dissolved rhenium in the yamuna river system and …sunil/46__sunil_tarun_yamuna_re.pdfjha et al.,...

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Dissolved rhenium in the Yamuna River System and the Ganga in the Himalaya:Role of black shale weathering on the budgets of Re, Os, and U in rivers and

CO2 in the atmosphere

TARUN K. DALAI , SUNIL K. SINGH,† J. R. TRIVEDI, and S. KRISHNASWAMI*Physical Research Laboratory, Ahmedabad 380 009, India

(Received January 16, 2001;accepted in revised form July 3, 2001)

Abstract—Extensive measurements of dissolved Re and major ion abundances in the Yamuna River System(YRS), a major tributary of the Ganga, have been performed along its entire stretch in the Himalaya, from itssource near the Yamunotri Glacier to its outflow at the foothills of the Himalaya at Saharanpur. In addition,Re analysis has been made in granites and Precambrian carbonates, some of the major lithologies of thedrainage basin. These data, coupled with those available for black shales in the Lesser Himalaya, allow anassessment of these lithologies’ contributions to the Re budget of the YRS.

The Re concentrations in the YRS range from 0.5 to 35.7 pM with a mean of 9.4 pM, a factor of�4 higherthan that reported for its global average concentration in rivers. Dissolved Re and�Cations* (�Na*�K�Ca�Mg) are strongly correlated in the YRS, indicating that they are released to these waters inroughly the same proportion throughout their course. The Re/�Cations* in most of these rivers are one to twoorders of magnitude higher than the (Re/Na�K�Mg�Ca) measured in granites of the Yamuna basin. Thisleads to the conclusion that, on average, granites/crystallines make only minor contributions to the dissolvedRe budget of the YRS on a basin-wide scale, though they may be important for rivers with low dissolved Re.Similarly, Precambrian carbonates of the Lesser Himalaya do not seem to be a major contributor to dissolvedRe in these rivers, as their Re/(Ca�Mg) is much less than those in the rivers. The observation that Reconcentrations in rivers flowing through black shales and in groundwaters percolating through phosphorite-black shale-carbonate layers in phosphorite mines are high, and that Re and SO4 are significantly correlatedin YRS, seems to suggest that the bulk of the dissolved Re is derived from black shale/carbonaceoussediments. Material balance considerations, based on average Re of 30 ng g�1 in black shales from the LesserHimalaya, require that its abundance in the drainage basin of the YRS needs to be a few percent to yieldaverage Re of 9.4 pM. Furthermore, the positive correlation between Re and�Cations* would require thatthese Re-rich sediments (e.g., black shales) and Re-poor lithologies (e.g., crystallines, Precambrian carbon-ates) contribute Re and cations in roughly the same proportion throughout the drainage basin. The availabledata on the abundance and distribution of black shales in the basin are not adequate to test if theserequirements can be met.

The annual fluxes of dissolved Re at the base of the Himalaya from the Yamuna are�150 mol at Batamandiand�100 mol at Saharanpur, compared to�120 mol from the Ganga at Rishikesh. The total flux from theYamuna and the Ganga account for�0.4% of the global riverine Re flux, much higher than their contributionto global water discharge. This is also borne out from the mobilization rate of Re:�1 to 3 g km�2 y�1 in theGanga and Yamuna basins in the Himalaya, compared to the global average of�0.1 g km�2 y�1.

Black shale weathering can also significantly influence the budgets of Os and U in rivers and CO2 in riversand the atmosphere. Using dissolved Re in rivers as a proxy, it is estimated that�(6–9)� 108 kg y�1 of blackshales are being weathered in the Ganga and Yamuna basins in the Himalaya. Weathering of such amountsof black shales can account for the reported concentrations of Os and U in these rivers. Furthermore, if theweathering results in the conversion of organic carbon in the black shales to CO2, it would release�2 � 105

mol of CO2 km�2 y�1 in the Yamuna and Ganga basins in the Himalaya, comparable to the CO2 consumptionfrom silicate weathering.Copyright © 2002 Elsevier Science Ltd

1. INTRODUCTION

The rivers draining the Himalaya contribute significantly towater, sediment, and elemental budgets of the oceans, therebyinfluencing the marine elemental and isotopic makeup. Weath-ering in the Himalaya has been suggested as an importantdriver in determining the steady rise of87Sr/86Sr and187Os/188Os in seawater through the Cenozoic (Richter et al., 1992;

Pegram et al., 1992). In support of this suggestion, it has beenfound that the rivers draining the Himalaya have Sr and Osisotopic compositions, which are generally more radiogenicthan other major rivers of the world (Palmer and Edmond,1989; Krishnaswami et al., 1992; Levasseur et al., 1999;Sharma et al., 1999). The187Os/188Os of rivers is determinedby the Re/Os ratios and age of the basins drained by them.Relatively higher187Os/188Os can be expected in rivers flowingthrough basins containing black shales, which are known tohave high Re/Os. Considering that weathering of black shalesby oxic river waters would also release Re to solution asperrhenate oxyanion (Brookins, 1986; Koide et al., 1986), the

* Author to whom correspondence should be addressed ([email protected]).† Present address: Centre de Recherches Petrographiques etGeochimiques-CNRS, B.P. 20, 54501 Vandoeuvre-les-Nancy, France.

Pergamon

Geochimica et Cosmochimica Acta, Vol. 66, No. 1, pp. 29–43, 2002Copyright © 2002 Elsevier Science LtdPrinted in the USA. All rights reserved

0016-7037/02 $22.00� .00

29

concentration of Re in such rivers is expected to be relativelyhigh. Hence, data on the abundance of Re in river waters in theHimalaya can aid not only in constraining their sources but alsoby providing a better understanding of the comparative geo-chemistry of Re and Os during weathering.

Knowledge of the sources of Re in river waters and under-

standing its geochemical behavior in the surficial weatheringenvironment have implications in the use of 187Re-187Os iso-tope pair for geochronology. The application of this pair for agedetermination requires, among other conditions, closed-systembehavior of Re and Os in the rock/sediment system to be dated(Ravizza and Turekian, 1989, 1991; Allegre et al., 1999; Cohen

Fig. 1. (a) Water sampling locations in the Yamuna River System. Samples were collected during October 1998 (postmonsoon), June 1999 (premonsoon), and September 1999 (monsoon). The sample numbers given are those from theOctober 1998 collection. (b) Lithologic map of the Yamuna catchment (Valdiya, 1980). Only some of the tributaries areshown (Fig. 1a). In the upper reaches, the Yamuna flows through HHC. The bulk of its drainage basin is in the LesserHimalaya, which is abundant in silicates of sedimentary origin and Precambrian carbonates. Many of these sedimentarydeposits are reported to have carbonaceous material in them. The streams in the lower reaches of the Yamuna, in particularthe tributaries Aglar, Bata, and Giri, flow through black shale occurrences.

30 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami

et al., 1999; Singh et al., 1999; Peucker-Ehrenbrink and Han-nigan, 2000). Studies of Re in rivers is one approach forlearning about the extent of its mobility from various rock typesduring surficial weathering and its possible consequences toRe-Os chronometry.

Some of these considerations, coupled with the availabilityof highly sensitive and precise techniques, such as the ID-

NTIMS and ICP-MS for Re measurements (Anbar et al., 1992;Colodner et al., 1993a, b), have contributed to recent studies ofRe in natural waters. Colodner et al. (1993b), in their recon-naissance study of the geochemical cycle of Re, observed thatrivers draining black shales, such as those in the VenezuelanAndes, have higher dissolved Re concentrations. Hodge et al.(1996), on the other hand, proposed carbonates to be an im-

Fig. 1. (b) (continued)

31Dissolved Re in the Yamuna River system, Himalaya

portant source of Re to groundwaters based on their observa-tion that Re/Mo/U ratios in groundwaters from Palaeozoiccarbonate aquifers in the Southern Great Basin (USA) andseawater are quite similar. This finding led them to suggestquantitative uptake of these elements from seawater by carbon-ates during their precipitation and their subsequent release togroundwater during dissolution.

