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

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Geochimica et Cosmochimica Acta, Vol. 66, No. 1, pp. 29–43, 2002 Copyright © 2002 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/02 $22.00 ϩ .00 PII S0016-7037(01)00747-5 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 its source 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 the drainage basin. These data, coupled with those available for black shales in the Lesser Himalaya, allow an assessment 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 ϳ4 higher than that reported for its global average concentration in rivers. Dissolved Re and ⌺Cations* (ϭ Na*ϩKϩCaϩMg) are strongly correlated in the YRS, indicating that they are released to these waters in roughly the same proportion throughout their course. The Re/⌺Cations* in most of these rivers are one to two orders of magnitude higher than the (Re/NaϩKϩMgϩCa) measured in granites of the Yamuna basin. This leads to the conclusion that, on average, granites/crystallines make only minor contributions to the dissolved Re 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 dissolved Re in these rivers, as their Re/(CaϩMg) is much less than those in the rivers. The observation that Re concentrations 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 correlated in YRS, seems to suggest that the bulk of the dissolved Re is derived from black shale/carbonaceous sediments. Material balance considerations, based on average Re of 30 ng gϪ1 in black shales from the Lesser Himalaya, require that its abundance in the drainage basin of the YRS needs to be a few percent to yield average Re of 9.4 pM. Furthermore, the positive correlation between Re and ⌺Cations* would require that these Re-rich sediments (e.g., black shales) and Re-poor lithologies (e.g., crystallines, Precambrian carbonates) contribute Re and cations in roughly the same proportion throughout the drainage basin. The available data on the abundance and distribution of black shales in the basin are not adequate to test if these requirements can be met. The annual fluxes of dissolved Re at the base of the Himalaya from the Yamuna are ϳ150 mol at Batamandi and ϳ100 mol at Saharanpur, compared to ϳ120 mol from the Ganga at Rishikesh. The total flux from the Yamuna and the Ganga account for ϳ0.4% of the global riverine Re flux, much higher than their contribution to global water discharge. This is also borne out from the mobilization rate of Re: ϳ1to3gkmϪ2 yϪ1 in the Ganga and Yamuna basins in the Himalaya, compared to the global average of ϳ0.1gkmϪ2 yϪ1.

Black shale weathering can also signiﬁcantly inﬂuence the budgets of Os and U in rivers and CO2 in rivers and the atmosphere. Using dissolved Re in rivers as a proxy, it is estimated that ϳ(6–9) ϫ 108 kg yϪ1 of black shales are being weathered in the Ganga and Yamuna basins in the Himalaya. Weathering of such amounts of black shales can account for the reported concentrations of Os and U in these rivers. Furthermore, if the ϳ ϫ 5 weathering results in the conversion of organic carbon in the black shales to CO2, it would release 2 10 Ϫ2 Ϫ1 mol of CO2 km y in the Yamuna and Ganga basins in the Himalaya, comparable to the CO2 consumption from silicate weathering. Copyright © 2002 Elsevier Science Ltd

1. INTRODUCTION Pegram et al., 1992). In support of this suggestion, it has been found that the rivers draining the Himalaya have Sr and Os The rivers draining the Himalaya contribute significantly to isotopic compositions, which are generally more radiogenic water, sediment, and elemental budgets of the oceans, thereby influencing the marine elemental and isotopic makeup. Weath- than other major rivers of the world (Palmer and Edmond, ering in the Himalaya has been suggested as an important 1989; Krishnaswami et al., 1992; Levasseur et al., 1999; 187 188 driver in determining the steady rise of 87Sr/86Sr and 187Os/ Sharma et al., 1999). The Os/ Os of rivers is determined 188Os in seawater through the Cenozoic (Richter et al., 1992; by the Re/Os ratios and age of the basins drained by them. Relatively higher 187Os/188Os can be expected in rivers flowing through basins containing black shales, which are known to * Author to whom correspondence should be addressed (swami@ have high Re/Os. Considering that weathering of black shales prl.ernet.in). † Present address: Centre de Recherches Petrographiques et by oxic river waters would also release Re to solution as Geochimiques-CNRS, B.P. 20, 54501 Vandoeuvre-les-Nancy, France. perrhenate oxyanion (Brookins, 1986; Koide et al., 1986), the 29 30 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami

Fig. 1. (a) Water sampling locations in the Yamuna River System. Samples were collected during October 1998 (post monsoon), June 1999 (premonsoon), and September 1999 (monsoon). The sample numbers given are those from the October 1998 collection. (b) Lithologic map of the Yamuna catchment (Valdiya, 1980). Only some of the tributaries are shown (Fig. 1a). In the upper reaches, the Yamuna flows through HHC. The bulk of its drainage basin is in the Lesser Himalaya, which is abundant in silicates of sedimentary origin and Precambrian carbonates. Many of these sedimentary deposits are reported to have carbonaceous material in them. The streams in the lower reaches of the Yamuna, in particular the tributaries Aglar, Bata, and Giri, flow through black shale occurrences. concentration of Re in such rivers is expected to be relatively standing its geochemical behavior in the surficial weathering high. Hence, data on the abundance of Re in river waters in the environment have implications in the use of 187Re-187Os iso- Himalaya can aid not only in constraining their sources but also tope pair for geochronology. The application of this pair for age by providing a better understanding of the comparative geo- determination requires, among other conditions, closed-system chemistry of Re and Os during weathering. behavior of Re and Os in the rock/sediment system to be dated Knowledge of the sources of Re in river waters and under- (Ravizza and Turekian, 1989, 1991; Allegre et al., 1999; Cohen Dissolved Re in the Yamuna River system, Himalaya 31

