Geochimica et Cosmochimica Acta, Vol. 69, No. 9, pp. 2221–2240, 2005Copyright © 2005 Elsevier Ltd

Printed in the USA. All rights reserved

doi:10.1016/j.gca.2004.11.019

Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in theheadwaters of the Ganges

MIKE J. BICKLE,1,* HAZEL J. CHAPMAN,1 JUDITH BUNBURY,1 NIGEL B. W. HARRIS,2 IAN J. FAIRCHILD,3 TALAT AHMAD,4

AND CATHERINE POMIÈS1

1Department of Earth Sciences, Downing Street, Cambridge CB2 3EQ, United Kingdom2Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom

3School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom4Department of Geology, University of Delhi, Delhi 110 007, India

(Received December 16, 2003; accepted in revised form November 30, 2004)

Abstract—Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater87Sr/86Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated87Sr/86Sr ratios and there is disagreement as to how much of the 87Sr flux is derived from silicate weathering.Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- andcarbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head watersof the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. Thecompositions of tributaries draining catchments restricted to a limited range of geological units can bedescribed by two-component mixing of silicate and carbonate-derived components and lie on a plane inmulticomponent composition space. Key elemental ratios of the carbonate and silicate components aredetermined by the intersection of the tributary mixing plane with the planes Na � 0 for carbonate and constantCa/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated bymass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrockimplies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mgtrace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, compo-sition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratiodiagrams. However the calculations are not sensitive to transformations of the compositions by incongruentdissolution or precipitation processes provided that the transformed silicate and carbonate component vectorsare constrained. Silicates are calculated to provide �50% of the dissolved Sr flux from the head waters of theGanges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arisefrom addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondarycalcite deposition, this estimate would be increased. Moreover, when 87Sr/86Sr ratios of the Sr inputs areconsidered, silicate Sr is responsible for 70 � 16% (1�) of the 87Sr flux forcing changes in seawaterSr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of thetotal Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weatheringcan be greatly underestimated if the processes that decouple the water cation ratios from those of the source

0016-7037/05 $30.00 � .00

rocks are not properly evaluated. Copyright © 2005 Elsevier Ltd

1. INTRODUCTION

The Phanerozoic seawater Sr-isotope record exhibits a 50 to100 Ma episodicity attributed principally to variations in thecontinental riverine Sr flux and its 87Sr/86Sr ratio (e.g., Palmerand Edmond, 1989). The size of the calculated variations in thesilicate-derived Sr flux (Richter et al., 1992) imply correspond-ing variations in solid-Earth CO2 degassing of sufficient mag-nitude to alter global climate (e.g., Bickle, 1996). The associ-ation between the marked rise in seawater 87Sr/86Sr ratios since40 Ma and the uplift and erosion of the Himalayan-Tibetanorogen provides the key evidence that continental processesimpact on the seawater Sr-isotope record. However, several ofthe major rivers draining the Himalayan-Tibetan region exhibithigh 87Sr/86Sr ratios at relatively high Sr concentrations (Ed-mond, 1992; Palmer and Edmond, 1992) and, as first postulatedby Palmer and Edmond (1992), metamorphosed carbonate

* Author to whom correspondence should be addressed (mb72@esc.cam.ac.uk).

2221

rocks exhumed by the Himalayan orogen have elevated 87Sr/86Sr ratios due to exchange with silicate minerals (Bickle et al.,2001). If much of the radiogenic Sr-isotope signal in runofffrom the Himalayas is derived from carbonate then changes inseawater Sr-isotope composition would not necessarily reflectchanges in the sources or magnitudes of silicate weatheringfluxes.

Published estimates of the relative proportions of HimalayanSr fluxes derived from carbonate and silicate sources are vari-able and authors disagree as to whether carbonate or silicatesources dominate the Sr-isotopic signal (e.g., Krishnaswami etal., 1992, 1999; Quade et al., 1997, 2003; McCauley andDePaolo, 1997; Blum et al., 1998; Harris et al., 1998; Singh etal., 1998; Krishnaswami and Singh, 1998; Galy et al., 1999;English et al., 2000; Jacobson et al., 2002; Dalai et al., 2003;Oliver et al., 2003). In part, the variation in estimates of thefraction of silicate-derived Sr may reflect real differences inoutputs from different tributary systems and seasonal changesin the sources of Sr-inputs which have not been properly taken

into account (e.g., Bickle et al., 2003). However most of the

2222 M. J. Bickle et al.

previous studies rely on the assumption that the Sr to cationratios of the bedrocks can be used to apportion Sr betweensilicate and carbonate sources, an assumption we show to bemisleading.

This study develops a new methodology for calculating therelative inputs of Sr and 87Sr to rivers from silicate and car-bonate rocks. It is based on a database that collates publishedand new samples from the headwaters of the Ganges and theupper reaches of the Marsyandi River in Nepal. We havepreviously calculated the relative inputs of Sr from the majorgeological units in the headwaters of the Ganges (Bickle et al.,2003) and studied the Sr and Sr-isotopic chemistry of thebedrocks (Ahmad et al., 2000; Bickle et al., 2001). We dem-onstrate here that 1) most of the chemical variation in sets oftributaries draining catchments of restricted bedrock geologycan be modeled by two-component mixing trends betweencarbonate and silicate like sources, 2) incongruent dissolutionand precipitation reactions, and possibly addition of a tracecalcite from silicate rocks, cause the chemistry of the waters toexhibit Sr/cation ratios which differ significantly from those ofthe source rocks, and 3) it is possible to calculate the Sr/cationratios of the carbonate and silicate inputs to the rivers, use theseto estimate the relative fractions, with uncertainties, of Sr and87Sr derived from silicate and carbonate sources and to correctthese estimates for additional factors such as trace calcite insilicate rocks. The calculated inputs of Sr and 87Sr from car-bonate and silicate sources from the major geological units arecombined with a revised calculation of the fluxes of Sr from themajor geological units to provide robust estimates, with uncer-tainties, of the sources of Sr and 87Sr in the headwaters of theGanges.

2. STUDY AREA

The headwaters of the Ganges in the Garhwal Himalayacross the three major lithotectonic units of the Himalaya (Fig.1). The major river (Dhauli Ganga) rises in the Tibetan Sedi-mentary Series which comprises folded siliclastic and carbon-ate Paleozoic and Mesozoic rocks. The Dhauli Ganga thenflows across the main metamorphic unit of the Himalayas, thelargely silicate metasediments and gneisses of High HimalayanCrystalline Series that is emplaced on the north-east dippingMain Central Thrust (MCT) along its south-western margin andin normal fault contact with the Tibetan Sedimentary Seriesalong its north-eastern margin. The major tributaries, the Saras-wati and Bhagirathi rise in the High Himalayan CrystallineSeries. The Dhauli Ganga (called the Alaknanda below theconfluence with the Saraswati) and the Bhagirathi then flowacross the Lesser Himalayan Series comprising a complexthrust sequence of low grade schists, phyllites, quartzites, calc-silicates, limestones and dolomites bounded to the south by theMain Boundary Thrust which outcrops at the margin of theGanges foreland basin. The mainstream is called the Gangesbelow the confluence of the Bhagirathi and the Alaknanda atDeopryag, within the Lesser Himalayan Series. The geologicalsetting and the substantial previous work on the chemistry andSr-isotopic systematics of these river systems are reviewed inmore detail by Bickle et al. (2003). Published data have beensupplemented by sampling during August 2003. The tributaries

draining the Tibetan Sedimentary Series are not accessible in

the Dhauli Ganga catchment for political reasons. To investi-gate controls on river chemistry draining Tibetan SedimentarySeries rocks on the southern margin of the Tibetan plate wesampled tributaries draining equivalent Tibetan SedimentarySeries rocks in the upper Marsyandi valley in Nepal duringMay 2002. The chemistry of the Marsyandi and Dhauli Gangarivers in their Tibetan Sedimentary Series catchments are re-markably similar.

Sr-isotope systematics have been investigated in detail forthe four catchments for which Bickle et al. (2003) calculatedthe relative contributions to the total Sr flux in the Alaknandaand for one additional catchment in the Lesser HimalayanSeries. These are the Tibetan Sedimentary Series which theDhauli Ganga leaves at Malari, the High Himalayan CrystallineSeries which the Alaknanda leaves at Helong, the DeobanFormation which the Alaknanda leaves at Nandapryag, theDeopryag catchment of Lesser Himalayan Series rocks down toDeopryag and the Kanwana catchment partly on younger Mus-soorie Group rocks (Fig. 1). The Deoban catchment is treatedseparately from the rest of the Lesser Himalayan Series becauseit contains the calc-silicate rocks with extremely high 87Sr/86Sr(up to 1.2, Bickle et al., 2001). The analysis of outputs fromthese catchments allows extrapolation to the whole of theGanges catchment above Rishikesh on the basis of the similar-ity of bedrock geology.

3. SAMPLING AND ANALYTICAL METHODS

Sampling, analytical methods and the water analyses for most of thesamples from the Dhauli Ganga, Alaknanda, Bhagirathi and upperGanges are given by Bickle et al. (2003). Additional analyses are givenin Table 1. Samples from the Marsyandi (Table 1) were collected inMay 2002, filtered on site through 0.1 �m cellulose nitrate filters (30mL samples) or 0.2 �m cellulose nitrate filters (1 L samples), acidifiedwith quartz-distilled HCl, and analyzed for Ca, Mg, Na, K, Si, Sr andS by AES in Cambridge and Cl, F, PO4 and SO4 by Dionex at the

Fig. 1. Geological map of headwaters of Ganges (modified fromBickle et al., 2003). River catchment boundaries shown as dashed lines.Location of Marsyandi shown on inset map.

University of Keele using the same methods as described by Bickle etal. (2003). 87Sr/86Sr isotope analyses of the Marsyandi waters and the

2223Silicate and carbonate inputs to riverine Sr

August 2003 samples by TIMS in Cambridge followed Bickle et al.(2003) and analyses of NBS987 for the 2002 samples gave 0.710231� 11 (2�, n � 8) and for the August 2003 samples gave 0.710255 � 17(2�, n � 38). Samples collected in August 2003 (Table 1) were filteredthrough 0.2 �m nylon filters, acidified with quartz-distilled HNO3 andanalyzed for cations by AES in Cambridge and anions by Dionex at theOpen University using the methods described by Bickle et al. (2003).

