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Research paper

Controls on intense silicate weathering in a tropical river, southwestern India

G.P. Gurumurthy a, K. Balakrishna a,⁎, Jean Riotte b,c, Jean-Jacques Braun b,c, Stéphane Audry c,H.N. Udaya Shankar a, B.R. Manjunatha d

a Department of Civil Engineering, Manipal Institute of Technology, Manipal University, Manipal 576104, Indiab Indo-French Cell for Water Sciences, Joint IRD-IISc Laboratory, Indian Institute of Science, Bangalore 560012, Indiac GET UMR 5563, Université Paul Sabatier, IRD and CNRS, 14, avenue E. Belin, 31400 Toulouse, Franced Department of Marine Geology, Mangalore University, Mangalagangothri 574199, India

a b s t r a c ta r t i c l e i n f o

Article history:Received 5 April 2011Received in revised form 15 December 2011Accepted 14 January 2012Available online 25 January 2012

Edited by B. Sherwood Lollar

Keywords:Silicate weatheringGranitic–gneissic terrainTropicalNethravati RiverCO2 consumption

The Silicate Weathering Rate (SWR) and associated Carbon dioxide Consumption Rate (CCR) in tropical sili-cate terrain is assessed through a study of the major ion chemistry in a small west flowing river of PeninsularIndia, the Nethravati River. The specific features of the river basin are high mean annual rainfall and temper-ature, high runoff and a Precambrian basement composed of granitic-gneiss, charnockite and minor meta-sediments. The water samples (n=56) were collected from three locations along the Nethravati River andfrom two of its tributaries over a period of twelve months. Chemical Weathering Rate (CWR) for the entirewatershed is calculated by applying rainwater correction using river chloride as a tracer. Chemical Weather-ing Rate in the Nethravati watershed is estimated to 44 t.km−2.y−1 encompassing a SWR of 42 t.km−2.y−1

and a maximum carbonate contribution of 2 t.km−2.y−1. This SWR is among the highest reported forgranito-gneissic terrains. The assessed CCR is 2.9·105 mol.km−2.y−1. The weathering index (Re), calculatedfrom molecular ratios of dissolved cations and silica in the river, suggests an intense silicate weathering lead-ing to kaolinite–gibbsite precipitation in the weathering covers. The intense SWR and CCR could be due to thecombination of high runoff and temperature along with the thickness and nature of the weathering cover.The comparison of silicate weathering fluxes with other watersheds reveals that under similar morpho-climatic settings basalt weathering would be 2.5 times higher than the granite–gneissic rocks.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The interaction of water, atmospheric CO2 and rocks at thesurface of the continents results in the dissolution of solublesalts of the primary rock forming minerals, which are trans-ported by rivers into the oceans. The dissolution of CO2 inwater provides the necessary protons for the dissolution of pri-mary rock forming minerals, resulting in the formation of sec-ondary clays and clay minerals (White and Brantley, 1995, andreferences therein). Chemical weathering of silicate rocks is ofprimary importance in the long-term global climate change be-cause for every mole of silicate rock weathered an equal amountof CO2 is withdrawn from the atmosphere and sequestered insediments (Eqs. (1) and (2)) (Garrels and Mackenzie, 1971;Berner et al., 1983; Berner, 1991). On the other hand, themajor ion chemistry of most of the world rivers are dominatedby weathering of carbonate rocks where there is no significantatmospheric CO2 drawdown (Gaillardet et al., 1999); that is,CO2 utilized for the dissolution of rock mineral will be further

released to the atmosphere during precipitation of carbonatesin the ocean (Eqs. (3) and (4)).

CaSiO3 þ 2CO2 þ H2O→Ca2þ þ 2HCO

−3 þ SiO2 ð1Þ

Ca2þ þ 2HCO3→CaCO3↓ þ H2O þ CO2↑ ð2Þ

CaCO3 þ CO2 þ H2O↔Ca2þ þ 2HCO

−3 ð3Þ

Ca2þ þ 2HCO3→CaCO3↓ þ H2O þ CO2↑ ð4Þ

Understanding the forcing factors and retroaction loops on thelong-term Chemical Weathering Rates (CWR) and associated Carbondioxide Consumption Rate (CCR) is a major challenge. CWR is regulat-ed by multiple co-dependent factors such as lithology, relief (i.e. tec-tonics), climate (runoff and temperature) and vegetation (see e.g.Garrels and Mackenzie, 1967; Drever and Zobrist, 1992; Bluth and

Chemical Geology 300-301 (2012) 61–69

⁎ Corresponding author. Tel.: +91 820 2924724; fax: +91 820 2571071.E-mail address: k.balakrishna@manipal.edu (K. Balakrishna).

0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2012.01.016

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Kump, 1994; Gaillardet et al., 1995; White and Blum, 1995; Stallard,1995; Viers et al., 1997).

Worldwide, the studies are based on the examination of small tomedium watersheds (1 to 100 km2) with, as much as possible, homo-geneous lithology (e.g. granito-gneiss or basalt) or on large river basinsincluding variable lithologies. In the Indian subcontinent, studies of thechemical weathering mostly focused on the large Himalayan River ba-sins (e.g. Sarin et al., 1989; Pande et al., 1994; Galy and France-Lanord,1999; Singh et al., 2005) and on smaller rivers draining the Deccan ba-salts (Dessert et al., 2001; Das et al., 2005). The rivers and streamsdraining the Precambrian silicate basement of Peninsular India have re-ceived much less attention, particularly the rivers originating from theWestern Ghats and flowing towards the Arabian Sea. These rivers arecharacterized by both high runoff and temperature. Their combineddischarge is about 200 km3, which corresponds to an average surfacerunoff of 1775 mm, and accounts for 12.5% of the entire Indian riverswater discharge (www.nih.ernet.in; Krishnaswami and Singh, 2005).The studies on watershed having high runoff and warmer temperatureare scarce in the present day worldwide database (Oliva et al., 2003and West et al., 2005). Therefore, the study of the west flowing riversof India would give a new insight to the chemical weathering models.