Fig. 1. (b) (continued) et al., 1999; Singh et al., 1999; Peucker-Ehrenbrink and Han- NTIMS and ICP-MS for Re measurements (Anbar et al., 1992; nigan, 2000). Studies of Re in rivers is one approach for Colodner et al., 1993a, b), have contributed to recent studies of learning about the extent of its mobility from various rock types Re in natural waters. Colodner et al. (1993b), in their recon- during surficial weathering and its possible consequences to naissance study of the geochemical cycle of Re, observed that Re-Os chronometry. rivers draining black shales, such as those in the Venezuelan Some of these considerations, coupled with the availability Andes, have higher dissolved Re concentrations. Hodge et al. of highly sensitive and precise techniques, such as the ID- (1996), on the other hand, proposed carbonates to be an im- 32 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami portant source of Re to groundwaters based on their observa- In the Lesser Himalaya, occurrences of grayish-black, black, tion that Re/Mo/U ratios in groundwaters from Palaeozoic and bleached shales are reported in the Infra Krol, the Lower carbonate aquifers in the Southern Great Basin (USA) and Tal, the Deoban, and the Mandhali Formations (Gansser, 1964; seawater are quite similar. This finding led them to suggest Valdiya, 1980). These are exposed at a number of locations in quantitative uptake of these elements from seawater by carbon- the Yamuna and the Tons catchment (Fig. 1b), the largest being ates during their precipitation and their subsequent release to at Maldeota and Durmala around Dehradun, where phosphorite groundwater during dissolution. is mined economically (Singh, 1999). In the Tons catchment, In this paper we report Re abundances in the Yamuna (the black shales occur in areas around Tiuni and Lokhandi areas on major tributary of the Ganga) and in many of its major as well the Chakrata-Tiuni road. The Krol dolomites are known to as minor tributaries in the Himalaya (Fig. 1a) sampled during contain pockets of gypsum (Valdiya, 1980), the one in Sha- three different periods. In addition, we have analyzed various hashradhara near Dehradun being economically workable. source rocks from the catchment, such as carbonates and gran- There are occurrences of geothermal springs in and around the ites, and a few groundwaters percolating through a phosphorite- source region, Janaki chatti and Yamunotri, upstream of Ha- black shale-carbonate sequence in the Maldeota phosphorite numan chatti (Fig. 1a). The Tons is the major tributary to the mine in the Lesser Himalaya. For comparison, a few water Yamuna in the Himalaya (Fig. 1a). It rises beyond the high- samples from the Ganga, collected at Rishikesh at the base of altitude valley of Har-ki-dun and drains the western part of the the Himalaya, were also measured for Re. This work, which Yamuna catchment. It flows mainly through crystallines and forms the first comprehensive study on the geochemistry of Re sedimentary silicates until around Tiuni, downstream of which in a river system in the Himalaya, intends to (1) delineate the it drains carbonates (Fig. 1b). The Tons merges with the source(s) of Re to these rivers, (2) determine dissolved Re flux Yamuna at Kalsi northwest of Dehradun (Fig. 1a). (There is from the Yamuna and the Ganga at their outflow at the foothills another stream that is part of the Asan river also called the of the Himalaya (an important dataset to assess the role of Tons; we have labeled this stream, flowing near Dehradun, the weathering in the Himalaya in contributing to Re budget of the Tons to distinguish between the two). oceans), and (3) understand the implications and influence of Based on the lithologic maps of the drainage basin, among black shale weathering on riverine Re, Os, and U budgets and the streams sampled, the Didar Gad, a tributary of the Yamuna, on the global carbon cycle. and the Godu Gad, a tributary of the Tons, drain silicates predominantly, whereas the Barni Gad, a tributary of the 2. LITHOLOGY OF THE YAMUNA CATCHMENT Yamuna, drains carbonates predominantly. These lithologic characteristics are reflected in their respective water chemis- The Yamuna, the largest tributary to the Ganga (Fig. 1a), tries (Dalai, 2001). originates at the Yamunotri glacier in the Higher Himalaya and drains the western part of the Ganga catchment. The river, 3. SAMPLING AND ANALYSIS along with its major tributaries (Tons, Giri, Aglar, Bata and Asan Rivers) (Fig. 1a, b) constitutes the Yamuna River System Water and sediment samples were collected along the entire (YRS) in the Himalaya (Negi, 1991). It has a drainage area of stretch of the Yamuna, from Hanuman Chatti, ϳ10 km south of 9600 km2 and an annual water discharge of ϳ10.8 ϫ 1012 ᐉ at Yamunotri, to Saharanpur at the foothills of the Himalaya, and Tajewala (a few tens of kilometers downstream of Batamandi; from many of its tributaries (Fig. 1a). Sampling was carried out Jha et al., 1988), both of which are a factor of ϳ2 less than that during October 1998, June and September 1999, which corre- of the Ganga at Rishikesh. The YRS drains a variety of lithol- sponded to the postmonsoon, premonsoon, and monsoon peri- ogies along its course (Gansser, 1964; Valdiya, 1980) (Fig. 1b). ods. Major ions, Sr isotopes, ␦D, and ␦18O and Re were The Yamuna, near its source in the Higher Himalaya, drains measured in these samples (Dalai, 2001). For Re, water samples mainly crystallines of the Ramgarh and the Almora Group, were filtered through 0.4␮ Nucleopore within 4 to5hofsampling, consisting of large masses of granodioritic-quartz dioritic rocks acidified to pH Ͻ 2 with ultrapure nitric acid (Seastar Baseline), with abundant biotite and quartz, granites rich in tourmaline and stored in precleaned polyethylene bottles for analysis. In the and muscovites. The Almora granites, at their northern border, laboratory, typically ϳ100 g of water was spiked with 185Re and are reported to have graphitic horizons of schistose graphitoid stored as such for at least 24 h for sample-spike equilibration. The quartzites (Gansser, 1964). The metamorphics in the Almora samples were then dried, digested with ultrapure HNO3, and Re and Ramgarh Groups have occurrences of graphitic/carbona- was extracted and purified using anion exchange in nitric acid ceous schists and carbonaceous schist-marble alternations medium (Trivedi et al., 1999). (Valdiya, 1980). Southwest of the source region, the river In addition to river water samples, granites and Precambrian drains the Berinag-Nagthat Formation, consisting mainly of carbonates from the drainage basin and ground water flowing metamorphosed quartz arenite, which grades into sericite- through the phosphorite-black shale-carbonate layers of a phos- quartz schist. Further downstream, the catchment is character- phorite mine at Maldeota were also analyzed for their Re ized by slates, conglomerates, limestones, and dolomites of the content. Granite samples, freshly chipped from outcrops, were Mandhali and Deoban Formations, followed by graywackes, collected in and around Hanuman Chatti. Approximately1gof siltstones, shales, and phyllites of the Chakrata and the Chand- these sample powders (Ϫ100 mesh) was spiked with 185Re pur Formations (Fig. 1b) (Valdiya, 1980). Downstream of the before dissolution in HF-HNO3, followed by separation and Lesser Himalaya, the Yamuna flows through the Siwaliks and purification of Re through anion-exchange procedure. Precam- finally the Indo-Gangetic Alluvium. The Yamuna joins the brian carbonate samples were selected for Re analysis from the Ganga at Allahabad in the plains. collection of Singh et al. (1998). These included calcites, do- Dissolved Re in the Yamuna River system, Himalaya 33 lomites, and those with high radiogenic Sr isotopic composition malaya in Table 2. In the YRS, dissolved Re varies by about (Singh et al., 1998). The latter were selected to examine the two orders of magnitude during the three sampling periods, influence of recrystallization and metamorphic remobilization from 0.5 to 35.7 pM (Table 1; Fig. 2), with an average of 9.4 on their Re content. The carbonate sample powders (ϳ2g) pM (ϳ1.8 ng ᐉϪ1). The average Re in the YRS, and in the were leached in 1N HNO3, centrifuged, and the supernatant Yamuna and the Ganga samples at Batamandi and Rishikesh, used for Re analysis; they were spiked, dried, and Re was respectively, near the foothills of the Himalaya, is significantly separated from the residue by anion exchange. The Re concen- higher than the global average value of 2.1 pM reported by trations in mine waters were measured in unfiltered samples. Colodner et al. (1993 b). Their average value of 2.3 pM was Two of the four samples analyzed had no visible particulate reduced by 10% to correct for spike calibration (Colodner et al., material in them. All of the mine water samples were stored for 1995). Rhenium concentrations of the Ganga sampled at Rish- several weeks before analysis. Unfiltered mine waters weighing ikesh varied from 5.3 to 7.9 pM during the three seasons, 20 to 50 g, referred as MW in Table 1, were decanted into clean compared to ϳ14 pM for the Yamuna at Batamandi (down- Teflon wares, spiked, equilibrated, dried, and Re from them stream of the Bata confluence). The Re concentration of the was purified following the procedures described for the river Ganga at Rishikesh is marginally lower than the value of 8.2 waters. Pure Re fractions from all these samples were loaded on pM reported near its outflow at Aricha Ghat, Bangladesh before degassed high-purity Pt filaments with specpure Ba(NO3)2 and the confluence of the Ganga and the Brahmaputra (data cor- measured by NTIMS (Creaser et al., 1991; Trivedi et al., 1999). rected for spike calibration; Colodner et al., 1993b, 1995). Several samples were run in replicates to ascertain the repro- Dissolved Re concentrations show a significant positive corre- ducibility of the results. lation with ⌺Cations* (Fig. 3) and total dissolved solids (TDS) The 185Re spike used in this study was diluted from the in these rivers, albeit with some scatter (⌺Cations* ϭ concentrated spike described in Trivedi et al. (1999) and has a Na*ϩKϩCaϩMg, where Na* is Na corrected for cyclic com- concentration of 1.193 ng gϪ1 (Dalai, 2001). The blank con- ponent; Sarin et al., 1989). Considering that the Yamuna and its tribution of Re to the signals was ascertained mainly through tributaries drain different subbasins with their own character- incremental analysis. In this approach, three or four aliquots of istic lithology and, hence, Re/major ion ratios, such a scatter is the same sample, ranging in size from 10 to 200 g, were not unexpected. For example, the tributaries sampled in and analyzed for Re. The results, when plotted between the weight around the foothills of the Himalaya have, in general, higher Re of the sample analyzed vs. the amount of Re measured, yield an than the streams flowing in the upper reaches, probably because intercept, which equals the total procedural blank. In addition, of more widespread occurrences of organic-rich sedimentary independent determination of the Re blank was made by mea- rocks at the foothills of the Himalaya. Another contributing suring its contribution from the chemical procedure carried out factor for the scatter in Figure 3 could be seasonal variations in with all of the reagents in quantities similar to those used for Re/⌺Cations*. The intensity of weathering of different lithol- the samples. The total Re blank in the present study was ogies in the drainage basin and, hence, Re/⌺Cations* of the determined to be ϳ4 pg, based on six sets of incremental rivers, could be season dependent. It is also observed that analyses and six reagent blank measurements. This value was Re/⌺Cations* in the Tons and its tributaries are generally used for blank correction. A part of total Re blank results from higher than those in the Yamuna and its tributaries. the contribution of Re from Pt filaments. This contribution, The strong positive correlation between Re and ⌺Cations* measured by running Re spike loaded on Pt filaments with Ba (Fig. 3) is an indication that both are released to the rivers in