In this paper we report Re abundances in the Yamuna (themajor tributary of the Ganga) and in many of its major as wellas minor tributaries in the Himalaya (Fig. 1a) sampled duringthree different periods. In addition, we have analyzed varioussource rocks from the catchment, such as carbonates and gran-ites, and a few groundwaters percolating through a phosphorite-black shale-carbonate sequence in the Maldeota phosphoritemine in the Lesser Himalaya. For comparison, a few watersamples from the Ganga, collected at Rishikesh at the base ofthe Himalaya, were also measured for Re. This work, whichforms the first comprehensive study on the geochemistry of Rein a river system in the Himalaya, intends to (1) delineate thesource(s) of Re to these rivers, (2) determine dissolved Re fluxfrom the Yamuna and the Ganga at their outflow at the foothillsof the Himalaya (an important dataset to assess the role ofweathering in the Himalaya in contributing to Re budget of theoceans), and (3) understand the implications and influence ofblack shale weathering on riverine Re, Os, and U budgets andon the global carbon cycle.

2. LITHOLOGY OF THE YAMUNA CATCHMENT

The Yamuna, the largest tributary to the Ganga (Fig. 1a),originates at the Yamunotri glacier in the Higher Himalaya anddrains the western part of the Ganga catchment. The river,along with its major tributaries (Tons, Giri, Aglar, Bata andAsan Rivers) (Fig. 1a, b) constitutes the Yamuna River System(YRS) in the Himalaya (Negi, 1991). It has a drainage area of9600 km2 and an annual water discharge of �10.8 � 1012 � atTajewala (a few tens of kilometers downstream of Batamandi;Jha et al., 1988), both of which are a factor of �2 less than thatof the Ganga at Rishikesh. The YRS drains a variety of lithol-ogies along its course (Gansser, 1964; Valdiya, 1980) (Fig. 1b).The Yamuna, near its source in the Higher Himalaya, drainsmainly crystallines of the Ramgarh and the Almora Group,consisting of large masses of granodioritic-quartz dioritic rockswith abundant biotite and quartz, granites rich in tourmalineand muscovites. The Almora granites, at their northern border,are reported to have graphitic horizons of schistose graphitoidquartzites (Gansser, 1964). The metamorphics in the Almoraand Ramgarh Groups have occurrences of graphitic/carbona-ceous schists and carbonaceous schist-marble alternations(Valdiya, 1980). Southwest of the source region, the riverdrains the Berinag-Nagthat Formation, consisting mainly ofmetamorphosed quartz arenite, which grades into sericite-quartz schist. Further downstream, the catchment is character-ized by slates, conglomerates, limestones, and dolomites of theMandhali and Deoban Formations, followed by graywackes,siltstones, shales, and phyllites of the Chakrata and the Chand-pur Formations (Fig. 1b) (Valdiya, 1980). Downstream of theLesser Himalaya, the Yamuna flows through the Siwaliks andfinally the Indo-Gangetic Alluvium. The Yamuna joins theGanga at Allahabad in the plains.

In the Lesser Himalaya, occurrences of grayish-black, black,and bleached shales are reported in the Infra Krol, the LowerTal, the Deoban, and the Mandhali Formations (Gansser, 1964;Valdiya, 1980). These are exposed at a number of locations inthe Yamuna and the Tons catchment (Fig. 1b), the largest beingat Maldeota and Durmala around Dehradun, where phosphoriteis mined economically (Singh, 1999). In the Tons catchment,black shales occur in areas around Tiuni and Lokhandi areas onthe Chakrata-Tiuni road. The Krol dolomites are known tocontain pockets of gypsum (Valdiya, 1980), the one in Sha-hashradhara near Dehradun being economically workable.There are occurrences of geothermal springs in and around thesource region, Janaki chatti and Yamunotri, upstream of Ha-numan chatti (Fig. 1a). The Tons is the major tributary to theYamuna in the Himalaya (Fig. 1a). It rises beyond the high-altitude valley of Har-ki-dun and drains the western part of theYamuna catchment. It flows mainly through crystallines andsedimentary silicates until around Tiuni, downstream of whichit drains carbonates (Fig. 1b). The Tons merges with theYamuna at Kalsi northwest of Dehradun (Fig. 1a). (There isanother stream that is part of the Asan river also called theTons; we have labeled this stream, flowing near Dehradun, theTons to distinguish between the two).

Based on the lithologic maps of the drainage basin, amongthe streams sampled, the Didar Gad, a tributary of the Yamuna,and the Godu Gad, a tributary of the Tons, drain silicatespredominantly, whereas the Barni Gad, a tributary of theYamuna, drains carbonates predominantly. These lithologiccharacteristics are reflected in their respective water chemis-tries (Dalai, 2001).

3. SAMPLING AND ANALYSIS

Water and sediment samples were collected along the entirestretch of the Yamuna, from Hanuman Chatti, �10 km south ofYamunotri, to Saharanpur at the foothills of the Himalaya, andfrom many of its tributaries (Fig. 1a). Sampling was carried outduring October 1998, June and September 1999, which corre-sponded to the postmonsoon, premonsoon, and monsoon peri-ods. Major ions, Sr isotopes, �D, and �18O and Re weremeasured in these samples (Dalai, 2001). For Re, water sampleswere filtered through 0.4� Nucleopore within 4 to 5 h of sampling,acidified to pH � 2 with ultrapure nitric acid (Seastar Baseline),and stored in precleaned polyethylene bottles for analysis. In thelaboratory, typically �100 g of water was spiked with 185Re andstored as such for at least 24 h for sample-spike equilibration. Thesamples were then dried, digested with ultrapure HNO3, and Rewas extracted and purified using anion exchange in nitric acidmedium (Trivedi et al., 1999).

In addition to river water samples, granites and Precambriancarbonates from the drainage basin and ground water flowingthrough the phosphorite-black shale-carbonate layers of a phos-phorite mine at Maldeota were also analyzed for their Recontent. Granite samples, freshly chipped from outcrops, werecollected in and around Hanuman Chatti. Approximately 1 g ofthese sample powders (�100 mesh) was spiked with 185Rebefore dissolution in HF-HNO3, followed by separation andpurification of Re through anion-exchange procedure. Precam-brian carbonate samples were selected for Re analysis from thecollection of Singh et al. (1998). These included calcites, do-


lomites, and those with high radiogenic Sr isotopic composition(Singh et al., 1998). The latter were selected to examine theinfluence of recrystallization and metamorphic remobilizationon their Re content. The carbonate sample powders (�2 g)were leached in 1N HNO3, centrifuged, and the supernatantused for Re analysis; they were spiked, dried, and Re wasseparated from the residue by anion exchange. The Re concen-trations in mine waters were measured in unfiltered samples.Two of the four samples analyzed had no visible particulatematerial in them. All of the mine water samples were stored forseveral weeks before analysis. Unfiltered mine waters weighing20 to 50 g, referred as MW in Table 1, were decanted into cleanTeflon wares, spiked, equilibrated, dried, and Re from themwas purified following the procedures described for the riverwaters. Pure Re fractions from all these samples were loaded ondegassed high-purity Pt filaments with specpure Ba(NO3)2 andmeasured by NTIMS (Creaser et al., 1991; Trivedi et al., 1999).Several samples were run in replicates to ascertain the repro-ducibility of the results.

The 185Re spike used in this study was diluted from theconcentrated spike described in Trivedi et al. (1999) and has aconcentration of 1.193 ng g�1 (Dalai, 2001). The blank con-tribution of Re to the signals was ascertained mainly throughincremental analysis. In this approach, three or four aliquots ofthe same sample, ranging in size from 10 to 200 g, wereanalyzed for Re. The results, when plotted between the weightof the sample analyzed vs. the amount of Re measured, yield anintercept, which equals the total procedural blank. In addition,independent determination of the Re blank was made by mea-suring its contribution from the chemical procedure carried outwith all of the reagents in quantities similar to those used forthe samples. The total Re blank in the present study wasdetermined to be �4 pg, based on six sets of incrementalanalyses and six reagent blank measurements. This value wasused for blank correction. A part of total Re blank results fromthe contribution of Re from Pt filaments. This contribution,measured by running Re spike loaded on Pt filaments with Ba(NO3)2, was in the range of 0.2 to 1.9 pg at currents similar toor marginally higher than those applied during the sample runs.The blank correction in most of the samples was � 5% of themeasured Re signals; only in four (out of 60) was the correc-tion � 10%. The precision of Re measurements, based onseveral repeat analyses, was better than 5%. The range in theRe concentrations in the samples analyzed in this study wasmuch larger than the blank correction and precision of themeasurements.

