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, a nd 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 t he Punjab rivers, however, (Na+ + K+) and (SO42- + Cl-) predominate. Cyclic salts, important locally for Na+ in dilute headwater tributaries, constitu te about 5% of the annual solutes transported by the Indus. Weathering of t wo lithologies, sedimentary carbonates and crystalline rocks, controls the dissolved inorganic carbon (DIC) concentrations and its carbon isotope syst ematics throughout the Indus, but turbulent flow and lower temperatures in the headwaters, and storage in reservoirs in the middle and lower Indus pro mote some equlibration with atmospheric carbon dioxide. Combined evidence f rom sulfur and oxygen isotopic composition of sulfates refutes the proposit ion that dissolution of these minerals plays a significant role in the IRE hydrochemistry and suggests that any dissolved sulfates were derived by oxi dation of sulfide minerals. In the upper Indus, silicate weathering contributes as much as 75% (or even higher in some tributaries) of the total Na+ and K+, declining to less tha n 40% as the Indus exits the orogen. In contrast, about two-thirds of Ca2and Mg2+ in the upper Indus lover 70% in some tributaries) and three-fourth in the lower Indus, are derived from sedimentary carbonates. The Sr-87/Sr- 86 ratios tend to rise with increasing proportions of silicate derived cati ons 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 m outh, the discharge weighted average contribution of silicate derived Ca2+ Mg2+ and silicate derived Na+ + K+ are, respectively, about one-fourth an d two-thirds of their total concentrations. (C) 2000 Elsevier Science B.V. All rights reserved.