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PII S0016-7037(97)00381-5

Chemical and strontium, oxygen, and carbon isotopic compositions of carbonates from theLesser Himalaya: Implications to the strontium isotope composition of the

source waters of the Ganga, Ghaghara, and the Indus rivers

SUNIL K. SINGH, J. R. TRIVEDI, K. PANDE, R. RAMESH, and S. KRISHNASWAMI*Physical Research Laboratory, Navrangpura Ahmedabad 380 009, India

(Received July31, 1997;accepted in revised form October28, 1997)

Abstract—Samples of Precambrian carbonate (mostly dolomite) outcrops collected across the Lesser Hima-laya have been analysed for their mineralogy, chemical composition, and isotope ratios of Sr, O, and C toassess the extent of their preservation and their role in contributing to the high radiogenic strontium isotopecomposition of the source waters of the Ganga, Ghaghara, and the Indus. Their Sr concentrations range from20 to 363 ppm,d18OPDB 21.4 to212.8‰ and Mn 11–2036 ppm. The petrography of the samples, their lowSr concentrations, and wide range ofd18O values are suggestive of their postdepositional alteration. The87Sr/86Sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from0.7064 to 0.8935 and are generally more radiogenic than that of contemporaneous seawater.

Comparison of the87Sr/86Sr and Sr/Ca ratios among the carbonates and silicates from the Lesser Himalayaand the source waters of the Ganga, Ghaghara, and the Indus shows that the values for the source watersoverlap with those of the silicates but are much higher than those in carbonates. An upper limit of carbonateSr in the various source waters is calculated to be between 6% and 43%, assuming thatall the Ca in the riversis of carbonate origin. The results show that on the average, weathering of the Precambrian carbonates isunlikely to be a major contributor to the highly radiogenic strontium isotope composition of these sourcewaters; however, they can be a dominant supplier of radiogenic Sr to some rivers on a regional scale.

The silicate Sr component in some of the source waters of the Ganga (Bhagirathi, Bhilangna, Alaknanda,and Ganga), Ghaghara (Kali and Sarju), and the Indus (Sutlej) was calculated from the Ca/Na, Sr/Na ratios,and strontium isotope compositions of these rivers and the silicate endmember. These calculations suggest that33–89% of Sr in the Bhagirathi, Bhilangna, Alaknanda, Ganga, and Sarju rivers is of silicate origin, whereasin the Kali and the Sutlej it is much lower , only;8%. The remaining Sr to all these waters has to be suppliedfrom other sources such as weathering of carbonates and evaporites. This study underscores the importanceof weathering of silicates, carbonates, and evaporites in contributing to the Sr mass balance and87Sr/86Sr ofthe source waters of the Ganga, Ghaghara, and the Indus. The present day silicate and carbonate Srcontributions to the Sr budget of the rivers vary considerably, but among the major source waters of the Ganga,silicate Sr exerts a more dominant control on their Sr abundance and87Sr/86Sr. Copyright © 1998 ElsevierScience Ltd

1. INTRODUCTION

It is well established that among the major rivers of theworld, those draining the Himalayan-Tibetan Plateau, particu-larly the Ganga-Brahmaputra (G-B) and a few tributaries of theIndus, are characterised by high87Sr/86Sr (.0.7300) and highSr concentration (Palmer and Edmond, 1989; Krishnaswami etal., 1992; Palmer and Edmond, 1992; Pande et al., 1994;Trivedi et al., 1995). The source(s) of the high radiogenic Sr inthese rivers remains equivocal. It has been suggested thatchemical weathering and erosion in the Himalaya have con-tributed significantly to global climate change during the Ce-nozoic (Raymo and Ruddiman, 1992). The coupling betweenchemical weathering in the Himalaya and climate change canbe better assessed if the contributions to the strontium isotopemass balance in these rivers from various source rocks (sili-cates, carbonates, and evaporites) and their temporal variationscan be constrained. The high87Sr/86Sr in the G-B system hasbeen ascribed to weathering of granites/gneisses (Edmond,

1992; Krishnaswami et al., 1992; Pande et al., 1994), metamor-phosed carbonates (Palmer and Edmond, 1992), and metasedi-ments (Harris, 1995). France-Lanord et al. (1993) suggestedthat the weathering of Higher Himalayan Crystalline Series isthe major source of detrital material to the sediments of the Bayof Bengal based on their isotopic and clay mineralogicalstudies.

We have carried out measurements of strontium, oxygen,and carbon isotopes and elemental abundances in a number ofPrecambrian carbonate deposits from the Lesser Himalaya(Singh et al., 1996) to evaluate their contribution to the stron-tium isotope composition of the source waters of the Ganga,Ghaghara, and the Indus rivers and thereby constrain the sourcefor the high87Sr/86Sr in these rivers. This study builds on ourearlier work on the major element composition and strontiumisotope systematics of the rivers draining the southern slopes ofthe Himalaya (Krishnaswami et al., 1992; Sarin et al., 1992;Pande et al., 1994; Trivedi et al., 1995).

2. MATERIALS AND METHODS

A significant fraction of the drainage basins of the source waters ofthe Ganga, the Ghaghara, and the Indus lies in the Lesser Himalaya.

* Author to whom correspondence should be addressed([email protected]).