The major cations in river water samples (Table 1) weremeasured in filtered and acidified waters by ICP-AES andAAS, and SO4 in filtered unacidified samples by ion chroma-tography. The analyses of mine waters were made on decantedunacidified samples. The precision of SO4 measurements,based on repeat analysis of standards and samples, was betterthan 6%. Details of these measurements are given in Dalai(2001).

4. RESULTS AND DISCUSSION

The concentrations of dissolved Re in the Yamuna and itstributaries are given in Table 1, and the Re abundances in thegranites and the Precambrian carbonates from the Lesser Hi-

malaya in Table 2. In the YRS, dissolved Re varies by abouttwo orders of magnitude during the three sampling periods,from 0.5 to 35.7 pM (Table 1; Fig. 2), with an average of 9.4pM (�1.8 ng ��1). The average Re in the YRS, and in theYamuna and the Ganga samples at Batamandi and Rishikesh,respectively, near the foothills of the Himalaya, is significantlyhigher than the global average value of 2.1 pM reported byColodner et al. (1993 b). Their average value of 2.3 pM wasreduced by 10% to correct for spike calibration (Colodner et al.,1995). Rhenium concentrations of the Ganga sampled at Rish-ikesh varied from 5.3 to 7.9 pM during the three seasons,compared to �14 pM for the Yamuna at Batamandi (down-stream of the Bata confluence). The Re concentration of theGanga at Rishikesh is marginally lower than the value of 8.2pM reported near its outflow at Aricha Ghat, Bangladesh beforethe confluence of the Ganga and the Brahmaputra (data cor-rected for spike calibration; Colodner et al., 1993b, 1995).Dissolved Re concentrations show a significant positive corre-lation with �Cations* (Fig. 3) and total dissolved solids (TDS)in these rivers, albeit with some scatter (�Cations* �Na*�K�Ca�Mg, where Na* is Na corrected for cyclic com-ponent; Sarin et al., 1989). Considering that the Yamuna and itstributaries drain different subbasins with their own character-istic lithology and, hence, Re/major ion ratios, such a scatter isnot unexpected. For example, the tributaries sampled in andaround the foothills of the Himalaya have, in general, higher Rethan the streams flowing in the upper reaches, probably becauseof more widespread occurrences of organic-rich sedimentaryrocks at the foothills of the Himalaya. Another contributingfactor for the scatter in Figure 3 could be seasonal variations inRe/�Cations*. The intensity of weathering of different lithol-ogies in the drainage basin and, hence, Re/�Cations* of therivers, could be season dependent. It is also observed thatRe/�Cations* in the Tons and its tributaries are generallyhigher than those in the Yamuna and its tributaries.

The strong positive correlation between Re and �Cations*(Fig. 3) is an indication that both are released to the rivers inroughly the same proportion throughout the drainage basin.Samples from the Kempti Fall, the Asan river, and the Tons atDehradun (Table 1) plot significantly outside the trend set bythe bulk of the samples (Fig. 3). The �Cations* of thesestreams seem to have a significant evaporite component (Dalai,2001), which may be contributing to their low (Re/�Cations*)ratios. The observation that the Shahashradhara sample(RW99-60, Table 1) having the highest SO4 concentration(15.4 mM) has only �16 pM Re attests to the idea thatevaporites are not a significant source of Re (Colodner et al.,1993b). The regression line (Fig. 3, r � 0.90) plotted throughthe data (excluding Kempti Fall, Asan, and Tons at Dehradun)yields a (Re/�Cations*) slope of �9.9 pM/mM (�60 pg Remg�1 Cations*). The implication of this, in determining thesource of Re to these river waters, is discussed in the followingsections.

4.1. Re Contribution From Crystallines

Granites and granodiorites, as already mentioned, constitutea significant proportion of the Yamuna catchment, particularlyin its higher reaches (Fig. 1b). Previous studies (Pierson-Wick-mann et al., 2000) have reported Re concentrations ranging


Table 1. Dissolved Re in the Yamuna River System, the Ganga and mine waters.

Codea River Location SeasonbSO4

(�M)Rec

(pM)

�Cat*d

(�M) (mg ��1)

Yamuna mainstreamRW98-16 Yamuna Hanuman Chatti PM 113 5.3 0.2 581 20.7RW98-20 Yamuna Downstream of Paligad Bridge PM 100 4.6 0.1 605 21.2RW98-25 Yamuna Barkot PM 76 5.4 0.4 649 22.2RW98-22 Yamuna Upstream of Naugaon PM 76 4.1 0.2 671 23.0RW98-15 Yamuna Upstream of Barni Gad’s confluence PM 72 3.5 0.1 794 27.2RW98-14 Yamuna Downstream of Barni Gad’s confluence PM 63 3.7 0.2 845 28.8RW98-12 Yamuna Downstream of Nainbag PM 66 3.6 0.1 796 27.1RW98-9 Yamuna Downstream of Aglar’s confluence PM 220 5.6 0.4 1040 34.9RW99-51 Yamuna Downstream of Aglar’s confluence M 40 1.7 0.1 495 17.7RW98-6 Yamuna Upstream of Ton’s confluence PM 216 5.9 0.2 1097 37.1RW99-64 Yamuna Upstream of Ton’s confluence M 154 4.2 0.2 899 31.2RW99-31 Yamuna Downstream of Ton’s confluence S 405 7.9 0.1 1174 39.5RW99-53 Yamuna Downstream of Ton’s confluence M 93 3.5 0.1 555 19.4RW98-1 Yamuna Rampur Mandi, Paonta sahib PM 101 6.2 0.2 780 26.8RW99-58 Yamuna Rampur Mandi, Paonta sahib M 100 4.9 0.1 692 24.2RW98-4 Yamuna Batamandi PM 333 14.6 0.3 1763 58.7RW99-55 Yamuna Batamandi M 288 14.1 0.4 1521 51.3RW98-33 Yamuna Yamuna Nagar, Saharanpur PM 268 10.0 0.2 1594 53.4RW99-7 Yamuna Yamuna Nagar, Saharanpur S 321 15.0 0.2 1667 55.8RW99-54 Yamuna Yamuna Nagar, Saharanpur M 192 7.7 0.2 822 28.2TributariesRW98-17 Jharjhar Gad Hanuman Chatti-Barkot Road PM 27 2.7 0.1 321 11.5RW98-18 Didar Gad Hanuman Chatti-Barkot Road PM 11 0.50 0.02 175 6.1RW98-19 Pali Gad Pali Gad Bridge PM 61 3.9 0.2 601 21.6RW98-13 Barni Gad Kuwa PM 27 4.8 0.1 1333 44.8RW98-21 Purola Between Naugaon and Pirola PM 32 1.7 0.2 924 30.9RW98-24 Gamra Gad Near the bridge over it PM 26 1.5 0.1 988 32.1RW98-26 Godu Gad Purola-Mori Road PM 19 2.7 0.2 312 10.2RW98-27 Tons Mori PM 62 7.0 0.4 307 10.7RW98-28 Tons Downstream of Mori PM 58 5.7 0.5 322 11.1RW98-29 Tons Tiuni PM 62 5.4 0.7 371 12.8RW98-31 Shej Khad Minas PM 159 18.7 0.8 1093 37.8RW98-30 Tons Minas PM 100 10.8 0.6 701 24.0RW98-5 Amlawa Kalsi-Chakrata Road PM 127 5.1 0.2 1147 39.4RW99-62 Amlawa Kalsi-Chakrata Road M 150 4.6 0.2 1188 40.7RW98-32 Tons Kalsi, Upstream of confluence PM 581 9.7 0.2 1395 46.9RW99-29 Tons Kalsi, Upstream of confluence S 777 10.4 0.4 1389 46.9RW99-63 Tons Kalsi, Upstream of confluence M 340 7.7 0.2 992 34.1RW98-7 Kemti Fall Dehradun-Mussourie Road PM 2913 23.8 0.4 4587 151RW98-8 Aglar Upstream of Yamuna Bridge PM 1052 18.9 0.9 2199 70.6RW99-52 Aglar Upstream of Yamuna Bridge M 1139 17.9 0.6 2163 70.8RW98-2 Giri Rampur Mandi PM 1115 33.3 0.5 2984 98.6RW99-3 Giri Rampur Mandi S 2046 35.7 1.2 3728 119RW99-57 Giri Rampur Mandi M 881 24.9 0.6 2398 80.3RW98-3 Bata Upstream of Batamandi PM 395 17.8 0.2 1655 54.7RW99-56 Bata Upstream of Batamandi M 386 17.8 0.3 1768 59.3RW98-10 Tons Tons Pol, Dehradun PM 1226 10.7 0.4 2651 87.9RW99-65 Tons Tons Pol, Dehradun M 707 8.0 0.1 2239 76.8RW98-11 Asan Simla Road Bridge PM 975 5.7 0.3 3125 104RW99-61 Asan Simla Road Bridge M 822 5.9 0.1 3013 101RW99-60 Spring Shahashradhara M 15400 16.1 0.2 17642 640GangaRW98-34 Ganga Rishikesh PM 165 6.7 0.2 767 26.0RW99-6 Ganga Rishikesh S 204 7.9 0.1 749 25.5RW99-59 Ganga Rishikesh M 145 5.3 0.2 639 22.0Mine Waters and Misc. samplesRW99-8 Bandal Near Maldeota S 909 32.2 2.3 2612 82.9MW-1 Maldeota mines S 4123 61.9 0.9 — —MW-2 Maldeota mines S 4750 7.5 0.3 — —MW-3 Maldeota mines S 2571 111 1 — —MW-4 Maldeota mines S 6854 86.9 1.2 — —

