Study On The Response Mechanism Of Redox-sensitive Elements At The Sediment-seawater Interface In The Low-oxygen Zone Of The Yangtze River Estuary

As an important area of economic development in China,the Yangtze river estuary has been concerned about its Marine environmental problems.The low oxygen zone formed in the bottom water outside the mouth of the Yangtze river estuary in summer is also one of the hot spots.The phenomenon of low oxygen in water has negative effects on the water environment and organisms,and the series of biogeochemical processes caused by it at the sediment-seawater interface are worth further study.In this paper,the redox sensitive elements(RSE)(Fe?Mn?Mo?U?V)in the sediment-seawater interface of the Yangtze river estuary and its adjacent waters are studied,and the main controlling factors affecting their distribution and their indicating significance were discussed in detail,and the diffusive benthic flux and authigenic mass accumulation rate were calculated.This study provides a scientific basis for understanding the early diagenesis of sediment-seawater interface RSE and the geochemical cycling process of trace metal elements in the low-oxygen area of the Yangtze river estuary.The following conclusions are drawn from the research:(1)The concentration of dissolved trace metal elements(Cu,Zn,Ni,Co,Cd)in the seawater of the Yangtze river estuary and its adjacent waters has a strong correlation with salinity.The source of dissolved metal is related to river input,but several trace elements behave in different ways due to different biogeochemical processes in estuaries.Among them,Cu,Zn,Ni and Cd show non-conservative behavior,while Co shows conservative behavior gradually diluted by seawater.The concentration of dissolved Pb has no correlation with salinity,and its source is related to atmospheric sedimentation.(2)According to the season(summer and winter)and two different years(2017,2018),the pore water dissolved Fe,Mn,Mo,U,V is the result of the vertical profile distribution reflects the sampling position sediment-water interface of oxidation-reduction environment,strengthening its reductive environment order should be: A14 in the winter of 2018 < A15 in the summer of 2017 < A14 in the summer of 2018 < A13 in the summer of 2017 < A15 in the summer of 2018.It is more reductive in summer than in winter,more reductive in summer 2018 than in summer 2017,and more reductive the closer the location is to the hypoxic frequent area.(3)According to the vertical profile distribution results of redox-sensitive elements(Mn,Mo,U,V)and S in sediments in two seasons(summer and winter)and in different years(2017 and 2018),and the enrichment degree of relative background values,the response of redox-sensitive elements to low oxygen area at the sampling site was analyzed.B3 < A13 < A14 < A15 was the order of increasing sediment reduction environment at each site.The closer the location is to the hypoxic frequent area,the more reductive the sedimentary environment is.(4)According to the diffusion flux results of sediment-seawater interface in two seasons(summer and winter)and the authigenic mass accumulation rate results of sediments in two regions(hypoxic and non-hypoxic),the material exchange of sediment-seawater interface in the seasonal hypoxic region of the Yangtze river estuary was analyzed.The diffusion flux of Mn across the boundary to the upper water body in summer is larger than that at the interface in winter,while the diffusion flux of Mo at the interface in summer is smaller than that in winter,which is consistent with the strong reducibility of sediment-seawater interface in low-oxygen zone.In the hypoxic zone,Mo and V are buried downward with large flux,while in the non-hypoxic zone,Mo and V are diffused upward into water bodies with small flux.