Chemical balance of the Yellow River source region, the northeastern Qinghai-Tibetan Plateau: Insights about critical zone reactivity

Abstract

Rivers play a vital role in geochemical processes of the critical zone. As the Third Pole, the geochemical processes of the Qinghai-Tibetan Plateau are vital to global element cycles. The Yellow River source region is located in the northeastern Qinghai-Tibetan Plateau, yet there has been little research concerning water chemistry in this region. In this study, river water and rain samples were collected to analyze the stable isotopes and major ion chemistry in the Yellow River source region. The δ18O and δD values ranged from −12.9‰ to −4.5‰ and from −95.7‰ to −36.7‰, respectively. The stable isotopic values of tributaries were lower than those in the main stream, and the construction of the dam could result in the enrichment of δ18O and δD. The stable isotopic values showed that precipitation was the main source of river water. The total dissolved solids (TDS) in river water ranged from 214.9 to 563.2 mg/L, and calcium and bicarbonate were the dominant ions. The average contributions of atmospheric input, evaporites, silicates, and carbonates to the dissolved load of river water were 1.8%–12.5%, 12.4%–15.9%, 18.1%–18.4%, and 56.7%–64.3%. The silicate and carbonate weathering rates were 2.3–2.5 t/km2/yr and 17.4–19.6 t/km2/yr, respectively. The chemical denudation rate in the study area was about 23.0–26.8 t/km2/yr, comparable to the world average value. The CO2 consumption rates by silicate and carbonate weathering were 91–94 × 103 moL/km2/yr and 185–208 × 103 moL/km2/yr, respectively. The TDS flux in the study area was 6.1 × 109 kg/yr, which accounted for 38–47% of that of the Yellow River into the sea. This study showed that although as an alpine arid area, the Yellow River source region had a comparable chemical denudation rate to the world average value, and even higher CO2 consumption rates by silicate and carbonate weathering than some of the large rivers in the world.