(NO3)2, was in the range of 0.2 to 1.9 pg at currents similar to roughly the same proportion throughout the drainage basin. or marginally higher than those applied during the sample runs. Samples from the Kempti Fall, the Asan river, and the Tons at The blank correction in most of the samples was Ͻ 5% of the Dehradun (Table 1) plot significantly outside the trend set by measured Re signals; only in four (out of 60) was the correc- the bulk of the samples (Fig. 3). The ⌺Cations* of these tion Ͼ 10%. The precision of Re measurements, based on streams seem to have a significant evaporite component (Dalai, several repeat analyses, was better than 5%. The range in the 2001), which may be contributing to their low (Re/⌺Cations*) Re concentrations in the samples analyzed in this study was ratios. The observation that the Shahashradhara sample much larger than the blank correction and precision of the (RW99-60, Table 1) having the highest SO4 concentration measurements. (15.4 mM) has only ϳ16 pM Re attests to the idea that The major cations in river water samples (Table 1) were evaporites are not a significant source of Re (Colodner et al., measured in filtered and acidified waters by ICP-AES and 1993b). The regression line (Fig. 3, r ϭ 0.90) plotted through

AAS, and SO4 in filtered unacidified samples by ion chroma- the data (excluding Kempti Fall, Asan, and Tons at Dehradun) tography. The analyses of mine waters were made on decanted yields a (Re/⌺Cations*) slope of ϳ9.9 pM/mM (ϳ60 pg Re Ϫ1 unacidified samples. The precision of SO4 measurements, mg Cations*). The implication of this, in determining the based on repeat analysis of standards and samples, was better source of Re to these river waters, is discussed in the following than 6%. Details of these measurements are given in Dalai sections. (2001). 4.1. Re Contribution From Crystallines 4. RESULTS AND DISCUSSION Granites and granodiorites, as already mentioned, constitute The concentrations of dissolved Re in the Yamuna and its a significant proportion of the Yamuna catchment, particularly tributaries are given in Table 1, and the Re abundances in the in its higher reaches (Fig. 1b). Previous studies (Pierson-Wick- granites and the Precambrian carbonates from the Lesser Hi- mann et al., 2000) have reported Re concentrations ranging 34 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami

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

⌺Cat*d c SO4 Re Codea River Location Seasonb (␮M) (pM) (␮M) (mg ᐉϪ1)

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

a RW-river water, MW-mine water. b S ϭ summer, M ϭ monsoon, PM ϭ post-monsoon. c Re values are blank corrected, errors are Ϯ2␴. d ⌺Cat* ϭ (Na*ϩKϩMgϩCa), Na* is Na corrected for chloride (Data from Dalai, 2001). Dissolved Re in the Yamuna River system, Himalaya 35

Table 2. Re abundances in granites and Precambrian carbonates.

Code Locationa Re (pg gϪ1)

