Weathering processes in the Indus River Basin: implications from riverine carbon, sulfur, oxygen, and strontium isotopes

This study deals with the major ions and isotope systematics for C, O, S, and Sr in the Indus River Basin (IRB). Major ion chemistry of the Indus, and most of its headwater tributaries, follow the order Ca2+>Mg2+>(Na++K+) and HCO3−>(SO42−+Cl−)>Si. In the lowland tributaries and in some of the Punjab rivers, however, (Na++K+) and (SO42−+Cl−) predominate. Cyclic salts, important locally for Na+ in dilute headwater tributaries, constitute about 5% of the annual solutes transported by the Indus. Weathering of two lithologies, sedimentary carbonates and crystalline rocks, controls the dissolved inorganic carbon (DIC) concentrations and its carbon isotope systematics throughout the Indus, but turbulent flow and lower temperatures in the headwaters, and storage in reservoirs in the middle and lower Indus promote some equlibration with atmospheric carbon dioxide. Combined evidence from sulfur and oxygen isotopic composition of sulfates refutes the proposition that dissolution of these minerals plays a significant role in the IRB hydrochemistry and suggests that any dissolved sulfates were derived by oxidation of sulfide minerals. upper Indus, silicate weathering contributes as much as 75% (or even higher in some tributaries) of the total Na+ and K+, declining to less than 40% as the Indus exits the orogen. In contrast, about two-thirds of Ca2+ and Mg2+ in the upper Indus (over 70% in some tributaries) and three-fourth in the lower Indus, are derived from sedimentary carbonates. The 87Sr/86Sr ratios tend to rise with increasing proportions of silicate derived cations in the headwater tributaries and in the upper and middle Indus, but are out of phase or reversed in the lower Indus. Finally, close to the river mouth, the discharge weighted average contribution of silicate derived Ca2++Mg2+ and silicate derived Na++K+ are, respectively, about one-fourth and two-thirds of their total concentrations.