SPECIAL SECTION: WATER Chemical weathering in the river basins of the Himalaya, India S. Krishnaswami* and Sunil K. Singh Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India Rivers transport weathered materials from land to the ocean. The chemistry of river waters is dictated by supply of various elements from both natural and anthropogenic sources. Among the natural sources, chemical weathering of the drainage basin is the dominant component, a process which consumes atmospheric CO2. On timescales of millions of years, atmospheric CO2 balance and hence global climate is influenced by chemical weathering process, silicate weathering in particular. The suggestion that silicate weathering in the Himalaya may be a driver of global cooling during the Cenezoic1–3 has prompted many studies on rivers draining the Himalaya, especially the source waters of the Ganga– Brahmaputra. This article reviews some of these studies and presents the current thinking on this topic. Keywords: Chemical weathering, Himalaya, CO2 drawdown, Ganga–Brahmaputra, Deccan. CONTINENTAL weathering and erosion are major components of the exogenic cycles of elements on the earth. Weathering breaks down rocks and the resulting dissolved and particulate materials are transported by rivers to the sea. Chemical weathering of rocks and minerals determines the flux of dissolved materials carried by rivers whereas physical weathering regulates the particulate transport. This makes the study of dissolved and particulate components of rivers important to characterize and quantify weathering and erosion. The study of chemical composition of river water is important not only for determining erosion rates, but also to learn about sources of elements to rivers, mineral weathering and elemental mobility and uptake of CO2 during chemical weathering. In addition, information on river chemistry is essential to assess water quality for domestic, agricultural and industrial usage. During the past 2–3 decades, many studies have been reported on the chemical and isotopic composition of Indian rivers4–10. Among the river basins studied, those from the Himalaya have received more attention mainly to elucidate the coupling between tectonics, weathering and climate. In recent years, there have also been studies of rivers draining the Deccan traps11,12 to determine the role of their weathering in contributing to global riverine fluxes and atmosphere CO2 draw-down. In this article, some of these issues, particu*For correspondence. (e-mail: swami@prl.ernet.in) CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 larly those pertaining to silicate weathering rates of Indian river basins, associated CO2 consumption and factors influencing them are reviewed based on available data on major ion chemistry of selected rivers. Data source India has a number of rivers all of which are fed mainly by water from monsoon rains. In addition, the rivers draining the Himalaya receive water from glaciers/snow melt during summer. Table 1 summarizes physical characteristics of a few selected rivers from India13,14. The rivers in India drain a total area of ~ 3.1 × 106 km2 and annually discharge ~ 1650 km3 of water. This translates into a mean run off of about 500 mm yr–1 for the entire India. The water discharge from India accounts for ~ 4.5% global river discharge. The pattern of monthly discharge of rivers mimics that of rainfall, with maximum for most rivers during July–August, coinciding with the peak of the more intense south-west monsoon rainfall (Figure 1). Some of the rivers draining the eastern and peninsular part of India also receive water from NE monsoon rains, their discharge therefore show effect of these rains as well (Figure 1). Water chemistry Sources of major ions to rivers The chemistry of river water is dictated by a number of sources. These include: Rain/precipitation. The primary