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Geochimica et Cosmochimica Acta, Vol. 62, No. 5, pp. 743–755, 1998Copyright © 1998 Elsevier Science LtdPrinted in the USA. All rights reserved

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The two main lithotectonic units of this region are the sedimentaryrocks (autochthonous and allochthonous) and the crystallines (Gansser,1964; Valdiya, 1980). The autochthonous sediments (Damtha andTejam groups) are made of greywacks, shales, slates, limestones, anddolomites. The dominant lithologies of allochthonous sediments (KrolNappe) are shales, slates, phyllites, conglomerates, carbonates, andquartzites. Gneisses, granites, schists, calc-silicates, and amphibolitesmake up the crystallines (Ramgarh and Almora group). Carbonates ofthe Tejam group and Krol Nappe are exposed extensively in Kumaunand Himachal of the Lesser Himalaya (Fig. 1). These carbonates,occurring as several extensive bodies, were sampled from variousoutcrops during three field campaigns conducted in 1992, 1994, and1995. The sampling was designed to span the water sheds of the sourcewaters of the Ganga, Ghaghara, and the Indus. Many of these sourcewaters (e.g., the Ramganga, Sarju, and Kali belonging to the Ghaghara,the Alaknanda and Pindar of the Ganga, and the Sutlej of the Indus)flow through the Tejam group of rocks dominated by Precambriancarbonates (Fig. 1). The drainage basins of the Bhagirathi, theBhilangna, and their tributaries (of the Ganga system) are dominated bysilicates from crystallines and sediments with minor amounts of car-bonates dispersed in them. The carbonate samples were collected fromboth the inner (Deoban5 Shali and Mandhali5 Sor Formations of theTejam group) and the outer (Blaini and Krol Formations of the KrolNappe) belts of the Lesser Himalaya (Valdiya, 1980). These carbonatesconsist predominantly of limestones, dolomites, and a variety of cal-carenites. The Deoban Formation is an extensive deposit of stromatolitebearing cherty dolomite, dolomitic limestone with intercalations ofblue limestone and gray slate and minor magnesite (Valdiya, 1980).The stromatolites in these deposits have led Valdiya (1980) to infer thatthey were formed during the Middle Riphean (1300–1000 Ma). Thefour major exposures of this Formation are at Thal-Tejam-Girgaon,Gangolihat-Pithoragarh, Shali- Bilaspur, and Chakrata-Deoban-Tiuniareas (Fig. 1). These carbonates show varying degrees of metamor-phism/postdepositional alteration. Bhattacharya (1982) has reported agradual increase in metamorphic effects of the carbonate rocks ofThal-Tejam area from south to north resulting from the proximity of theMain Central Thrust. Evidence for diagenetic processes of dolomitiza-tion and silicification are reported from the oolitic horizon of theGangolihat dolomite (Kumar and Tewari, 1978). Based on the sedi-mentary structures and associated stromatolites Valdiya (1980), Kumarand Tewari (1978) have suggested an intertidal carbonate flat as thedepositional environment for the carbonates from Gangolihat. Eightsamples were analysed from the Deoban Formation (Table 1); threefrom Gangolihat-Pithoragarh and one from Thal-Tejam areas, whichare a part of the drainage basins of the rivers Sarju, Ramganga, andKali, tributaries of the Ghaghara (Fig. 1). The other four samples arefrom Chakrata-Deoban-Tiuni region through which the rivers Yamunaand Tons flow. The sample KU92-26 is from the northern Tejamregion, where metamorphic effects are more pronounced. The samplesKU92-13, 43, and UK95-19 are from regions near Gangolihat whereevidence for carbonate diagenesis has been reported. The samplesHP94-41, 42, and 43, from the drainage basin of Sutlej, are from theShali (5Deoban) Formation. The carbonates from the Sutlej basin aregenerally intercalated with slates; however, the samples collected inthis study are well within the carbonate zone.

The Mandhali (5 Sor) Formation lies above the Deoban Formation,and the passage from Deoban to Mandhali throughout Kumaun isperfectly transitional (Valdiya, 1980). The major components of theMandhali Formation are blue and black limestones, pelitic slate, car-bonaceous and pyritic slates, and conglomerates. The study of stro-matolites in these rocks has constrained the time of their deposition tobe between Upper Riphean to early Vendian (Valdiya, 1980). MandhaliFormation is exposed mainly in three regions, Thal-Tejam, Pithoragarh,and Basantpur (Fig. 1). The sample KU92-22 is from the southern part,and KU92-36 and UK95-12 are from northern part of the Thal-Tejamexposure. The sample KU92-36 was in contact with augen gniesses.UK95-12 is from Pipalkoti, a part of the drainage basin of theAlaknanda, where massive deposits of rhythmites (intercalations ofshales/slates and carbonates) are exposed. The sample KU92-46 is fromMandhali Formation exposed in Pithoragarh region and collected at thecontact of carbonate and slate.

The Blaini Formation is a distinctive unit of the Krol belt made ofcarbonates, siltstone, shale, quartzite, and mudstone. Sedimentological

studies of these deposits suggest that they were deposited in a shallowtidal sea (Singh, 1978). The sample VBL-1 from Mussoorie belongs tothe Blaini Formation.