Granites GR98-1(A) Yamuna bank, H. C. 45.8 Ϯ 1.9 GR98-1(B) Yamuna bank, H. C. 41.6 Ϯ 2.7 GR98-2 2 km down stream J. Gad 26.9 Ϯ 2.8 GR99-1 5 km down stream H. C. 17.4 Ϯ 1.7 GR99-2 7 km downstream H. C. 13.7 Ϯ 1.7 Mean 26 Carbonatesb KU92-9 (D) Dasaithal 54.0 Ϯ 5.1 KU92-43 (D) Kanalichina 30.4 Ϯ 0.9 KU92-48 (D) Nainital 35.6 Ϯ 1.0 HP94-42 (C) Salapar ϳ0 UK94-77 (D) Jari 225 Ϯ 3 UK94-45 (C) Dehradun 5.0 Ϯ 3.5 UK94-50 (C, D) Mussoorie 35.9 Ϯ 2.1 UK95-23 (D) Nainital 29.7 Ϯ 0.2 Mean 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. 3. Scatter diagram of Re vs. ⌺Cations* in the YRS samples ⌺ ϭ from 26 to ϳ1430 pg gϪ1 (geometric mean ϳ270 pg gϪ1)in analyzed. The Shahashradhara sample is not plotted. Cations* Na*ϩKϩCaϩMg, Na* is sodium corrected for cyclic component. The granites and gneisses from the Central Nepal Himalaya. Our data show a strong positive correlation (r ϭ 0.90, p Ͻ 0.005), suggest- results on four granites from the Yamuna catchment yield 14 to ing that Re and (Na*ϩKϩMgϩCa) are released to the rivers in roughly 46 pg gϪ1 of Re (Table 2). The average Re in these four the same proportion along their entire length (⌺Cations* data from samples is approximately an order of magnitude lower than the Dalai, 2001; regression analysis excludes Kempti Fall (KF), Tons at Dehradun (T), and Asan (A) rivers, which fall away from the trend set mean Re in granites and gneisses from Nepal Himalaya; how- by other rivers; see text). ever, a critical comparison may not be appropriate, considering that the samples are from different locations and that the number of samples analyzed are few. Taking a value of 50 pg Ϫ 2), it is estimated that ϳ35 g would have to be dissolved/ g 1 Re for granites, (close to the maximum concentration leached per liter of Yamuna water to yield the average dis- measured in samples from the Yamuna catchment; see Table solved Re of 1.8 ng ᐉϪ1. 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 mentioned earlier, is ϳ9.9 pM/mM. This is more than two orders of magnitude higher than the [Re/(Na ϩ K ϩ Mg ϩ Ca] ratio in crystallines of ϳ0.05 pM/mM (0.3 pg mgϪ1). Comparison of these two ratios would suggest that crystallines can account only for a small fraction of dissolved Re in the YRS (Table 3) if Re and (NaϩKϩMgϩCa) from them are released to the rivers in the same proportion as their abundance. The above estimate, based on the assumption that all the dissolved major cations are derived from crystallines, would be an upper limit, as a significant fraction of Ca and Mg in these waters is from carbonates and/or evaporites (Dalai, 2001). Therefore, reassess- ment of Re contribution from the crystallines was made using Na as an index of silicate weathering. Comparison of Re/Na ratios in crystallines with Re/Na* in rivers reinforces our earlier inference that weathering of crystallines is not an important source of Re to the Yamuna waters (Table 3; Fig. 4). This conclusion would be valid even if a much higher average Re concentration is used for the crystallines, e.g., ϳ270 pg gϪ1 (geometric mean of the values reported by Pierson-Wickmann et al., 2000) or ϳ400 pg gϪ1, the value estimated for the upper Fig. 2. Frequency distribution of dissolved Re content in the Yamuna continental crust (Esser and Turekian, 1993). and its tributaries. The concentrations range from 0.5 to 35.7 pM with an average of 9.4 pM (1.8 ng ᐉϪ1). The average Re concentration in the The role of preferential release of Re to solution from crys- YRS is a factor of ϳ4 higher than the global value (Colodner et al. tallines is more difficult to assess. One approach is to assume 1993b). that all the suspended matter in the Yamuna system is derived 36 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami

Table 3. Contribution of granites and Pc carbonates to dissolved Re estimate of Galy and France-Lanord (2001) on the total erosion in the YRS. in the Himalaya is considered. Their calculations suggest that the contribution of bed load and flood plain deposition is Element/Ratio Granitesa) Pc carbonatesa) YRSb) comparable to the sum of suspended and dissolved loads. It is Re 50 225 1800 also borne out that crystallines are not an important source of Re/Na 0.7 — 690 dissolved Re for the Ganga at Rishikesh if the granites mea- ϩ ϩ ϩ Re/Na K Ca Mg 0.3 — 43 sured in this work (Table 2) are taken to be representative of its Re/CaϩMg — 0.7d) 50 catchment. Thus, based on the granites analyzed (Table 2) it Re contributionc) (pg lϪ1) can be said that, on average, they can make only a minor contribution to the dissolved Re budget of the Yamuna and the (i) 2–15 30 Ganga Rivers on a basin-wide scale. However, for rivers with (ii) 200 45 low Re (e.g., Didar Gad, 0.5 pM, Table 1), the contribution by a) Re concentration are maximum values (pg gϪ1, Table 2, see text). preferential leaching can become significant if a large fraction The ratios (pg mgϪ1) are mean for granite samples in Table 2 (Dalai, of Re from their suspended matter and bed load can be mobi- 2001). Ϫ1 Ϫ1 lized. b) Mean Re concentration (pg l ). The ratios (pg mg ) are mean The strong positive correlation between Re and ⌺Cations* in values of samples in Table 1 (excluding Shahashradhara, Kempti Fall, Tons at Dehradun and Asan). the Yamuna system (Fig. 3) is intriguing in light of the above c) Mean of calculated values for samples in Table 1 (see text). (i) for calculations and conclusions and suggests the need for a source granites two values are given. First based on Re/Na* and the second with significantly higher Re/(Na ϩ K ϩ Mg ϩ Ca) or Re/Na ϩ ϩ ϩ ϩ Re/(Na K Ca Mg), for carbonates based on Re/(Ca Mg) (ii) esti- compared to those in the granites of the Yamuna catchment mate of Re released assuming its preferential release from suspended and bed load. For granites ϳ4glϪ1 with of 50 pg gϪ1 Re. For (Table 2). Pierson-Wickmann et al. (2000) have reported one Ϫ1 carbonates, assuming 5% abundance in the suspended and bed load gneiss sample with ϳ1430 pg g Re from Nepal Himalaya. with 225 pg gϪ1. More analysis of Re in crystallines is necessary to assess the Ϫ1 d) Based on maximum Re value, 225 pg g . The mean, calculated distribution of Re-rich granites and gneisses in the Yamuna and for individual samples, reduces to 0.2. the Ganga catchments and to determine if they are abundant and typical. from crystallines and that all Re in them is released to solution during weathering. Both these assumptions are quite exagger- 4.2. Re Contribution From Precambrian Carbonates ated (cf. Peucker-Ehrenbrink and Blum, 1998). However, the Precambrian carbonates, calcites, and dolomites, occurring estimate based on them can place useful constraints. Using a Ϫ in the drainage basins of the Yamuna and the Ganga in the suspended matter concentration of ϳ2gᐉ 1 (Subramanian and Ϫ Lesser Himalaya, significantly influence their water chemistry, Dalavi, 1978; Hay, 1998) with 50 pg g 1 Re in them, the Ϫ contributing the bulk of the total cations in them (Sarin et al., maximum Re supply would be ϳ100 pg ᐉ 1of Re (ϳ0.6 pM), 1989; Dalai, 2001). It is tempting to infer from this observation only a few percent of the average Re abundance in these rivers. that these carbonates might also be contributing significantly to This contribution may increase by a factor of ϳ2 if the recent dissolved Re in these waters. The Re abundances in the Pre- cambrian carbonates from the Lesser Himalaya (both calcites and dolomites), based on the analysis of nitric acid leach of eight samples, vary from ϳ0 to 225 pg gϪ1, with an average of 52 pg gϪ1 (Table 2). Pierson-Wickman et al. (2000) reported values of 187, 273, and 1160 pg gϪ1 Re for three whole-rock limestone/marble samples from the Nepal Himalaya. The dif- ferences in the two sets of values can be because of reasons such as leach vs. whole-rock concentrations, the extent and nature of metamorphism of the carbonates, and abundance of Re-rich phases in them. Among the samples analyzed (Table 2), the two calcites have the lowest Re concentrations: Յ 5pg gϪ1. If the data in Table 2 can be considered typical of calcites and dolomites, then it would seem that dolomites incorporate in them measurable quantities of Re during their formation or that they contain minor amounts of Re-rich phases. From the lim- ited results, it can also be seen that the Re concentration does not show any discernible trend with 87Sr/86Sr; the Re content of the sample KU92-43 with a high 87Sr/86Sr of 0.8786 (Singh et al., 1998) is similar to that in other samples with lower 87Sr/ 86Sr. This is an indication that metamorphic alteration of these Fig. 4. Frequency distribution of Re/Na* in the water samples. The carbonates may not have caused substantial modification of ratio varies between 0.01 and 0.21 pM/␮M with a mean of 0.08. their Re abundances. Using the highest measured concentration Typical ratios for granites from the Yamuna catchment (G), Precam- Ϫ1 brian carbonates (C), and black shales from the Lesser Himalaya (B) of 225 pg g as typical of Re in these carbonates, it can be are also shown (Table 5). Data for Kempti Fall is not plotted. estimated that Re from ϳ8 g is needed to be supplied per liter Dissolved Re in the Yamuna River system, Himalaya 37