source of water for rivers is rainfall and snow melt; which makes their composition an important component of river water chemistry. It is their chemistry which forms the base line for the evolution of river water composition. Rain water composition is locationdependent, near the coasts it is dominated by sea salt and in these regions the elemental ratios in rains are more similar to those in ocean. In inland regions, sea salt, continental dust, biogenic and anthropogenic inputs contribute to chemistry of rains. The relative significance of marine contribution to rain decreases with distance away from the coast and generally levels off to a constant low value inland. Na+ and Cl– are the dominant components of coastal rains, this changes to Ca+2, HCO3– and SO–2 4 inland. Typical major ion 841 SPECIAL SECTION: WATER Table 1. Drainage area and water discharge of major rivers of India10,11,13,14 River Area (105 km2) Station Ganga Ganga Yamuna Gomati Chambal Betwa Ken Yamuna Ghaghara Son Gandak Kosi Ganga Devprayag Batamandi Nr. Confluence with Ganga Nr. Confluence with Yamuna Hamirpur Nr. Confluence with Yamuna Allahabad Nr. Confluence with Ganga Nr. Confluence with Ganga Nr. Confluence with Ganga Nr. Confluence with Ganga Farakka 0.20 0.10 0.30 0.25 0.46 0.28 3.66 1.28 0.71 0.46 0.75 9.35 22 11 7 30 10 11 93 94 32 52 62 380 Brahmaputra Tsangpo Siang Dibang Lohit Brahmaputra Subansiri Burhi Dihing Jia Bhareli Dhansiri Kopili Manas Brahmaputra Yangcun Pasighat Sadeya Sadeya Dibrugarh Nr. Confluence with Brahmaputra Nr. Confluence with Brahmaputra Nr. Confluence with Brahmaputra Nr. Confluence with Brahmaputra Nr. Confluence with Brahmaputra Nr. Confluence with Brahmaputra Bahadurabad 6.36 2.46 0.13 0.24 3.23 0.33 0.08 0.12 0.12 0.16 0.38 6.30 29 200 63 60 323 54 14 26 20 28 32 670 Indus Jhelum Chenab Beas Sutlej Indus Mangla Merala Mandi Rupar Thatta 0.35 0.26 0.18 – 11.65 28 29 16 17 197 Peninsular rivers Mahanadi Godavari Krishna Bhima Pennar Cauvery Sabarmati Narmada Tapi Mouth Rajamundri Vijayawada Raichur Nellore Lower Anicut Mouth Bharuch Kathor 1.42 3.13 2.59 0.77 0.55 0.88 0.22 0.99 0.65 67 105 68 13 3 21 3 41 18 chemistry of rains over select regions in India is given in Table 2. Chemical weathering in drainage basins. Chemical reactions occurring in the drainage basins are the primary source of solutes to rivers. These reactions are of two types: (i) Dissolution reactions NaCl → Na+ + Cl–, (1) CaSO4 → Ca+2 + SO–2 4 . (2) 842 Discharge (km3 yr–1) These reactions are congruent dissolution and can be important source of ions to rivers draining terrains containing evaporites and saline/alkaline soils. (ii) Reactions requiring protons. The most common source of protons (H+) in rivers is carbonic acid generated by solution of CO2 from atmosphere in rains and from soil gas in river waters. The partial pressure of CO2 in soil gas is generally much higher than that of CO2 in the atmosphere because of oxidation of biogenic matter in the upper layers of soil, as a result rivers derive most of their carbonic acid from this source. In regions where vegetation is sparse, such CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 SPECIAL SECTION: WATER Table 2. Chemical composition of rain water over India µEq l –1 Location Na K Mg Ca Cl NO3 SO4 F HCO3 Coastal Goa Chembur Kalyan Kalyan Colaba Alibag Thumba Bombay 115 96 103 147 179 220 207 115 3 28 26 6 6 5 5.6 3.6 30 57 39 48 59 64 38 24 46 175 93 130 155 133 46 36 135 141 112 134 171 236 228 138 6 – 31 66 34 9 – – 32 421 108 110 52 36 14 10 – 5 6 – – – – – – – – – – – – – 7 117 21 14 12 8 – – 11 34 34 35 35 35 36 36 3 4 36 4 4 7.6 44 17 14 20 43 94 45.6 70 68 43 119 183 153 56.1 134 44 43 47 32 38 32 140 18 21 10 25 43 23 67 23 20 32 213 19 36 90 – – – 5 29 18 – – – 22 – 44 – – 26 3 13 15 48 40 26 37 38 39 34 38 34 34 7 4 10 – 68 – 9 Inland Pune Silent valley (Nilgiris) Hyderabad Korba