The Krol Formation represents a sequence of limestone, slate, andsiltstone (Lower Krol) with massive deposits of dolomite in the upperpart (Upper Krol; Valdiya, 1980). The Krol is underlain by Infra Kroland overlain by Tal Formations. Krol sediments are inferred to beshallow water deposits based on sedimentological studies and investi-gations of stromatolites and oolites in them. The stromatolite studiesalso suggest that the Krol sediments were deposited during the LatePrecambrian (Singh and Rai, 1978). The Krol samples for this studywere collected from two sections, Dehradun-Mussoorie and Nainital(Table 1). Two of these samples, UK94-44 and 45, were procured froma cement factory in Dehradun.

In addition to carbonates, one sample of gypsum from the SpitiValley (SPT-5, Table 1) was also analysed.

In the laboratory, whole rock powders were prepared from the fieldsamples. Towards this, the field samples were broken into chips ofwhich about 50–100 g were finely powdered using a TEMA agate mill.The powdered samples were used for various analyses. Strontiumisotopes were measured in both the whole rock and their mild acidleachates. For the whole rock analyses, about 100 mg of powderedsamples were brought into solution by digesting them in HF-HNO3.The solutions were spiked with87Rb, 84Sr tracers, and the Sr, Rbfractions were separated and purified following ion-exchange proce-dures practiced in our laboratory (Trivedi et al. 1995). To determine thestrontium isotope ratios of the carbonate fraction, the whole rocksamples were mildly leached using either dilute acetic acid or hydro-chloric acid. For the acetic acid leach, about 100 mg of powderedsamples were treated with 20 mL of 5% acid at 60°C for;1 h. Theslurry was cooled, let stand for 2–3 h and centrifuged. For the hydro-chloric acid leach;100 mg of the powdered samples (or in some cases;1 gm of a few mm size chips) were treated with;20 mL of 0.1 NHCl at room temperature for 1–2 h and centrifuged. The leachates wereprocessed for strontium isotopes following standard procedures(Trivedi et al. 1995). The purified Rb and Sr fractions were run for theirisotopic composition using a 90 radius single focusing mass spectrom-eter. NBS-987 standard was run periodically to check the long termstability of the machine, the mean87Sr/86Sr value of several runs overthe past few years being 0.710256 0.00005 (Trivedi et al., 1995). Theoverall precision in the determination of87Sr/86Sr is 6 0.0005 (twostandard error of the mean based on;50 ratio measurements).

The oxygen and carbon isotopes were measured in the CO2 liberatedfrom ;5 mg of samples by treating them with 100 % phosphoric acidfor ;72 h. The measurements were made using a VG 903 massspectrometer with respect to a laboratory standard calibrated againstV-PDB. The precision ford 18O andd13C determinations are60.2‰.Thed 18O andd 13C data are expressed with respect to V-PDB, withoutany additional fractionation correction ford 18O in dolomite.

The elemental abundances were determined both in the whole rocksamples and in the carbonate fraction (i.e., mild acid leaches). For thewhole rock analysis, about 500 mg samples were brought into completesolution by HF-HNO3 digestion. To measure the elemental abundancesin the carbonate fraction, about 500 mg of samples were treated with;100 mL of 0.1 N HCl for 4–6 h at room temperature. The slurry wascentrifuged and the composition of the leaches were determined. Cal-cium, magnesium, aluminum, manganese, and strontium were mea-sured in the solution using either a Perkin Elmer AAS or a Jobin YvonICP-AES (model 38S). USGS rock standards were also measured alongwith samples to check the accuracy and precision of the analyses(Singh, 1998).

X-ray diffraction and thin section microscopic studies were alsomade to identify various minerals present in the samples and to char-acterise them.

3. RESULTS AND DISCUSSION

Studies of the composition of ancient sedimentary carbon-ates provide a means to constrain the chemical and isotopicevolution of the seawater through time. The measured isotopicand chemical composition of ancient carbonates is a result oftheir original composition (i.e., at the time of their deposition)

744 S. K. Singh et al.

Fig. 1. Sampling locations of Precambrian carbonates from the Lesser Himalaya. Samples were collected from outcrops of various formations. Northwest of Tiuni is Himachal Lesser Himalaya andKumaun Lesser Himalaya lies southeast of Tiuni. The sample numbers are abbreviated, e.g., K9 corresponds to KU92-9 (Table 1), U97 is UK94-97 (Table 1).

745S

risotopes

inP

recambrian

LesserH

imalayan

carbonates

and postdepositional overprinting on their original signatures.Many of the earlier studies (cf. Veizer 1983; Hall and Veizer1996) based on the covariation trends in the elemental andstable isotope systematics have been successful in identifyingand selecting the least altered carbonates for paleoceanographicresearch. We have followed some of these approaches to obtainqualitative information on the alteration effects in the samplesanalysed, though the primary goal of our study is to assess therole of carbonate weathering in contributing to the present daystrontium isotope systematics of the rivers flowing through theLesser Himalaya. This goal can be achieved through measure-ments of the present day composition of carbonates and com-paring them with those of the modern rivers without addressingthe issues pertaining to the origin and post depositional alter-ation effects on the composition of the carbonates.

3.1. Mineralogy, Composition, Oxygen, and CarbonIsotopes

The mineralogical and chemical composition of the samplesanalysed are given in Table 1, their strontium, oxygen, andcarbon isotopic data are given in Table 2. Microscopic andXRD studies and chemical composition measurements suggest

that the predominant lithology of the samples analysed isdolomite, the few exceptions being the limestones from Shali-Bilaspur in the inner belt and Dehradun-Mussoorie in the outerbelt (Table 1).

