Strontium isotopes and major ion chemistry in the Chambal River system, India: implications to silicate erosion rates of the Ganga

Abstract

The dissolved concentrations of major ions and Sr isotopes (⁸⁷Sr/⁸⁶Sr) were measured in the headwaters of the Chambal river and its tributaries draining the Deccan Trap basalts and the Vindhyan sediments of peninsular India. The total dissolved solids (TDS) ranged from 181 to 547 mg L^-1; much higher than the global "mean" river water. A significant fraction of solute abundance in the Chambal river is derived from sodium salts, unlike the Himalayan rivers which exhibit dominance of (Ca + Mg) salts. It is estimated that the Chambal river supplies about one-third of sodium via the Yamuna to the Ganga at Rajshahi (Bangladesh), with only ~ 6.5% of water discharge. The presence of Na salts not associated with chloride in the Chambal headwaters constraints the application of Nalow asterisk (Na corrected for Cl) as an index of silicate derived component. This finding brings out the need to revisit the estimates of silicate erosion rate (SER) and associated CO₂ consumption in the Ganga basin, downstream Allahabad, based on Nalow asterisk as an index. The Sr concentration in the Chambal tributaries varied from 1.9 µM to 5.9 µM and ⁸⁷Sr/⁸⁶Sr ratio from 0.70923 to 0.71219. Unlike the Himalayan Rivers, Sr isotope composition in the Chambal river is far less radiogenic as the major sources of Sr to the Chambal are the Deccan Trap basalts and the Vindhyan sediments, which are low in ⁸⁷Sr/⁸⁶Sr. The Sr isotope budget of the Ganga, based on available data of the Chambal, Betwa, Ken, Yamuna and the Ganga shows that, weathering of the Deccan Trap basalts and the Vindhyan sediments (the drainage basin of the Chambal, Betwa and the Ken) contribute ~ 70% of the dissolved Sr to the Ganga at Varanasi. This study highlights the key role of peninsular rivers draining the Deccan and the Vindhyan regions in the major ion and Sr budget of the Ganga.