of Yamuna waters to give rise to its average concentration of Table 4. Corg, Re, Os and U in black shales from the 1.8 ng ᐉϪ1. As in the case of crystallines discussed above, this Lesser Himalaya.* requirement is almost impossible to attain. It can be estimated Outcrop Mine All samples from the Ca and Mg abundances of the Yamuna waters that on average, Ͻ 5% of the dissolved Re in the Yamuna can be from Element n range n range n (X/Re)@ Precambrian carbonates. A similar conclusion also results for ϳ ϫ 7 Re in the Ganga at Rishikesh. Furthermore, selective leaching Corg (wt %) 19 0.21–5.85 14 1.01–7.28 26 2 10 Re (ng gϪ1) 14 0.22–16.9 14 3.05–264 28 — of Re from carbonates in suspended and bed loads is also not a Os (ng gϪ1) 16 0.02–4.13 14 0.17–13.5 25 0.05 significant source of dissolved Re, as their carbonate content is U(␮ggϪ1) —— 8 16.6–94.8 8 1900 quite low (0.1–7%; Dalai, 2001). Thus, the present study of Re ϭ in Precambrian carbonates of the Lesser Himalaya shows that *Corg, Re, Os from Singh et al. (1999), U this study, n number of samples. they are not a major contributor to the Re budget of the @ working ratios used in calculations. Corg/Re is molar ratio, Os/Re Yamuna River System. If, however, there are widespread oc- and U/Re in ng ngϪ1. currences of carbonates with high Re abundance (Pierson- Wickmann et al., 2000) in the Yamuna and the Ganga catchments, their leaching can contribute significantly to the 1980), many of which form part of the drainage basins of the dissolved Re in these rivers. Giri, the Aglar, and the Bata, the tributaries of the Yamuna. Hodge et al. (1996), based on the similarity in Re/Mo/U These rivers have dissolved Re concentrations about a factor of ratios in groundwater draining carbonate aquifers from the 2 to 4 higher (Table 1) than the average for the YRS. The river Southern Great Basin (USA) and in seawater, suggested that Bandal, draining black shale deposits near Maldeota mines, has these elements are sequestered quantitatively by carbonates Re as high as 32.2 pM (Table 1). The black shales in the Lesser during their precipitation from seawater and are subsequently Himalaya have Re concentrations spanning a wide range: 0.2 to released to groundwater during weathering. The results, ob- 264 ng gϪ1 (Table 4). (Singh et al. 1999 designated all shale tained in this study on Re abundances in Precambrian calcites samples they analyzed as black shales, though some had Corg and dolomites of the Lesser Himalaya and in rivers draining of Ͻ 1%.wt. we follow the same convention.) Among these them, lead to the conclusion that at least for these rivers, such samples, those from the underground mines, which are better Ϫ1 carbonates are not a significant source of Re. The average preserved, have higher Re (3.1–264 ng g ) and Corg (1– Re/Ca in the Precambrian carbonates analyzed in this study 7.3%.wt.), compared to surface outcrops (Re: 0.2–16.9 ng gϪ1; ϳ ( 0.05 pM/mM) and that in calcareous oozes (Colodner et al., Corg: 0.2–5.9%.wt), indicating that Re and Corg are lost during 1993b) from the major ocean basins (ϳ0.025 pM/mM) is weathering. This inference is also supported by the more recent nearly two orders of magnitude lower than that in seawater (ϳ4 results of Peucker-Ehrenbrink and Hannigan (2000) on Re pM/mM). This, coupled with the observation that burial of Re abundances in black shale weathering profiles from Utica in suboxic and anoxic marine sediments is its primary sink in Shale, Quebec. The mobility of Re during weathering is also the ocean (Colodner et al., 1993b; Morford and Emerson, 1999; borne out from the analysis of water percolating through the Crusius and Thomson, 2000), points out the need to reevaluate phosphorite-black shale-carbonate layers of Maldeota mines the suggestion that Re is scavenged quantitatively by carbon- near Dehradun (MW-1–MW-4, Table 1). Three of the four ates during their precipitation from seawater and that they form mine waters have dissolved Re factors 2 to 3 higher than the an important source of Re to groundwaters (Hodge et al., 1996). maximum river water Re concentration measured in this study (Table 1), with the highest value ϳ21 ng ᐉϪ1 (111 pM). All 4.3. Re Contribution From Black Shales these data provide evidence that black shales can release high concentrations of Re to waters draining them. As black shales The hypothesis that the weathering of black shales can be an (and other reducing sediments) are often associated with pyrite, important source of dissolved Re to rivers comes from the work their oxidative weathering produces sulfuric acid that attacks of Colodner et al. (1993b). Their hypothesis is based on the these sediments as well as the other rocks associated with them, observations of high Re concentrations in some of the tributar- releasing SO4 and a host of cations to the waters. Hence, Re, ies of the Orinoco draining black shales and bituminous lime- SO4, and the cations released to the waters from the weathering stones and the strong correlation between Re and SO4 in the of these reducing sediments are very likely to show positive waters. The knowledge that Re is highly enriched in black correlations with each other (Colodner et al., 1993b). Indeed, a shales and in other reducing sediments relative to its crustal significant correlation between Re and SO4 in the Yamuna abundance (Ravizza and Turekian, 1989; Morford and Emer- waters has been observed (Fig. 5), supporting the idea that son, 1999; Crusius and Thomson, 2000) and that these organic black shales are an important source of dissolved Re to these rich sediments are more easily weatherable, further supports rivers. (Figure 5 is a log-log plot as Re and SO4 concentrations their suggestion. range over two to three orders of magnitude.) It is seen from In the present study, there is evidence of the dominant role of Figure 5 that the data from three of four mine water samples are weathering of black shales and other reducing sediments in also consistent with the trend of Yamuna waters. Statistical contributing dissolved Re to the headwaters of the Yamuna and analysis of the data in Figure 5 yields a correlation coefficient the Ganga in the Himalaya. First, there are reports of exposure of 0.84 (n ϭ 53), which becomes 0.83 if the four mine waters of grayish-black and black shales in the Lesser Himalaya are excluded. The scatter in Figure 5 probably results from the