Gopalpura (near Agra) Dayalbagh Delhi 39 46 40 21 21 18.4 82 Himalaya Nepal Himalaya 10 2 12 44 NH4 Ref. as bare mountains or glaciated areas, atmospheric contribution of CO2 to carbonic acid could be dominant. Examples of weathering reactions involving carbonic acid are: CaCO3 + (CO2 + H2O) ‡ Ca+2 + 2HCO–3 (3) CaSiO3 + (2CO2 + 3H2O) ‡ Ca+2 + 2HCO–3 + Si(OH)4 (4) 2NaAlSi3O8 + 2CO2 + 11H2O ‡ 2Na+ + 4Si(OH)4 (albite) + Al2Si2O5(OH)4 + 2HCO–3. (kaolinite) (5) Another source of protons for these reactions is sulphuric acid from the oxidation of pyrites 4FeS2 + 15O2 + 8H2O → 2Fe2O3 + 8H2SO4. (6) The protons liberated from H2SO4 reacts with various minerals, yielding the corresponding cations and SO–24. For example, weathering of albite by H2SO4 can be written as 2NaAlSi3O8 + H2SO4 + 9H2O → 2Na+ + SO–24 + 4Si(OH)4 + Al2Si2O5(OH)4. Figure 1. Monthly variations in rainfall and river discharge. In rainfall diagram the dashed line is for peninsular region, which experience both SW and NE monsoons and the solid line is for North–West region. Data from refs 13, 14, 42. Pennar river experiences NE monsoon and the Ganga SW monsoon. These discharges follow their rainfall patterns. CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 (7) Pyrites are more common in organic-rich sediments and hence in river basins containing them H2SO4 can be an important source of H+. In regions where there are abundant pyrites and other sulphides, there can be significant production of H2SO4. In such cases kaolinite and oxides of Fe (eqs (6) and (7)) would also react with H2SO4, producing other secondary minerals. 843 SPECIAL SECTION: WATER A third source of acids for chemical weathering is organic acids generated from partial oxidation of vegetation in the upper layers of soil. These include the humic and fulvic acids and others such as oxalic acid15. These acids weather rocks by solubilizing and complexing various elements including Fe and Al. These soluble complexes enter rivers and soil water. Many of these organic anions undergo oxidation in rivers to CO2: 4H2C2O4 + 2O2 + 7H2O + 2NaAlSi3O8 → 2Na+ + 2HCO–3 + 4Si(OH)4 + Al2Si2O5(OH)4 + 6CO2. (8) The end products of reactions (5) and (8) are nearly the same, except for CO2 produced from oxidation of oxalic acid in (8). Thus, from river water chemistry, it is sometimes difficult to quantitatively assess the role of organic acids in chemical weathering. Weathering reactions mediated by acids can be both congruent dissolution (eq. (3)) and incongruent dissolution. Many silicate weathering reactions fall in the latter category, resulting in the formation of secondary solids such as clays and oxides of Fe/Al. Anthropogenic input. Major ion abundances in rivers can be modified by anthropogenic inputs such as discharge of sewage, industrial and mining effluents, supply from fertilizers, etc. This input can be an important source for Na, Cl (NaCl in sewage, mining of sodium salts, solution of road salt, etc.), SO4 (fertilizers, mining of pyrites, industrial wastes, atmospheric deposition from fossil fuel burning, etc.) and nutrients (nitrogen and phosphorus compounds, mainly from fertilizers). It is estimated that on average ~ 30% of Na, Cl, SO4 and nutrients can be of anthropogenic origin16, for individual rivers, however, the contribution from this source can be significantly different from the mean. In addition, there can be two other potential suppliers of major elements to rivers. One is organic matter which during their growth incorporate elements such as N, P and K. Decay of organic matter can release these elements to rivers. Among these, the nutrients (N and P) are recycled and are generally reconverted to organic matter by plant uptake. Potassium, concentrated in plant leaves is from weathering of silicates. Table 3. Element Na K Ca Mg HCO3 Cl SO4 N species 844 Another supplier of major elements to