The average bulk composition of dolomites and limestonesfrom the inner and outer belts are given in Table 3. Broadly,among the dolomites, the samples from the inner belt havehigher Al, Rb, and87Sr/86Sr (Table 3). The high Al content ofthe inner belt samples is consistent with field observations thatmany of them occur as intercalations of carbonates and shales/slates (rhythmites) with significant spatial variability in theirrelative thicknesses.

The chemistry of the bulk samples (Table 1) and their mildacid leaches (Singh, 1998) shows that most (.90%) of the Ca,Mg, Sr, and Mn are leachable from the bulk rock suggestingthat these elements reside almost entirely in the carbonatephase. In contrast, the fraction of leachable Al shows a widescatter (3–64%), averaging;30% (Singh, 1998). Among theelements in the carbonate fraction, the abundances of Sr andMn are determined by the extent of alteration of the samples(Viezer 1983; Hall and Viezer 1996). During alteration, thecarbonate matrix loses Sr and gains Mn, and their covariation


trend provides a measure of the preservation of their originalsignatures. The mean concentration of Sr in the dolomites andthe limestones from the inner belt are 48 and 148 ppm, respec-tively, in the outer belt samples these are about a factor of twohigher (Table 3). In general, the concentration of Sr in thecarbonates from the Lesser Himalaya are significantly lowerthan those in recent marine carbonates.

The meand18O andd13C data for the dolomites and lime-stones from the inner and outer belts overlap with each other,with mean values of; 29‰ and;1‰, respectively (Table 3).The d18O data show a wide range,21.4‰ to212.8‰ (Table2) similar to those reported for the Krol carbonates (Bhatta-charya et al., 1996; Sarkar et al., 1996) and for the Precambriancarbonates from other geographical regions (Viezer and Hoefs,1976; Hall and Viezer, 1996). It is known that oxygen isotopesshift towards lighterd18O values during dolomitization involv-ing dissolution-reprecipitation reactions occurring at elevatedtemperatures and/or in presence of meteoric waters. Thed13Cvalues, in comparison show a narrow range,21‰ to 13.3‰

(Table 2), as the carbon isotope pool is dominated by thecarbonates.

3.2. Strontium Isotope Systematics

The strontium isotopic composition of the bulk carbonaterocks and their mild acid leaches show considerable variation,from 0.7064 to 0.8935 (Table 2, Fig. 2). Most of the samples,however, have87Sr/86Sr in the range of 0.7064–0.7300, withtwo samples from the Gangolihat-Pithoragarh region havingextremely high values, 0.8535 and 0.8935 (Table 2). The87Sr/86Sr of the leaches are similar to those of the bulk samples(Table 2) consistent with that expected as most of the Sr in thebulk rock is leachable. While this work was in progress, Sarkaret al. (1996) reported87Sr/86Sr in the carbonate fraction of afew samples from Krol-Tal formations, their87Sr/86Sr num-bers, 0.70976–0.72918, bracket our data. Besides this, data for87Sr/86Sr in carbonates from the Lesser Himalaya are sparse;Derry and France-Lanord (1996) report values of;0.8 with

747Sr isotopes in Precambrian Lesser Himalayan carbonates

low Sr and Quade et al. (1997) values of;0.73 for detritalcarbonates. Among the samples analysed, those from the innerbelt have higher87Sr/86Sr ratios. The samples having the mostradiogenic ratios are KU92-43 and 46 (Table 2) which alsohave high Al content (Table 1). Both these samples are ryth-mites made of layers of carbonates and silicates. The strontiumisotope ratio of the carbonate fraction of these samples also arehighly radiogenic with values in excess of 0.85 (Table 2).These results lead us to conclude that the strontium isotopesystematics of carbonates from this region have been consid-erably modified by postdepositional addition of radiogenic Sr.Two other bulk samples which have87Sr/86Sr in excess of 0.73are KU92-26 and 36 (Table 2). KU92-26 is from the northernThal-Tejam region an area which is reported to be affected byenhanced metamorphism (Bhattacharya, 1982), and KU92-36was collected at the contact with gneiss. The strontium isotopecomposition of mild acid leaches of these samples is also quiteradiogenic, attesting to the influence of alteration processes inthe redistribution of strontium isotopes between silicates andcarbonates.

The 87Sr/86Sr of the carbonate fraction of many of thesamples analysed are significantly higher than that of contem-peraneous seawater, 0.705–0.707 (Veizer, 1989) suggestingpostdepositional addition of radiogenic Sr to the carbonates.Widespread metamorphism and magmatism in the Himalayainvolving large-scale fluid transport could have caused loss ofSr from the carbonate and redistribution of strontium isotopesbetween coexisting silicate and carbonates resulting in theformation of carbonates with low Sr content and high87Sr/86Sr.