a RW-river water, MW-mine water.b S � summer, M � monsoon, PM � post-monsoon.c Re values are blank corrected, errors are 2�.d �Cat* � (Na*�K�Mg�Ca), Na* is Na corrected for chloride (Data from Dalai, 2001).


from 26 to �1430 pg g�1 (geometric mean �270 pg g�1) ingranites and gneisses from the Central Nepal Himalaya. Ourresults on four granites from the Yamuna catchment yield 14 to46 pg g�1 of Re (Table 2). The average Re in these foursamples is approximately an order of magnitude lower than themean Re in granites and gneisses from Nepal Himalaya; how-ever, a critical comparison may not be appropriate, consideringthat the samples are from different locations and that thenumber of samples analyzed are few. Taking a value of 50 pgg�1 Re for granites, (close to the maximum concentrationmeasured in samples from the Yamuna catchment; see Table

2), it is estimated that �35 g would have to be dissolved/leached per liter of Yamuna water to yield the average dis-solved Re of 1.8 ng ��1. This requirement, as discussed below,is difficult to meet; hence, the source of Re must lie elsewhere.

The slope of the Re-�Cations* regression line, as mentionedearlier, is �9.9 pM/mM. This is more than two orders ofmagnitude higher than the [Re/(Na � K � Mg � Ca] ratio incrystallines of �0.05 pM/mM (0.3 pg mg�1). Comparison ofthese two ratios would suggest that crystallines can accountonly for a small fraction of dissolved Re in the YRS (Table 3)if Re and (Na�K�Mg�Ca) from them are released to therivers in the same proportion as their abundance. The aboveestimate, based on the assumption that all the dissolved majorcations are derived from crystallines, would be an upper limit,as a significant fraction of Ca and Mg in these waters is fromcarbonates and/or evaporites (Dalai, 2001). Therefore, reassess-ment of Re contribution from the crystallines was made usingNa as an index of silicate weathering. Comparison of Re/Naratios in crystallines with Re/Na* in rivers reinforces our earlierinference that weathering of crystallines is not an importantsource of Re to the Yamuna waters (Table 3; Fig. 4). Thisconclusion would be valid even if a much higher average Reconcentration is used for the crystallines, e.g., �270 pg g�1

(geometric mean of the values reported by Pierson-Wickmannet al., 2000) or �400 pg g�1, the value estimated for the uppercontinental crust (Esser and Turekian, 1993).

The role of preferential release of Re to solution from crys-tallines is more difficult to assess. One approach is to assumethat all the suspended matter in the Yamuna system is derived

Table 2. Re abundances in granites and Precambrian carbonates.

Code Locationa Re (pg g�1)

GranitesGR98-1(A) Yamuna bank, H. C. 45.8 1.9GR98-1(B) Yamuna bank, H. C. 41.6 2.7GR98-2 2 km down stream J. Gad 26.9 2.8GR99-1 5 km down stream H. C. 17.4 1.7GR99-2 7 km downstream H. C. 13.7 1.7Mean 26Carbonatesb

KU92-9 (D) Dasaithal 54.0 5.1KU92-43 (D) Kanalichina 30.4 0.9KU92-48 (D) Nainital 35.6 1.0HP94-42 (C) Salapar �0UK94-77 (D) Jari 225 3UK94-45 (C) Dehradun 5.0 3.5UK94-50 (C, D) Mussoorie 35.9 2.1UK95-23 (D) Nainital 29.7 0.2Mean 52

a H. C.: Hanuman Chatti, J. Gad: Jharjhar Gad (#17, Fig. 1a). (A) and(B) are replicates.

b Samples from Singh et al. (1998), C: calcite, D: dolomite.

Fig. 2. Frequency distribution of dissolved Re content in the Yamunaand its tributaries. The concentrations range from 0.5 to 35.7 pM withan average of 9.4 pM (1.8 ng ��1). The average Re concentration in theYRS is a factor of �4 higher than the global value (Colodner et al.1993b).

Fig. 3. Scatter diagram of Re vs. �Cations* in the YRS samplesanalyzed. The Shahashradhara sample is not plotted. �Cations* �Na*�K�Ca�Mg, Na* is sodium corrected for cyclic component. Thedata show a strong positive correlation (r � 0.90, p � 0.005), suggest-ing that Re and (Na*�K�Mg�Ca) are released to the rivers in roughlythe same proportion along their entire length (�Cations* data fromDalai, 2001; regression analysis excludes Kempti Fall (KF), Tons atDehradun (T), and Asan (A) rivers, which fall away from the trend setby other rivers; see text).


from crystallines and that all Re in them is released to solutionduring weathering. Both these assumptions are quite exagger-ated (cf. Peucker-Ehrenbrink and Blum, 1998). However, theestimate based on them can place useful constraints. Using asuspended matter concentration of �2 g ��1 (Subramanian andDalavi, 1978; Hay, 1998) with 50 pg g�1 Re in them, themaximum Re supply would be �100 pg ��1of Re (�0.6 pM),only a few percent of the average Re abundance in these rivers.This contribution may increase by a factor of �2 if the recent

estimate of Galy and France-Lanord (2001) on the total erosionin the Himalaya is considered. Their calculations suggest thatthe contribution of bed load and flood plain deposition iscomparable to the sum of suspended and dissolved loads. It isalso borne out that crystallines are not an important source ofdissolved Re for the Ganga at Rishikesh if the granites mea-sured in this work (Table 2) are taken to be representative of itscatchment. Thus, based on the granites analyzed (Table 2) itcan be said that, on average, they can make only a minorcontribution to the dissolved Re budget of the Yamuna and theGanga Rivers on a basin-wide scale. However, for rivers withlow Re (e.g., Didar Gad, 0.5 pM, Table 1), the contribution bypreferential leaching can become significant if a large fractionof Re from their suspended matter and bed load can be mobi-lized.

