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

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 ~4 higher than 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 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/Na+K+Mg+Ca) 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/(Ca+Mg) 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 g^-1 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~150 mol at Batamandi and ~100 mol at Saharanpur, compared to ~120 mol from the Ganga at Rishikesh. The total flux from the Yamuna and the Ganga account for ~0.4% 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: ~ to 3 g km^-2 y^-1 in the Ganga and Yamuna basins in the Himalaya, compared to the global average of ~0.1 g km^-2y^-1. 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 ~(6-9) × 10⁸ kg y^-1 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 weathering 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 consumption from silicate weathering.