3.3. Carbonate Weathering and87Sr/86Sr of the Ganga,Ghaghara, and the Indus Source Waters

The idea that the high87Sr/86Sr of the G-B rivers in theirupper reaches results from the weathering of carbonates en-riched in radiogenic Sr was suggested by Palmer and Edmond

(1992). Their suggestion stems from the observation that themajor ion chemistry of these rivers is dominated by carbonateweathering and that the covariation trend between87Sr/86Sr and(1/Sr) predicts a value of 0.7209 for the high Sr endmember,generally taken to be carbonates. Interestingly the mean87Sr/86Sr of the bulk carbonates analysed in this study, 0.725(Table 3) is very similar to that estimated by Palmer andEdmond (1992). The plot of87Sr/86Sr -vs- (1/Sr) for the sourcewaters of the Bhagirathi, Alaknanda, Ghaghara, Sutlej, andtheir tributaries (Fig. 3a) shows considerable scatter, suggestingthat the strontium isotope composition of all these rivers isdifficult to explain by a common mixing line with two distinctendmembers. The likely cause for such a scatter could bevariations in the87Sr/86Sr of the silicates in the drainage basinsthrough which these individual tributaries flow. However,the general inverse correlation between Sr concentration and87Sr/86Sr observed among the larger rivers of the G-B system(Krishnaswami et al., 1992; Palmer and Edmond, 1992) seemsto suggest that they smooth out the variations in87Sr/86Sr of thedrainage basins of the individual tributaries. This trend alsosuggests that the strontium isotope composition of these largerrivers can be described in terms of a two component mixing(Fig. 3b).

Figure 2 is a comparison of the frequency distribution of87Sr/86Sr in the source waters of the Bhagirathi, Alaknanda,Ghaghara, and the Indus and those in the carbonates and wholerock granites/gneisses from the Lesser Himalaya. We have usedthe strontium isotopic data of whole rocks from the LesserHimalaya for comparison, as a large fraction of the drainagebasins of the Bhagirathi, Alaknanda, Ghaghara, and Sutlej is inthis region. Many of the tributaries of these rivers predomi-nantly drain the Lesser Himalaya which would suggest thattheir chemistry and strontium isotope composition are likely tobe controlled by rocks from this region. France-Lanord et al.(1993) concluded that the primary source of the sediments to


the Bay of Bengal are the higher Himalayan Crystallines (or itsclose analogue with only minor contributions from the LesserHimalaya) based on studies of clay mineralogy, strontium,neodymium, and oxygen isotope systematics in them. Morerecently, Derry and France-Lanord (1996), while attesting tothe earlier conclusions of France-Lanord et al. (1993) regardingthe provenance of detritus in the Bay of Bengal sediments,alluded to the possibility of a relative increase in the contribu-tion of the Lesser Himalaya materials to the Bay of Bengalsediments during the Pliocene. The observation that the Bha-girathi (at Gangotri) and Alaknanda (at Badrinath) have high87Sr/86Sr with moderately high Sr concentration before theyenter the Lesser Himalayan basin (Krishnaswami et al. 1992)also points to the important influence the rocks in the HigherHimalaya may have in regulating the strontium isotope com-position of these rivers. Currently, however, the relative con-tributions from the drainage basins of the Higher and LesserHimalaya to the Sr flux and isotopic composition of these riversare uncertain as data on their water flux along the flow path isunavailable.

The strontium isotope composition of the source watersoverlaps with those of the granites/gneisses, but are signifi-cantly higher than that in most of the carbonates. A similarpattern is also seen in the comparison of the Sr/Ca ratios (Fig.2), the source waters have a mean Sr/Ca ratio (1.4160.69)nmol/mmol that is much higher than that of the carbonates(0.2060.15) nmol/mmol. An upper limit of Sr contributionfrom carbonate weathering can be derived by assuming that (1)all the dissolved Ca in rivers is of carbonate origin, (2) Sr andCa are weathered congruently from the carbonates, and (3) themean composition of samples measured in this study (Table 3)is representative of composition of carbonates being weatheredby these rivers. Using the mean Sr/Ca (nmol/mmol) abundanceratio of 0.2060.15 in the carbonates and the Ca concentrationin rivers (Sarin et al. 1992; Pande et al., 1994; Trivedi et al.,1995) the upper limit of carbonate component of dissolved Srin these rivers is estimated to range between (665)% to(43632)%, with a mean of;18%. Considering that Ca canalso be supplied to these rivers from the weathering of silicates,evaporites, and phosphates the carbonate component of Sr in

Fig. 2. Frequency distribution of87Sr/86Sr in source waters, carbonates, and silicates (whole rock granites/gneisses). Thecarbonate data are for twenty-five bulk samples and leaches from six others for which bulk samples were not analysed(Table 2). The Sr isotopic composition and (Sr/Ca) of the source waters are significantly higher than those measured in mostof the carbonates. Data for silicates are from Le Fort et al., 1980, 1983; Rao, 1983; Trivedi, 1990; and Sharma et al., 1992.Source waters data are for rivers of the Ganga system (Krishnaswami et al, 1992), Ghaghara (Trivedi et al., 1995), andSutlej, Beas, Chandra, Bhaga, and Dharcha of the Indus (Pande et al., 1994).


these rivers is likely to be lower than that estimated aboveassuming that all the Ca in these rivers is of carbonate origin.These estimates indicate that bulk of the Sr in most of these

rivers has to originate from sources other than carbonates suchas weathering of silicates, evaporites, and phosphates.

