Water geochemistry of the rivers around the Taklimakan Desert (NW China): Crustal weathering and evaporation processes in arid land

Abstract

Major-ion and Sr 2+ concentrations and Sr isotopic compositions of four major rivers in the Tarim Basin, NW China, were measured. These rivers in the arid lands have high total cation concentrations (TZ+) ranging from 2.3 to 46.4 meq l −1, which are significantly higher than those of the global river average (TZ+=0.725 meq l −1). Ca 2+, Mg 2+ and Na + in these rivers are the major cations, together accounting for >95% of the total cations. The total dissolved solids in the Keriya and Gaz Rivers gradually increase from their upper to lower reaches. These general features of these rivers show a strong effect of evaporation and the dissolution of the evaporates in the desert areas. The chemical and isotopic compositions of these rivers are largely controlled by the geological signatures of their drainage areas. The rivers at the northern foot of the Kunlun Mountains, an old terrain, show higher 87Sr 86Sr ratios of 0.7111–0.7154, compared to the Aksu River ( 87Sr 86Sr =0.7103−0.7105 ) draining a relatively young area. The end-member identification by the intercorrelations between chemical and isotopic ratios shows three main sources, which are derived from weathering of silicates, carbonates and evaporates. The Yurongkash River, Keriya River and the small rivers flowing from the northern foot of the Kunlun Mountains have high total ion concentrations and are characterized by high Na Ca , Mg Ca and K Ca mole ratios (1.4–30, 0.5–3.5 and 0.06–0.7, respectively), showing a strong influence of soil salts. On the other hand, the Aksu and Gaz Rivers draining regions in the western part of the Tianshan Mountains and Kunlun Mountains are dilute with respect to the amount of total dissolved solids and show low Na Ca mole ratio from 0.2 to 2, Mg Ca from 0.15 to 2 and K Ca from 0.03 to 2, suggestive of a strong influence of carbonate weathering.