Here, we report new data on the chemical weathering rates andassociated carbon dioxide consumption from a west-flowing riverdraining granito-gneissic bedrocks, the River Nethravati. The presentstudy focuses on the (1) present day chemical weathering rate andcarbon dioxide consumption in the Nethravati River, (2) the possiblecontrols on the silicate weathering rate, by comparing the resultswith west-flowing rivers draining basalts and with other smallstreams draining silicates.

2. Study area

The Nethravati River originates at an elevation of 1000 m amsl, inthe densely forested Western Ghats near Samse of Chickamagaloredistrict. It lies between the latitude of 12°29′11″N and 13°11′11″Nand longitude of 74°49′08″ E and 75°47′53″E (Fig. 1). It is surroundedon the north by the Tunga–Bhadra River system, on the east by Cau-very basin, on the south by Payaswani basin and on the west by Ara-bian Sea. It flows southwest for 147 km and meets the Arabian Seaforming a common estuary with Gurupur River at Mangalore. Thetotal drainage area of the river is 3657 km2, which together with

Gurupur River represents about 8% of the total discharge of waterby the west flowing rivers of Peninsular India (Rao, 1979;Manjunatha and Shankar, 1992). The major tributaries of the Nethra-vati River are Kumaradhara, Shishila hole, Gundiya hole and Neriyahole (Fig. 1). The upper catchment is overlaid by dense thick greenforest cover and the lower one, the Konkan coastal plain, by higherpopulation density, i.e. agriculture accompanied by small scaleindustries.

Geologically, the Nethravati watershed falls in the metamorphictransition zone of theWest Dharwar Craton composed of granodiorit-ic gneissic complex of Archean age (Trondhjemite–tonalite–granodi-orite suite; Naqvi and Rogers, 1987). The high peaks of the WesternGhats and the southern part of plateau are composed of granuliticrocks. A metamorphic transition zone separates the southern andnorthern parts of the Dharwar Craton: in the northern part, gneissicrock with schist belts are metamorphosed to grades lower than am-phibolite facies whereas in the southern part, both gneissic and schistbelts are largely metamorphosed to granulite grade resulting in theformation of charnockites, pyroxene granulites and high grade am-phibolite assemblages (John et al., 2005). The Nethravati River drainsboth sides of the metamorphic transition zone. The major lithologiesof the River Nethravati encompass meta-sediments, banded iron for-mations, amphibolitic facies gneisses, and foliated charnockites trans-formed into pyroxene-bearing granulites through influx of CO2 richfluid (Radhakrishna and Vaidyanadhan, 1997). The gneisses containquartz, hornblende, hypersthene, plagioclase, biotite and the char-nockite contains quartz, hornblende, hypersthene, plagioclase and or-thoclase as major minerals (Sharma and Rajamani, 2000). TheNethravati watershed is composed of about 83% migmatites andgranodiorites, 5% of charnockites, about 6% metasediments and 2%amphibolites (Fig. 1).

In the Konkan coastal plain, the basement is topped by Tertiary toQuaternary alluviums on which thick lateritic formation has devel-oped. On the slopes of the Western Ghats scarp the weatheringcover is also composed of a lateritic formation and is developed insitu, on the basement rocks. The thickness of the weathered zone isin the range 20–40 m (Rajesh, 1999) and the basement rocks belowthis zone hosts groundwater in fractures, joints etc. The NethravatiRiver discharges annually 12·109 m3 of water and 14·105 t of sedi-ment into the Arabian Sea (Karnataka Irrigation Department, 1986;Subramanian et al., 1987). The intensity of flow depends on seasons,

Fig. 1. Location map of the Nethravati watershed showing the sampling stations and geology of the area.

62 G.P. Gurumurthy et al. / Chemical Geology 300-301 (2012) 61–69

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with 94% of the total discharge during the period of southwest mon-soon (June–October).

The Nethravati watershed is characterized by high humidity(>70%), and heavy rainfall (3600–4200 mm.y−1; data from IndianMeteorological Department, Government of India, http://www.imd.gov.in). The average runoff over the last 16 years is 3300 mm.y−1

(data from Central Water Commission, Government of India) andthe Potential Evapo-Transpiration (PET) is estimated at1400 mm.y−1 (Rao and Jagannathan, 1994) along the coast. Themean temperature is around 30 °C (±5), with extremely small varia-tions throughout the year (Murthy et al., 1988; Fig. 2). The vegetationcover is essentially composed of tropical rain forest.

3. Sampling and analysis

River samples were collected monthly at five stations, from roadbridges, from October 2006 to September 2007 (Fig. 1). Sampleswere filtered through 0.22 μm pore size Whatman Nuclepore (poly-carbonate) membrane filters and stored in pre-cleaned polypropyl-ene bottles. The pH, temperature, conductivity, dissolved oxygenand alkalinity were measured on the field using HACH multipara-meter probes and a Merck alkalinity test kit. In the laboratory thesamples were analyzed for anions using Dionex ion chromatographICS90 and DX 600 at NITK Surathkal and the Indo-French Cell forWater Sciences (IFCWS), Indian Institute of Science (Bangalore)with a detection limit better than 1 μmol.L−1 and a relative precisionbetter than 10%, bicarbonates were analyzed with a Metrohm® Auto-titrator, major cations were analyzed with an Agilent ICP-MS at GET,Toulouse (France) and Atomic Absorption Spectrophotometer atNITK Surathkal. Silica was measured using BRAN+LUEBBE spectro-photometer at GET, Toulouse. The validity of measurements waschecked with the reference standard ION 915. For all samples exceptfour, the charge balance (NICB) was within 12% (Table 1).

4. Results

The chemical compositions of Nethravati main stream and its tribu-taries are presented in Table 1. The pH varies from 5.2 to 7.4, withlower values during the monsoon and higher during the dry season(base flow). These values are comparable to the values reported for riversdraining granitic terrains (e.g. Oliva et al., 2003). The electrical conductiv-ity (EC) varies from 29 to 87 μS.cm−1 while the river total dissolved salts(TDS) varies from 29 to 66 mg.L−1 (average 43mg.L−1); lower valuesare recorded during the monsoon and higher values during base flow.