4. CALCULATION OF SILICATE- AND CARBONATE-DERIVED SR FRACTIONS

Knowledge of the relative fractions of Sr derived fromsilicate and carbonate sources is necessary to evaluate theclimatic significance of the seawater Sr-isotope record and thepotential climatic impact of the Himalayan-Tibetan orogen.Previous published estimates of the fractions of Himalayanriverine Sr derived from silicate and carbonate rocks are largelybased on various elemental ratios (Sr/Ca, Sr/Na, Na/Ca) in thesource rocks (e.g., Krishnaswami and Singh, 1998; Singh et al.,1998; Galy et al., 1999; Krishnaswami et al., 1999; English etal., 2000; Jacobson et al., 2002). However these authors alldraw attention to the potential for incongruent mineral disso-lution in the weathering reactions which control the river chem-istry. For example Krishnaswami and Singh (1998) and Galy etal. (1999) show that Sr budgets calculated from source rockelement ratios frequently underestimate the total Sr budget.English et al. (2000) speculate that kinetic controls release Srfaster than Ca during weathering of carbonate whereas Galy etal. (1999) consider that discrepancies in the Sr flux calculationsmay be due to precipitation of secondary calcite which sub-stantially increases the end-member Sr/Ca ratios in the waters.Jacobson et al. (2002) present compelling evidence for super-saturated waters precipitating calcite and increasing waterSr/Ca ratios. Singh et al. (1998) suggest that it should bepossible to use the two-component mixing trends displayed bywater compositions to place constraints on the end-membercompositions, but the scatter in their data precluded robustconstraints. Millot et al. (2003) show that tributary composi-tions from the Mackenzie River basin exhibit two-componentmixing but these authors invert for silicate and carbonate inputsusing the a priori inversion method of Négrel et al. (1993)which is based on presumed rock element and isotope ratios.

In this paper we calculate the relative silicate and carbonateinputs to the dissolved Sr flux by using silicate and carbonateend member cation ratios constrained by arrays of tributarycompositions from geologically restricted catchments in a man-ner similar to that suggested by Singh et al. (1998). Thisapproach corrects for incongruent dissolution or precipitationreactions which cause element ratios in the water to differ fromthose in their source rocks.

To use the arrays of tributary chemistry to constrain carbon-ate and silicate end-members it is necessary to 1) establish thatthe water chemistry is dominated by two-component mixing, 2)use an appropriate method for fitting mixing planes in theappropriate composition space and 3) use the best fit planes tocalculate carbonate and silicate end-member compositions. Inthe discussion below we first discuss the corrections for rainand hot-spring inputs and the statistical method of fitting thedata to two-component mixing trends. The quality and signif-icance of the fits and problems that arise from interpretation of

the covariation of cation ratios are then illustrated by a discus-

sion of the data from the Deopryag catchment. This is followedby derivation of the methodology to calculate the relativefractions of Sr from carbonate and silicate sources.

4.1. Correction for Rainfall and Hot-Spring Inputs

Water compositions have been corrected for rainwater inputsby assuming that the rain water has the Cl content of thetributaries with the lowest Cl content and that the rain ischaracterized by the cation/Cl ratios interpolated from thecompilation of Galy and France-Lanord (1999) (Table 2). TheHigh Himalayan Crystalline Series tributaries have Cl as low as7.5 �molar, Deoban Catchment tributaries as low as 10 �mo-lar, Lesser Himalayan Series Catchment �20 �molar at lowflow (1996, 1997–1 and 1997–2 collections) and �14 �molarat high flow (1998). The Marsyandi Tibetan Sedimentary Serieswaters have minimum Cl � 7 �molar. These minimum Clconcentrations are within the range of Himalayan rain watercompositions published by Galy and France-Lanord (1999).

Cl in excess of rain water inputs is presumed to be derivedfrom hot-springs (cf. Evans et al., 2001). Himalayan hot-springcompositions show a wide range of cation/Cl ratios (Evans etal., 2001; Bickle et al., 2003; Becker et al., 2003). In part thismay result from dilution with cold ground water of similarcomposition to river waters in which case the hot-spring end-member would have the lowest cation/Cl ratios. However manyof the hot-springs have high Cl-corrected Na/Ca and Na/Mgratios attributed to high temperature spring water-rock reac-tions by Bickle et al. (2003). Table 2 lists the mean andstandard error of 11 Himalayan hot-springs with Na/Cl �1.2which excludes those that may be dominated by groundwatermixing or water-rock reactions. However correcting watercompositions with less saline springs (average of all springsgives Na/Cl � 2) makes little difference to the fits discussedbelow although it does result in a larger number of correctedcompositions with negative cation concentrations. Water com-positions have been corrected first for rain water inputs andthen for hot-spring inputs, assuming that all Cl remaining afterrain Cl has been subtracted is derived from hot-springs. Theuncertainty on the final corrected composition, used in thecalculation of Sr inputs below, comprises a 5% presumedanalytical uncertainty plus the propagated uncertainties in rainwater and spring water composition from Table 2. The area-weighted mean tributary compositions of the major catchmentsand subcatchments have been recalculated here (as in Bickle etal., 2003) including the new data, and corrected for rain andhot-spring inputs. The combined uncertainties arising fromsampling and corrections are given in Table 3.

4.2. Modeling Two-Component Mixing

The central premise on which the calculations of the relativesilicate and carbonate Sr inputs are calculated is that the sets oftributary water compositions should lie on two-componentmixing trends, provided samples are restricted to catchments ofsufficiently uniform geology and weathering processes andcorrected for rain and hot-spring inputs. Because river watersare variably diluted by rainfall or concentrated by evaporation,a two-component mixing trend will be spread out into a plane

or hyperplane through the origin in three or higher dimensional

Table 1. Water analyses.

East North Na K Ca Mg Si HCO3 Cl F SO4 NO3

Sample Locationa Source Date Decimal degrb �mol/L Sr (nmol/L) 87Sr/86Sr pH T °C Catchc

AK18 Mana Nadi 19/5/96 79.4944 30.7703 55 26 172 21 86 336 8 11 48 17 134 0.751945 7.99 4.7 HAK19 Mana Saraswati 19/5/96 79.4944 30.7703 50 13 158 18 89 245 17 17 60 16 137 0.752250 7.75 5.2 HAK28 Jumagwa Juma Gad 21/5/96 79.8168 30.6557 91 22 610 358 114 1413 13 3.2 304 15 1836 0.717931 na na HAK44 Huial tributary 2/5/97 78.3883 30.1371 315 40 1222 888 181 3009 42 5.4 732 59 3600 0.731389 8.40 21.3 KAK46 Yala C. Spring 2/5/97 78.4954 30.0765 155 26 360 184 178 1165 42 4.2 28 4 1033 0.756906 8.15 23.9 KAK171 Gangalsi tributary 9/9/98 78.4937 30.0776 169 35 908 614 197 2565 42 3.1 293 54 2251 0.717604 8.45 26.1 KAK172 Baral Huial 9/9/98 78.3897 30.1372 213 33 797 618 202 2355 29 3.5 342 8 2177 0.730670 8.54 25.8 KAK175 Rishikesh Alaknanda 20/9/98 78.3105 30.1224 75 35 348 116 113 793 15 8.1 102 18 404 0.742086 8.54 20.1 GAK176 Chameli Gular Gad 20/9/98 78.4342 30.1163 81 22 166 71 161 397 40 1.9 42 55 590 0.734553 8.33 20.6 KAK177 Dhanota small trib 13/8/03 78.5728 30.1000 117 16 215 74 186 553 46 3.1 29 51 368 0.741612 7.53 22.6 KAK178 Deopryag Bagirathi 13/8/03 78.5979 30.1456 70 36 292 94 106 585 15 8.8 128 16 311 0.748825 8.40 18.5 BAK179 Deopryag Alaknanda� 13/8/03 78.5979 30.1456 58 50 408 118 91 955 10 6.8 87 17 349 0.738090 8.51 18.6 AAK180 S Deopryag small trib W 14/8/03 78.5846 30.1221 181 31 592 339 212 1889 73 6.0 32 44 1698 0.747311 8.35 23.9 KAK181 Bharpur small trib W 14/8/03 78.5775 30.1116 78 18 215 108 151 629 38 3.2 22 28 568 0.752140 7.94 25.3 KAK182 S Deopryag Bhandi Nadi 14/8/03 78.5891 30.1188 257 18 313 100 229 958 73 4.4 22 23 836 0.741266 8.31 26.6 KAK183 S Deopryag Ganges 14/8/03 78.5866 30.1165 68 51 387 105 97 868 12 7.8 102 14 372 0.738619 8.46 18.4 GAK184 Deopryag Alaknanda� 14/8/03 78.5979 30.1456 62 40 419 129 97 973 10 7.2 97 15 420 0.736776 8.44 18.5 AAK185 Pauri Catch Kathulsyn 14/8/03 78.7388 30.2152 357 24 503 152 236 1336 185 5.5 39 88 970 0.739280 8.34 24.7 DPAK186 Bhatali small trib S 15/8/03 78.8610 30.2391 249 31 481 106 228 1261 102 5.2 35 16 833 0.739282 8.15 24.7 DPAK187 Dikholi small trib S 15/8/03 78.9436 30.2429 195 17 390 125 206 1108 63 4.1 29 13 642 0.744409 7.82 24.8 DPAK188 Rudrapryag small trib S 15/8/03 78.9834 30.2840 55 13 184 98 158 575 20 2.0 13 11 221 0.723543 7.92 24.8 DPAK189 Nandapryag Alaknanda� 15/8/03 79.3161 30.3321 55 36 449 143 66 928 8 7.8 159 15 564 0.731962 8.51 16.0 AAK190 Nandapryag Nandakini 15/8/03 79.3161 30.3321 69 40 378 126 121 991 13 8.3 47 14 345 0.752773 8.18 18.6 DPAK191 Nandapryag Alaknanda� 15/8/03 79.3161 30.3321 48 33 448 134 59 939 5 6.5 141 13 558 0.729990 8.40 15.1 AAK192 Bheem Fall small trib S 16/8/03 79.3593 30.4066 82 105 310 93 179 825 16 4.0 59 30 229 0.730547 0.00 0.0 DBAK193 Helong Alaknanda 16/8/03 79.5009 30.5242 51 28 411 104 53 739 5 5.9 177 7 651 0.727274 8.21 14.3 AAK194 Nr Helong Kalpa Ganga 16/8/03 79.5018 30.5323 44 33 237 42 89 491 7 4.3 61 10 256 0.737822 7.88 17.3 HAK195 Joshimat Alaknanda 16/8/03 79.5541 30.5664 51 28 387 107 54 655 5 5.8 195 13 637 0.728183 8.29 14.3 AAK197 Segri Dhauli Ganga 16/8/03 79.7672 30.5676 65 21 532 201 40 1029 6 4.0 251 12 1296 0.721133 8.47 10.3 DGAK198 Pawanrsa Gari Gadhera 17/8/03 79.7646 30.5647 20 20 134 13 53 283 4 0.5 16 17 252 0.742194 8.02 12.0 HAK199 Suraithota Talma Gad 17/8/03 79.7422 30.5302 40 26 220 29 79 441 4 1.8 53 12 368 0.749023 7.40 14.2 HAK200 Lata Rishi Ganga 17/8/03 79.6953 30.4841 40 43 549 160 51 776 4 3.7 354 9 737 0.735961 8.18 13.0 HAK201 Helong Alaknanda 17/8/03 79.5009 30.5242 52 29 387 97 52 686 7 6.6 169 12 599 0.727978 8.46 15.1 AAK202 Hat small trib E 17/8/03 79.4298 30.4192 64 23 707 757 136 2667 15 3.0 154 24 368 0.945341 8.54 20.2 DBAK203 Nandapryag Alaknanda� 17/8/03 79.3161 30.3321 54 33 439 123 56 910 15 7.7 133 14 554 0.729934 8.51 16.2 AAK204 Rudrapryag Alaknanda� 18/8/03 79.9792 30.2881 55 34 424 123 83 965 11 7.1 93 15 454 0.734028 8.44 18.8 AAK205 Rudrapryag Mandakini 18/8/03 79.9792 30.2881 77 29 215 49 144 513 12 6.3 45 13 229 0.753185 8.13 21.0 DPAK206 Rampur small trib NW 18/8/03 78.6784 30.2227 462 11 631 239 261 2000 127 8.1 31 20 1133 0.751616 8.30 24.5 DPAK207 Deopryag Alaknanda� 18/8/03 78.5979 30.1456 52 34 393 113 83 908 9 6.1 82 13 393 0.735943 8.59 18.8 AAK208 Deopryag Bhagirathi 19/8/03 78.5979 30.1456 64 39 256 86 90 477 12 10.9 139 12 289 0.752509 8.08 18.9 BAK209 Koudiol small trib N 19/8/03 79.5044 30.0731 75 33 344 195 155 888 48 4.5 71 105 1708 0.737367 8.13 24.3 KAK210 E Gular Gad small trib N 19/8/03 78.4342 30.1163 145 24 546 316 214 1435 36 4.8 169 82 1105 0.737803 8.22 24.6 KAK211 Rishikesh Alaknanda 19/8/03 78.3105 30.1223 69 37 381 121 98 859 12 7.4 108 17 439 0.738466 8.29 20.2 GMarsyandi Tibetan sedimentary series riversCT9 Kyupar Naur Khola 25/4/02 84.2598 28.5543 184 29 1317 792 61 2288 26.2 8.5 1053 12 3937 0.715236 8.30 11.6 MCT10 Chame Marsyandi 25/4/02 84.2405 28.5524 113 29 914 481 44 1914 25.0 1.9 492 16 2427 0.719352 8.65 11.4 MCT12 W. Dhikur Trib from S. 26/4/02 84.1666 28.6050 40 38 1380 210 65 3064 7.7 0.9 92 12 1151 0.728101 na 7.2 MCT13 Pisang tributary 27/4/02 84.1486 28.6150 33 25 880 384 40 2283 8.6 1.2 140 21 1786 0.728299 7.30 7.8 MCT14 Pisang Beshi Trib from S. 27/4/02 84.1392 28.6199 115 27 1433 624 116 3404 8.6 2.4 418 13 2987 0.729372 8.49 na MCT16 E. Hongde Trib from S. 27/4/02 84.1073 28.6341 21 10 950 278 47 2052 8.8 1.3 207 14 1071 0.726526 8.48 2.7 MCT18 W. Hongde Trib from S. 27/4/02 84.0699 28.6451 84 35 961 754 49 2444 20.2 0.7 538 18 2814 0.718677 8.42 11.7 M

