The Response Of Sedimentary Records In The Mud Area Of The Changjiang Estuary To Hypoxia In The Last 300 Years

Due to the combined effects of human activities and global climate change,there have been frequent occurrences of hypoxia in many large estuaries all over the world since the 1950s.The red tide outbreaks caused by eutrophication and seasonal hypoxia in coastal waters have also become typical ecological disasters in the Changjiang Estuary and adjacent waters.Due to the relatively short time scale of the observational data,the understanding about the hypoxic degree and the reconstruction history of the hypoxia evolution in the Changjiang Estuary on a century-scale is insufficient.It's possible to explore the evolutionary history of the sedimentary environment and the development of history hypoxia on a longer time scale in the Changjiang Estuary and adjacent waters by analyzing the sedimentary records.In this study,we analyze sediment grain size,trace elements,biogenic element indicators such as total organic carbon(TOC),total organic nitrogen(TN),the stable carbon and nitrogen isotopes(?¹³C??¹⁵N),the abundance,species richness and diversity of sedimentary diatoms of the sediment core C2 retrieved from the modern hypoxic zone of the Changjiang subaqueous delta and dated by ²¹⁰Pb?¹³⁷Cs and Optically Stimulated Luminescence(OSL),in order to explore the redox status of the sedimentary environment and reconstruct the relationship between the historical stage of hypoxia and eutrophication in the Changjiang Estuary and adjacent waters in the past?300 years.The results are as follows:(1)The average sedimentation rate of core C2 was about 0.36 cm·y^-1 and the time span of this core was about 1739 to 2018.The average sedimentation rate can be roughly divided into two stages:The average sedimentation rate obtained by ²¹⁰Pb and¹³⁷Cs(0?60 cm section)was about 0.65 cm·y^-1 and the time span was about 1919 to2018;The average sedimentation rate(60?100 cm section)obtained by Bacon age model was about 0.22 cm·y^-1 and the time span was about 1739 to 1919.It is speculated that the change of average sedimentation rate at station C2 may be related to the change of sediment flux into the sea since the 1950s.(2)The results about the content of redox sensitive elements(RSE)show that:Fe-Mn remigration process,organic matter adsorption effect,particle control effect have relatively weak influence on the enrichment of RSEs in this area.Consequently,Mo/Ti and As/Ti are more suitable indicators to indicate the bottom water redox environment change in the Changjiang Estuary hypoxic zone.The vertical distribution of TOC,TN,?¹³C,?¹⁵N indicate that the sources of organic matter in the Changjiang estuarine muddy area are mixture of land and sea sources.The contribution of land-source organic matter is relatively higher,reaching more than 65%.It is speculated that the decline about the contribution of terrestrial organic matter during 1980s to early 21st century may be related to the decrease in the sediment flux from the Changjiang River to the sea.The sediment flux from the Changjiang River into the sea still showed a decreasing trend after 2000,but terrestrial organic matter is still the main type of organic matter in the sediment,which may be related to the rapid increasing of nutrient flux caused by human activities.The increase in the contribution of sea-sourced organic matter may be related to the proliferation of phytoplankton and eutrophication in the water which caused by the outbreak of red tides during this period.The vertical distribution of absolute abundance,species diversity and abundance about sedimentary diatoms show that the biomass of phytoplankton represented by diatoms has been continuously increasing since the 1970s,corresponding to the increasing frequency of red tides ecological disasters.In the beginning of the 1980s,the changes in the concentration and structure of dissolved nutrients have become the main factors which affected the productivity of water bodies and the abundance of diatoms.In some extent,it can attributed to the high nutrient discharge flux which caused by human activities and changes in the trophic structure of water bodies.(3)We reconstruct the historical hypoxia evolution in the muddy area of Changjiang subaqueous delta in the past?300 years by redox sensitive elements(Mo/Ti,As/Ti),biogenic elements(TOC,TN,?¹³C,?¹⁵N),the vertical variation characteristics of the dominant algae species of sedimentary diatoms,benthic foraminifer community,Cd/Al,and biological silicon in nearby stations.The results show that the evolution of the Changjiang estuarine hypoxic zone during this period can be roughly divided into four stages:a possible hypoxic environment in this area during 1740?1800,it's speculated that there may be a hypoxic environment between 1780?1790,and the formation of hypoxia may be related to the massive outbreak of phytoplankton which caused by the eutrophication;lessened hypoxia in 1800?1850;rising frequency of hypoxia in1850?1970 together with the emerging impact of human activities,during 1890?1900,1920?1930,and 1955?1960,three hypoxic events have occurred near the station C2,and the leading factors for the formation of these hypoxic events are not exactly same;and significantly increased occurrence rate and area of hypoxia after the 1970s,coupled with the increase of nutrients inputs into the Changjiang Estuary and the integrated influence of factors such as higher surface sea temperature and changes in sediment fluxes.(4)The changes in the nutrient structure of the water body caused by the input of external nutrients,eutrophication and red tide outbreaks are closely related to the evolution of coastal ecosystems and phytoplankton communities and the formation of the hypoxic environment in the Changjiang Estuary and adjacent waters.In addition,intensified water stratification,global climate change,local topographic differences and changes in flow fields will all have an impact on the interannual variability of the coastal hypoxic zone.