The TDS of Nethravati River is relatively low compared to the globalriver average (283 mg.L−1; Gaillardet et al., 1999), but comparable toother rivers draining orogenic zones such as the Amazon River(44 mg.L−1), Orinoco (82mg.L−1), Brahmaputra (71 mg.L−1) (Galyand France-Lanord, 1999), Congo and Niger (35 mg.L−1 and59 mg.L−1; Gaillardet et al., 1999). At comparable morpho-climatic set-tings, the Nethravati River exhibits slightly lesser TDS than thewest flow-ing rivers draining Deccan traps of India (75–91 mg.L−1; Das et al.,2005).

Sodium is the dominant dissolved cation, with concentrationsranging from 86 to 260 μmol.L−1. It is followed by Ca (38 to157 μmol.L−1), Mg (26 to 121 μmol.L−1), and K (10 to40 μmol.L−1). The lowest concentrations are observed in the Shishila-hole tributary, located in the footsteps of the Western Ghats. Overall,major cation concentrations in the Nethravati watershed decreaseduring monsoon while spatially, it increases further downstream.The Kumaradhara tributary, which partly drains the hypersthene-rich charnockites, exhibits slightly higher Ca/Na and Mg/Na molar ra-tios than the Nethravati upper watershed (Uppinangadi station,Fig. 1).

The bicarbonate concentrations range from 170 to 540 μmol.L−1

and account for an average of 70% of the total anion budget. Chlorideconcentrations range from 57 to 137 μmol.L−1. Like cations, the lowestbicarbonate and chloride values were observed during the monsoonseason. In all sampling stations the SO4 concentrations are rangingfrom 8 to 21 μmol.L−1 which are about 10 times lesser than the bicar-bonates. The NO3 concentrations range from 2 to 18 μmol.L−1 withmaximum values observed during the onset of monsoon when fertil-izers are added to freshly sown agricultural crops. Chloride and SO4

concentrations are not correlated to NO3 variations. Monthly dis-charges and concentrations of dissolved species of the NethravatiRiver at BC Road, Bantwala (Table 1) were used to calculate theweight-ed mean concentrations of major ions and to deduce the annual flux ofmajor ions discharged by this river into the Arabian Sea.

The chemical composition of local rainfall is presented in Table 2.The weighted average chloride concentration is 47 μmol.L−1, Na45 μmol.L−1, Ca 20 μmol.L−1, K 5 μmol.L−1, Mg 7 μmol.L−1 and SO4

9 μmol.L−1. The molar Na/Cl ratio (~0.95) in the rain water is corre-sponding with the marine ratio suggesting the rain water composi-tion is dominated by sea salts. However, enrichment of Ca and Mgwith respect to sea water composition is noticed. According toHegde (2007) who studied the rainfall and aerosol composition inand around Mangalore City, these enrichments would originatefrom continental dusts and/or anthropogenic sources.

Oct Nov Dev Jan Feb Mar Apr May Jun Jul Aug Sep

Mon

thly

PE

T, r

ainf

all &

run

off

(mm

)

0

200

400

600

800

1000

1200

Tem

pera

ture

0 C

15

20

25

30

35

Dis

char

ge m

3 .s-1

0

200

400

600

800

1000

1200PrecipitationRunoffPET Temperature Discharge

Fig. 2. Average monthly temperature, rainfall, PET, runoff and discharge of Nethravati River.

63G.P. Gurumurthy et al. / Chemical Geology 300-301 (2012) 61–69

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Table 1Stream chemical composition at the 5 stations of the Nethravati River and tributaries.

Date ofsampling

Dischargea T pH DO TDS EC Na K Ca Mg F Cl NO3− SO4

2− HCO3− SiO2 TZ+ TZ− NICB

(m3/s) (°C) (mg/L) (μS/cm) (μmol/L) (μeq/L) (%)

Nethravati at Bantwala, BC Road (Lat: 12°52′47.63″ Long: 75°02′27.53″)Oct 26, 06 356 30 6.8 7.2 34 54 151 15 65 56 1 85 4 10 260 204 407 370 10Nov 29, 06 133 29 6.7 5.7 36 52 136 17 68 55 1 88 6 11 290 206 399 407 −2Dec 21, 06 35 25 6.9 7.0 39 51 125 15 63 53 1 91 2 11 330 210 371 446 −18Jan 19, 07 9 28 6.7 6.5 – 54 – – – – – – – – – – – – –

Feb 27, 07 3 29 7.0 7.2 47 64 141 18 105 64 2 104 – 12 420 190 497 550 −10Mar 29, 07 2 32 7.1 6.7 53 74 197 26 123 92 2 127 – 15 480 173 653 640 2Apr 27, 07 1 33 7.1 5.9 60 82 260 37 124 121 2 136 – 15 540 191 788 708 11May 25, 07 11 32 6.6 6.3 52 80 190 38 94 91 2 128 – 21 450 178 599 622 −4Jun 29, 07 386 26 6.0 7.5 29 32 104 19 46 40 1 87 14 21 180 140 294 323 −9Jul 27, 07 1129 27 6.8 7.5 29 35 107 13 49 41 1 84 9 11 220 186 302 335 −11Aug 31, 07 1139 27 7.0 5.5 27 35 96 13 50 38 2 72 8 10 210 170 286 310 −8Sep 21, 07 510 27 6.6 7.1 28 36 103 12 46 41 1 74 6 9 220 182 289 318 −10Weighted mean 108 14 51 42 1 79 8 11 221 178 309 331

Kumaradhara at Uppinangadi (Lat: 12°50′ 10.16″ Long: 75°14′34.33″)Oct 26, 06 235 27 6.8 7.3 31 34 102 16 54 45 1 73 8 11 240 168 317 344 −8Nov 29, 06 66 29 6.7 6.0 34 41 124 17 62 56 1 83 6 10 270 191 377 380 −1Dec 21, 06 13 25 7.1 6.8 37 47 118 13 77 55 3 84 3 10 310 207 396 420 −6Jan 19, 07 0 27 6.7 6.1 38 50 – – 80 – 1 87 2 9 350 205 160 459 –