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2225Silicate and carbonate inputs to riverine Sr

composition space and the carbonate and silicate end memberswill lie on vectors through the origin (e.g., Fig. 2). A necessarytest for two-component mixing is that rain- and hot-spring-corrected tributary compositions should lie within error of aplane through the origin in composition space defined by ele-ments that exhibit conservative behavior in rivers. In this studywe consider the cations Na, K, Ca, Mg, Si and Sr although notall of these are invariably conservative.

The general equation of a plane which passes through theorigin in n-dimensions is

�1x1 � �2x2 . . . �nxn � 0 (1)

where the terms �i are the n coefficients that describe therelationship between the n compositional components, xi (i.e.,the n cation concentrations in each sample). The coefficients, �i

have been calculated by the linear least-squares routines fromKent et al. (1990) (see also Albarède, 1995, chap. 4) whichassumes that the composition of each component of each watersample xj (i.e., elements, xj1, xj2. . .xjn) is independently andnormally distributed. The variances of each element in eachanalysis are estimated from the propagated uncertainties in theanalyses and rain- and hot-spring-corrected compositions asdiscussed above and we have assumed that the covariances arezero because there are insufficient data to assume otherwise.The goodness of fit is given by the mean squared weighteddeviate (MSWD � �2/(j-n) for j samples and n elements) whichshould be of order 1 for a fit within experimental error. Wherethe MSWD is greater than unity the computed uncertainties onthe fit coefficients, �i, have been increased by MSWD to allowfor underestimation of the compositional errors. The lengths ofresiduals (i.e., the misfit) and weightings of each point havebeen computed to establish outliers which may unduly influ-ence the fits. Samples in which the cumulate errors fromcorrections for rain and hot-spring inputs exceed 50% of ele-ment concentrations and samples which have high residualsand unduly influence the fits have been excluded as discussedbelow.

Central to our calculation of the fractions of Sr derived fromsilicate and carbonate inputs is the premise that the compositionof the silicate and carbonate end members can be constrainedby the two-component mixing relation exhibited by the tribu-tary sets. Carbonate minerals contain only trace Na and there-fore the carbonate end-member vector is taken to have Na � 0.Silicate rocks mostly have Ca/Na molar ratios between �0.1and 0.5 and the silicate vector is therefore characterized byCa/Na ratios in this range with allowance for incongruentdissolution or precipitation reactions. The intersections of thetwo-component mixing plane with the planes on which thecarbonate and silicate end-members are constrained have beenconstrained in three dimensional sub-spaces (e.g., Fig. 2). Wehave solved for these intersections in the four three-dimen-sional subspaces of the system Sr-Ca-Mg-Na. The hypothesisthat the mixing planes pass through the origin has been testedby fitting the general equation of a plane (Eqn. 1 plus a constantterm) and the fits always give the value of the constant withinerror of zero.

The quality of the fits is described by the MSWD parameterdiscussed above. Because it is difficult to visually portray the

fits in three or more dimensions, we illustrate the quality of fitsC

T1

CT

2C

T2

CT

2C

T2

CT

2C

T2

CT

2C

T2

CT

3C

T3

CT

3C

T3

CT

3 a

Ge b c

003) wi

2226 M. J. Bickle et al.

on cation-ratio diagrams (e.g., Fig. 3). Note that the end-member cation ratios define the end-member carbonate andsilicate vectors in composition space (Fig. 2).

4.2.1. Tributary chemistry: Lesser HimalayanDeopryag catchment

Figure 3 illustrates Si/Ca, K/Ca, and Sr/Ca ratios vs. Ca/Naratios and Ca/Na ratios vs. Mg/Na ratios for tributaries to theLesser Himalayan Deopryag catchment (Fig. 1). Cation/Caratios plotted against Na/Ca ratios will lie on straight lines fortwo-component mixing where the intercept at Na/Ca � 0 givesthe cation/Ca ratio of the carbonate end member. However wehave chosen to plot data against Ca/Na ratio because this betterillustrates the structure of the low-Na carbonate-dominatedwater samples. The least-squares fits and fit parameters to thecation/Ca to Na/Ca ratio plots are shown on Figure 3.

All four cation plots in Figure 3 exhibit reasonable approx-imations to two-component mixing of silicate and carbonate-derived components, excluding outliers, as discussed below.Mean squared weighted deviates (MSWD) between 5.6 to 20imply scatter of only 2.4 to 4.5 times the estimated errors. TheSi/Ca and K/Ca regressions against Na/Ca imply that the Si/Caand K/Ca ratios of the carbonate end member are close to zero.The scatter of samples about the regressions might be expectedto exceed that predicted by the estimated errors because of 1)scatter in cation ratios of rocks within the catchments, 2)variable extents of incongruent dissolution and precipitationand 3) the influence of three or more end-members. Where theadditional scatter is limited we argue that it is reasonable tomodel the trends by two component mixing. The K/Ca, and toa lesser extent Si/Ca, trends exhibit a number of samples which

Table 2. Cation-chloride mo

Na/Cl 1� K/Cl 1� Ca/Cl 1�

Raina 1.33 0.10 0.29 0.10 3.00 0.42Hot springsb 0.84 0.07 0.08 0.01 0.19 0.03

a From compilation by Galy and France-Lanord (1999).b Mean of 11 springs from Evans et al. (2001) and Becker et al. (2c Sr/Cl nmol/�mol.

Table 3. Mean area-weighted catchment t

Na 1�a K 1� Ca 1�

TSSc 112.6 13.2 23.5 1.6 664.7 21.4Deopryag 77.4 16.2 38.0 4.0 450.2 23.6Deoban 59.9 12.1 51.7 8.3 613.7 56.3Kanwana 129.9 24.4 20.2 2.6 475.5 93.3

HHCS

Vaikrita 37.6 5.3 42.4 6.4 417.4 73.4Saraswati 57.6 11.7 20.0 1.3 192.9 8.6Munsiari 55.0 3.6 60.3 3.2 393.9 21.1

a 1� uncertainty for catchments includes sampling after Bickle et aSaraswati and Munsiari sub-catchments based on scatter of sub-catcuncertainty on 87Sr/86Sr � 104.

b nmolar.c

of Malari.

scatter below the best fit line and we suspect that there may besignificant biomass uptake of K and Si.

In detail the cation-ratio trends of the waters show somesignificant deviations from carbonate and silicate rock compo-sitions. The composition of the carbonate end-member in theDeopryag tributary waters, inferred from intercepts as Na/Catends to zero, may be compared with analyses of massivecarbonate and calc-silicate rocks in the Lesser Himalayas(Singh et al., 1998; Bickle et al., 2001). In the Lesser Himala-yas, massive carbonates are found principally in the Precam-brian Deoban Formation, the Krol Formation and the Mussoo-rie Group although thinner units are scattered widely throughthe Lesser Himalayan meta-sedimentary formations (Valdiya,1980). The Deopryag catchment drains mainly Inner LesserHimalayan Series. Ratios of Sr, Ca and Mg for water carbonateend members and rocks are given in Table 4. The Sr/Ca ratio1

inferred from the water compositions for the end-member car-bonate in the Deopryag catchment is 0.56 � 0.06 (nmol/�mol)which compares with mean Sr/Ca from the Inner Lesser Hima-layan Series for limestones of 0.22 � 0.02 (1 se, 4 samples) anddolomites of 0.15 � 0.02 (18 samples).

4.2.2. Processes that may modify water chemistry

Several processes may result in the water cation ratios devi-ating from two-component mixtures of silicate and carbonaterock sources. Evaporites, phosphorites and basic siliceouslithologies might result in additional compositional complexi-ties. The lack of a correlation between Cl and SO4 (Fig. 4)

1 Sr/cation ratios are all cited as nmol/�mol.

s of rain water and springs.

l 1� Si/Cl 1� Si/Clc 1� 87Sr/86Sr 1�

0.05 0.10 0.05 2.16 0.43 0.714 0.0040.01 0.07 0.01 1.31 0.10 0.77 0.02

th Na/Cl � 1.2.

compositions and uncertainties (�mol).