The role of these carbonates in contributing to the high

Fig. 3. (a) Scatter digram of87Sr/86Sr vs. 1/Sr for Bhagirathi, Alaknanda, Ghaghara, Sutlej, and their tributaries. The datashow an overall covariation trend with significant scatter, suggesting that Sr isotopic data for all these rivers is difficult toexplain by a unique two component mixing line. (b) Data for the larger rivers, however, seem to fit better on a twocomponent mixing line. Data from Krishnaswami et al., 1992; Pande et al., 1994; and Trivedi et al., 1995.


strontium isotope composition of the source waters is governedby their 87Sr/86Sr and their supply of Sr to the rivers. Thus,rivers draining carbonates with87Sr/86Sr of ' 0.85, such as

those from Gangolihat-Pithoragarh (Table 2) would have87Sr/86Sr in excess of 0.74, even if the fraction of carbonate Srin them is only;18%, the balance Sr being supplied from

Fig. 4. Property plots for rivers of the Ganga, Ghaghara, and the Indus source waters to assess their covariation trends.(a) 87Sr/86Sr vs. Na*/Sr. The silicate endmember is characterised by high87Sr/86Sr and Na*/Sr. (b) Sr/Na* vs. Ca/Na*. Thenonsilicate endmember (carbonates, evaporites) has high values of these ratios.


silicates with87Sr/86Sr of 0.720. From the analyses carried outin this work, it is, however, seen that carbonates with such high87Sr/86Sr are not common in the Lesser Himalaya and that mostof the samples have87Sr/86Sr # 0.730. This leads us to con-clude that although some of the carbonates of the LesserHimalaya can serve as a source for high radiogenic Sr to thestreams on a regional scale (e.g., for those flowing through theinner belt carbonates with high radiogenic strontium isotopecomposition), on the average they are unlikely to be a majorcontributor to the high present day87Sr/86Sr of the sourcewaters on a basinwide scale. This conclusion arises from theobservation that the87Sr/86Sr and Sr/Ca ratios of most of thecarbonates analysed in this study, collected across the LesserHimalaya, are significantly lower than the present day values inmany of the source waters (Fig. 2).

Sarkar et al. (1996) and Quade et al. (1997) have recentlyproposed that carbonates are important and perhaps domi-nant source of high87Sr/86Sr in the rivers of the Himalaya.Sarkar et al. (1996) arrived at this conclusion based on theanalysis of Krol-Tal carbonates from the Garhwal (Kumaun)Himalaya whereas the suggestion of Quade et al. (1997)relies on the87Sr/86Sr measurements of soil carbonates fromthe Siwalik and detrital carbonates from the rivers of Nepal.The strontium isotope data measured by these two groupsand in the present study in the various carbonates overlapwith each other and show that many of these are quiteradiogenic in composition. This by itself, however, does notmake the carbonates to be a dominant source of the high87Sr/86Sr in rivers, as it would also depend on their contri-bution to the Sr budget of the rivers. Our estimate based onthe Sr/Ca ratio of the carbonates suggest that on the averagethey account for only about a fifth of the Sr in the sourcewaters. Thus as mentioned earlier, from the87Sr/86Sr of thePrecambrian carbonates and their contribution to Sr budgetof rivers we infer that they are unlikely to be a major sourceof the high 87Sr/86Sr to the rivers on a basinwide scale;however, they could be significant for particular streams.

In order to better constrain the contribution of Sr fromvarious source rocks, silicates, carbonates, and evaporites, it isnecessary to chemically and isotopically characterise theseendmembers. Towards this, we have followed two approaches:(1) based on the statistical analysis of Ca, Na, and Sr abun-dances in rivers assuming two component mixing and (2) usingthe chemical and strontium isotope composition of rivers andwhole rocks of the Lesser Himalaya (cf. Negrel et al., 1993).

The values of the Ca/Na, Sr/Na, and87Sr/86Sr of the silicateendmember can be derived from Ca, Na, Sr, and87Sr/86Sr data ofthe rivers by assuming that their abundances in water resultfrom two component mixing, silicates, and nonsilicates (i.e.,carbonates1evaporites1phosphates). Such an approach providesbest fit endmember values for the source waters, though theassumption that all the rivers have the same two endmembers maybe an oversimplification considering the scatter in the87Sr/86Sr-vs- 1/Sr and other property plots (Figs. 3, 4). The mass balanceequations for Ca, Sr, and87Sr/86Sr in the rivers are

Car5fCans1~12f )Cas (1)

Srr5f Srns1~12f )Srs (2)

Rr 5 [Rnsf Srns 1 Rs~12f )Srs]/Srr (3)

where R is 87Sr/86Sr, subscripts s, ns, and r are silicates,nonsilicates, and dissolved in rivers, respectively;f is the con-tribution from nonsilicates to the dissolved load of rivers. Thesilicate contribution to the dissolved load of rivers can bedetermined by assuming that their Na* abundance (Na* issodium concentration of rivers corrected for contributions fromcyclic salts and evaporites based on their chloride content) isentirely of silicate origin,

f 5 [12(Na*r/Nas)] (4)

From Eqns. 1 through 4,

Srr 5 a[Na*r] 1 b[Car] (5)

Rr 5 [kNa*r 1 e Car]Srr (6)

where

a5[(Sr/Na)s 2 (Sr/Ca)ns(Ca/Na)s]

b5(Sr/Ca)ns

k5(Sr/Na)sRs 2 (Ca/Na)s(Sr/Ca)nsRns

e5Rns(Sr/Ca)ns

By regressing the measured riverine Sr concentration with alinear combination of the riverine Na* and Ca concentrations,a andb can be determined. Likewisek ande can be derived byregressing87Sr concentration in the rivers ( i.e., SrrRr) withNa* and Ca concentrations. The (Sr/Na) and (87Sr/86Sr) of thesilicate endmember can be deduced from the coefficientsa andk, using an average values of (Ca/Na)s and for the nonsilicateendmember fromb ande directly. The values for the variousendmembers (Table 4) calculated based on the data of all riversof the Ganga, Ghaghara, and the Indus source waters (n541)have significant uncertainties associated with them. These er-rors are primarly due to the scatter in the data. Attempts tobetter constrain the endmember values based on the data ofmajor rivers (n57) were also unsuccessful.

