00937-7 Major ion chemistry in the headwaters of the Yamuna river system: Chemical weathering, its temperature dependence and CO 2 consumption in the Himalaya T. K. DALAI, S. KRISHNASWAMI* and M. M. SARIN Physical Research Laboratory, Navrangpura, Ahmedabad - 380 009, India (Received November 21, 2001; accepted in revised form May 1, 2002) 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) CO 2 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. CO 2 drawdown resulting from silicate weathering in the YRS basin in the Himalaya during monsoon ranges between (4 to 7)  10 5 moles km -2 y -1 . This is higher than that estimated for the Ganga at Rishikesh for the same season. The CO 2 consumption rates in the Yamuna and the Ganga basins in the Himalaya are higher than the global average value, suggesting enhanced CO 2 drawdown in the southern slopes of the Himalaya. The impact of this enhanced drawdown on the global CO 2 budget may not be pronounced, as the drainage area of the YRS and the Ganga in the Himalaya is small. The CO 2 drawdown by silicates in the YRS basin is marginally higher than the reported values of CO 2 release from oxidation of organic rich sediments, estimated using Re as a proxy. This comparison shows the need to constrain CO 2 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 10 mm ky -1 and on the Ganga basin, it is 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. Copyright © 2002 Elsevier Science Ltd 1. INTRODUCTION Studies on chemical and isotopic composition of rivers pro- vide important information on the sources of major ions to them, chemical weathering rates of basins and on fluxes of dissolved materials from continents to oceans. Knowledge of chemical weathering rates is essential for determination of CO 2 consumption rates and their impact on global climate. Among various rivers of the world, those draining the Himalaya have received special attention in recent years because of possible connection between Himalayan uplift and Cenozoic climate change (Raymo and Ruddiman, 1992). In this “tectonics- weathering-climate” hypothesis, the uplift of the Himalaya has been suggested as a major driver of Cenozoic cooling as it can promote rapid silicate weathering and thereby enhance CO 2 drawdown from the atmosphere. The validity of this hypothesis can be tested by determining silicate weathering rates, an exercise which requires extensive data on the chemical com- position of rivers draining the Himalaya. The work described in this paper is a part of our continuing studies on rivers draining the Himalaya and is focussed on the headwaters of the Yamuna, one of the large river systems draining the southern slopes of the Himalaya. The Yamuna is a major tributary of the Ganga and they merge together at Alla- habad in the Gangetic plains. Previous work on rivers, draining the southern slopes of the Himalaya, include those on the major ion chemistry of the Ganga and its headwaters, the Indus and the Ghaghara (Sarin et al., 1989, Sarin et al., 1992; Pande et al., 1994 and Krishnaswami et al., 1999). These studies have brought out the dominant role of carbonate weathering in contributing to the dissolved load of these rivers and have provided estimates of CO 2 consumption rates via silicate weathering in their drainage basins. Galy and France-Lanord (1999), based on studies of major ions and  13 C of dissolved inorganic carbon in the Narayani watershed in Nepal, have * Author to whom correspondence should be addressed (swami@prl. ernet.in). Pergamon Geochimica et Cosmochimica Acta, Vol. 66, No. 19, pp. 3397–3416, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 + .00 3397