Feb 27, 07 0 29 7.0 6.6 47 61 148 19 93 76 2 98 – 10 440 208 505 560 −10Mar 29, 07 1 32 7.1 6.4 54 73 192 27 107 98 2 109 – 12 500 207 628 636 −1Apr 27, 07 0 33 7.4 5.6 58 84 185 34 152 103 4 113 – 12 530 225 728 669 8May 25, 07 9 33 7.4 6.2 47 75 176 35 79 95 2 101 – 18 410 202 559 549 2Jun 29, 07 189 25 5.7 7.6 27 31 87 17 43 38 1 74 18 12 170 135 266 287 −7Jul 27, 07 488 26 6.7 7.8 27 33 96 12 44 41 1 79 9 10 200 177 277 308 −11Aug 31, 07 677 27 7.0 5.6 26 36 90 12 45 40 1 79 9 9 200 162 273 307 −12Sep 21, 07 187 27 7.1 8.5 26 34 86 11 44 39 1 70 9 8 200 165 264 296 −12Weighted mean 94 13 47 42 1 77 10 10 206 166 284 314

Nethravati at Uppinangadi (Lat: 12°50′28.69″ Long: 75°14′ 34.33″)Oct 26, 06 121 29 6.7 7.2 34 41 165 16 67 52 1 87 4 9 240 210 419 352 18Nov 29, 06 68 28 6.8 6.3 35 41 126 21 80 45 1 83 7 12 260 198 398 375 6Dec 21, 06 22 25 6.9 7.0 39 54 137 15 72 49 1 95 3 12 330 234 393 452 −14Jan 19, 07 9 27 6.8 6.7 40 63 – – 98 26 1 98 7 13 350 216 247 482Feb 27, 07 3 29 6.9 6.5 44 58 157 19 93 59 2 108 4 14 380 202 481 521 −8Mar 29, 07 1 32 7.1 6.4 44 67 198 26 91 77 2 123 – 15 440 196 560 596 −6Apr 27, 07 1 33 7.3 6.7 58 75 231 40 146 81 2 137 1 20 500 212 726 684 6May 25, 07 2 33 6.7 5.7 50 87 204 35 93 74 2 126 – 19 430 192 572 596 −4Jun 29, 07 197 25 6.3 8.1 28 31 97 16 40 34 2 73 8 11 180 140 262 285 −8Jul 27, 07 640 26 6.6 7.5 30 35 122 13 51 40 2 81 7 9 220 196 318 328 −3Aug 31, 07 462 27 7.0 5.6 26 46 93 12 48 33 1 69 7 10 200 169 266 296 −11Sep 21, 07 323 27 6.6 7.6 28 36 106 12 49 37 1 76 8 9 220 186 290 322 −11Weighted mean 112 14 52 38 1 77 7 10 216 183 305 321

Nethravati at Dharmasthala (Lat: 12°57′ 51.07″ Long: 75°21′53.55″)Oct 26, 06 30 6.9 7.3 59 45 156 13 76 48 1 90 2 8 350 283 418 460 −9Nov 29, 06 29 6.6 7.4 45 54 159 14 93 51 1 91 2 9 350 260 461 462 0Dec 21, 06 26 7.0 7.7 46 59 137 12 85 47 1 94 1 10 400 263 413 516 −22Jan 19, 07 28 6.7 5.7 43 59 – 10 99 29 2 106 1 13 390 235 265 525 –

Feb 27, 07 31 6.9 6.3 49 64 178 19 112 58 2 115 1 12 430 235 537 571 −6Mar 29, 07 34 7.1 6.2 55 74 251 35 157 85 2 116 3 12 470 223 769 614 22Apr 27, 07 34 6.4 5.2 52 72 226 30 108 68 1 118 – 13 460 229 607 606 0May 25, 07 25 5.8 7.6 26 32 89 16 54 33 3 74 10 12 180 135 280 291 −4Jun 29, 07 25 6.2 8.1 30 37 117 12 47 41 1 76 7 8 240 203 306 341 −11Jul 27, 07 25 6.5 7.6 29 37 105 12 52 37 1 76 10 9 220 178 295 324 −9Aug 31, 07 27 6.7 8.3 33 44 123 11 61 42 1 74 – 9 260 219 339 353 −4

Shishilahole at Parpikal (Lat: 12°52′59.93″ Long: 75°25′03.44″)Dec 21, 06 24 7.0 7.8 35 39 – – 56 – 1 111 – 10 280 222 113 412 –

Jan 19, 07 26 6.6 7.4 29 37 95 12 58 34 1 81 3 11 240 169 291 347 −18Feb 27, 07 30 6.8 6.9 29 37 103 12 58 36 1 83 – 12 230 151 305 338 −10Mar 29, 07 31 7.2 7.3 30 38 125 20 77 42 0 96 2 15 220 163 381 348 9Apr 27, 07 31 6.6 7.5 34 41 144 18 57 45 1 89 – 12 270 197 365 384 −5May 25, 07 25 5.2 9.0 24 29 89 12 38 31 2 63 6 10 190 158 238 281 −17Jun 29, 07 25 6.4 8.8 27 29 98 10 44 34 1 67 4 8 210 195 264 298 −12Jul 27, 07 27 6.8 8.9 25 31 91 10 45 33 1 57 4 9 200 180 256 280 −9Aug 31, 07 28 5.9 9.4 26 30 101 11 45 37 1 60 – 12 210 187 276 295 −7

T °C=water temperature measured in the field.TDS=Total dissolved solids (Na+K+Ca+Mg+Cl+SO4+NO3+HCO3+SiO2).TZ+=total dissolved cations.TZ−=total dissolved anions.NICB=TZ++TZ− /mean×100%.

a Monthly discharge from Central Water Commission and Irrigation Department.

