Magnesium isotopic compositions and calcium and magnesium concentrations of river waters from the Huanghe basin in China

The magnesium (Mg) isotopic composition of river waters is mainly controlled by riverine Mg sources and geochemical processes. It is meaningful to characterize Mg isotope behavior for us in understanding chemical weathering and other geochemical processes, such as secondary mineral formation linked to environmental conditions. The Huanghe basin was chosen to investigate the behavior of Mg isotopes during river erosion in temperate-semiarid and arid climate. Dissolved Mg shows a wide range of d26Mg values, ranging from - 1.53 per mil to - 0.11 per mil, with most values being close to - 1.09 per mil. By comparison, most of the suspended loads are enriched in heavy Mg isotope, with d26Mg values ranging from - 2.00 per mil to - 0.62 per mil, which is controlled by mineralogy. The Mg isotopic composition in the upstream is mainly determined by the mixture between carbonate and silicate sources. When the Huanghe flows through the Loess Plateau, the dissolved Mg decreased due to the dissolution of easily dissoluble phases in loess, such as MgCl2 or MgSO4. Carbonates weathering and evaporites dissolution are the major controlling factor of the evolution of Mg isotopic composition in dissolved phase, though evaporation-related precipitation of secondary carbonates also influences the dissolved Mg isotope composition in some tributaries. The strong relationships between d26Mg and pH, temperature, and d18O suggest that the Mg isotope system bears significant information about the hydrogeochemical characteristics and climate conditions of the Huanghe basin.