The strong positive correlation between Re and �Cations* inthe Yamuna system (Fig. 3) is intriguing in light of the abovecalculations and conclusions and suggests the need for a sourcewith significantly higher Re/(Na � K � Mg � Ca) or Re/Nacompared to those in the granites of the Yamuna catchment(Table 2). Pierson-Wickmann et al. (2000) have reported onegneiss sample with �1430 pg g�1 Re from Nepal Himalaya.More analysis of Re in crystallines is necessary to assess thedistribution of Re-rich granites and gneisses in the Yamuna andthe Ganga catchments and to determine if they are abundantand typical.

4.2. Re Contribution From Precambrian Carbonates

Precambrian carbonates, calcites, and dolomites, occurringin the drainage basins of the Yamuna and the Ganga in theLesser Himalaya, significantly influence their water chemistry,contributing the bulk of the total cations in them (Sarin et al.,1989; Dalai, 2001). It is tempting to infer from this observationthat these carbonates might also be contributing significantly todissolved Re in these waters. The Re abundances in the Pre-cambrian carbonates from the Lesser Himalaya (both calcitesand dolomites), based on the analysis of nitric acid leach ofeight samples, vary from �0 to 225 pg g�1, with an average of52 pg g�1 (Table 2). Pierson-Wickman et al. (2000) reportedvalues of 187, 273, and 1160 pg g�1 Re for three whole-rocklimestone/marble samples from the Nepal Himalaya. The dif-ferences in the two sets of values can be because of reasonssuch as leach vs. whole-rock concentrations, the extent andnature of metamorphism of the carbonates, and abundance ofRe-rich phases in them. Among the samples analyzed (Table2), the two calcites have the lowest Re concentrations: � 5 pgg�1. If the data in Table 2 can be considered typical of calcitesand dolomites, then it would seem that dolomites incorporate inthem measurable quantities of Re during their formation or thatthey contain minor amounts of Re-rich phases. From the lim-ited results, it can also be seen that the Re concentration doesnot show any discernible trend with 87Sr/86Sr; the Re content ofthe sample KU92-43 with a high 87Sr/86Sr of 0.8786 (Singh etal., 1998) is similar to that in other samples with lower 87Sr/86Sr. This is an indication that metamorphic alteration of thesecarbonates may not have caused substantial modification oftheir Re abundances. Using the highest measured concentrationof 225 pg g�1 as typical of Re in these carbonates, it can beestimated that Re from �8 g is needed to be supplied per liter

Table 3. Contribution of granites and Pc carbonates to dissolved Rein the YRS.

Element/Ratio Granitesa) Pc carbonatesa) YRSb)

Re 50 225 1800Re/Na 0.7 — 690Re/Na�K�Ca�Mg 0.3 — 43Re/Ca�Mg — 0.7d) 50

Re contributionc) (pg l�1)

(i) 2–15 30(ii) 200 45

a) Re concentration are maximum values (pg g�1, Table 2, see text).The ratios (pg mg�1) are mean for granite samples in Table 2 (Dalai,2001).

b) Mean Re concentration (pg l�1). The ratios (pg mg�1) are meanvalues of samples in Table 1 (excluding Shahashradhara, Kempti Fall,Tons at Dehradun and Asan).

c) Mean of calculated values for samples in Table 1 (see text). (i) forgranites two values are given. First based on Re/Na* and the secondRe/(Na�K�Ca�Mg), for carbonates based on Re/(Ca�Mg) (ii) esti-mate of Re released assuming its preferential release from suspendedand bed load. For granites �4 g l�1 with of 50 pg g�1 Re. Forcarbonates, assuming 5% abundance in the suspended and bed loadwith 225 pg g�1.

d) Based on maximum Re value, 225 pg g�1. The mean, calculatedfor individual samples, reduces to 0.2.

Fig. 4. Frequency distribution of Re/Na* in the water samples. Theratio varies between 0.01 and 0.21 pM/�M with a mean of 0.08.Typical ratios for granites from the Yamuna catchment (G), Precam-brian carbonates (C), and black shales from the Lesser Himalaya (B)are also shown (Table 5). Data for Kempti Fall is not plotted.


of Yamuna waters to give rise to its average concentration of1.8 ng ��1. As in the case of crystallines discussed above, thisrequirement is almost impossible to attain. It can be estimatedfrom the Ca and Mg abundances of the Yamuna waters that onaverage, � 5% of the dissolved Re in the Yamuna can be fromPrecambrian carbonates. A similar conclusion also results forRe in the Ganga at Rishikesh. Furthermore, selective leachingof Re from carbonates in suspended and bed loads is also not asignificant source of dissolved Re, as their carbonate content isquite low (0.1–7%; Dalai, 2001). Thus, the present study of Rein Precambrian carbonates of the Lesser Himalaya shows thatthey are not a major contributor to the Re budget of theYamuna River System. If, however, there are widespread oc-currences of carbonates with high Re abundance (Pierson-Wickmann et al., 2000) in the Yamuna and the Ganga catch-ments, their leaching can contribute significantly to thedissolved Re in these rivers.

Hodge et al. (1996), based on the similarity in Re/Mo/Uratios in groundwater draining carbonate aquifers from theSouthern Great Basin (USA) and in seawater, suggested thatthese elements are sequestered quantitatively by carbonatesduring their precipitation from seawater and are subsequentlyreleased to groundwater during weathering. The results, ob-tained in this study on Re abundances in Precambrian calcitesand dolomites of the Lesser Himalaya and in rivers drainingthem, lead to the conclusion that at least for these rivers, suchcarbonates are not a significant source of Re. The averageRe/Ca in the Precambrian carbonates analyzed in this study(�0.05 pM/mM) and that in calcareous oozes (Colodner et al.,1993b) from the major ocean basins (�0.025 pM/mM) isnearly two orders of magnitude lower than that in seawater (�4pM/mM). This, coupled with the observation that burial of Rein suboxic and anoxic marine sediments is its primary sink inthe ocean (Colodner et al., 1993b; Morford and Emerson, 1999;Crusius and Thomson, 2000), points out the need to reevaluatethe suggestion that Re is scavenged quantitatively by carbon-ates during their precipitation from seawater and that they forman important source of Re to groundwaters (Hodge et al., 1996).

4.3. Re Contribution From Black Shales

The hypothesis that the weathering of black shales can be animportant source of dissolved Re to rivers comes from the workof Colodner et al. (1993b). Their hypothesis is based on theobservations of high Re concentrations in some of the tributar-ies of the Orinoco draining black shales and bituminous lime-stones and the strong correlation between Re and SO4 in thewaters. The knowledge that Re is highly enriched in blackshales and in other reducing sediments relative to its crustalabundance (Ravizza and Turekian, 1989; Morford and Emer-son, 1999; Crusius and Thomson, 2000) and that these organicrich sediments are more easily weatherable, further supportstheir suggestion.

