The Water-sediment Regulation Scheme At Xiaolangdi Reservoir And Its Impact On Sulfur Cycling In The Yellow River Basin

Sulfate(SO₄^2-)input by rivers is an important source of marine SO₄^2-,and its flux and isotopic compositions(δ³⁴S_SO4andδ¹⁸O_SO4)affect sulfate isotope compositions in marine.SO₄^2-flux together withδ³⁴S_SO4andδ¹⁸O_SO4 values output by rivers were not only controlled by SO₄^2-sources,but affected by the internal environmental changes of rivers,including sulfate bacterial reduction and sulfide reoxidation.However,the degree of these effects is still unclear,particularly the influences of human damming and water-sediment regulation scheme for sediment discharge on sulfur cycling in watershed were still unknown.The discharged water from Xiaolangdi Reservoir in the Yellow River was selected to solve this problem,and hydrochemical compositions,water isotope compositions(δD_H2O andδ¹⁸O_H2O),andδ³⁴S_SO4andδ¹⁸O_SO4 were determined to indicate the formation process of sulfate from pyrite/organic sulfur oxidation caused by the large amount of sediment discharge during water–sediment regulation scheme of Xiaolangdi Reservoir in the Yellow River,and its influence on the sulfate output of the river by analysing the variation characteristics of SO₄^2-contents,δ³⁴S_SO4andδ¹⁸O_SO4 compositions in discharged water before and after and during the process of water–sediment regulation scheme.Furthermore,the impact of water-sediment regulation scheme at Xiaolangdi Reservoir on sulfur cycling in the Yellow River Basin was revealed.The main results are as follows:（1）The discharged water had averageδD_H2O andδ¹⁸O_H2O values of-60‰and-8.5‰during the water-regulation period in 2019,and averageδD_H2O andδ¹⁸O_H2O values of-63‰and-9.0‰during the sediment-regulation period in 2019,and averageδD_H2O andδ¹⁸O_H2O values of-66‰and-9.5‰during the normal schedule period,respectively.The discharged water during the water-regulation period and the sediment-regulation period in 2020 had averageδD_H2O andδ¹⁸O_H2O values of-64‰and-9.2‰,-67‰and-9.3‰,respectively.During the sediment-regulation period,the average values of hydrogen and oxygen isotopes decreased.（2）The discharged water during the water-regulation period and the sediment-regulation period in 2019 and the normal schedule period had average SO₄^2-contents of1.48 mmol·L^-1,1.37 mmol·L^-1 and 1.49 mmol·L^-1,and averageδ³⁴S_SO4 values of 6.8‰,5.9‰and 7.5‰,and averageδ¹⁸O_SO4 values of 6.3‰,5.6‰and 4.1‰,respectively.The discharged water during the water-regulation period and the sediment-regulation period in 2020 had average SO₄^2-contents of 1.59 mmol·L^-1 and 1.30 mmol·L^-1,and averageδ³⁴S_SO4 values of 6.9‰and 6.5‰,and averageδ¹⁸O_SO4 values of 3.2‰and 2.2‰,respectively.During the sediment-regulation period,the average SO₄^2-contents and the mean values ofδ³⁴S_SO4andδ¹⁸O_SO4 showed a decreasing trend.（3）The discharged water mainly comes from the surface water of Xiaolangdi Reservoir during the water-regulation period,while the discharged water mainly comes from the bottom water of the Xiaolangdi Reservoir and the discharged water from the upstream reservoir during the sediment-regulation period.This difference causes the discharged water to carry more information about the bottom water of the reservoir and sediment pore water during the sediment-regulation period.Sulfate bacterial reduction reduces SO₄^2-concentration,resulting in negative oxygen isotope composition of sulfate generated by sulfide reoxidation.During the sediment-regulation period in 2019,the discharged water was also mixed with pyrite oxidation sulfate scoured by upstream rainwater.The superposition of the two causes the decrease of sulfate concentration and the decrease of sulfur and oxygen isotope values in discharged water at this stage.During the sediment-regulation period in 2020,due to less superposition of pyrite oxidation sulfate scoured by upstream rainwater,the sulfate sulfur isotope value has little change,while the significant decrease of oxygen isotope value is related to the sulfide reoxidation to sulfate in reservoir sediments.Therefore,the sediment discharge process of Xiaolangdi Reservoir leads to the decrease of sulfate oxygen isotope value,which is closely related to the reoxidation of sulfide and organic sulfur in sediments.（4）The sulfate flux of the entire hydrological year from June 21,2019 to June 20,2020 is 0.061 Tmol·yr^-1,and the proportion of sulfate flux during the sediment-regulation period is 24.6%.The flow-weighted averageδ³⁴S_SO4 andδ¹⁸O_SO4 values of SO₄^2-into the ocean are 6.7‰and 5.3‰,respectively.