rivers is springs/ groundwaters. Many rivers receive water from springs and groundwater, particularly during lean stages of their flow. The primary source of major ions to spring and groundwaters is chemical weathering of aquifer rocks. The importance of springs/groundwater on the abundances of major ions in rivers, though is recognized, it is difficult to quantify. More recently, Ge has been used as a tracer17 to estimate the water flow from springs to rivers in the Narayani basin of the Nepal Himalaya. Table 3 summarizes the various sources of major ions to rivers. Rivers integrate the major ion contributions from the sources discussed above. Table 4 lists the chemical composition of select rivers. Several features are evident from the data. Total dissolved solids (TDS) show a range of over one order of magnitude, from 35 to 587 mg l–1. These values are within the range recommended for potable water18. Among the various sources contributing major ions to rivers, supply from chemical weathering depends on the lithology of the basin. River basins, particularly those of medium and large size rivers, are multi-lithological comprising silicates/ carbonates and lesser amounts of evaporites. Chemical weathering of river basins would supply major ions to so- a b Sources of various dissolved elements Source Rain, halites, saline/alkaline soils, anthropogenic, silicate Rain, silicates, biogenic, fertilizers Silicates, carbonates, evaporites, fertilizers Silicates, carbonates Silicates, carbonate weathering, CO2 Rain, halites, anthropogenic Rain, evaporites, pyrites, fertilizers Rain, fertilizers Figure 2. Ternary diagram for major cation and anion abundances in select Indian rivers. The cations cluster around Ca apex and the anions around HCO3. The major ion abundances suggest predominance of carbonate weathering. CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 SPECIAL SECTION: WATER Table 4. Chemical composition of select Indian rivers µM River Na K Ca Mg F Cl Badrinath Rishikesh Arichaghat Apr–89 Apr–89 Apr–89 Apr–89 Apr–89 Apr–89 Apr–89 Apr–89 Apr–89 Apr–89 Oct–90 Apr–89 Nov–83 141 98 68 42 249 294 89 184 132 119 44 143 614 64 83 34 51 29 28 46 31 61 36 19 51 84 291 373 179 229 555 394 288 160 388 310 175 496 923 103 93 52 60 190 139 54 82 187 108 23 242 410 62 36 38 19 – – 26 42 16 37 36 18 – 13 6 11 9 30 55 13 45 56 27 5 27 175 5 11 15 19 – – 14 – 4 – – 13 – 364 460 46 45 69 94 120 24 33 47 76 210 – 174 174 427 543 1590 1144 524 601 1261 853 302 1241 2851 189 194 131 102 217 254 122 197 195 170 – 126 – 79 91 52 59 151 123 69 67 120 88 35 136 – 6 6 6 6 6 6 6 6 6 6 6 6 5 Indus Indus Sutlej Beas Beas Chandra Bhaga Darcha Shyok Tangstze Loma Salapar Hanogimata Beas Kund Tandi Tandi Darcha Sumur Tangstze Feb–92 Feb–92 Feb–92 Feb–92 Feb–92 Feb–92 Feb–92 Feb–92 Feb–92 452 84 48 22 24 28 24 341 410 34 82 25 14 21 19 20 68 78 522 622 148 61 353 332 189 567 818 200 193 39 18 76 114 54 197 285 – – – – – – – – – 210 25 29 – 13 11 7 249 208 – – – – – – – – – 128 306 50 – 123 187 117 144 339 1486 1313 347 – 916 594 322 1381 1833 – – – – – – – – – – – – – – – – – – 8 8 8 8 8 8 8 8 8 Yamuna Yamuna Yamuna Giri Bata Tons Tons Kemti fall Aglar Asan Batamandi Hanuman chatti Rampurmandi Batamandi Mori Kalsi Near Mussorie Yamuna bridge Simla road bridge Oct–98 Oct–98 Oct–98 Oct–98 Oct–98 Oct–98 Oct–98 Oct–98 Oct–98 255 67 259 275 64 152 54 154 330 51 56 49 42 45 52 26 28 46 1019 388 1658 954 182 808 2520 1099 1893 497 100 1065 460 35 411 2035 957 1080 8 16 10 7 11 11 10 11 60 30 48 77 19 29 49 39 225 35 333 14 113 58 1115 39 395 14 62 21 581 19 2913 28 1052 164 975 2369 780 2834 1867 383 1395 2340 2115 3840 211 92 231 241 116 186 142 217 351 254 88 399 229 49 202 587 318 479 10 10 10 10 10 10 10 10 10 Near origin Panchaganga con. Sep–02 Sep–02 Aug–81 174 484 257 2957 10 20 – 59 182 454 225 350 110 270 123 325 – – – – 133 287 286 – 19 39 – – 5 90 – – 