64 G.P. Gurumurthy et al. / Chemical Geology 300-301 (2012) 61–69

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5. Discussion

5.1. Sources of major ions

For the estimation of SWR and CCR in the Nethravati River basin,the quantum of contribution of major ions from evaporites, anthropo-genic sources and atmospheric/sea salts have to be considered. Sinceevaporites are absent amongst the bedrocks of the Nethravati water-shed, the contribution from them are ruled out. Anthropogenic chlo-ride in the river water could result in an increase in Cl content and adecrease in the Na/Cl ratio. This phenomenon is not observed in theNethravati watershed. Though the weighted average chloride con-centrations in the river are 1.5 times higher than the rainfall, it hasalso correspondingly increased the Na/Cl molar ratio (Fig. 3), withthe highest values corresponding to the base flow. This could beexplained by simple evapotranspiration process in the watershed,and negligible influence of anthropogenic chloride. In order to quan-tify the chemical fluxes coming only from bedrock weathering contri-butions from atmosphere have to be corrected. . This is achievedusing chloride as a proxy and relying on the understanding that Clin the Nethravati River originates only from seawater through atmo-sphere (rainfall). The sea salt corrected concentration of an element(Xr*) is calculated following the relation (Stallard and Edmond,1981; Négrel et al., 1993; Eq. (5)):

X�r ¼ Xriver−Clriverð Þ X=Clð Þrainfall ð5Þ

where, Xriver and Clriver are the molar concentrations in the riverwater and (X/Cl)rainfall is the X/Cl molar ratio in the local rainfall.Based on Eq. (5), the proportion of atmospheric inputs accounts for70% for Na, 65% for Ca and 65% for K and 20% for Mg. The correctionapplied to SO4 led to slightly negative values, which indicates thatthe entire dissolved sulfate observed in the river could be of atmo-spheric origin. There is no significant difference between local rainfalland seawater corrections on Na fluxes. However, the impact of the localrainfall correction on the SO4, Ca, K and Mg fluxes is much higher thanthe seawater; this is because the local rainfall is enriched with these ele-ments compared to seawater. For instance, the use of seawater composi-tion as a reference for atmospheric inputs would lead to a correction ofonly 1% for Ca and 7% forMg. This means that the choice of the referencefor atmospheric inputs correction impacts, at least in this watershed, theweathering fluxes of SO4, Ca, Mg and K. The weighted concentrations(and related specificfluxeswithin parentheses) attributed toweatheringat the outlet of the watershed are 32 μmol.L−1 for Na* (1.1·105

mol.km−2.y−1), 17 μmol.L−1 for Ca* (0.5·105 mol.km−2.y−1), 31μmol.L−1 for Mg* (1.0·105 mol.km−2.y−1), and 5 μmol.L−1 for K*(0.2·105 mol.km−2.y−1).

In addition to atmospheric inputs, part of Ca and Mg fluxes couldbe derived from dissolution of disseminated calcite from the bedrocksthat may lead to an overestimation of silicate weathering rate and as-sociated carbon dioxide consumption in the watershed (White et al.,1999; Jacobson and Blum, 2000;White et al., 2005). The occurrence ofcalcite and Mg-rich calcite crystals was reported in mafic and ultra-mafic rocks of the Dharwar craton (Braun et al., 2009). The dissolu-tion of Ca–Mg calcite is much faster than the Ca–Mg–Na silicatesand should force enrichment of Ca and Mg relative to Na+ in thestream (higher Ca*/Na* and Mg*/Na*) compared to the bedrock.This contribution is usually estimated by comparing the Ca*/Na* andMg*/Na* molar ratios of the stream with those of the local bedrockand the relative “excess” of Ca and Mg is then attributed to carbonatedissolution (Krishnaswami et al., 1999). The part of Ca and Mg associ-ated to Ca–Mg silicates may be estimated according to the followingEqs. (6) and (7):

Casil ¼ Na�⋅ Ca=Nað Þrock ð6Þ

Mgsil ¼ Na�⋅ Mg=Nað Þrock ð7Þ

where Na* (Nasil) is the weighted mean sodium concentration of theriver water corrected from atmospheric inputs, subscripts “sil” refersto silicate origin and “rock” refers to the bedrock composition (seeTable 3). As the watershed encompasses a wide range of lithologies,the calculations were performed using two different parent rock com-positions (averages for both gneiss and charnockite): the concentra-tions of Ca and Mg that is derived from silicate weathering rangefrom 11 μmol.L−1 (gneiss) to 15 μmol.L−1 (charnockite) for Ca andfrom 8 μmol.L−1 (charnockite) to 13 μmol.L−1 (gneiss) for Mg.These values are compared with the rainfall corrected values of therespective elements; the difference is small for Ca with2–6 μmol.L−1 but significant for Mg, with 18–24 μmol.L−1. Thesevalues indicate that (1) Ca-carbonate dissolution in the watershedwould be minor or Ca would be leached out at the same rate as Naduring silicate weathering and (2) more than 2/3 of Mg would origi-nate from Mg carbonate dissolution. The latter interpretation is incontradiction with the absence of Mg-carbonates in the bedrock(Braun et al., 2009). Moreover, the plot of Ca*/Na* and Mg*/Na*molar ratios of the river water and bedrocks (Fig. 4) shows that allthe river water samples fall close to the silicate end-member definedby Gaillardet et al. (1999) with a slight orientation towards the Mg-silicate mineral composition. The composition of the tributary drain-ing partly the charnockites and amphibolite (Kumaradhara at Uppi-nangadi; Fig. 1) tends towards the Mg-silicate mineral compositionwhereas the upstream samples and Nethravati at Uppinangadi

Table 2Chemical composition of the rainwater at Mangalore.