1� Si 1� Srb 1� 87Sr/86Sr 1�

5.8 44.1 2.9 2066 157 0.7189 214.0 129.1 10.0 442 51 0.7489 9

103.9 115.8 8.8 319 32 0.8395 29875.6 175.4 19.4 1357 266 0.7398 6

hments

32.1 67.1 5.0 627 96 0.7392 25.9 78.0 8.9 228 18 0.7421 133.7 110.7 5.6 396 21 0.7526 5

3) and corrections for rain and hot spring inputs. 1� uncertainty forcompositions and includes rain and hot spring input corrections. 1�

lar ratio

Mg/C

0.340.02

ributary

Mg

383.8206.9562.1305.4

subcatc

137.628.270.4

l. (200hment

Average Dhauli Ganga at Malari weighted for seasonal output and corrected for HHCS inputs for samples collected short distances downstream

ets in (

2227Silicate and carbonate inputs to riverine Sr

implies that evaporites are probably not a significant source ofSO4 and that the sulfate is probably derived by oxidation ofpyrite (cf. Galy and France-Lanord, 1999). Average molarSO4/Ca ratios are less than 0.3 which limits the possible evap-orite input. None of the waters has significant phosphate. Basiclithologies are too rare (� 1% of outcrop) for high Ca/Nasources to make a significant contribution and we note that the

Fig. 2. Water composition mixing relations illustrated in Sr-Ca-Naspace. Tributary water compositions (black dots) define plane (gray)through origin. The carbonate end-member defines vector perpendicu-lar to Na axis (in plane Na � 0) at set Sr/Ca ratio. The silicateend-member defines vector of given Ca/Na ratio (�0.2). Proportions ofcarbonate and silicate components are given by magnitude of compo-nents of appropriately orientated vectors which sum to mean watercomposition, as illustrated.

Fig. 3. Correlation diagrams for tributary water composibars 1� after correction for rainwater and hot-spring inpEquations of best fit lines, given on figures, are calculatedafter Kent et al. (1990). Dashed line and equation in brack

Eqn. 2). Samples plotted as open symbols excluded from fits.

Si/Na ratio of the silicate component in the waters is alwayswithin error of, or less than, 2.0, the value expected for weath-ering of plagioclase to kaolinite.

Incongruent dissolution and precipitation processes may re-sult in the dissolved cation ratios differing from the source rockratios. Deposition of low-Sr and low-Mg carbonate in pedo-genic, spelean and riverine spring settings will cause elevationof Sr/Ca and Mg/Ca ratios in the water (Galy et al., 1999;Fairchild et al., 2000; Jacobson et al., 2002). Also limestonesdissolve faster than dolomites. However the Sr/Ca ratio of theInner Lesser Himalayan Series limestones is only �50% higherthan the Sr/Ca ratio of the dolomites and both are less than theSr/Ca ratio of 0.56 in the water carbonate component. Bothpreferential dissolution of limestone and deposition of second-ary calcite are necessary to explain the Sr/Ca and Mg/Ca ratiosin the dissolved component. The observed Sr/Ca and Mg/Caratios in the water carbonate end member may be attained bydissolution of limestone to dolomite in the ratio 1.6 � 0.4 to 1(1� uncertainty) (the source rocks are present in the ratiolimestone to dolomite 0.25:1), giving an input with Mg/Ca� 0.23 and Sr/Ca � 0.20. Loss of �65 � 9% of the Ca tosecondary calcite would elevate the cation ratios in the water tothe observed Mg/Ca ratio of 0.71 and Sr/Ca ratio of 0.56.

The Ca-Mg-Na correlation of the Deopryag catchment trib-utaries implies negative silicate end-member Mg/Na ratios atCa/Na ratios �1.1 (Fig. 3d). Himalayan silicate rock Ca/Naratios are always �1 and usually average �0.25 (e.g., Galy etal., 1999; Ahmad et al., 2000; Jacobson et al., 2002). Thisdiscrepancy could arise from systematic displacement of theCa/Na to Mg/Na relationship by loss of Mg, preferential loss ofCa from the higher Ca/Na samples or by systematic addition ofa high Ca component to the silicate end-member.

om Lesser Himalayan Deopryag catchment (Fig. 1). Errore text) and are not shown where smaller than symbols.least squares best fits to planes in three component spacec) is best fit from four 3D plane fits in Sr-Ca-Mg-Na (see

tions fruts (sefrom

2228 M. J. Bickle et al.

Systematic loss of Mg is not thought likely since smectitesform a minor proportion of the clay mineral fraction. It ispossible that Ca from the silicate end-member is enhanced bydissolution of trace calcite in the silicate rocks (cf. Drever andHurcomb, 1986; Mast et al., 1990; Stauffer, 1990; Turk andSpahr, 1991; Blum et al., 1998; White et al., 1999). This wouldincrease Ca/Na of an apparent silicate plus trace-calcite endmember without changing Mg/Na (Fig. 5). Given that silicateCa/Na is likely to lie between �0.2 and 0.5, this would implythat 50 to 80% of the Ca, in the component which has silicate-like Mg/Na, is trace calcite-derived. To preserve the observedcorrelations, the silicate and trace-calcite components wouldneed to be mixed in similar proportions in each water sample orscatter would be observed on Mg/Ca vs. Ca/Na diagrams.

An alternative explanation is that Ca loss by precipitation ofsecondary calcite correlates with Ca/Na ratio so as to causerotation of the Ca/Na vs. Mg/Na trend and apparent elevation ofthe silicate component (Fig. 5). Given the Ca/Na and Mg/Naratios of the more silicate-dominated waters, the more carbon-ate-dominated samples would need to have lost �50% of theirCa to rotate the Ca/Na to Mg/Na trend through a ‘silicate-like’end member. The transformation must have been linear (withinerror) to preserve the planar disposition in Sr-Ca-Mg-Na space,and if the Ca loss was proportional to the Ca/Na ratio thetributary composition plane would continue to pass through theorigin. However calcite saturation indices in the Deopryagcatchment (Fig. 6) show no correlation with Ca/Na, althoughthe sampled saturation indices measured after calcite precipi-

Fig. 4. Cl vs. SO4 for 115 water samples modeled in this study

Table 4. Comparison of cation ratios of c

Molarratio

Water carbonate end memberInn

Deopryag 1� Kanwana 1�Limestone

(n � 3)

Mg/Ca 0.71 0.09 0.79 0.06 0.03Sr/Cab 0.56 0.06 1.04 0.30 0.22

a 1se � standard error � 1�/n0.5, where n is number of samples.b nmol/�mol. Analyses from Singh et al. (1998) and Bickle et al.

correlations.

excluding 10 samples with high Cl or SO4. Note lack of any correlationbetween Cl and SO4.

tation may bear no relation with the amount of secondarycalcite precipitation.

4.2.3. Calculation of Sr inputs to the Deopryag catchment

Published methods for apportioning riverine Sr between car-bonate and silicate sources rely on estimates of Ca/Na, Sr/Caand/or Sr/Na ratios of silicates and Sr/Ca and Mg/Ca ratios ofcarbonates (e.g., Blum et al., 1998; Krishnaswami and Singh,1998; Harris et al., 1998; Singh et al., 1998; Galy et al., 1999;Krishnaswami et al., 1999; English et al., 2000; Dalai et al.,2003; Oliver et al., 2003). 87Sr/86Sr ratios in Himalayan car-bonates are elevated by metamorphic exchange with old silicateminerals (Bickle et al., 2001) which precludes the use ofSr-isotope ratios to discriminate carbonate and silicate inputs.Jacobson et al. (2002) and Oliver et al. (2003) have attempteda correction for secondary calcite deposition.

Here the calculation of the proportions of strontium derivedfrom carbonate and silicate sources is based on the composition

Fig. 5. Relationships suggested to explain high Ca/Na at low Mg/Na.(1) Composition-dependent loss of Ca varying from 50% for carbonateend-member to 0% for most silicate-rich samples at Ca/Na � 2 rotatesCa/Na to Mg/Na covariation from initial trend (dashed line) to ob-served solid line, or (2) Low-Mg trace calcite may mix with silicate togive silicatelike end member with high Ca/Na ratio. Shaded ellipseindicates 1� uncertainty adopted for composition of silicate-derivedcation ratios. Note figure shows only lower Ca/Na and Mg/Na samples

e rock and water carbonate end member.

er Himalayas Outer Lesser Himalayas

Dolomite(n � 18) 1 sea

Limestone(n � 4) 1 sea

Dolomite(n � 12) 1 sea

0.83 0.05 0.08 0.03 0.82 0.080.15 0.02 0.40 0.07 0.22 0.05

. Sr/Ca and Mg/Ca ratios calculated from Sr-Ca-Na and Mg-Ca-Na

arbonat

er Less

1 sea

0.010.02

(2001)

from Deopryag catchment, and error bars are removed from somesamples for clarity (see Fig. 3d).

2229Silicate and carbonate inputs to riverine Sr

of the carbonate and silicate end-members constrained by thetwo-component mixing planes exhibited by the tributary sets.

The parameters which define the carbonate vector (Sr/Cac,Sr/Mgc, Mg/Cac given Na/Cac � 0) are calculated from thefour best fit planes

�1Sr � �2Ca � Na

�1Sr � �2Mg � Na

�1Ca � �2Mg � Na

1Sr � 2Mg � Ca (2)

given Na � 0 and the maximum likelihood solution to theover-determined set of equations. Similarly the parameters thatdefine the silicate vector (Sr/Nas and Mg/Nas at given Ca/Nas)are calculated from Eqn. 2 by a least squares solution.

The fractions of silicate and carbonate-derived cations are

Fig. 6. Calcite saturation indices (Drever, 1997, p. 25) vs. molarCa/Na for tributaries sets. Note that Deoban, Deopryag, Kanwana andTibetan Sedimentary Series catchments are mostly saturated and HighHimalayan Series catchments range from saturated to undersaturated.

Fig. 7. Illustration of carbonate (solid line) and silicate (dashed lines)end-member vectors in 2D dimensional Sr-Ca space for Deopryagcatchment. Vertical lines show magnitudes of silicate- and carbonate-derived Sr. Carbonate and silicate Sr/Ca ratios calculated from best fitto planes in four 3D subspaces of Sr-Ca-Mg-Na space (Eqn. 2). Silicatevectors calculated at Ca/Nas � 0.05, 0.5 and 1.5. Note that for Ca/Nas

�0.5 choice of Ca/Nas makes little difference to magnitude of silicate

Sr fraction, and increasing silicate Ca/Nas to 1.5 only increases silicateSr component by �25%.

then determined by solution of the four mass balance equations,given the carbonate and silicate vectors calculated as above.