In the present study, there is evidence of the dominant role ofweathering of black shales and other reducing sediments incontributing dissolved Re to the headwaters of the Yamuna andthe Ganga in the Himalaya. First, there are reports of exposureof grayish-black and black shales in the Lesser Himalaya(Valdiya, 1980; Fig. 1b) and association of carbonaceous mat-ter with the crystallines and carbonates (Ganser, 1964; Valdiya,

1980), many of which form part of the drainage basins of theGiri, the Aglar, and the Bata, the tributaries of the Yamuna.These rivers have dissolved Re concentrations about a factor of2 to 4 higher (Table 1) than the average for the YRS. The riverBandal, draining black shale deposits near Maldeota mines, hasRe as high as 32.2 pM (Table 1). The black shales in the LesserHimalaya have Re concentrations spanning a wide range: 0.2 to264 ng g�1 (Table 4). (Singh et al. 1999 designated all shalesamples they analyzed as black shales, though some had Corg

of � 1%.wt. we follow the same convention.) Among thesesamples, those from the underground mines, which are betterpreserved, have higher Re (3.1–264 ng g�1) and Corg (1–7.3%.wt.), compared to surface outcrops (Re: 0.2–16.9 ng g�1;Corg: 0.2–5.9%.wt), indicating that Re and Corg are lost duringweathering. This inference is also supported by the more recentresults of Peucker-Ehrenbrink and Hannigan (2000) on Reabundances in black shale weathering profiles from UticaShale, Quebec. The mobility of Re during weathering is alsoborne out from the analysis of water percolating through thephosphorite-black shale-carbonate layers of Maldeota minesnear Dehradun (MW-1–MW-4, Table 1). Three of the fourmine waters have dissolved Re factors 2 to 3 higher than themaximum river water Re concentration measured in this study(Table 1), with the highest value �21 ng ��1 (111 pM). Allthese data provide evidence that black shales can release highconcentrations of Re to waters draining them. As black shales(and other reducing sediments) are often associated with pyrite,their oxidative weathering produces sulfuric acid that attacksthese sediments as well as the other rocks associated with them,releasing SO4 and a host of cations to the waters. Hence, Re,SO4, and the cations released to the waters from the weatheringof these reducing sediments are very likely to show positivecorrelations with each other (Colodner et al., 1993b). Indeed, asignificant correlation between Re and SO4 in the Yamunawaters has been observed (Fig. 5), supporting the idea thatblack shales are an important source of dissolved Re to theserivers. (Figure 5 is a log-log plot as Re and SO4 concentrationsrange over two to three orders of magnitude.) It is seen fromFigure 5 that the data from three of four mine water samples arealso consistent with the trend of Yamuna waters. Statisticalanalysis of the data in Figure 5 yields a correlation coefficientof 0.84 (n � 53), which becomes 0.83 if the four mine watersare excluded. The scatter in Figure 5 probably results from thesupply of SO4 to the waters from evaporite dissolution and/orthe presence of multiple end members with different Re/SO4

Table 4. Corg, Re, Os and U in black shales from theLesser Himalaya.*

Element

Outcrop Mine All samples

n range n range n (X/Re)@

Corg (wt %) 19 0.21–5.85 14 1.01–7.28 26 �2 � 107

Re (ng g�1) 14 0.22–16.9 14 3.05–264 28 —Os (ng g�1) 16 0.02–4.13 14 0.17–13.5 25 0.05U (�g g�1) — — 8 16.6–94.8 8 1900

* Corg, Re, Os from Singh et al. (1999), U this study, n � number ofsamples.

@ working ratios used in calculations. Corg/Re is molar ratio, Os/Reand U/Re in ng ng�1.


ratios (Dalai et al., 1999, 2000). The wide range of Re/SO4 inthe mine waters (Table 1) is an indication of the occurrence ofvarious end members and/or a dilution effect caused by mixingwith SO4 from evaporites.

An important consideration in assessing the role of blackshales as a major source of Re to the YRS is their abundanceand distribution in their drainage basins. There is no quantita-tive data on area coverage, abundances, and distribution ofblack shales in the Yamuna and the Ganga river basins. Thepaucity of such data makes it difficult to attest the inferencemade earlier, based on geochemical evidence, that black shalesare a major supplier of Re to the Yamuna waters. The geo-chemical data, however, allow us to make rough estimates onthe quantity of black shales that needs to be weathered toaccount for the measured dissolved Re in the Yamuna waters.The average Re concentration in black shales in the LesserHimalaya, based on the outcrop and underground mine sam-ples, is �30 ng g�1 (Singh et al., 1999). Taking this as thetypical Re concentration in black shales of the Yamuna basin,it can be estimated that, on average, Re from �60 mg blackshales would have to be released per liter of river water to yield1.8 ng ��1 Re. If Re is mobilized preferentially, and theweathered black shales are added to the particulate load of therivers, it would make up 1 to 2% of the total abundance of theirsuspended and bed loads. These estimates require that blackshales occur at levels of a few percent in the drainage basin.The available data, as mentioned earlier, is not sufficient todetermine if they occur in such abundance. Pierson-Wickmannet al. (2000), in attempting to balance Os isotope compositionin river bedloads from the Nepal Himalaya, also came to theconclusion that the abundance of black shales in the catchmenthas to be at a level of a few percent, a requirement that theyindicate may be difficult to realize. Furthermore, to explain the

strong positive correlation between Re and �Cations* (Fig. 3),it would require that black shales be and other reducing sedi-ments dispersed throughout the drainage basin such that Re and�Cations* are released to rivers in roughly the same proportionalong their entire length in the Himalaya.

In addition to black shales, other reducing sediments andRe-rich phases (e.g., sulfides) could also serve as source(s) ofdissolved Re. In this context, recent studies of Re in mildlyreducing suboxic sediments show that they are a dominant sinkfor Re in the oceans (Morford and Emerson, 1999; Crusius andThomson, 2000). The role of such sediments in contributing tothe Re budgets of rivers in the Himalaya needs to be assessed.It is also interesting to note that even at Hanuman Chatti, nearthe origin of the Yamuna (Fig. 1b), where the predominantlithology is of granitic-granodioritic composition, the Yamunawater has Re concentration as high as 1 ng ��1 (Table 1).Calculations based on Re in granites (Table 2) do not allowsuch high Re concentrations to be achieved from their weath-ering and suggest the need for Re-rich phases. There are reportsof organic matter associated with these rocks (Valdiya, 1980)and sulphide mineralisation at places upstream of HanumanChatti (Jaireth et al., 1982), either or both of which may becontributing Re to these waters; however, their analysis isrequired to verify such a speculation.

The role of crystallines, Precambrian carbonates, and blackshales in determining the composition and Re abundances ofthe Yamuna River System is summarized in the Ca/Na* vs.Re/Na* plot (Fig. 6a). The plot also presents the average Ca/Naand Re/Na ratios of these lithologies from the Lesser Himalaya(Table 5).

Considering that bulk of the drainage of the YRS lies in theLesser Himalaya, for mixing calculations, the average abun-dances of Ca and Na in the Lesser Himalaya crystallines havebeen used (Krishnaswami et al., 1999). Other data listed inTable 5, such as Re in LH crystallines and Pc carbonates, arefrom the present study; Na and Ca in the Precambrian carbon-ates are from Mazumdar (1996) and Singh et al. (1998), and allthe black shale data are from Singh (1999). A more detaileddiscussion about the data in Table 5 and the calculation ofvarious ratios are given in Dalai (2001). It is seen from Figure6a that Ca/Na* in the waters is by and large a mixture of �40to 70% Precambrian carbonate end member with the balancefrom an end member having LH crystalline composition. TheRe/Na* in this carbonate-crystalline mixture is only �5 �10�4, about two orders of magnitude lower than those mea-sured in river waters (�5 � 10�2). Addition of 10 to 30% ofan end member having Re/Na, as in LH black shales, to thiscarbonate-crystalline mixture would yield Re/Na* values sim-ilar to the measured ratios in waters (Fig. 6a). Thus, it is seenthat a contribution of several percent from black shale endmember is required to reproduce the Re/Na* measured in theYamuna and the Ganga waters. These estimates rely on theassumption that Ca/Na and Re/Na are released to waters fromthe crystallines, carbonates, and black shales in the same ratioas their abundances. Considering that Re is a redox-sensitiveelement and that in reducing sediments it may be associatedwith organic matter (Peucker-Ehrenbrink and Hannigan, 2000;Crusius and Thomson, 2000), it is possible that during weath-ering, Re is more rapidly and readily lost compared to Na. If,for example, the Re/Na released to the waters from black shale

Fig. 5. Re-SO4 scatter plot on a log-log scale. Data for the YRS(excluding Shahashradhara sample) and mine waters are presented.Three of the four mine waters analyzed also fall in the trend set by theYRS data. The significant correlation (r � 0.84, p � 0.005) between Reand SO4 is supportive of the hypothesis that black shale weathering isa major source of dissolved Re in these waters.


weathering is five times their abundance ratio, then the propor-tion of black shales required to match the measured range ofRe/Na in the YRS reduces to �1 to 5% (Fig. 6b). Data onRe/Na and Ca/Na in streams draining monolithologic unitsand/or laboratory leaching experiments with various rock typeswould help in better constraining the sources of dissolved Re ofrivers.