Location Samplingdate

Rain fall F Cl SO42− NO3

− Na K Ca Mg

(mm) (μmol/L)

Manipal Jul 08, 09 40a bdl 103 26 1 85 17 66 16Manipal Jul 02, 09 23a 0.4 50 7 2 44 3 15 7Parpikal,Mangalore

Jul 29, 09 24a 0.5 26 6 4 39 3 12 6

Mangalore Jul 19, 09 61a 0.7 48 11 0 44 2 22 9Mangalore Jun 25, 09 35a 0.5 72 9 0 64 4 29 13Mangaloreb Jul 3, 10 34 bdl 56 8 4 57 7 19 7Mangaloreb Jul 18, 10 83 bdl 21 4 2 22 3 6 3Mangaloreb Aug 11, 10 21 bdl 55 6 3 59 7 8 6Mangaloreb Sep 09, 10 41 bdl 22 5 1 24 4 8 3Mangaloreb Sep 21, 10 2.1 bdl 22 5 3 22 5 6 3Weightedmean

47 9 2 45 5 20 7

a TRMM data (http://disc.sci.gsfc.nasa.gov/precipitation/).b Unpublished data.

Fig. 3. Scatter plot of Na/Cl vs Cl (not corrected for sea salts) in the sampling stations.The horizontal line stands for the “marine Na/Cl” molar ratio.

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(Fig. 1) samples fall close to the Biotite–Gneiss composition. Thisquestions the pertinence of a carbonate correction for Mg, especiallywhen the watershed bedrock contains weatherable ferromagnesiansilicate minerals such as biotite, hypersthene or amphiboles. There-fore, it is relevant to consider the atmospheric inputs corrected con-centration of Mg as an upper limit of Mg-silicates weathering in theNethravati watershed.

5.1.1. Silicate weathering rate in the Nethravati RiverThe silicate weathering rate (SWR) (t.km−2.y−1) in the Nethra-

vati watershed is calculated based on the following relationship(Eq. (8)):

SWR ¼ Q=A⋅ ∑ Naþ þ Kþ þMg2þ þ Ca2þ� �

silþ SiO2

� �ð8Þ

where Q is the discharge in m3.s−1, A is the surface area of the water-shed in km2 and subscript ‘sil’ refers to silicate-derived concentra-tions (Krishnaswami and Singh, 2005). The SWR in the Nethravatiwatershed is estimated using the atmospheric input and carbonatecorrected concentrations of riverine cations in mg.L−1. Silicate de-rived cations are calculated based on both atmospheric and carbonaterock corrections. The atmosphere corrected values are considered asupper limit of SWR, while the latter is considered as the lower limitof SWR, based on the discussions made in Section 5.1. In these

conditions, the silicate weathering rate at the outlet of the Nethravatiwatershed would range from 42 (lower limit) to 44 t.km−2.y−1

(upper limit) (Table 4), which means that the maximum carbonatecontribution would be 2 t.km−2.y−1.

The silicate weathering fluxes per unit area are comparable to one ofthe highest reported silicate weathering fluxes for the well-studiedwatersheds, Rio Icacos (40 t.km−2.y−1; West et al., 2005) and higherthan well studied watershed Nsimi, Cameroon, which reported lesser sil-icate weathering flux (7 t.km−2.y−1; Braun et al., 2005). Further, the sil-icate weathering flux in Nethravati watershed is higher than theorogenic belt rivers such as Yamuna (28 t.km−2.y−1; Dalai et al.,2002), Bhagirathi and Alaknanda (14 t.km−2.y−1; Krishnaswami et al.,1999) and the watershed draining shields such as Amazon(13 t.km−2.y−1), Mackenzie (1.8 t.km−2.y−1), Parana (5 t.km−2.y−1),Mekong (14.3 t.km−2.y−1), Congo–Zaire (4.2 t.km−2.y−1) and Orinoco(9.5 t.km−2.y−1) (Gaillardet et al., 1999). When compared with the wa-tershed having similar morpho-climatic settings draining basaltic rocksof peninsular India (53 t.km−2.y−1; Das et al., 2005), the Nethravatihas slightly lesser silicate weathering fluxes.

5.1.2. Degree of silicate-rock weathering in the Nethravati watershedThe degree of silicate-rock weathering occurring in the Nethravati

River basin is determined using Re index proposed by Tardy (1971)and modified by Boeglin and Probst (1998). This index is based onthe silicate-derived dissolved cations and silica concentrations inthe river. The coefficients used in the Eq. (9) correspond to averagegranite containing feldspar, mica and Mg-silicate minerals such asamphiboles (Boeglin and Probst, 1998). Such composition is similarto the mean bedrock composition of the Nethravati watershed,which makes the above equation suitable for estimating the intensityof rock weathering within the watershed. Re is expressed as:

Re ¼ 3Naþ þ 3Kþ þ 1:25Mg2þ þ 2Ca2þ−SiO2

0:5Naþ þ 0:5Kþ þ 0:75Mg2þ þ Ca2þð9Þ

where Na, K, Mg, Ca and SiO2 are the concentrations of each chemicalspecies attributed to silicate weathering. It is assumed to be equiva-lent to the molar ratio SiO2/Al2O3 remaining in the weathering pro-file: if Re=0, the dominant weathering process is the genesis ofgibbsite (called ‘allitization’), if Re=2, kaolinite is essentially formed(‘monosiallitization’), if Re=4, the weathering products are mainlysmectites (‘bisiallitization’). The well drained terrain results in theformation of gibbsite. It indicates that the silicate weathering is in-tense and proceeding to gibbsite phase. Whereas, as Re of ≥2 indi-cates relatively lesser intensity of weathering leading to theformation of kaolinite–smectites in the weathering profile.

Table 3Major element abundance in the main rock types of the Nethravati watershed.

