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

Abstract

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 CO2 outgassing during the adjustment of the WSRS. To test this hypothesis, we used the isotopic composition of the DIC (δ13CDIC) 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 δ13CDIC values decreased from −7.01 to −11.53‰ and then increased to −6.82‰; and the partial pressure of the CO2 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 δ13CDIC 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 CO2 drawdown via the burial of organic carbon in the geological timescales.