585 1468 – – 309 334 97 271 75 168 – – 11 11 40 41 Dibrugarh Goalpara Apr–82 Apr–82 73 80 56 46 370 318 163 148 – – 19 29 – – 113 99 1021 884 172 118 107 92 5 5 Dobni Dholpur Hamirpur Mar–82 Mar–82 Mar–82 1149 1971 2377 99 68 53 794 610 502 695 555 690 – – – 258 594 426 – – – 255 336 71 3644 3095 4238 210 83 237 347 333 388 5 5 5 Deccan rivers Bhima Krishna Mahanadi Godavari Brahmaputra Brahmaputra Brahmaputra Rivers in plains Gomati Chambal Betwa Gangotri Gangotri Pilang Uttarkashi Nagun Seansu Ghuttu Jola Nailchami lution from all these lithologies. In the Himalaya, high TDS are in rivers of the Yamuna system particularly in its lower reaches and in the rivers from the plains. Some of these samples also have high Cl and SO4 indicative of major ion supply from weathering of evaporites, alkaline/saline soils and anthropogenic inputs5,10. Figure 2 a and b are ternary plots of cation and anion abundances in the rivers listed in Table 4. It is seen from Figure 2 a that most of the rivers from the Himalaya cluster around the Ca apex leaning CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 NO3 SO4 HCO3 Si TDS (mg l –1) Date Ganga Bhagirathi Kedarganga Pilang gad Asiganga Nagun gad Seansu gad Bhilangana Jola gad Nailchami gad Balganga Alaknanda Ganga Ganga Location Ref. towards Mg. In the anion diagram (Figure 2 b) these data points are close to alkalinity apex and lie along a mixing line with Cl and SO4. The distribution of data in Figure 2 is an indication that major ion abundances in these waters are dominated by carbonate weathering brought about mainly by CO2. This inference is also attested from the Mg/Na– Ca/Na ratio plot (Figure 3). The three end members presented in Figure 3 are the oceanic end member, silicates and carbonates from the Himalaya19. The data show that, rain 845 SPECIAL SECTION: WATER water ratios fall along the mixing line between oceanic end member and carbonates, whereas the ratios of the rivers from the Himalaya follow silicate–carbonate mixing. The Ca/Na ratios of rivers can be affected by removal of Ca by calcite precipitation. Calcite supersaturation is common among many rivers from the Himalaya, especially in its lower reaches20 and those from the plains. If this results in calcite precipitation, it would shift the Ca/Na data to the left of the mixing line. The observed scatter in Figure 3 seems to support such a removal process, however, it is difficult to confirm this removal because of uncertainties in the end member compositions. Silicate weathering rates Silicate weathering rate (SWR) per unit area is the riverine flux of dissolved major cations and silica derived from silicates in the basin SWR = Q∑(Na + K + Mg + Ca)sil + SiO2 = Q{TDS(s)}, where (X)sil is the dissolved riverine concentration of cations derived from silicate weathering and Q is the water discharge per unit area. To derive silicate weathering rates of river basins, it is necessary to extract the contributions of cations and silica from silicates to the measured major ion composition of rivers. This is one of the challenging problems in the determination of silicate weathering rates and associated CO2 consumption. The estimation of (X)sil from the measured major ion abundances is model dependent and relies on the use of a suitable proxy. A common proxy is Nasil, which is calculated as 1000 100 Car Mg/Na 10 Rain Ganga Indus Yamuna Deccan Brahmaputra Plain 1 Sil 0.1 Oceanic 0.01 0.01 0.1 1 10 100 1000 Ca/Na Figure 3. Mg/Na and Ca/Na ratio plots for rain and river water samples. The rain samples follow the mixing trend between oceanic and carbonate end members. The rivers follow the silicate–carbonate mixing trend; points falling off the mixing line. This could be because of variability in end member compositions and/or precipitation of Ca as calcite. 