Nas � Nac � Nam

Srs � Src � Srm

Cas � Cac � Cam

Mgs � Mgc � Mgm (3)

where Cc and Cs are the concentrations of cation C derivedfrom carbonate and silicate respectively and Cm is the meanconcentration of cation C in the tributary output from thecatchment. This is equivalent to finding the lengths of thecarbonate and silicate vectors in Figure 2, knowing their direc-tions (see also Fig. 7). Substituting the carbonate and silicatevectors determined from Eqn. 2 into Eqn. 3 gives

Nas � Nam

� Ca

Nas� · Nas � Cac � Cam

� Sr

Nas� · Nas � � Sr

Cac� · Cac � Srm

�Mg

Nas� · Nas � �Mg

Cac� · Cac � Mgm

which are solved for Nas and Cac by singular value decompo-sition (Press et al., 1986). The concentrations of the othercomponents are then calculated by substitution and errors arepropagated by a Monte-Carlo routine. Results of a calculationare shown in Table 5.

Uncertainty in the Ca/Na ratio of the silicate input is onelimit on the precision of the calculation of relative silicate andcarbonate inputs although the limiting factor is the nature of theincongruent dissolution or precipitation processes discussedbelow. The parent silicate component Ca/Na ratios may rangebetween �0.15 and 0.5 (e.g., Galy and France-Lanord, 1999;Ahmad et el., 2000; Jacobson et al., 2002). Loss of Ca tosecondary calcite, as discussed above, might reduce the Ca/Naratio of the silicate end member to �0.075 in the Deopryagcatchment waters. Sr/Na ratios will be less sensitive to depo-sition of secondary calcite but are highly variable in rocks. Forexample, for the rocks analyzed by Ahmad et al. (2000) fromthe Alaknanda catchment, the Lesser Himalayan silicates haveSr/Na � 3.2 � 0.5 (1se) and the High Himalayan silicatesSr/Na � 2.1 � 0.4, which compare with mean values of 1.1 and0.9 for the Lesser Himalayan and High Himalayan rocks com-piled by Jacobson et al. (2002). The correlation between thewater Sr/Na and Ca/Na for the Deopryag catchment (Fig. 3)gives Sr/Na between �2.2 and 2.5 (� 0.3 1�) for Ca/Nabetween 0.05 and 0.5. If low-Sr trace-calcite is mixed into theapparent silicate component, as discussed below, or if theCa/Na trend is rotated by variable Ca loss, the silicate Sr/Nacould be as high as 3.0.

Figure 7 illustrates the vector addition calculation in two-dimensions (Sr-Ca). The magnitude of the silicate Sr fractionscan be seen to be insensitive to the assumed silicate Ca/Naratio, and thus silicate Sr/Ca ratio, because the latter is steep

and variations in its gradient make little difference to the length

.

2230 M. J. Bickle et al.

of the Sr component. This diagram does illustrate that thecalculation is very sensitive to the carbonate Sr/Ca ratio andalso that calculation of the Ca partitioning between carbonateand silicate sources is sensitive to both the assumed silicateCa/Na ratio and any incongruent dissolution and precipitationprocesses. The insensitivity of the calculated Sr partition to theassumed silicate Ca/Na ratio carries through to the calculationin four dimensions (Sr-Ca-Mg-Na) (Fig. 8).

4.2.4. Influence of secondary calcite and trace calcite on Srpartition calculations

The high Ca/Na ratios of the waters at zero Mg/Na impliessome systematic perturbation from simple mixing of pure sil-icate and carbonate components. As discussed above, this mayreflect 1) systematic loss of Mg, 2) rotation of the Ca/Na toMg/Na trend by composition-dependent Ca loss by precipita-tion of secondary calcite, or 3) uniform mixing of trace calcitewith silicate rock resulting in a high Ca/Na silicate plus tracecalcite end member (Fig. 5). Although the Lesser HimalayanSeries rocks are lower grade than amphibolite-facies gneissesstudied by Blum et al. (1998) and granites analyzed by White

Fig. 8. Illustration of calculation of silicate Sr as percentage of totalSr by (1) removing massive carbonate fraction with Sr/Ca ratio definedby regression through tributary compositions (solid line) and (2) byremoving trace calcite assuming that silicate–trace-calcite mixture hasCa/Na � 2.0 (the lowest Ca/Na sample from Deopryag catchment)(dashed line). Numbers below line show percentage of Sr from silicateas a function of assumed silicate Ca/Na, assuming no trace calcite

Table 5. Deopryag catchment. Model fit

Calculated from

Silicate 1� Carbonate

Na 87.0 7.7 0.0Ca 174.1 15.3 256.2Sr 310.4 27.1 142.7Mgb 36.4 9.3 179.1

a Expressed as �mol/L of Na, Ca and Mg and nmol/L of Sr.b Note that silicate Ca/Na less than 1.6 implies negative silicate Mg

addition. Numbers above line show percentage of silicate Sr assumingtrace calcite with Sr/Ca � 0.2.

et al. (1999), calcite veining is common in such deformedrocks.

Transformations of the composition data would not affect thecalculated Sr partition provided the location of the transformedend-members can be constrained. Simple loss of Mg would notchange the silicate Ca/Na ratio and thus not alter the calculatedSr partition. If the mixing trends are rotated by composition-dependent Ca loss then the silicate end-member is displaced toa Ca/Na ratio between �1.5 and 2 (Fig. 5) constrained by thesamples with lowest Mg/Na and the condition that Mg/Na �0.2in the silicate component. This increases the estimated propor-tion of silicate Sr to �64% over the 49% calculated at a silicateCa/Na ratio of 0.25 in the Deopryag catchment.

The alternative assumption that the ‘apparent’ silicate end-member comprises both silicate and trace calcite requires anestimate of Sr supplied by trace calcite to calculate the Srderived from silicate. This assumption generally gives lowerestimates for the silicate Sr fraction than modeling the variationtrends by composition-dependent secondary calcite deposition.The bulk composition of the combined trace-calcite and silicatecomponent must have a Ca/Na ratio less than 2, the minimumvalue of tributary waters, and Ca/Na greater than �1.5 � 0.3 tobe consistent with a silicate Mg/Ca ratio �0.3 as discussedabove. It is necessary to assume a Sr/Ca ratio of the tracecarbonate to estimate how a trace calcite would alter the cal-culated partition of Sr between carbonate and silicate sources.Trace calcite in silicates has a relatively low Sr/Ca ratio of�0.2 according to Blum et al. (1998) and Komor (1995) and�0.4 in the granites analyzed by White et al. (1999). However,if Ca is lost to secondary calcite, the Sr/Ca ratio of the tracecarbonate component on the Sr/Ca to Ca/Na mixing diagramswould be correspondingly increased.

To illustrate the consequences of addition of trace calcite(Sr/Ca � 0.2), estimated percentages of silicate Sr as a functionof the assumed silicate Ca/Na ratio are shown on Figure 8compared to control by carbonate with Sr/Ca � 0.56 andMg/Ca � 0.71 appropriate for the Deopryag tributaries. Thecalculation is performed in two stages. First the fractions of Ca,Sr and Mg in the massive carbonate end-member and Na, Sr,Ca and Mg in the silicate plus trace-calcite end-member withassumed Ca/Na � 2, are calculated as described above. Thencomponents of the Ca/Na � 2 end-member are apportioned toa silicate-derived component of given Ca/Na and a trace-calcitecomponent with the assumed Sr/Ca ratio and zero Mg. Figure8 shows that if the dissolved trace calcite had a Sr/Ca of 0.2after removal of secondary calcite, then estimated silicate Sr

Ca, Sr and Mg for silicate Ca/Na � 2.

Observed

Total 1� Tributary 1�

87.0 7.7 77.4 16.2430.3 13.0 450.2 23.6453.1 15.6 442.2 50.5215.5 6.5 206.9 14.0

to Na,

fita

1�

29.323.37.2

percentages would be increased from �49% to �62% at Ca/Na

2231Silicate and carbonate inputs to riverine Sr

� 0.25. If the trace calcite had a Sr/Ca ratio as high as 0.8 theestimated silicate Sr would be reduced to �42% at Ca/Na� 0.25.

Assuming the trace-calcite Sr/Ca � 0.5 � 0.3, the silicate–trace-calcite mix had a Ca/Na ratio � 1.75 � 0.25 and thesilicate Ca/Na � 0.25 (range 0.05 to 0.5), the best estimate ofsilicate Sr in the Lesser Himalayan waters is �49 � 12%. Theerror includes an uncertainty of �4% from modeling the data,� 2.5% from choice of silicate Ca/Na ratios between 0.05 and0.5, � 10% if trace calcite had Sr/Ca ratios between 0.2 and 0.8(after deposition of secondary calcite) and �3.7% if the Ca/Naratio of the silicate–trace-calcite component lay between 1.5 to2.0. If the cation-ratio correlations are systematically rotated bycomposition-dependent Ca loss to secondary calcite, the silicateSr percentage could be as high as 69 � 5%. If systematic Mgloss occurred the silicate Sr would be �49 � 8% for Ca/Nasilicate between 0.05 and 0.5. The apportionment of Sr com-ponents and uncertainties, as discussed below, are listed inTable 6.

4.2.5. Estimates of uncertainties

Uncertainties from the best fit 3D planes, the calculatedcarbonate vectors, and the calculated silicate vectors at givensilicate Ca/Na have been propagated as discussed above. Cal-culation of the uncertainties arising from 1) the possible rangeof silicate Ca/Na, allowing for secondary calcite deposition, 2)the Ca/Na of the silicate–trace-calcite component and 3) therange of possible Sr/Ca ratios of the trace-calcite component,modified by secondary calcite deposition, is less straightfor-ward. We have assumed that the total range of each of theseparameters is equivalent to �1� about the mean and the totaluncertainty listed in Table 6 is the square root of the sum of thesquares of each error.

The uncertainties arising from solution of Eqn. 2 and 3 atgiven Ca/Nas and Sr/Ca for trace calcite are relatively small at� �4.4%. The uncertainties arising from the estimates of the

Table 6. Deopryag, Kanwana, Deoban and Ma

Deopryag Kanwana

Value 1� Value

Sr/Cac 0.56 0.06 1.04Mg/Cac 0.71 0.09 0.79Sr/Nas

a 2.48 0.34 7.78Mg/Nas �0.81 0.29 �0.46%Srs

b 47–69c 13 69–79d

%Srsf 49 12 72

Srsg 222 54 1019

Srtcg 26–171 44–175

Srcg 153 24 302

Sr total 453 16 1430

a Silicate cation ratios calculated at Ca/Na � 0.25, not allowing forb Silicate Sr% assuming no trace calcite and uncertainty is that at fic Ca/Na silicate 0.1 to 3.0.d Ca/Na silicate 0.1 to 1.5.e Ca/Na silicate 0.1 to 3.0.f %Srs assuming trace calcite addition and silicate plus trace-calciteg Range of Sr given in trace calcite. Src, Srs and Srtc are calculated c

assumption.

