Characteristics of carbonate, evaporite and silicate weathering in Huanghe River basin: A comparison among the upstream, midstream and downstream

To systematically study chemical weathering in the entire Huanghe River basin, we divided the basin into three parts with significant differences in geology and climate. We collected 38 water samples from the main channel and its main tributaries, spread between the upstream (above Lanzhou), midstream (from Lanzhou to Huayuankou) and downstream (from Huayuankou up to river mouth) segments of the river. The concentrations of major elements and H, O isotopic compositions were obtained from the samples, and the total dissolved solids (TDS) and, CO2 consumption budget were calculated from the aggregated data of each of the three segments. The results demonstrate that: the TDS are mainly derived from carbonate weathering in upper Huanghe River; evaporite dissolution is predominately occurred in the midstream; and there is almost no additional contribution from rock weathering in the downstream. An increasing trend of CO2 consumption rate by silicate weathering is observed, from 0.14 × 105 mol/km2/yr in the upstream to 5.62 × 105 mol/km2/yr in the downstream, and the budget of CO2 consumption by silicate weathering is estimated to be 26.2 × 109 mol/yr. In contrast, the CO2 consumption rate by carbonate weathering decreases from 3.04 × 105 mol/km2/yr in the upstream to near zero in the downstream, and the budget of CO2 consumption is estimated to be 100.5 × 109 mol/yr. As a whole, in the entire Huanghe River basin, the CO2 consumption budget and TDS yield are estimated to be 126.7 × 109 mol/yr, and 27.5 × 106 t/yr, respectively. These results indicate that evaporite dissolution in the midstream is responsible for the high TDS in the Huanghe River basin, while carbonate weathering in the upstream plays the most significant role in CO2 consumption.