The second approach is to characterise the silicate endmem-ber values. In Fig. 4a and 4b the87Sr/86Sr -vs- Na*/Sr andCa/Na* -vs- Sr/Na* of the source waters are presented. Thedata in Fig. 4a seem to show a two component mixing trendwith high 87Sr/86Sr and Na*/Sr values representing the silicateendmember. It is difficult to assign a unique87Sr/86Sr, Sr/Na,and Ca/Na for the silicate endmember, as these parameters varyamong the silicates of the drainage basins, and the data in Fig.4a and 4b show significant scatter. An approach to constrainthese values, would be to identify tributaries whose chemicaland isotopic composition are dominated by silicate weathering.Among the source waters of Ganga, the Jolagad seems to suitthis requirement best, it has the highest87Sr/86Sr among all thesource waters, 0.798660.0008 (Krishnaswami et al. 1992),highest Na*/Ca suggesting dominant silicate weathering andquite low SO4/HCO3

2 (Sarin et al. 1992) indicating minorsupply, if any, from weathering of evaporites. The strontiumisotope ratio of Jolagad is very similar to the average87Sr/86Srfor the whole rocks from the Lesser Himalaya 0.7760.07 (Srabundance weighted mean of sixty-nine granite/gneisses hav-ing 87Sr/86Sr ,1.0, Fig. 2). Considering these, we have as-signed a value of 0.7860.02 for the87Sr/86Sr of the silicate


endmember. The Sr/Na ratios in the Lesser Himalayan granites/gneisses show a wide range, 0.35–4.53, (Rao, 1983), the Sr/Na* of Jolagad waters, 1.96, falls within this range. We haveassigned a value of 2.561.0 (nmol/mmol) for the Sr/Na ratio ofthe silicate endmember. The Ca/Na in the granites/gneissesrange between 0.07 and 1.33 (Rao, 1983). For calculating theCa contribution from silicate weathering, we have used a valueof 1.060.5 (mmol/mmol) for Ca/Na, consistent with the Ca/Na*of the Jolagad water, 1.15. Using these endmember values andthe measured Na and chloride concentrations in the rivers, wehave estimated the silicate and carbonate components of Sr tothe larger rivers. These results are given in Table 5. Thecalculation shows that the contribution from silicate weatheringto the dissolved Sr of these rivers ranges between (663)% and(89635)%. This compares with the rough estimate of;30%(Krishnaswami, et al. 1992) for the source waters of the Gangabased on their major element composition. The current esti-mates (Table 5) are better constrained as the endmember valuesare assigned based on more stringent criteria. The carbonate Srcalculated from calcium in the waters, after accounting forsilicate Ca contribution based on Na/Ca ratio of granites/gneis-

ses, falls in the range of 6–32 %. These again are likely to beupper limits as part of the Ca can be from other sources, i.e.,evaporites and phosphates. These estimates, however, showthat in the Bhagirathi, Bhilangna, Alaknanda, and Ganga, thecarbonate Sr is less than the silicate Sr and for the other threerivers they are comparable (Table 5).

Among the rivers listed in Table 5, Sr balance could only beachieved for the Bhilangna. For four other rivers (Bhagirathi,Alaknanda, Ganga, and Sarju) 43–70% of the dissolved Srcould be accounted for, though with the uncertainties associ-ated with these estimates they could be stretched to balance theSr budget. Alternatively a part of the Sr in these waters couldbe from other sources such as weathering of gypsum/anhydriteswhich have high Sr concentration (700–2500 ppm) and Sr/Caratio (Veizer, 1978). The single gypsum sample analysed in thisstudy (Table 2) has a Sr concentration of 1027 ppm with aSr/Ca ratio of 1.91 (nmol/mmol), about an order of magnitudehigher than those in the Precambrian carbonates of the LesserHimalaya. In Kali and Sutlej, which have high dissolved Srconcentration the estimated contribution of silicates and car-bonates together account only for;20% of the dissolved Sr.


The source of the additional;80% Sr to these rivers is unclear;it could be from weathering of evaporites or phosphates orbecause the endmembers values we have assigned may not beapplicable to these rivers.

The87Sr/86Sr of these major rivers were calculated based onthe estimated Sr contribution from silicates and assuming thatthe balance Sr is supplied from carbonates and evaporites.Towards this,87Sr/86Sr endmember values of 0.7860.02 and0.71560.01 were used for the silicate and combined(carbonate1evaporite) components, respectively. These calcu-lated values show reasonable agreement with the measuredstrontium isotope composition in these waters (Table 5).