Ca/Na ratio of the silicate component and Ca/Na of the silicate

plus trace calcite component are similarly small (�2.0 and�2.5%). The possible range in Sr/Ca ratio of trace calcite,especially where this may have been elevated by secondarycalcite deposition, dominates the quoted uncertainties at�10%. However the major uncertainty is the cause of thediscrepancies in Ca:Mg:Na covariation and if this arises fromrotation of Sr/Ca to Mg/Ca trends by composition-dependentCa loss then the Deopryag catchment silicate Sr input may be�66% rather than 49%. These formal estimates of uncertaintiesshould be treated with some caution where regressions dependon a limited number of samples.

4.2.6. Sr inputs in the Kanwana catchment

The Kanwana catchment comprises a heterogeneous groupof rocks including the outer units of the Lesser HimalayanSeries below Deopryag and the late Precambrian to CambrianMussoorie Group comprising a major carbonate horizon, theKrol Formation, interbedded with a range of quartz-rich meta-sedimentary rocks (Azmi and Paul, 2004). The MussoorieGroup rocks include gypsum and phosphates and Singh et al.(1998) considered that these rocks might be the source of highSr/Ca ratios in some rivers. However the lack of correlationbetween Sr/Ca ratios and SO4 or Cl suggest that evaporites arenot a controlling influence.

The catchment exhibits cation ratios distinct from those ofthe Deopryag catchments (Fig. 9, Table 6) with most sam-ples exhibiting higher Sr/Ca (1.04 � 0.30) and slightlyhigher Mg/Ca (0.79 � 0.06) in the carbonate end-member.The three samples with lower Sr/Ca have Sr/Ca and Ca/Nawithin error of the Deopryag catchment correlation. Thecarbonate end-member Sr/Ca and Mg/Ca ratios are consis-tent with limestone to dolomite dissolution in the ratio 1.4 �0.6 to 1 followed by loss of 66 � 10% of Ca to secondarycalcite.

The high Ca/Na ratios (�1.3) at silicate-like Mg/Na is mod-eled by addition of a trace-calcite component. The Ca/Na ratio

TSS catchments cation ratios and Sr partition.

Deoban TSS

Value 1� Value 1�

0.44 0.05 1.44 0.141.36 0.16 0.53 0.031.65 0.53 15.8 2.7

�3.84 0.85 �0.27 0.6935–63e 6.00 49–66c 11

40 18 57 11108 50 1230 38525–98 21–112100 18 841 154275 10 2134 216

arbonate or transformation of silicate Sr %.a/Na)s.

� 2.0. Uncertainty reflects trace calcite Sr/Ca from 0.2 to 0.8.e, silicate and trace calcite Sr concentrations in nmol/l for trace calcite

rsyandi

1�

0.300.060.190.0178

99

8756

trace cxed (C

Ca/Naarbonat

of the silicate–trace-calcite mixture must lie between Ca/Na

rs) are

2232 M. J. Bickle et al.

�1.3, the intercept at Mg/Na � 0, and Ca/Na �2.0, the lowestCa/Na sample. Trace calcite has been modeled with Sr/Cabetween 0.2 and 0.8 to allow for secondary calcite deposition.The calculated percentage of silicate Sr is most sensitive to theSr/Ca ratio adopted for the trace-calcite mixture and the bestestimate of silicate Sr is 72 � 8% (Table 6). If the Ca/Na toMg/Na trend is rotated by composition-dependent secondarycalcite deposition the estimated silicate Sr contribution couldbe as high as 79 � 3%.

Fig. 9. Si/Ca, Sr/Ca and K/Ca vs. Ca/Na and Ca/Na vssamples excluded from regressions. Regressions (1� errorwhere smaller than symbols.

Fig. 10. Correlation diagram for Deoban samples. Opeexcluded from Sr-Ca-Na fit. Correlations shown (1� erro

where smaller than symbols.

4.2.7. Sr inputs in the Deoban catchment

The Deoban catchment contains the highly radiogenic car-bonates structurally immediately underlying the MCT zone.Figure 10 illustrates Si/Ca, K/Ca and Sr/Ca vs. Ca/Na, andCa/Na vs. Mg/Na for the Deoban catchment samples. Thecalculated Sr/Ca and Mg/Ca ratios of the water carbonateend-member (0.44 � 0.05 and 1.4 � 0.1) are, on average, afactor of 3 � 0.4 higher than the mean of 20 Deoban carbonates

for Kanwana catchment samples. Open symbols denoten for individual three-cation fits. 1� error bars not shown

ols samples excluded from fits. AK79 (Ca/Na � 62) isfor individual three-cation fits. 1� error bars not shown

. Mg/Nas) show

n symb

2233Silicate and carbonate inputs to riverine Sr

(0.16 � 0.02 and 0.44 � 0.11, 1 standard error, Bickle et al.,2001) implying significant secondary calcite deposition. TheCa/Na to Mg/Na correlation gives negative Mg concentrationsat plausible silicate Ca/Na ratios which might either reflectcomposition-dependent secondary calcite precipitation or addi-tion of a low-Mg trace-calcite component to the silicate. TheCa/Na ratio of this mixture is less well constrained than for theDeopryag and High Himalayan Crystalline Series catchmentsbecause the most silicate-rich sample has a Ca/Na � 4.0 andMg/Ca �0 for Ca/Na �3.2 � 0.6. The silicate Sr percentagehas been calculated by assuming 1) the addition of trace calcitewith Sr/Ca ratios between 0.2 and 0.8, Ca/Na ratio of the tracecalcite–silicate component mixture between 2 and 3.5 and forCa/Na silicate between 0.05 and 0.5 and 2) the systematicdisplacement of the Ca/Na trend by composition-dependentsecondary calcite deposition such that the silicate Ca/Na ratio isincreased to 3.5 (Table 6). The percentage of silicate-derived Sris 40 � 18% for the trace-calcite model with a maximum of 63� 4% for the composition-dependent displacement. If theCa/Na to Mg/Na trend was displaced by systematic loss of Mg,the estimated percentage of silicate-derived Sr would be re-duced to 26 � 7%. The main uncertainty arises from the natureof the incongruent dissolution or precipitation processes.

4.2.8. Calculation of Sr inputs in the High HimalayanCrystalline catchment

Figure 11 illustrates Si/Ca and Sr/Ca vs. Ca/Na, andCa/Na vs. Mg/Na correlations for the High Himalayan Crys-talline catchment between Malari and Helong. Si/Ca toCa/Na illustrates coherent variation with a silicate Si/Naratio of 2.7 � 0.4, slightly in excess of that expected forplagioclase to kaolinite weathering. Plots of ratios of Sr, Ca,Mg and Na exhibit more scatter interpreted as resulting from

Fig. 11. Si/Ca to Ca/Na, Sr/Ca to Ca/Na and Ca/Ncatchments. Open symbols denote samples excluded from1� error bars not shown where smaller than symbols.

variability of carbonate and silicate end-members within the

terrain. Fits to the covariation of selected ratios have there-fore been made to data sets from individual catchment areaswithin the High Himalayan Crystalline Series. The maingeological division is between the Vaikrita Formation, com-prising principally amphibolite-facies quartzites, subsidiarypelites, orthogneisses and Himalayan leucogranites, and theMunsiari Formation of diverse orthogneisses. A kilometer-thick unit of calc-silicates outcrops within the Vaikrita For-mation in the catchment of the Saraswati River. The Mun-siari and Saraswati Catchments have therefore beendistinguished from other catchments of the Vaikrita Forma-tion.

Massive carbonates have been sampled in the Saraswaticatchment and have mean Sr/Ca ratio of 0.50 � 0.02 (1 se)and a mean Mg/Ca ratio of 0.067 � 0.002 (Bickle et al.,2001). The Sr/Ca ratio of the carbonate end-member waterfrom the Saraswati catchment is poorly constrained by theregression because of the limited spread of the data (Fig. 11)but must be less than the Sr/Ca � 0.7 of the most carbonate-dominated water composition. The carbonate end-memberhas therefore been set at Sr/Ca � 0.51. If the silicate endmember is mixed with trace calcite the estimated fraction ofsilicate Sr is 55 � 19% where most of the uncertainty arisesfrom the assumed range of trace calcite Sr/Ca ratio between0.2 and 0.8 (Table 7). If trace calcite is not a factor theestimated silicate Sr fraction of 60 � 9% (at silicate Ca/Nabetween 0.1 and 1) is very similar to that calculated withtrace-calcite. The Vaikrita catchment data exhibit most scat-ter and only the Sr-Ca-Na and Ca-Mg-Na correlations havebeen used. Again calculations with or without trace-calciteaddition give very similar silicate Sr fractions at �35%. TheMunsiari catchment data are the best constrained and con-sistently give �70% silicate Sr irrespective of assumptions

g/Na diagrams for High Himalayan Crystalline Seriessions. Regressions shown for individual three-cation fits.

a to Mregres

regarding trace-calcite contributions.

2234 M. J. Bickle et al.

The silicate-derived output of Sr is 48 � 12% after weight-ing the High Himalayan Crystalline Series sub-catchment out-puts by their areas and Sr fluxes.

4.2.9. Calculation of Sr inputs in the Tibetan SedimentarySeries catchment

Because tributaries to the Dhauli Ganga are inaccessible forpolitical reasons, modeling the output of the Dhauli Gangafrom the Tibetan Sedimentary Series is based on a tributarysuite collected from the upper Marsyandi River in central Nepalduring May 2002. The analogy is justified by the similarity ofthe respective mainstream compositions (Fig. 12) and the ob-servation that the weighted average of the Dhauli Ganga atMalari (Table 3) lies within error of the Marsyandi cation ratioand 87Sr/86Sr regressions (Figs. 13 and 14). The water compo-

Table 7. High Himalayan catch

Vaikrita 1� S

Sr/Cac 1.14 0.32Mg/Cac 0.56 0.15Sr/Nas

a 4.83 2.79Mg/Nas �2.60 1.30%Srs

b 26–39c 15%Srs

f 35 14Srs

g 214 84Srtc

g 11–56Src

g 443 116Sr total 627 96

a Silicate cation ratios calculated at Ca/Na � 0.25, not allowing forb Silicate Sr% assuming no trace calcite and uncertainty is that at fic Ca/Na silicate 0.1 to 2.0.d Ca/Na silicate 0.1 to 1.0.e Ca/Na silicate 0.1 to 3.5.f %Srs assuming trace calcite addition and silicate plus trace-calciteg Range of Sr given in trace calcite. Src, Srs and Srtc are calculated ca

assumption.

Fig. 12. Cation ratios of rain and hot-spring input-corrected outputfrom Tibetan Sedimentary Series in Marsyandi and Dhauli Ganga atMalari compared with other catchments. Note that Sr/Ca ratio is di-vided by two to fit scale. Tibetan Sedimentary Series outputs aresimilar, both being characterized by high Sr/Ca, low Na/Ca, K/Ca and

Si/Ca. Kanwana catchment in low Lesser Himalayas is compositionallymost similar to Tibetan Sedimentary Series outputs.

sitions are distinguished by surprisingly high Sr/Ca and Sr/Naratios.