Sample SiO2 CaO MgO K2O Na2O Ca/Na Mg/Na n Reference

(wt.%) (molar)

Charnockite, Tamil Nadu 69.8 3.8 0.8 1.7 4.0 0.52 0.15 1 Sharma and Rajamani (2000)Charnockite (mafic granulite) 72.6 4.3 1.0 0.7 4.3 0.55 0.18 1 Rajmani et al. (2009)Charnockite, Kerala 68.8 2.5 1.0 5.0 3.2 0.44 0.24 6 Soman (2002)Charnockite, Kerala 65.1 2.1 1.2 5.4 2.5 0.47 0.37 1 Soman (2002)Meana charnockites 68.9 2.8 1.0 4.2 3.3 0.47 0.24Gneiss, Halagur Karnataka 59.5 4.7 2.1 3.4 3.7 0.70 0.44 1 Sharma and Rajamani (2000)Biotite Gneiss Satnur Karnataka 51.2 4.3 3.2 3.5 3.7 0.64 0.66 1 Sharma and Rajamani (2000)Gneiss, Mulehole (avg.) 68.2 2.0 2.6 1.7 4.5 0.25 0.45 25 Braun et al. (2009)Gneiss, Kerala 67.6 1.3 1.6 3.9 2.6 0.28 0.47 13 Soman (2002)Gneiss, Kerala 68.7 2.4 1.0 4.5 2.4 0.54 0.32 11 Soman (2002)Gneiss, Kerala 67.8 3.6 1.6 1.1 4.4 0.45 0.28 7 Soman (2002)Meana gneiss 67.7 2.20 2.0 2.7 3.6 0.35 0.41Granites, Satnur Karnataka 68.7 1.1 0.2 7.3 3.4 0.18 0.03 1 Sharma and Rajamani (2000)Amphibolites, Mulehole (avg.) 46.3 2.0 3.2 0.1 0.8 1.38 3.08 35 Braun et al. (2009)

a The mean takes into account of the number of samples analyzed (n).

Fig. 4.Mixing diagrams of atmospheric input corrected Mg/Na vs. Ca/Na molar ratios ofNethravati River. The silicate and carbonate end-memberdomains are taken fromGaillardetet al. (1999). The bedrock compositions are taken from Sharma and Rajamani (2000), Soman(2002), Braun et al. (2009). Stationsmarked in blue and green shows detailed data ofNethra-vati waters, and stations marked in red shows the discharge weighted average for each sta-tion. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

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The Re calculated for the Nethravati River at the outlet rangesfrom −0.6 (gibbsite formation) during the peak flow season to 2.4(kaolinite formation) during the dry season. Using the weightedmean concentrations, the annual average Re value is 0.14, suggestingthe formation of gibbsite in the watershed regolith which is usual forhumid tropical climate. The calculated Re values, using data fromGaillardet et al. (1999) for Amazon (2.1), Congo–Zaire (2.1), Orinoco(1.6), Parana (1.4), Ganges (2.2), Brahmaputra (2), Mekong (1.7) sug-gests kaolinite formation in their weathering cover. In contrast toBoeglin and Probst (1998), who observed a slight increase of Realong the Niger main stream, the Re values are quite uniform in theNethravati watershed, suggesting no difference in the weathering re-gime between the upper and lower parts of the watershed.

5.1.3. Carbon dioxide consumption during rock weatheringSince the dissolved SO4 in the Nethravati River originates from atmo-

spheric inputs and not from sulfide oxidation, CO2 may be considered asthe only weathering agent in the watershed. Then, the carbon dioxideconsumption rate (CCR, expressed in mol.km−2.y−1) corresponds tothe flux of cations originating from silicate weathering:

CCR ¼ Q=A⋅∑ Naþ þ Kþ þMg2þ þ Ca2þ� �

ð10Þ

where, Q is the discharge in m3.s−1 and A the surface area of the water-shed in km2.

The CCR in Nethravati watershed is 3·105 mol.km−2.y−1 at sta-tion BC road (Bantwala; Table 4). This value is higher than other trop-ical rivers draining shields such as Amazon (0.5·105 mol.km−2.y−1),Parana (0.9·105 mol.km−2.y−1), Orinoco (0.6·105 mol.km−2.y−1),Congo–Zaire (0.5·105 mol.km−2.y−1), (Gaillardet et al., 1999), butlesser than rivers draining mountainous ranges such as Yamuna(5·105 mol.km−2.y−1; Dalai et al., 2002) or Bhagirathi–Alaknanda(4·105 mol.km−2.y−1; Krishnaswami et al., 1999), though their SWRis slightly lower than that ofNethravati River. This apparent contradictionmay be explained by the difference in the degree of silicate-rock weath-ering in the Himalayan andNethravati watersheds. The limited degree ofsilicate rockweathering in Himalayas (Re~3, i.e. betweenmonosiallitiza-tion and bisiallitization, for Yamuna Head waters, data from Dalai et al.,2002) is explained by a high dissolved cation and relatively less silicafluxes whereas the high degree of weathering in the Nethravati isexplained by a relatively lesser cation flux and high silica fluxes.

5.2. Factors controlling the intensity of silicate weathering in theNethravati watershed

The Nethravati watershed is characterized by several importantmeteorological, geological and geomorphological features such as

high runoff (3300 mm.y−1), high mean temperature (30 °C), varyinglithologies and steep slopes. The importance of these factors in control-ling the SWR in the Nethravati watershed is investigated. The combina-tion of high runoff and high temperature is a common feature of basinsexhibiting intense silicate weathering (Fig. 5), like for instance Rio Ica-cos (40 t.km−2.y−1, West et al., 2005) and other west flowing riversdraining the Deccan basalts (53 t.km−2.y−1, Das et al., 2005). In con-trast, the watersheds having high temperature and low runoff (Nsimi,Cameroon; Braun et al., 2005) or low temperature and high runoff(British Columbia; reviewed in West et al., 2005), exhibit limited sili-cate weathering rate, i.e. below 15 t.km−2.y−1. This can be explainedin terms of higher kinetic reaction at high temperature combinedwith fast renewal of regolith solutions that prevents oversaturation ofsoil solutions.

The possible influence of two additional factors, namely the basinslope and nature of bedrock on the weathering of silicate rocks, is

Table 4Dissolved chemical fluxes, silicate weathering rate and CO2 consumption in the Nethravati River, its tributaries and in the other west-flowing rivers of Peninsular India. Silicateweathering rates of other west-flowing rivers of Peninsular India are recalculated from data in Das et al. (2005) and Trivikramaji and Joseph (2001).

