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 carbon- ates) 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.