An important consequence of Re mobility from black shalesis that the closed system assumption required for their chro-nology by Re-Os systematics may not be satisfied in all cases.That differential mobility of Re and Os from black shalesduring weathering can disturb the Re-Os isochron was borneout from the studies of Peucker-Ehrenbrink and Hannigan(2000) and Jaffe et al. (2000).

4.4. Re Flux From the Yamuna and Ganga Basins

The dissolved Re fluxes from the Yamuna and Ganga at thefoothills of the Himalaya have been determined from the datain Table 1. These calculations, based on Re concentrations inSeptember (peak flow), yield �150 and �100 mol y�1 Re fromthe Yamuna at Batamandi and Saharanpur, respectively, and120 mol y�1 from the Ganga at Rishikesh. These values differfrom the earlier reported values of Dalai et al. (2000) for tworeasons. First, the fluxes of the Yamuna and the Ganga in Dalaiet al. (2000) are presented in reverse order; they should havebeen reported as 200 and 120 mol y�1 instead of 120 and 200mol y�1 as given. The value of �200 mol y�1 from theYamuna basin is based on its Re concentration at Batamandiand water discharge at Okhla-Delhi (13.7 � 1012 � y�1). TheYamuna (at Batamandi) and the Ganga (at Rishikesh) togethercontribute 270 mol Re per year at their outflow at the foothillsof the Himalaya (Table 6). This constitutes �0.4% of global Reflux (Colodner et al., 1993b), about four times their contribu-tion to the global water discharge (�0.1%). The flux calcula-tion also shows that the Re is released from the catchment ofthese rivers in the Himalaya at a rate of �6 to 16 mmol km�2

y�1, an order of magnitude higher than the global average(Table 6). These estimates of Re flux from the Yamuna andGanga at the foothills of the Himalaya though are dispropor-tionately high compared to their contribution to water dis-charge, its impact on a global scale is not pronounced, as thedrainage area and water discharge of these rivers are only asmall fraction of the global value. Sarin et al. (1990) reportedthat the weathering rate of uranium in the Himalaya is about afactor of 10 higher than the global average. Similarly, based onOs abundances in the Ganga waters (Levasseur et al., 1999), itcan be estimated that Os is released by weathering from theHimalaya at a rate of about three times the global average.These estimates show that chemical weathering in the Yamunaand Ganga basins liberates Re, Os, and U in amounts dispro-portionately higher than their contribution to global water dis-charge and drainage area.

In the previous sections it was put forward that black shalesexert dominant control on the Re budgets of the Yamuna andGanga Rivers. This makes it possible to use Re as a proxy toestimate the quantity of black shales being weathered in theYamuna and Ganga basins. Based on Re flux (Table 6) andusing 30 ng g�1 Re as an average, it is estimated that �(6–9) �

Fig. 6. (a) Re/Na* vs. Ca/Na* plot for the waters analyzed (Ca andNa* data from Dalai, 2001). The points plot within the mixing spacebound by the three end members: crystallines (G), Precambrian car-bonates (C), and LH black shales (B). The dotted evolution line iscalculated on the basis that Ca/Na and Re/Na are released to waters inthe same ratio as their abundances in the three end members. TheCa/Na* in the waters is governed largely by mixing between crystal-lines and carbonates, whereas Re/Na* is controlled predominantly bycontribution from black shales. The solid line represents the evolutionin which the Ca/Na ratio released to the waters from the granites istwice their abundance ratio. The proximity of the solid and dotted linesis an indication that the Ca/Na in the granite-carbonate mixture is notaffected significantly by the preferential release of Ca (over Na) fromgranites. (b) Three end member mixing diagrams as in Figure 6a.Mixing calculations performed by assuming preferential release of Reover Na from the black shales, the Re/Na in solution being five timestheir abundance ratio. It is seen that the proportion of black shalesneeded to explain the Re/Na* in the river waters decreases to 1 to 5%from 10 to 30% (Fig. 6a).


108 kg of black shales are being weathered annually in theYamuna and Ganga basins in the Himalaya.

4.5. Black Shale Weathering: Implications to RiverineTrace Metal Budgets and Carbon Cycle

Black shales, in addition to Re, are abundant in carbon, PGE,and redox-sensitive metals such as V, U, and Mo (Horan et al.,1994; Peucker-Ehrenbrink and Hannigan, 2000). Their oxida-tive weathering can also release these elements and CO2 inaddition to Re to the river waters draining them (Petsch et al.,2000; Peucker-Ehrenbrink and Hannigan, 2000). We evaluatedthe influence of black shale weathering on the budgets of someof these elements in the Yamuna and Ganga Rivers based onavailable data on Re, Os, and U in rivers and Re, Os, U, andorganic carbon in black shales (Table 4; Singh et al., 1999).

The Os/Re weight ratios in black shale samples from theoutcrops and underground mines overlap each other and centeraround a value of 0.05 0.03 (Singh et al., 1999). If Re and Osare supplied to rivers in the same ratio as their abundance inblack shales, it can be estimated from the Os/Re ratio in themand the Re content of the Ganga at Rishikesh (1.0 ng ��1;Table 1) that black shales would contribute �20 to 80 pg of Osper liter of water. This estimate is significantly higher than theOs measured in the Ganga water at Rishikesh (6.2 pg ��1;Levasseur et al., 1999), indicating that black shale weatheringcan account for the dissolved Os levels in the Ganga. It is alsointeresting to note that the estimated Os value is similar to thedesorbable Os concentration, as determined by leaching theGanga bed sediments at Patna (30 pg ��1; Pegram et al., 1994).The Os/Re in the Ganga at Rishikesh and at Rajashahi, basedon Os data of Levasseur et al. (1999) and Re from the presentstudy and Colodner et al. (1993b), is �0.006, an order ofmagnitude lower than those in the Lesser Himalayan blackshales. This would suggest that Os is less mobile than Reduring weathering of black shales and/or that the Os released isremoved by scavenging onto the sediment surfaces by Fe-Mn

oxyhydroxides. The geochemical behavior of Re, i.e., inertnessand stability of ReO4

� in an oxygenated aqueous environment(Koide et al., 1986; Colodner et al., 1993b) and the particlereactivity of Os (Williams et al., 1997; Levasseur et al., 2000),would favour the latter hypothesis that it is removed fromdissolved phase to particulates to explain the low Os/Re inrivers. The presence of significant desorbable component of Osin river sediments further attests to this hypothesis (Pegram etal., 1994; Pierson-Wickmann et al., 2000). The available dataon Os and Re abundances in black shale weathering profilesshow evidence for differential mobility of Re and Os (Peucker-Ehrenbrink and Hannigan, 2000; Jaffe et al., 2000). It is,however, difficult to determine from these results whether Re ismore mobile than Os or vice versa. The results of Peucker-Ehrenbrink and Hannigan (2000) show that in three of the fourprofiles, Os is lost from black shales more than Re duringweathering, whereas the data of Jaffe et al. (2000) indicate thatRe is more mobile than Os. More importantly, it is borne outfrom the above calculations that even if the Os mobility fromblack shales during weathering is lower than that of Re, it maystill account for the reported dissolved Os concentration in theGanga. The high radiogenic Os isotopic composition of theGanga at Rishikesh (187Os/188Os � 2.65; Levasseur et al.,1999) compared to other major rivers, provides support to thehypothesis that black shales can be an important source of Osto this river.

Following an approach similar to that adopted for Os, it canbe estimated from the U/Re of �1900 in black shales (Table 4)and dissolved Re of �1.8 ng ��1, that their weathering cancontribute on an average �3 �g ��1 of U to the rivers. Theestimated uranium concentration is very similar to the valuesreported for some of these rivers and the Ganga headwaters(Sarin et al., 1990, 1992), indicating that black shales can be asignificant source of dissolved uranium to them. The significantcorrelation between U and �Cations* in the Ganga watersprompted Sarin et al. (1990, 1992) to suggest that weathering of

Table 5. Ca, Na and Re abundances in various lithologies from the Lesser Himalaya.