Cation ratio plots for the Marsyandi suite are illustrated inFigure 13. The Sr/Ca of the water carbonate end member of1.4 � 0.2 is within error of the mean Sr/Ca of 1.0 � 0.2 (1se)of Tibetan Sedimentary Series carbonate rocks compiled byJacobson et al. (2002). The Mg/Ca of the water carbonateend-member of 0.53 � 0.03 is much higher than the Mg/Ca of0.09 � 0.02 from carbonate pebbles derived from the TibetanSedimentary Series, sampled by English et al. (2000). Thecarbonate end-member water Sr/Ca and Mg/Ca ratios couldreflect dissolution of limestone (Sr/Ca � 1) to dolomite (Sr/Ca� 0.2) in the ratio 1.25 � 0.16 and loss of 47 � 6% of the Cato secondary calcite. However dolomites appear relatively rarein the Tibetan Sedimentary Series and dolomite would beexpected to weather more slowly than limestone. It is possiblethat the published average Tibetan Sedimentary Series lime-stone compositions have higher Sr/Ca than the Marsyandicarbonates. At a silicate-like Ca/Na � 0.25 in the waters, Sr/Caequals 63 � 10 and Sr/Na equals 15.8 � 2.7. These values arehigh and compare with silicate Sr/Ca between 6 and 31 andSr/Na between 1.5 and 8 for the other catchments in theAlaknanda (Tables 5 and 6).

For silicate Ca/Na between 0.1 and 3, estimated silicate Srpercentages in the Dhauli Ganga at Malari range from 47 to69 � 7%. This range of Ca/Na would be expected to en-compass the results of most incongruent precipitation reac-tions likely to displace the water compositions from that ofthe silicate source. Alternatively, if a low Mg/Ca tracecalcite is added to the silicate (the trace-calcite silicatemixture is constrained to Ca/Na between �2.0 and 3.6 by thelocation of the Mg/Na negative intercept and the samplewith the lowest Ca/Na ratio) the percentage of silicate-derived Sr would be 49 � 12% (Table 6). The relatively highsilicate contribution to the Sr flux from the carbonate-dom-inated Tibetan Sedimentary Series is a surprising result thathas not recognized by previous studies (Harris et al., 1998;

cation ratios and Sr partition.

ti 1� Munsiari 1�

0.24 0.170.02 0.11 0.020.14 5.52 0.590.006 0.26 0.062.6 77–83e 15

19 71 1440 287 41

19–783 75 52

18 396 21

arbonate or transformation of trends./Nas.

� 2.0. Uncertainty reflects Sr/Catc from 0.2 to 0.8., silicate and trace calcite Sr concentrations in nmol/L for trace calcite

ments:

araswa

0.510.121.53

�0.01251–68d

5510926–10227

228

trace cxed Ca

Ca/Narbonate

Galy et al., 1999; English et al., 2000; Jackson et al., 2002;

2235Silicate and carbonate inputs to riverine Sr

Fig. 13. Correlation diagram for tributaries draining Tibetan Sedimentary Series in upper Marsyandi Valley, Nepal.Open-symbol samples excluded from fits. Star shows season-weighted average of Dhauli Ganga at Malari. Correlations

shown (1� errors) are for individual three-cation fits. 1� error bars not shown where smaller than symbols.

Fig. 14. 87Sr/86Sr variation with Ca/Na for Himalayan catchments. Curves are model fits (Eqns. 5 and 6) with calculatedsilicate and carbonate end-member 87Sr/86Sr ratios and 1� errors as shown. Silicate 87Sr/86Sr ratios calculated at Ca/Na �

0.25. 1� error bars shown except where smaller than symbols. Fit to Deopryag catchment calculated with carbonate87Sr/86Sr ratio set � to 0.720. Dashed curve in (a) shows calculated mixing trend using rock 87Sr/86Sr ratios (Table 8).

ata by J

2236 M. J. Bickle et al.

Oliver et al., 2003). A similar result (49%) of silicate-derived Sr is calculated for the Marsyandi.

4.3. Constraints on 87Sr/86Sr Ratios of Carbonate andSilicate End-Members

The variation of 87Sr/86Sr ratios with Ca/Na ratios placesconstraints on the Sr-isotopic composition of the carbonate andsilicate end-members. The presence of significant Sr in a trace-calcite component potentially complicates the mixing relation-ships, although it is probable that the trace calcite has similar87Sr/86Sr ratios to the silicate phases in the metamorphic rocks.Scatter of 87Sr/86Sr ratios in plots of tributary compositions vs.Ca/Na is presumed to reflect heterogeneity in the bedrock87Sr/86Sr ratios and imposes a limit on the precision of theinferred end-member isotopic compositions. Bedrock compo-sitions exhibit similar heterogeneities and below we use boththe bedrock data and the water data to estimate consistentvalues of the 87Sr/86Sr ratios of the silicate and carbonateend-members.

The 87Sr/86Sr ratio of a tributary varies with the fractions ofsilicate and carbonate Sr (Srs

w and Srcw), as

87Sr ⁄ 86Sr � Srsw . 87Sr/86Srs � Src

w . 87Sr/86Src (5)

where Srsw � 1 � Src

w, and Srsw and Src

w may be calculated fromthe silicate and carbonate Sr contents of the water, Srs and Src.These are given by the water Na/Ca ratio (Na/Caw), the end-member Sr/Ca ratios, Sr/Cac and Sr/Cas defined by the covaria-tion of Sr/Ca and Na/Ca ratios for the tributary set at assumedsilicate Na/Ca ratio, Na/Cas. Srs

w is given by

Srsw �

� Na

Caw� � Sr

Cas�

Sr

Na(6)

Table 8. End-member 87Sr/86Sr rati

Catchment

87Sr/86Sr rock end membersa

Silicate Carbonate

1 se 1 se

Deopryag ILHS 0.896 0.083 0.74 0.06Kanwana OLHS 0.793 0.011 0.711 0.001Deobanb na 0.825 0.03Deobanb

HHCS Vaikritac 0.762 0.006 0.717 0.003HHCS Saraswatic 0.762 0.006 0.717 0.003HHCS Munsiaric 0.908 0.039 naTSS Marsyandi high 87Srd 0.727 0.012 0.718 0.002TSS Marsyandi low 87Srd 0.712 0.002

a Silicate rock data from Ahmad et al. (2000), carbonate rock datab Deoban split into two groups. High 87Sr/86Sr catchments and lo

Unsampled tributaries from north allocated Deopryag-like values as inc 87Sr/86Sr of carbonate fixed to give reasonable correlation throughd Marsyandi split into high 87Sr/86Sr and low 87Sr/86Sr groups as disc

et al., 2000), high value of carbonate (0.718) from three analyses of bedby Galy et al. (1999): low estimate from compilation of Himalayan d

�Caw� �Cas

�

The variation of water 87Sr/86Sr ratios therefore depends onboth the 87Sr/86Sr and Sr/Ca and Ca/Na ratios of the silicate andcarbonate end-members. Best fit curves shown in Figure 14have been calculated by least squares fits to Eqn. 5 and 6 usingthe Levenberg-Marquardt method (Press et al., 1986). The 1�uncertainties on the end-member 87Sr/86Sr ratios have beencalculated by increasing the calculated errors on the rain andhot-spring corrected 87Sr/86Sr ratios by a factor calculated togive the expected �2 for the appropriate degrees of freedom.The fit is also dependent on the Ca/Na ratio assumed for thesilicate end-member, but the shape of the curve changes slowlyas Ca/Nas varies in the range 0.05 to 2.0 (see range of resultsin Table 8). The 87Sr/86Sr ratio of the silicate end-member doesdepend on the value of the silicate Ca/Na which could providea constraint on the latter except the scatter of silicate rockcompositions and uncertainty in silicate end-member 87Sr/86Srratios generally encompass the likely range in silicate Ca/Na.The 87Sr/86Sr ratio of trace calcite is assumed to be the same asthat of the silicate (cf. Blum et al., 1998).

The 87Sr/86Sr ratios of tributaries from the Deopryag catch-ment (Fig. 14a) exhibit considerable scatter. The calculated twocomponent mixing curve based on the Sr/Ca to Ca/Na variation(Fig. 3c) and the rock 87Sr/86Sr ratios tabulated in Table 8passes above all the data presumably reflecting biased sam-pling. The fit shown was chosen to give a reasonable average ofthe water data while satisfying error bounds on the rock data.

The Deoban catchment exhibits two groups of samples (Fig.14c). AK78, 79, 140 and 202 come from small catchmentsdraining the very high 87Sr/86Sr ratio carbonates and exhibit acoherent trend. The rest of the samples except AK138 and 192are from the Birehi River and a small catchment underlain bysimilar geology to the north and give a coherent trend at lower87Sr/86Sr ratios. The adopted 87Sr/86Sr ratios are the area-weighted averages. In the calculations below, the inputs fromthe mostly unsampled tributaries from the north are assumed to

ilicate and carbonate components.

r/86Sr water end members Used in modeling

ate endmber

Carbonate endmember

Silicate endmember

Carbonate endmember

0.05 to 2.0 1� 1� 1�

2–0.763 0.720 c 0.78 0.015 0.72 0.013–0.753 0.716 0.008 0.79 0.01 0.712 0.0030–1.087 0.915 0.009 0.890 0.025 0.770 0.0253–0.816 0.747 0.0042–0.760 0.736 0.001 0.77 0.006 0.736 0.0010–0.749 0.720 0.002 0.765 0.006 0.720 0.003na 0.80 0.01 0.735 0.010

4–0.741 0.714 0.002 0.725 0.004 0.709 0.0045–0.723 0.709 0.004

ngh et al. (1998) and Bickle et al. (2001).r/86Sr Birehi River. Values used in modeling area weighted mean.et al. (2003).

n text. Silicate rock 87Sr/86Sr from TSS silicate rocks �0.725 (Najmand seven bedload carbonate leaches in Nepal near Marsyandi catchmentacobson et al. (2002).

os for s

87S

Silicme

Ca/Na

0.780.761.180.850.780.77

0.740.72

from Siwer 87S

Bickledata.

ussed irocks an

be similar to the Deopryag catchment (as also sample AK138)

2237Silicate and carbonate inputs to riverine Sr

as these tributaries drain typical Lesser Himalayan units (seeBickle et al., 2003).