846 Nasil = Nar – (Narain + Nas), where the subscripts sil, r, rain and s refer to silicate, river, rainwater and halite/salinealkaline soils/anthropogenic inputs. Narain is the rainwater concentration of Na, appropriately corrected for evapotranspiration. Often (Narain + Nas) is approximated as equal to measured Cl in rivers, Clr. This makes Nasil equal to Nasil ≈ Nar – Clr. This approximation requires that Nas is composed only of NaCl, the validity of which can be in doubt in basins with alkaline/saline soils and which receive anthropogenic inputs. In case of K, its dominant source is silicate weathering with minor supply from rains, therefore, Ksil can be calculated by subtracting the rainwater K contribution from measured values in rivers. For Ca and Mg, their silicate components (Casil, Mgsil) are difficult to estimate as they have several sources (Table 3). Casil and Mgsil, therefore, are calculated assuming that they are released to rivers from silicates in the basin in a fixed proportion relative to Na. For the Himalayan rivers, the (Ca/Na) and (Mg/Na) ratios released to rivers is assumed to range from 0.2 to 1.0 and 0.3 respectively19 based on their ratios in rocks, soil profiles and small streams predominantly draining silicates. The wide range in the Ca/Na ratio is because rocks of both the Higher and the Lesser Himalaya contribute Ca and Na to rivers draining them, the relative proportions of which are not well quantified. Further, the approaches used to derive the ratios also differ. Another approach to derive contributions from various lithologies to the river water concentrations, is by the inversion method21. In this mixing equations are formulated for various elements w.r.t. Na and solved iteratively with an initial set of a priori parameters. Sr isotopes, 87Sr/86Sr, can also place constraints on silicate/ carbonate weathering contributions to rivers. Carbonates and evaporites inherit their 87Sr/86Sr signatures from sea water from which they form. These values are in the range of 0.7070–0.7094 during the past22 500 Ma. The 87Sr/86Sr of silicates show very wide variations, depending on their Rb content and age. The granites and gneisses of the Himalaya are some of the highly radiogenic rocks in 87Sr/86Sr. The Lesser Himalaya silicates have 87Sr/86Sr in the range of 0.78 to 0.90, whereas in the Higher Himalayan crystallines it varies between23,24 0.73 and 0.76. In contrast, the Deccan basalts are quite non-radiogenic and cluster around values25 of ~ 0.706. Thus, rivers draining silicates of Lesser and Higher Himalaya are expected to have more radiogenic 87Sr/86Sr compared to those weathering carbonates, evaporites and Deccan basalts. The Sr concentration in rivers also reflects its abundance in silicates and carbonates/evaporites and the kinetics of their weathering. Generally, silicate weathering results in lower Sr and higher 87Sr/86Sr and carbonate/ evaporite weathering in higher Sr and lower 87Sr/86Sr. As CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 SPECIAL SECTION: WATER a 0.8 0.82 b Sil 0.8 0.78 Sr/86Sr 0.76 87 87 Sr/86Sr 0.78 0.76 0.74 0.74 0.72 0.72 Car 0.7 0.7 0 0.001 0.002 0.003 0.004 0.005 0 0.002 0.004 0.006 0.008 0.01 1/Sr (nM) Figure 4. Two end member mixing diagram for Sr in rivers from the Himalaya. a, Major rivers; b, Major rivers and tributaries. Larger scatter in (b) is a reflection of variability in 87Sr/86Sr values in the individual drainage basins. Data from refs 7, 8, 27. Table 5. Rivers Bhagirathi Alaknanda Ganga Yamuna Ganga–Brahmaputra Krishna West flowing rivers from Deccan Deccan Silicate cations and silicate weathering rates Location Area (103 km2) Devprayag Bhagwan Rishikesh Batamandi – Alamatti – – 7.8 11.8 19.6 9.6 1555 36.3 – 500 Discharge (1012 l yr–1) 8.3 14.1 22.4 10.8 1002 17.3 – – TDS(s) (mg l –1) 14 9 11 25 21 30 32 – SWR (ton km–2 yr–1) 15 10.2 12.9 26 13.6 14 53 – CO2 draw-down (105 moles km–2 yr–1) 4 4 4 5 3 – – 4* *For the entire Deccan11. the