In the above discussion the contribution of carbonates to thepresent day Sr budget and isotopic composition of rivers drain-ing the Himalaya has been assessed based on the assumptionsthat (1) the Precambrian carbonates of the Lesser Himalayaanalysed in this study are representative of the carbonates beingweathered by the various rivers and (2) the endmember valuesfor 87Sr/86Sr, Sr/Na, and Ca/Na assigned for silicates is typicalof the drainage basin. Uncertainities in the estimate of carbon-ate contribution to the Sr budget of rivers can arise if: (1) thereis a bias in our sampling. In addition to the Precambriancarbonates analysed in this study there are reports (Gansser,1964) of Tethyan sedimentary carbonates in the drainage basinsof some of the source waters of the Indus and the Ghaghara,particularly in their upper reaches. If these carbonates areabundant in the drainage basins and have higher Sr concentra-tion with more radiogenic strontium isotopic ratio than thePrecambrian carbonates analysed in this study then they couldbe an important source of high87Sr/86Sr to these rivers. Cur-rently, however, data on these carbonates are unavailable tomake a more quantitative assessment of their contribution tothe Sr budget of these rivers. (2) the Sr/Na and Ca/Na in thesilicates undergoing weathering are less than those used forcalculation (Table 5). In such a case, the Ca supplied to therivers via silicate weathering would be lower which in turnwould increase the fraction of nonsilicate Ca (and hence Sr) inthem. The effect of this, however, does not appear to besignificant for Sr, by decreasing the Ca/Na in silicates from 1.0to 0.5, the carbonate Sr increased by about 10% of the valueslisted in Table 5.

In this section our attempts to establish a mass balance forSr in the larger rivers of the Ganga-Ghaghara-Indus system,based on contribution from silicates and carbonates, is pre-sented. As evident, this is a difficult exercise as Ca and Sr inthese waters have multiple sources with their own charac-teristic 87Sr/86Sr. Our data and calculations show that sili-cate weathering exerts a more dominant control relative tocarbonate weathering on the present day Sr mass balance andtheir 87Sr/86Sr (Table 5) in the source waters of the Ganga,whereas in Kali and Sarju (of Ghaghara) and Sutlej (ofIndus) the contributions from silicate and carbonate weath-ering to their Sr budget seem comparable.

4. SUMMARY AND CONCLUSIONS

Identifying the source(s) for the highly radiogenic strontiumisotope composition in the Ganga-Brahmaputra-Indus sourcewaters is important to assess the role of weathering in theHimalaya in global climate change. The87Sr/86Sr of silicates

(granites, gneisses, and sedimentaries) in the drainage basins ofthese rivers are known to be quite radiogenic and are capable ofsupplying adequate quantities of Sr with high87Sr/86Sr to theserivers. There is, however, a suggestion that metamorphosedcarbonates in these basins could be enriched in87Sr/86Sr andcould also serve as a source for the high87Sr/86Sr in rivers. Inthis study, we examined the later possibility by analysingsamples of Precambrian carbonate outcrops collected across theLesser Himalaya for their chemical and isotopic composition.Thed18O values (21.4‰ to212.8‰), Sr abundance (20–363ppm), and87Sr/86Sr (0.7064–0.8935) all suggest that many ofthe carbonates have undergone extensive post depositional al-teration. These processes have caused redistribution of stron-tium isotopes between coexisting silicate and carbonate phasesresulting in the formation of carbonates with low Sr enrichedin 87Sr.

The mean (Sr/Ca) ratio in the source waters (1.4160.69nmol/mmol) is about an order of magnitude higher than themean in carbonates (0.2060.15 nmol/mmol). The87Sr/86Sr ofmost of the carbonates are,0.730, significantly less than thosereported in several of the source waters. The average carbonateSr in these rivers, calculated assuming all the Ca in them to beof carbonate origin, is only;18%. This itself is an upper limit,as part of the Ca in the waters is derived from silicates andevaporites. The results thus suggest that on a basin wide scalethese carbonates are unlikely to be a major contributor to thehigh 87Sr/86Sr in the source waters; however, they may repre-sent a significant source for particular streams, for example, forthose flowing through inner belt carbonates with high87Sr/86Sr.

Estimates of silicate Sr in some of the major rivers of thesource waters were made from their Ca, Na, and Sr abundancesand87Sr/86Sr and in silicate rocks. These computations suggestthat 33–89% of Sr in Bhagirathi, Bhilangna, Alaknanda,Ganga, and Sarju is of silicate origin, whereas in the Kali andSutlej it is,10%. The Sr budget of these rivers, based on a twocomponent (silicate and carbonate) mixing model, thoughcould be balanced for most of them within the uncertainties ofthe estimates, these calculations indicate the need for an addi-tional Sr component with higher (Sr/Ca) than those in Precam-brian carbonates.

The study addresses to the contributions of silicate andcarbonate weathering to the present day Sr abundance and87Sr/86Sr of the source waters of the Ganga, Ghaghara, and theIndus. The results show that their relative contributions to theSr budget of the various rivers vary widely; however, in manyof the major source waters, particularly those of the Ganga,silicate weathering exercises a more dominant control on theirpresent day Sr abundance and87Sr/86Sr.

Acknowledgments—We thank Mr. V. Dinakaran for his help withsampling during the 1992 field trip, Dr. K. K. Sharma for providing ussome of the samples analysed in this study and Dr. M. M. Sarin forguidance and help with the ICP-AES analysis. Profs. A. Chakrabartiand S. K. Tandon helped with thin section studies. Discussions withProf. K. S. Valdiya and Dr. K. K. Sharma and comments by Dr.Christian France-Lanord and an anonymous reviewer have helpedconsiderably in improving the manuscript.

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