(Valdiya, 1980; Fig. 1b) and association of carbonaceous mat- supply of SO4 to the waters from evaporite dissolution and/or ter with the crystallines and carbonates (Ganser, 1964; Valdiya, the presence of multiple end members with different Re/SO4 38 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami

strong positive correlation between Re and ⌺Cations* (Fig. 3), it would require that black shales be and other reducing sediments dispersed throughout the drainage basin such that Re and ⌺Cations* are released to rivers in roughly the same proportion along their entire length in the Himalaya. In addition to black shales, other reducing sediments and Re-rich phases (e.g., sulﬁdes) could also serve as source(s) of dissolved Re. In this context, recent studies of Re in mildly reducing suboxic sediments show that they are a dominant sink for Re in the oceans (Morford and Emerson, 1999; Crusius and Thomson, 2000). The role of such sediments in contributing to the Re budgets of rivers in the Himalaya needs to be assessed. It is also interesting to note that even at Hanuman Chatti, near the origin of the Yamuna (Fig. 1b), where the predominant lithology is of granitic-granodioritic composition, the Yamuna water has Re concentration as high as 1 ng ᐉϪ1 (Table 1). Calculations based on Re in granites (Table 2) do not allow such high Re concentrations to be achieved from their weathering and suggest the need for Re-rich phases. There are reports of organic matter associated with these rocks (Valdiya, 1980)

Fig. 5. Re-SO4 scatter plot on a log-log scale. Data for the YRS and sulphide mineralisation at places upstream of Hanuman (excluding Shahashradhara sample) and mine waters are presented. Chatti (Jaireth et al., 1982), either or both of which may be Three of the four mine waters analyzed also fall in the trend set by the contributing Re to these waters; however, their analysis is YRS data. The significant correlation (r ϭ 0.84, p Ͻ 0.005) between Re required to verify such a speculation. and SO4 is supportive of the hypothesis that black shale weathering is a major source of dissolved Re in these waters. The role of crystallines, Precambrian carbonates, and black shales in determining the composition and Re abundances of the Yamuna River System is summarized in the Ca/Na* vs. ratios (Dalai et al., 1999, 2000). The wide range of Re/SO4 in Re/Na* plot (Fig. 6a). The plot also presents the average Ca/Na the mine waters (Table 1) is an indication of the occurrence of and Re/Na ratios of these lithologies from the Lesser Himalaya various end members and/or a dilution effect caused by mixing (Table 5). with SO4 from evaporites. Considering that bulk of the drainage of the YRS lies in the An important consideration in assessing the role of black Lesser Himalaya, for mixing calculations, the average abun- shales as a major source of Re to the YRS is their abundance dances of Ca and Na in the Lesser Himalaya crystallines have and distribution in their drainage basins. There is no quantita- been used (Krishnaswami et al., 1999). Other data listed in tive data on area coverage, abundances, and distribution of Table 5, such as Re in LH crystallines and Pc carbonates, are black shales in the Yamuna and the Ganga river basins. The from the present study; Na and Ca in the Precambrian carbon- paucity of such data makes it difficult to attest the inference ates are from Mazumdar (1996) and Singh et al. (1998), and all made earlier, based on geochemical evidence, that black shales the black shale data are from Singh (1999). A more detailed are a major supplier of Re to the Yamuna waters. The geo- discussion about the data in Table 5 and the calculation of chemical data, however, allow us to make rough estimates on various ratios are given in Dalai (2001). It is seen from Figure the quantity of black shales that needs to be weathered to 6a that Ca/Na* in the waters is by and large a mixture of ϳ40 account for the measured dissolved Re in the Yamuna waters. to 70% Precambrian carbonate end member with the balance The average Re concentration in black shales in the Lesser from an end member having LH crystalline composition. The Himalaya, based on the outcrop and underground mine sam- Re/Na* in this carbonate-crystalline mixture is only ϳ5 ϫ ples, is ϳ30 ng gϪ1 (Singh et al., 1999). Taking this as the 10Ϫ4, about two orders of magnitude lower than those mea- typical Re concentration in black shales of the Yamuna basin, sured in river waters (ϳ5 ϫ 10Ϫ2). Addition of 10 to 30% of it can be estimated that, on average, Re from ϳ60 mg black an end member having Re/Na, as in LH black shales, to this shales would have to be released per liter of river water to yield carbonate-crystalline mixture would yield Re/Na* values sim- 1.8 ng ᐉϪ1 Re. If Re is mobilized preferentially, and the ilar to the measured ratios in waters (Fig. 6a). Thus, it is seen weathered black shales are added to the particulate load of the that a contribution of several percent from black shale end rivers, it would make up 1 to 2% of the total abundance of their member is required to reproduce the Re/Na* measured in the suspended and bed loads. These estimates require that black Yamuna and the Ganga waters. These estimates rely on the shales occur at levels of a few percent in the drainage basin. assumption that Ca/Na and Re/Na are released to waters from The available data, as mentioned earlier, is not sufficient to the crystallines, carbonates, and black shales in the same ratio determine if they occur in such abundance. Pierson-Wickmann as their abundances. Considering that Re is a redox-sensitive et al. (2000), in attempting to balance Os isotope composition element and that in reducing sediments it may be associated in river bedloads from the Nepal Himalaya, also came to the with organic matter (Peucker-Ehrenbrink and Hannigan, 2000; conclusion that the abundance of black shales in the catchment Crusius and Thomson, 2000), it is possible that during weath- has to be at a level of a few percent, a requirement that they ering, Re is more rapidly and readily lost compared to Na. If, indicate may be difficult to realize. Furthermore, to explain the for example, the Re/Na released to the waters from black shale Dissolved Re in the Yamuna River system, Himalaya 39