rivers integrate Sr contributions from both silicates and carbonates/evaporites, their data would fall on a mixing line if the end member compositions are unique. The 87Sr/ 86 Sr data for larger Himalayan rivers, indeed show an overall two component mixing trend (Figure 4 a). The trend, however, becomes less pronounced, as data from smaller tributaries are added to the plot (Figure 4 b). The increase in scatter reflects scatter in the end member 87Sr/86Sr values, particularly silicates, of the various basins. The above simple interpretation of two-end member mixing is often challenged, especially for the Himalayan rivers, because of potential contributions from disseminated calcites and metamorphized carbonates (calc-silicates) which are shown to have highly radiogenic Sr isotope composition26. The impact of these sources on the Sr isotope budget of rivers draining the Himalaya are still debated27,28. Krishnaswami and Singh29 have shown that in the Ganga head waters the source of high 87Sr/86Sr is silicate weathering, a similar conclusion has also been arrived by Bickle et al.28 and Dalai et al.27 for the Ganga and the Yamuna head waters. Recently, HCO3 and Mg concentrations in rivers have been used to derive silicate weathering rates of basalts11,12. The use of these proxies requires that they are supplied to rivers CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 only from silicate weathering, an assumption which may be violated if carbonate weathering is a source of dissolved inorganic carbon and Mg to these rivers. It is, however, demonstrated by Das et al.11 that for Deccan basalts, Mg is a good proxy to derive basalt weathering rates, and that (Ca/Mg) and (Na/Mg) ratios released to rivers are roughly the same as in Deccan basalt, suggestive of near congruent weathering of these elements in basalts. Table 5 presents TDS(s) in selected rivers and associated silicate weathering rates. The SWR for the head waters of the Ganga in the Himalaya (Bhagirathi, Alaknanda and Yamuna) is in the range of 10–25 tons km–2 yr–1 (4–10 mm k yr–1) The SWR for these rivers are factors of 2–5 higher than reported for world average attributable to enhanced weathering due to mountain uplift and monsoonal climate. Factors influencing silicate weathering rates A key issue in weathering studies is to understand the various factors especially the role of temperature in regulating weathering. This is because uptake of CO2 by silicate weathering serves as negative feedback for atmospheric 847 SPECIAL SECTION: WATER CO2. Weathering rate and temperature are related by the Arrhenius equation: SWR = Aexp(–Ea/RT), where Ea is the activation energy and T the temperature. Typical Ea for feldspars are in the range of 50–80 kJ/mol. For a temperature range of 20°C (10 to 30°C) and Ea of 60 kJ, the SWR change would be a factor of ~ 3. This difference in SWR should be measurable, if other factors remain nearly the same. Dalai et al.10 observed a strong temperature dependence of SWR for the Yamuna tributaries in the Lesser Himalaya (Figure 5). Similarly, Dessert et al.12 also reported temperature dependence for basalt weathering, based on a global database. In contrast, Edmond and Huh30, based on geochemical data from large rivers at different latitudes, concluded that there is no discernible temperature dependence on weathering and that physical mechanisms of exposure and transport dominate weathering processes. Thus, at present, this is a controversial topic, resolution of which is compounded by the fact that in field studies several factors regulate the weathering rates making it difficult to critically assess the temperature dependence. Runoff and physical erosion are the physical parameters which also contribute to enhanced chemical erosion31,32. Higher runoff and physical erosion helps in the transport of weathered materials and exposing fresh