End memberCa

(%. wt)Na

(%. wt)Re

(pg g�1)Ca/Naa

(�M/�M)Re/Naa

(pM/�M)

LH crystallines 1.04 1.76 26 0.34 0.0002Pc carbonates 24 0.03 52 460 0.02LH black shale 0.67 0.58 30000 0.68 0.66

a For details see Dalai, (2001).

Table 6. Re fluxes from the Yamuna and the Ganga at the base of the Himalaya.

River LocationDischarge(1012 �)

Area(103 km2)

Re(pM)

Flux

(moles y�1) (mmoles km�2 y�1)

Yamuna@ Batamandi 10.8 9.6 14.1 150 16Ganga Rishikesh 22.4 19.6 5.3 120 6Ganga* Aricha Ghat 450 975 8.2 3700 4Global* 36000 101000 2.1 75000 0.7

* Re concentrations for Ganga at Aricha Ghat and Global average from Colodner et al. (1993b)@ Based on RW99-55 (Table 1) and discharge at Tajewala, few tens of kilometers downstream of Batamandi.


silicates and uraniferous minerals could be important source(s)of U to these waters. Colodner et al. (1993b), though, proposedcarbonaceous shales as a possible source for U and Re in thesewaters; they also suggested that there could be additionalsources of U as the U/Re in the Ganga-Brahmaputra were muchhigher than those in typical black shales. Analysis of a fewblack shales from the Lesser Himalaya show that U/Re (wt.ratio) is �200 to 30000, with an average of �1900 (Table 4).If this ratio is typical of black shales in the region, it indicatesthat they can be candidates for contributing to high uraniumcontent in these rivers.

In addition to the discussion on the trace elements presentedabove, another important aspect of black shale weathering isthe fate of the organic carbon in them. Petsch et al. (2000),based on a study of black shale weathering profiles, proposedthat the rate of black shale weathering is controlled by the rateof physical erosion and their subsequent exposure to oxygen-ated surface waters. In the Himalaya, the black shale weather-ing rate is expected to be high because the basin is dominatedby physical erosion resulting from steep gradients and intenseprecipitation (rain and snow melt) throughout the year. TheCorg/Re molar ratio in the black shales in the Lesser Himalayarange from 0.23 to 24 � 107 (Singh et al., 1999). Using atypical value of 2 � 107 for Corg/Re (Table 4), it can beestimated that �1 to 3 � 105 mol km�2 y�1 of CO2 would bereleased from them in the Ganga and the Yamuna basins in theHimalaya (Table 7). This calculation assumes (1) that dissolvedRe in rivers can be used as an index to derive the quantity of theblack shales being weathered and (2) that all organic carbon inthe black shales is oxidized to CO2 during weathering. Theamount of CO2 release from the Yamuna basin is about a factorof 2 to 3 more than that from the Ganga (Table 7). This valueis higher than the preliminary estimate reported in Dalai et al.(2000), the cause for the increase being higher Corg/Re used inthe present calculation. The calculated CO2 release rate (alikely upper limit) is two to four times lower than the reportedCO2 consumption rate due to silicate weathering in theAlaknanda and Bhagirathi basins (headwaters of the Ganga inthe Himalaya) and that derived for YRS, assuming all silicateweathering is due to CO2 (Krishnaswami et al., 1999; Table 7).If, however, a significant component of silicate weatheringresults from other proton sources (e.g., H2SO4), then CO2

uptake by silicate weathering and CO2 release via black shaleoxidation could be comparable in the Ganga and the Yamunabasins in the Himalaya (Dalai, 2001).

5. SUMMARY AND CONCLUSIONS

The focus of this work has been to assess the importance ofblack shale weathering in contributing to dissolved Re in the

YRS and the Ganga and its impact on the budgets of severalother elements: Os, U, and C. This was performed through (1)a systematic study of dissolved Re in the YRS and the Ganga,(2) measurements of Re abundances in granites and Precam-brian carbonates, some of the major lithologies of their drain-age basins, and (3) the use of available data on Re and otherelements in black shales from the Lesser Himalaya and infor-mation on its behavior during weathering. The following ob-servations and conclusions result from this study:

1. The average dissolved Re in the YRS is 9.4 pM (�1.8 ng��1), significantly higher than the reported global averageriver water concentration of 2.1 pM (Colodner et al., 1993b).Re in the Yamuna and Ganga collected at Batamandi andRishikesh, locations near the foothills of the Himalaya, arealso factor �6 and 3 higher than the global average. Thefluxes of dissolved Re from the Yamuna (at Batamandi) andthe Ganga (at Rishikesh) are 150 and 120 mol y�1, respec-tively. These fluxes translate to an Re weathering rate of �1to 3 g km�2 y�1, an order of magnitude higher than theglobal average of �0.1 g km�2 y�1 (Colodner et al.,1993b). These results suggest that the dissolved Re fluxfrom the Yamuna and the Ganga are disproportionately highcompared to their water discharge and drainage areas. Theimpact of such high Re mobilization in the basins of theserivers, however, is not pronounced on the global riverine Refluxes, as the water discharge of the Yamuna and the Gangaat the foothills of the Himalaya is only �0.1% of the globaldischarge.

2. The Re abundances in the granites of the Yamuna basin andPrecambrian carbonates average �26 and �52 pg g�1,respectively. Calculations using �Cations* and (Ca�Mg)abundances in these waters to estimate the contribution ofRe from these lithologies show that the Re concentrations inthem are too low to make a significant impact on thedissolved Re budget of these rivers on a basin-wide scale.

3. The significant correlation between Re and SO4 in watersand higher Re in rivers flowing through known black shaleoccurrences and groundwaters dripping through black shale-phosphorite-carbonate layers, favour the idea that blackshales could be a major source of Re to these waters.Furthermore, the strong correlation between �Cations* andRe in these waters require that both are supplied to the YRSalong its entire length in roughly the same proportion. Animportant consideration in deciding whether black shalescan be a dominant source for dissolved Re in the YRS andthe Ganga is their abundance and distribution in the drainagebasins. The concentration of 1.8 ng ��1 in the YRS requiresthat, on average, Re from �60 mg black shales be releasedper liter of water, making up a few percent of the suspended

Table 7. Uptake and release of CO2 in the Yamuna and the Ganga basins in the Himalaya.

RiverArea

(103 km2)Discharge(1012 �)

Fluxa CO2 flux (moles km�2 y�1)b

Re (�Cations)sil Uptake Release

Ganga 19.6 22.4 6 4.5 � 105 4.5 � 105 1.2 � 105

Yamuna 9.6 10.8 16 7.2 � 105 7.2 � 105 3.1 � 105

a Re flux in units of mmoles km�2 y�1, (�Cations)sil is the flux of cations from silicate weathering in Eq km�2 y�1.b Uptake is the CO2 consumption due to silicate weathering and release is the flux of CO2 from black shale weathering.


and bed loads in these rivers. The available data is too sparseto determine if this requirement can be met.

4. Using dissolved Re as an index, it is shown that the reportedconcentrations of Os and U in these rivers can be suppliedvia black shale weathering if all these metals are released towater in the same proportion as their abundance. Extensionof these calculations to Corg in black shales shows that if itis oxidized entirely to CO2, its flux released to waters/atmosphere in the Yamuna and the Ganga basins in theHimalaya would be of same order as its consumption viasilicate weathering.

Acknowledgments—We thank R. Rengarajan for help in fieldwork andthank the authorities of Wadia Institute of Himalayan Geology forlogistics. The analysis of major ions has benefited from the guidanceand help of Dr. M. M. Sarin. Reviews by Prof. K. K. Turekian and Dr.R. B. Peucker-Ehrenbrink and comments of Prof. L. R. Kump werevery constructive and helped improve the manuscript.

Associate editor: L. R. Kump

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