Porewater dissolved inorganic carbon released due to artificial sediment scouring in the Yellow River.

The water-sediment regulation scheme (WSRS) in the Yellow River has been adjusted annually to maintain the optimal capacity of the Xiaolangdi Reservoir via the scouring of deposits downstream before the rainy season. The high-intensity sediment scouring in a short time has given rise to elevated dissolved inorganic carbon (DIC) exportation; however, the source of the DIC is still unclear. Here, we hypothesized that sediment scouring released porewater DIC into the river water, increasing the riverine DIC concentration and enhancing CO 2 outgassing during the adjustment of the WSRS. To test this hypothesis, we used the isotopic composition of the DIC (δ13C DIC) to constrain the sources and transformations of the DIC. River water samples were collected below the Xiaolangdi Reservoir dam before, during, and after the adjustment of the WSRS. In these three stages, the pH of the river water decreased from 8.2 to 8.0 and then increased to 8.4; and the DIC concentrations increased from 185.30 to 216.37 mg L−1, and then decreased to 199.93 mg L−1. The δ 13C DIC values decreased from −7.01 to −11.53‰ and then increased to −6.82‰; and the partial pressure of the CO 2 increased from 5.21 × 10−3 to 35.30 × 10−3 atm, and then decreased to 3.40 × 10−3 atm. The increase in the DIC concentrations with negative δ 13C DIC values during the adjustment of the WSRS resulted from the release of porewater DIC due to the sediment scouring, which was derived from the anaerobic oxidation of organic matter in the permeable sediment environment. More broadly, the sedimentary organic carbon-derived DIC released into the ocean would counteract CO 2 drawdown via the burial of organic carbon in the geological timescales. [Display omitted] • Artificial sediment scouring increased DIC concentrations and flux during the WSRS. • Riverine δ 13C DIC values were negative during the WSRS. • DO concentrations were as low as 1.6 mg/L, and d -excess values increased to 9‰ during the WSRS. • Porewater mixing has resulted in negative δ 13C DIC values and increased DIC concentrations. [ABSTRACT FROM AUTHOR]