The 87Sr/86Sr vs. Ca/Na variation of the High HimalayanCrystalline Series Vaikrita catchment shows moderately coher-ent behavior, excluding the Saraswati catchment. The carbon-ate end-member 87Sr/86Sr ratio of 0.7298 � 0.0002 is higherthan the mean calc-silicate sampled in the Saraswati catchment(0.717 � 0.003, 1se on nine samples) but the carbonate signalmay be dominated by thinner more widely dispersed units thathave undergone further exchange with adjacent silicates(Bickle et al., 2001). The estimated 87Sr/86Sr ratio of thesilicate end-member lies between 0.78 and 0.76 for silicateCa/Na ratios between 0.05 and 2.0 which compares with themean value of 0.762 � 0.006 (1se on 11 samples, Bickle et al.,2001). The Saraswati, less upstream samples AK18, 19 and 20,exhibits a good fit to calculated two-component mixing curvewith a carbonate end-member of 0.720 � 0.002 which com-pares with the mean of the calc-silicate rocks of 0.717 � 0.003.The calculated 87Sr/86Sr ratio of the silicate end-member liesbetween 0.77 and 0.75 again spanning the mean composition ofanalyzed Vaikrita silicates. There are insufficient samples fromthe Munsiari catchment and end-member 87Sr/86Sr ratios havebeen estimated from the available rock and water samples.

The Sr-isotope data from the Tibetan Sedimentary Series inthe Marsyandi splits into a high and a low 87Sr/86Sr ratio group(Fig. 14e). The high values are mainly from tributaries derivedfrom the Annapurna range which may include some lithologiesof the High Himalayan Crystalline Series. The 87Sr/86Sr andCa/Na ratios of the mainstem Marsyandi at Chame (where itleaves the Tibetan Sedimentary Series) and the values from theDhauli Ganga at Malari lie on the mixing trend defined by thelower 87Sr/86Sr ratio tributaries from the north which mustdominate the inputs.

5. CALCULATION OF SR AND 87SR/86SR FLUXES

The new tributary and mainstem data from the August 2003collection have been added to the previously published data andthe estimation of water, Sr and the 87Srexcess input fluxes fromthe major lithotectonic units in the headwaters of the Gangesabove Rishikesh have been recalculated using the methodologyof Bickle et al. (2003) (Table 9). 87Srexcess is the relative flux of87Sr in excess of 0.709 (seawater), that is 87Srexcess � (87Sr/86Sr � 0.709) � Sr, and measures the impact of that Sr flux onpresent-day seawater 87Sr/86Sr variation. The runoff from theKanwana catchment is assumed to be the same as from the

Table 9. Percentage inputs of Sr to the Ga

May 1996 1� May 1997

TSS 72 5 38HHCS 15 5 15Deoban 2 5 3Deopryag 8 8 32Kanwana 3 8 12Sample period (d) 340–125 149–203Percentage of annual

water flux 12 31

Deopryag catchment and Sr-input flux calculated accordingly.

As in Bickle et al. (2003), the sample periods have beenassigned to representative dates and weighted by the water fluxdata at Deopryag of Pal (1986) (Table 9). The August 2003input fluxes exhibit a relatively large contribution from theLesser Himalayan catchments, as did the 1997 September/October collection and this increases the estimated Sr contri-bution from these units from 34 � 8 to 50 � 10% (1�) andemphasizes the sensitivity of the estimates to inadequate sam-pling of average annual fluxes.

The estimates of the partition of Sr between silicate, tracecalcite and massive carbonate and the 87Sr/86Sr ratios of thesilicate and carbonate end-members (Table 8) have been com-bined with the estimates of the relative mean Sr outputs fromthe catchments (Table 9) to calculate the fractions of Sr and the87Srexcess flux contributed by silicate rock, massive carbonaterock, trace calcite, rain and hot-springs in the entire catchmentabove Rishikesh.

The results (Table 10) show that silicates contribute �50%of the total Sr flux and 70% of the 87Srexcess flux. Precipitationinputs 4% of the Sr and has a negligible impact on the 87Srexcess

flux. Hot-springs contribute �2% of the Sr flux and 3% to the87Srexcess flux, values much less than the estimated 30% con-tribution to the Sr flux of the Marsyandi by Evans et al. (2001).The relatively high silicate Sr-flux from the Tibetan Sedimen-tary Series is not reflected in a high 87Srexcess flux because ofthe relatively low 87Sr/86Sr ratio of the Tibetan SedimentarySeries output. The Lesser Himalayan Series catchments dom-inate the 87Srexcess flux because of their radiogenic isotopiccompositions, which reflects the late Archaean/Early Protero-zoic age of many of these units. It should also be noted thatcarbonate makes a significant contribution to the 87Srexcess fluxand that trace calcite may contribute a significant fraction ofthis although this is the least well-constrained component. If thediscrepancies in the Ca/Na to Mg/Na correlations of the tribu-tary compositions relate to composition-dependent secondarycalcite precipitation, the trace-calcite component should beadded to the silicate component although this slightly reducesthe estimated silicate 87Sr/86Sr ratio. Our estimates of thesilicate Sr fraction are therefore probably minimum values.

The estimate of the fraction of the strontium derived fromsilicate (Table 10) is compared with published estimates inFigure 15. Singh et al. (1998) and Krishnaswami et al. (1999)calculated the fractions of Sr derived from silicate and carbon-ate rocks and the calculations resulted in a significant fractionof Sr unaccounted for by the rock chemistry. Figure 15 shows

Rishikesh from major lithotectonic units.

Sept–Oct1997 1�

Aug–Sept1998 1�

August2003 1�

19 1 30 3 13 18 2 19 2 8 22 2 4 2 1 2

51 20 33 3 57 1020 22 13 4 22 11

275–339 246–274 204–245

9 24 24

nges at

1�

223

1010

the silicate-derived Sr fractions from these two papers as a

Tab

le10

.Sr

and

87Sr

exce

ssin

puts

from

silic

ate,

trac

eca

lcite

,ca

rbon

ate,

rain

,an

dho

tsp

ring

s.

Silic

ate

Tra

ceca

lcite

Mas

sive

carb

onat

eR

ain

Hot

spri

ngs

Tot

alSi

licat

eT

race

calc

iteM

assi

veca

rbon

ate

Rai

nH

otsp

ring

sT

otal

%Sr

1�%

Sr1�

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2238 M. J. Bickle et al.

percentage of the calculated silicate- plus carbonate-derivedfraction (total bar height) and as a fraction of the total Sr (filledbar height). Jacobson et al. (2002) and Oliver et al. (2003)concurred with earlier suggestions (Singh et al. 1998; Galy etal., 1999) that the ‘excess’ Sr arose because secondary calcitedeposition increased the Sr/Ca ratio of the carbonate end-member and corrected their estimates for this while usingestimates of rock Ca/Na and Sr/Na to constrain the silicateend-members. The principal reason for the difference betweenthe results of this paper (51% Sr derived from silicate) and themajority of the published estimates which lie between 20 and30% is that the end-member Sr/Ca and Sr/Na ratios defined bythe water compositions are consistently higher than rock val-ues. The water-defined silicate end-members have Sr/Ca ratiosbetween 3 and 63 and Sr/Na ratios between 1 and 16 at Ca/Na� 0.25. Most Himalayan silicate rocks have Sr/Ca approxi-mately between 1 and 10 and Sr/Na between 2 and 6 (e.g.,Ahmad et al., 2000; Jacobson et al., 2002). If the water Ca/Nais reduced by secondary-calcite deposition, the calculated sili-cate water end-member Sr/Ca ratios are proportionally higher.

The discrepancies between the rock silicate and carbonatecompositions and the end-members derived from the arrays ofwater compositions might arise from incongruent dissolution orprecipitation reactions or from additional sources of Sr. Whiletransformations of the water compositions do not alter themixing proportions, if the locations of the transformed end-members can be constrained, it remains possible that watercompositions may be systematically altered so as to bias cal-

Fig. 15. Published estimates of percentage of Sr derived from silicatein major Himalayan tributary systems compared with estimate in thispaper of 51 � 6%. For Singh et al. (1998) and Krishnaswami et al.(1999) total bar height shows percentage of silicate Sr to Sr derivedfrom silicate and carbonate-derived Sr and filled bar shows percentageof silicate-derived Sr to total Sr (see text). 1� uncertainty shown onestimate from this work.

culations. Until the processes that control the water composi-

TSS

HH

Deo

Deo

Kan

Tot

2239Silicate and carbonate inputs to riverine Sr

tions are understood all such estimates should be treated withcaution.

A final important conclusion concerning the significance ofHimalayan silicate weathering for control of the seawater Sr-isotope curve is that �70 � 16% of the 87Srexcess flux is silicatederived, reflecting the higher 87Sr/86Sr ratios of the silicaterocks. It would seem that silicate weathering is probably thedominant cause of the high 87Sr output from Himalayan riversalthough carbonate sources are not negligible. However itdoesn’t necessarily follow that such an increased silicate influ-ence on the seawater 87Sr/86Sr ratio reflects an increased sili-cate weathering flux, given the very high silicate 87Sr/86Srratios.

6. CONCLUSIONS

Dissolved Sr in the Himalayan catchment of the headwatersof the Ganges above Rishikesh is derived in equal proportionsfrom silicate and carbonate sources. The more radiogenic Sr-isotopic compositions of the silicate rocks means that thesilicate sources provide �70% of the 87Sr flux (87Srexcess flux)which controls variations in seawater 87Sr/86Sr isotope compo-sition. The marked difference between these conclusions andearlier estimates of the partition of Sr fluxes arises because theend members are constrained by using arrays of tributary watercompositions. Most sets of tributary water compositions arewell modeled by two-component mixing. Ca-Mg-Na covaria-tions which deviate from silicate rock compositions are mod-eled either by addition of a low-Mg trace calcite to the silicatecomponent or by rotation of the trends by composition-depen-dent secondary-calcite deposition. Transformations of the watercompositions do not invalidate the mass-balance modeling pro-vided that the transformed positions of the end-members can beconstrained. The modeling shows that a number of processes,which probably include secondary calcite deposition and pos-sibly incongruent dissolution, result in water compositions withsignificantly different element ratios from those of the sourcerocks. A full understanding of Sr budgets requires that theseprocesses be better understood. Much of the secondary calciteappears not to redissolve in the river waters within the moun-tainous catchments, possible because deposition takes place insoils and terraces adjacent to the rivers, and when eroded, thismaterial is transported rapidly downstream. Bickle et al. (2003)calculate that carbonate dissolution is insignificant over thetime-scales for transport of suspended load in the Ganges headwaters. Much further weathering takes place in the Gangesflood plain (Sarin et al., 1989; Chabaux et al., 2001; West et al.,2002) and evaluation of the full impact of Himalayan weath-ering will require that these processes are quantified.

Acknowledgments—This research has been support by the UK NaturalEnvironment Research Council and the Leverhulme Trust. The meth-odology was discussed with Albert Galy, Ed Tipper and Josh West.Tank Ojha is thanked for logistic support in Nepal. Christina de laRocha assisted with sample collection in 2003. Fatima Kahn assistedwith analyses. Graham Howell is thanked for help with anion analyses.We thank the associate editor, S. Krishnaswami, and three anonymousreviewers for insightful comments.

Associate editor: S. Krishnaswami

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