Location T Runoff SWR CCR Area Rainfall Reference Correction applied(°C) (mm.y−1) (t.km−2.y−1) (105 mol.km−2.y−1) (km2) (mm.y−1)

Nethravati River 29 3300a 42 2.8–2.9 3657 3600 This study Local rain water and Ca/Na(0.47) and Mg/Na (0.41)molar ratio

Kumaradhara River 28 2594a 30 1.6–1.7 1825 – This studyNethravati at Uppinangadi 28 2734a 38 2.8–2.9 1750 – This studyGurupur River 28 3425a 38 5.2 824 – This studyMuatupuzha River 30 2200a 27 5.5 2004 3385 Trivikramaji and

Joseph (2001)Gad River 25 1690b 40 5.7 981 2600 Das et al. (2005) Local rain water and Ca/Na

(1.30) and Mg/Na (1.0)molar ratio

Kajli River 26 1657b 48 5.8 762 2550 Das et al. (2005)Shashtri River 25 2117b 58 6.3 2174 3260 Das et al. (2005)Vashishti River 26 2198b 63 7.1 2238 3391 Das et al. (2005)Krishna River 24 477 19 4.2 36,268 – Das et al. (2005)Bhima River 25 215 12 3.3 33,916 – Das et al. (2005)

a Discharge data from Central Water Commission, Government of India.b Discharge data from www.nih.ernet.in.

0

10

20

30

40

50

60

70

-20

-10

010

2030

40

01000

20003000

4000

SW

R(t

.km

-2y-1

)

Tem

pera

ture

°C

Runoff (mm)

Nethravati & Muathapuzha rivers

Deccan basalt rivers

Small silicatewatersheds

Major world rivers

Fig. 5. Comparison of SWR with temperature and runoff for the Nethravati River, Dec-can basalt rivers (Das et al., 2005; Gupta et al., 2011, Table 4), Small worldwide silicatewatersheds (West et al., 2005), Muathapuzha River (Trivikramaji and Joseph, 2001,Table 4) and major world rivers (Gaillardet et al., 1999; Krishnaswami et al., 1999;Dalai et al., 2002). This plot confirms that the highest silicate fluxes result from thecombination of high runoff and warm temperature as observed by West et al. (2005)and White and Blum (1995).

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investigated. This can be better understood by comparing river basinsthat exhibit similar morpho-climatic conditions like temperature, el-evation and soil thickness and more or less similar runoff which isthe case for west flowing rivers of Peninsular India. This is achievedby plotting the sum of cations and silica concentrations attributed tosilicate weathering against the basin slope (Fig. 6). For a given lithol-ogy, the sum of cations and silica concentrations remain stable irre-spective of the basin slope. This means that, for this range ofconcentrations, the slope would not exert a significant control onthe silicate weathering fluxes. In the same figure (Fig. 6), the sum ofcations and silica concentrations define two clear domains relatedto nature of the watershed lithology: the west flowing rivers drainingbasalts exhibit concentrations ~2.5 times higher than those draininggneissic rocks. On the other hand in the low temperature watersheds(≤10 °C) the difference in concentrations of ions drained from basaltsand gneissic rocks is much larger (by 3.9 times) as calculated from thecompilations of West et al. (2005) for granite–gneissic lithologies andDessert et al. (2009) and Louvat (1997) for basaltic lithology. A possi-ble implication is that the difference of “weatherability” between gra-nitic–gneisses and basaltic rocks seems to be narrowed at high runoffand temperature, as against the pattern at low temperature water-sheds. Such conclusion deserves further studies on silicate weather-ing in varying temperature and runoff environments.

6. Conclusions

A systematic monitoring of the geochemistry of a small, tropicalwest flowing river of Peninsular India (Nethravati River), and its trib-utaries flowing across a hot and humid environment was carried outfor 12 months to determine the rate of silicate weathering and its as-sociated carbon dioxide drawdown from the atmosphere. The chemi-calweathering rate in theNethravati River is estimated to 44 t.km−2.y−1

and the silicate weathering rate of the watershed estimated to42 t.km−2.y−1, accounts for one of the highest ever reported. Due tothe absence of dissolved sulfate originating from sulfide oxidation, thecarbonic acid may be considered as the principal weathering agent intheNethravatiwatershed. The carbondioxide consumption rate associat-ed with silicate weathering is 2.9·105 mol.km−2.y−1. The degree ofweathering intensity within the watershed, calculated from the molecu-lar ratio of dissolved cations and silica concentrations in the river (Re),ranges seasonally from −0.6 to 2.4, with an annual mean value of 0.14.Such value suggests that allitisation, i.e. gibbsite formation, is currentlytaking place in the weathering profiles of the watershed as a result of

an intense chemical weathering. The intensity of silicate weathering inthe Nethravati watershed is primarily controlled, like the few other ba-sins in the world that exhibit extreme values, by the combination of in-tense runoff (3300 mm.y−1) and warm temperature. Though the basinslope does not seem to influence the weathering intensity of West flow-ing rivers of peninsular India, the nature of lithology still plays an impor-tant role as the chemical weathering rate of basaltic rocks is 2.5 timeshigher than that of gneissic silicate rocks under similar morpho-climatic settings. However, this lithology-dependence of silicate weath-ering rate is lessmarked compared to the cooler and less humid environ-ment, suggesting that intense runoff and high temperaturewould reducethe difference in weatherability of silicate lithologies.

Acknowledgment

Council of Scientific and Industrial Research (13/8062-A/2006-Pool), Ministry of Environment and Forests (19/36/2006-RE), Mani-pal Center for European Studies (through European Commissionfunding), INSU-EC2CO (CNRS), Embassy of France in India and IRD,France are thanked for funding this research work. We thank TIFAC-CORE, NITK, Surathkal for providing the ion-chromatography andAAS facilities. We thank the Central Water Commission (CWC), Gov-ernment of India for providing the discharge data. The authors aregrateful to two anonymous reviewers for their fruitful commentswhich helped in revising the manuscript. Tripti M at MIT Manipal,Prof. Gopal Mugeraya, Mr. Harish Poojary, and Ms. Lalithakumari atNITK Surathkal are thanked for the logistics.

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Slope (m.m-1)0.00 0.01 0.02 0.03 0.04 0.05

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SiO

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