weathering is ﬁve times their abundance ratio, then the proportion of black shales required to match the measured range of Re/Na in the YRS reduces to ϳ1 to 5% (Fig. 6b). Data on Re/Na and Ca/Na in streams draining monolithologic units and/or laboratory leaching experiments with various rock types would help in better constraining the sources of dissolved Re of rivers. An important consequence of Re mobility from black shales is 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 shales during weathering can disturb the Re-Os isochron was borne out 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 the foothills of the Himalaya have been determined from the data in Table 1. These calculations, based on Re concentrations in September (peak flow), yield ϳ150 and ϳ100 mol yϪ1 Re from the Yamuna at Batamandi and Saharanpur, respectively, and 120 mol yϪ1 from the Ganga at Rishikesh. These values differ from the earlier reported values of Dalai et al. (2000) for two reasons. First, the fluxes of the Yamuna and the Ganga in Dalai et al. (2000) are presented in reverse order; they should have been reported as 200 and 120 mol yϪ1 instead of 120 and 200 mol yϪ1 as given. The value of ϳ200 mol yϪ1 from the Yamuna basin is based on its Re concentration at Batamandi and water discharge at Okhla-Delhi (13.7 ϫ 1012 ᐉ yϪ1). The Yamuna (at Batamandi) and the Ganga (at Rishikesh) together contribute 270 mol Re per year at their outflow at the foothills of the Himalaya (Table 6). This constitutes ϳ0.4% of global Re flux (Colodner et al., 1993b), about four times their contribution to the global water discharge (ϳ0.1%). The flux calculation also shows that the Re is released from the catchment of these rivers in the Himalaya at a rate of ϳ6 to 16 mmol kmϪ2 yϪ1, an order of magnitude higher than the global average (Table 6). These estimates of Re flux from the Yamuna and Ganga at the foothills of the Himalaya though are disproportionately high compared to their contribution to water discharge, its impact on a global scale is not pronounced, as the Fig. 6. (a) Re/Na* vs. Ca/Na* plot for the waters analyzed (Ca and Na* data from Dalai, 2001). The points plot within the mixing space drainage area and water discharge of these rivers are only a bound by the three end members: crystallines (G), Precambrian car- small fraction of the global value. Sarin et al. (1990) reported bonates (C), and LH black shales (B). The dotted evolution line is that the weathering rate of uranium in the Himalaya is about a calculated on the basis that Ca/Na and Re/Na are released to waters in factor of 10 higher than the global average. Similarly, based on the same ratio as their abundances in the three end members. The Ca/Na* in the waters is governed largely by mixing between crystal- Os abundances in the Ganga waters (Levasseur et al., 1999), it lines and carbonates, whereas Re/Na* is controlled predominantly by can be estimated that Os is released by weathering from the contribution from black shales. The solid line represents the evolution Himalaya at a rate of about three times the global average. in which the Ca/Na ratio released to the waters from the granites is These estimates show that chemical weathering in the Yamuna twice their abundance ratio. The proximity of the solid and dotted lines and Ganga basins liberates Re, Os, and U in amounts dispro- is an indication that the Ca/Na in the granite-carbonate mixture is not affected significantly by the preferential release of Ca (over Na) from portionately higher than their contribution to global water dis- granites. (b) Three end member mixing diagrams as in Figure 6a. charge and drainage area. Mixing calculations performed by assuming preferential release of Re In the previous sections it was put forward that black shales over Na from the black shales, the Re/Na in solution being five times exert dominant control on the Re budgets of the Yamuna and their abundance ratio. It is seen that the proportion of black shales needed to explain the Re/Na* in the river waters decreases to 1 to 5% Ganga Rivers. This makes it possible to use Re as a proxy to from 10 to 30% (Fig. 6a). estimate the quantity of black shales being weathered in the Yamuna and Ganga basins. Based on Re flux (Table 6) and using 30 ng gϪ1 Re as an average, it is estimated that ϳ(6–9) ϫ 40 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami

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

Ca Na Re Ca/Naa Re/Naa End member (%. wt) (%. wt) (pg gϪ1) (␮M/␮M) (pM/␮M)

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

a For details see Dalai, (2001).

108 kg of black shales are being weathered annually in the oxyhydroxides. The geochemical behavior of Re, i.e., inertness Ϫ Yamuna and Ganga basins in the Himalaya. and stability of ReO4 in an oxygenated aqueous environment (Koide et al., 1986; Colodner et al., 1993b) and the particle 4.5. Black Shale Weathering: Implications to Riverine reactivity of Os (Williams et al., 1997; Levasseur et al., 2000), Trace Metal Budgets and Carbon Cycle would favour the latter hypothesis that it is removed from dissolved phase to particulates to explain the low Os/Re in Black shales, in addition to Re, are abundant in carbon, PGE, rivers. The presence of significant desorbable component of Os and redox-sensitive metals such as V, U, and Mo (Horan et al., in river sediments further attests to this hypothesis (Pegram et 1994; Peucker-Ehrenbrink and Hannigan, 2000). Their oxida- al., 1994; Pierson-Wickmann et al., 2000). The available data tive weathering can also release these elements and CO in 2 on Os and Re abundances in black shale weathering profiles addition to Re to the river waters draining them (Petsch et al., show evidence for differential mobility of Re and Os (Peucker- 2000; Peucker-Ehrenbrink and Hannigan, 2000). We evaluated Ehrenbrink and Hannigan, 2000; Jaffe et al., 2000). It is, the influence of black shale weathering on the budgets of some however, difficult to determine from these results whether Re is of these elements in the Yamuna and Ganga Rivers based on available data on Re, Os, and U in rivers and Re, Os, U, and more mobile than Os or vice versa. The results of Peucker- organic carbon in black shales (Table 4; Singh et al., 1999). Ehrenbrink and Hannigan (2000) show that in three of the four The Os/Re weight ratios in black shale samples from the profiles, Os is lost from black shales more than Re during outcrops and underground mines overlap each other and center weathering, whereas the data of Jaffe et al. (2000) indicate that around a value of 0.05 Ϯ 0.03 (Singh et al., 1999). If Re and Os Re is more mobile than Os. More importantly, it is borne out are supplied to rivers in the same ratio as their abundance in from the above calculations that even if the Os mobility from black shales, it can be estimated from the Os/Re ratio in them black shales during weathering is lower than that of Re, it may and the Re content of the Ganga at Rishikesh (1.0 ng ᐉϪ1; still account for the reported dissolved Os concentration in the Table 1) that black shales would contribute ϳ20 to 80 pg of Os Ganga. The high radiogenic Os isotopic composition of the 187 188 per liter of water. This estimate is significantly higher than the Ganga at Rishikesh ( Os/ Os ϭ 2.65; Levasseur et al., Os measured in the Ganga water at Rishikesh (6.2 pg ᐉϪ1; 1999) compared to other major rivers, provides support to the Levasseur et al., 1999), indicating that black shale weathering hypothesis that black shales can be an important source of Os can account for the dissolved Os levels in the Ganga. It is also to this river. interesting to note that the estimated Os value is similar to the Following an approach similar to that adopted for Os, it can desorbable Os concentration, as determined by leaching the be estimated from the U/Re of ϳ1900 in black shales (Table 4) Ϫ Ganga bed sediments at Patna (30 pg ᐉϪ1; Pegram et al., 1994). and dissolved Re of ϳ1.8 ng ᐉ 1, that their weathering can Ϫ The Os/Re in the Ganga at Rishikesh and at Rajashahi, based contribute on an average ϳ3 ␮g ᐉ 1 of U to the rivers. The on Os data of Levasseur et al. (1999) and Re from the present estimated uranium concentration is very similar to the values study and Colodner et al. (1993b), is ϳ0.006, an order of reported for some of these rivers and the Ganga headwaters magnitude lower than those in the Lesser Himalayan black (Sarin et al., 1990, 1992), indicating that black shales can be a shales. This would suggest that Os is less mobile than Re significant source of dissolved uranium to them. The significant during weathering of black shales and/or that the Os released is correlation between U and ⌺Cations* in the Ganga waters removed by scavenging onto the sediment surfaces by Fe-Mn prompted Sarin et al. (1990, 1992) to suggest that weathering of