surfaces for weathering. Both these parameters, however, may be linked to temperature. Lithology of the basins is another important variable determining the chemical weathering rate. Basalts are known to weather more rapidly than granites and gneisses33. However, the recent findings of Das et al.11 that under favourable conditions granites/gneisses of the Himalaya weather at rates similar to that of Deccan basalts suggest that differ- ences in kinetics of weathering of these rock types can be compensated by other physical parameters. Other factors such as vegetation, soil cover and frost shattering also contribute to variability in weathering rates in river basins. Thus, the dependence of weathering rates on a multitude of parameters continues to make the quantification of each of their roles a difficult issue. A focus of future research in continental weathering should be on this topic. CO2 consumption by silicate weathering One of the goals of estimating the silicate cation abundances in rivers is to obtain CO2 consumption rate by silicate weathering. Eqs (4) and (5) show that a definite relation exists between the CO2 consumption and major ions released to rivers, for every equivalent of silicate cations released, one mole of CO2 is consumed. Table 5 also lists the CO2 uptake by some of the rivers in India. The results show that the head waters of the Ganga in the Himalaya consume 4 × 105 mole CO2 km–2 yr–1. This is a factor of ~ 3 higher than the global average CO2 consumption by silicate weathering31. The uplift of the Himalaya, favourable monsoon climate, intense physical weathering all contribute to enhanced silicate weathering in this region and associated CO2 consumption. Analogous to these basins in the Himalaya, weathering of Deccan basalts also have high CO2 consumption rate, in the range of 0.5 to 10 × 105 moles km–2 yr–1. These results demonstrate that under favourable conditions, both grainites/gneisses and basalts weather roughly at the same rates with similar rates of CO2 consumption. This observation is significant in assessing the various factors influencing silicate weathering rates. Summary 3.55 Chemical and isotopic studies of Indian rivers have taken major strides during the last 1–2 decades. These studies have generated high quality data, which have provided better quantification of chemical and silicate weathering rates of various basins, associated CO2 consumption and factors contributing to their variations. Most of the conclusions, however, are based on limited sampling, particularly on temporal scales. More work needs to be done during peak discharge over several years to obtain weathering rates with less uncertainties. Another issue is the role of more easily weatherable minor lithologies in contributing to major ion chemistry and isotope systematics of the rivers. Developing suitable proxies to resolve this is important. The use of Re– Os, U isotopes, Ca, Mg isotopes to quantify erosion timescale, and weathering of organic rich sediments, silicates/carbonates should be informative. Figure 5. Arrhenius plot for Si in Yamuna tributaries10. The trend is suggestive of dependence of silicate weathering on temperature with an ‘average’ activation energy of 51 kJ mol–1. 1. Molnar, P. and England, P., Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg. Nature, 1990, 346, 29–34. 2.8 log Si (µ µM) 2.4 2 1.6 3.25 3.3 3.35 3.4 3.45 3.5 1/Tx1000 848 CURRENT SCIENCE, VOL. 89, NO. 5, 10 SEPTEMBER 2005 SPECIAL SECTION: WATER 2. Raymo, M. E., Ruddiman, W. F. and Froelich, P. N., Influence of Late Cenozoic mountain building on ocean geochemical cycle. Geology, 1998, 16, 649–653. 3. Raymo, M. E. and Ruddiman, W. F., Tectonic forcing of late Cenozoic climate. Nature, 1992, 359, 117–122. 4. Sarin, M. M. and Krishnaswami, S., Nature, 1984, 312, 538– 541. 5. Sarin, M. 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