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

Flux Discharge Area Re River Location (1012 ᐉ) (103 km2) (pM) (moles yϪ1) (mmoles kmϪ2 yϪ1)

Yamuna@ Batamandi 10.8 9.6 14.1 150 16 Ganga Rishikesh 22.4 19.6 5.3 120 6 Ganga* Aricha Ghat 450 975 8.2 3700 4 Global* 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. Dissolved Re in the Yamuna River system, Himalaya 41

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

a Ϫ2 Ϫ1 b Flux CO2 ﬂux (moles km y ) Area Discharge 3 2 12 ᐉ ⌺ River (10 km ) (10 ) Re ( Cations)sil Uptake Release

Ganga 19.6 22.4 6 4.5 ϫ 105 4.5 ϫ 105 1.2 ϫ 105 Yamuna 9.6 10.8 16 7.2 ϫ 105 7.2 ϫ 105 3.1 ϫ 105

a Ϫ2 Ϫ1 ⌺ Ϫ2 Ϫ1 Re flux in units of mmoles km y ,( Cations)sil is the flux of cations from silicate weathering in Eq km y . b Uptake is the CO2 consumption due to silicate weathering and release is the flux of CO2 from black shale weathering. silicates and uraniferous minerals could be important source(s) YRS and the Ganga and its impact on the budgets of several of U to these waters. Colodner et al. (1993b), though, proposed other elements: Os, U, and C. This was performed through (1) carbonaceous shales as a possible source for U and Re in these a systematic study of dissolved Re in the YRS and the Ganga, waters; they also suggested that there could be additional (2) measurements of Re abundances in granites and Precam- sources of U as the U/Re in the Ganga-Brahmaputra were much brian carbonates, some of the major lithologies of their drain- higher than those in typical black shales. Analysis of a few age basins, and (3) the use of available data on Re and other black shales from the Lesser Himalaya show that U/Re (wt. elements in black shales from the Lesser Himalaya and infor- ratio) is ϳ200 to 30000, with an average of ϳ1900 (Table 4). mation on its behavior during weathering. The following ob- If this ratio is typical of black shales in the region, it indicates servations and conclusions result from this study: that they can be candidates for contributing to high uranium 1. The average dissolved Re in the YRS is 9.4 pM (ϳ1.8 ng content in these rivers. Ϫ ᐉ 1), significantly higher than the reported global average In addition to the discussion on the trace elements presented river water concentration of 2.1 pM (Colodner et al., 1993b). above, another important aspect of black shale weathering is Re in the Yamuna and Ganga collected at Batamandi and the fate of the organic carbon in them. Petsch et al. (2000), Rishikesh, locations near the foothills of the Himalaya, are based on a study of black shale weathering profiles, proposed also factor ϳ6 and 3 higher than the global average. The that the rate of black shale weathering is controlled by the rate fluxes of dissolved Re from the Yamuna (at Batamandi) and of physical erosion and their subsequent exposure to oxygen- Ϫ the Ganga (at Rishikesh) are 150 and 120 mol y 1, respec- ated surface waters. In the Himalaya, the black shale weather- tively. These fluxes translate to an Re weathering rate of ϳ1 ing rate is expected to be high because the basin is dominated Ϫ Ϫ to3gkm 2 y 1, an order of magnitude higher than the by physical erosion resulting from steep gradients and intense Ϫ Ϫ global average of ϳ0.1gkm2 y 1 (Colodner et al., precipitation (rain and snow melt) throughout the year. The 1993b). These results suggest that the dissolved Re flux C /Re molar ratio in the black shales in the Lesser Himalaya org from the Yamuna and the Ganga are disproportionately high range from 0.23 to 24 ϫ 107 (Singh et al., 1999). Using a compared to their water discharge and drainage areas. The typical value of 2 ϫ 107 for C /Re (Table 4), it can be org impact of such high Re mobilization in the basins of these estimated that ϳ1to3ϫ 105 mol kmϪ2 yϪ1 of CO would be 2 rivers, however, is not pronounced on the global riverine Re released from them in the Ganga and the Yamuna basins in the fluxes, as the water discharge of the Yamuna and the Ganga Himalaya (Table 7). This calculation assumes (1) that dissolved at the foothills of the Himalaya is only ϳ0.1% of the global Re in rivers can be used as an index to derive the quantity of the discharge. black shales being weathered and (2) that all organic carbon in 2. The Re abundances in the granites of the Yamuna basin and the black shales is oxidized to CO during weathering. The Ϫ 2 Precambrian carbonates average ϳ26 and ϳ52 pg g 1, amount of CO release from the Yamuna basin is about a factor 2 respectively. Calculations using ⌺Cations* and (CaϩMg) of 2 to 3 more than that from the Ganga (Table 7). This value abundances in these waters to estimate the contribution of is higher than the preliminary estimate reported in Dalai et al. Re from these lithologies show that the Re concentrations in (2000), the cause for the increase being higher C /Re used in org them are too low to make a significant impact on the the present calculation. The calculated CO release rate (a 2 dissolved Re budget of these rivers on a basin-wide scale. likely upper limit) is two to four times lower than the reported 3. The significant correlation between Re and SO in waters CO consumption rate due to silicate weathering in the 4 2 and higher Re in rivers flowing through known black shale Alaknanda and Bhagirathi basins (headwaters of the Ganga in occurrences and groundwaters dripping through black shale- the Himalaya) and that derived for YRS, assuming all silicate phosphorite-carbonate layers, favour the idea that black weathering is due to CO (Krishnaswami et al., 1999; Table 7). 2 shales could be a major source of Re to these waters. If, however, a significant component of silicate weathering Furthermore, the strong correlation between ⌺Cations* and results from other proton sources (e.g., H SO ), then CO 2 4 2 Re in these waters require that both are supplied to the YRS uptake by silicate weathering and CO release via black shale 2 along its entire length in roughly the same proportion. An oxidation could be comparable in the Ganga and the Yamuna important consideration in deciding whether black shales basins in the Himalaya (Dalai, 2001). can be a dominant source for dissolved Re in the YRS and the Ganga is their abundance and distribution in the drainage 5. SUMMARY AND CONCLUSIONS basins. The concentration of 1.8 ng ᐉϪ1 in the YRS requires The focus of this work has been to assess the importance of that, on average, Re from ϳ60 mg black shales be released black shale weathering in contributing to dissolved Re in the per liter of water, making up a few percent of the suspended 42 T. K. Dalai, S. K. Singh, J. R. Trivedi, and S. Krishnaswami

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