Research On The Modern Sedimentary Characteristics And The Temporal And Spatial Distribution Of Heavy Metals In The Coastal Tidal Flats Of Northern Jiangsu

The tidal flat is widely distributed in the northern Jiangsu of Radial Tidal Sands?RTS?,which is the fastest deposition part in Jiangsu coast.It brings considerable amounts of reserved cultivated land for Jiangsu province,and provides a broad space for the future development of coastal zones.In recent years,however,with the reduction of sediment load from the abandoned Yellow River and Yangtze River estuary,and the large-scale reclamation activities in coastal tidal flat,the sedimentary environment shows greatly changed.At the same time,the tidal flat as the most typical area of land-sea interaction has been sensitive to environmental changes.With the development of coastal area economy in Jiangsu province,a large number of terrigenous pollutants were carried into the sea by industrial and domestic sewage,which also caused great pressure on the coastal environment.In this study,we collected three core samples in typical regions from the northern part?Xinyanggang,XYG?,the central part?Chuandonggang,CDG?and the southern part?Xiaoyangkou,XYK?.The²¹⁰Pb and¹³⁷Cs dating was used to estimate the modern sedimentary rate in the past 100 years.The characteristics of sediment composition,temporal and spatial variation of heavy metal in sediment and their response to human activities were analyzed,the evolution process of sedimentary environment and the history of heavy metal pollution were revealed,which provided the basis for the construction of modern sedimentary process,and the appropriate utilization of tidal flat resources and the protection of ecological environment.The results as follows:?1?The trend of sedimentary rate in typical regions of study area shows obvious different,which is mainly controlled by sediment supply.The average sedimentation rates of XYG,CDG and XYK were 1.27,1.75-1.85 and 2.43-3.3cm/yr,respectively,which reflecting increasing sedimentation rates from north to south due to controlled by the sediment source.The sedimentation rates have gradually declined in recent 100years,which were an inevitable trend under the background of decreasing sediment source.However,human reclamation activities affected the natural deposition process of tidal flat,which resulting in the increase of sedimentation rate.?2?The vertical distribution of ²¹⁰Pb_exx activity shows a general pattern?the decay segment and the background segment?at the core XYG,which indicated that the sedimentary environment was relatively stable.In the coastal reclamation before 2001,the sedimentary environment of tidal flat was relatively stable in XYK,theprofiles of ²¹⁰Pb_exx activity shows obvious attenuation trend.The sedimentary environment of CDG tidal flat was unstable due to the development of tidal channel,the regulation of vertical distribution of²¹⁰Pb_exx activity was not obvious.The sedimentation of the study area was mainly gradual suspension,reflecting the weaker hydrodynamic force and tidal sedimentation.?3?Heavy mineral assemblages of the three samples are relatively consistent,indicating possibly the same sediment source.Based on heavy mineral assemblages and grain size assemblages of sediment,combined with previous studies,we initially consider that the sediment source of XYG,CDG is from the abandoned Yellow River Delat,while the material of XYK is from the offshore RTS.?4?A high-resolution sedimentary record in the past hundred years has been established.The type of sediments was mainly sandy silt.The grain size of sediment in core XYG became coarser around 1980,which recorded the process of erosion from the abandoned Yellow River Delta to offshore slope in the 1980s.The grain size of sediment in core CDG was refined in 1957,which recorded the process of material source from the offshore RTS to the abandoned Yellow River delta during the late1950s when the Xiyang channel ran through to the sampling point.The sedimentary records in core XYK recorded traces of hydrodynamic and reclamation activities.The grain size of sediment was coarsened in 1985,which was related to the strengthening of nearshore tidal current.The grain size was refined in 2001,which was closely related to reclamation activities.In general,the grain size of study area followed the coarsening trend.The tidal current was the dominant factor in the evolution of the sedimentary environment of tidal flats,and human reclamation activity also played an important role.?5?The environmental records and pollution history of study area had been established over the past 100 years.The vertical variation of heavy metal was basically compatible with the sedimentary characteristics.The content of heavy metals in XYG changed from decreasing to increasing around 1975.The content of heavy metals in CDG changed around 1966,and XYK changed from decreasing to increasing in 1985.The environmental quality of the study area was good and was in a low potential ecological risk.In recent years,there has been some contamination of individual elements,and Cr in XYG core reaches moderate pollution and significant pollution.Pb?Zn and Cu in core CDG were slightly polluted.Cu in core XYK achieved moderate pollution and significant pollution,and As was slightly polluted.?6?The content of heavy metal elements in sediments was mainly controlled by the abandoned Yellow River Delta and the offshore Radial Tidal Sands.Cr in core XYG was controlled by external industrial sources;Pb?Zn and Cu in core CDG were mainly controlled by external industrial sources,local agricultural sources and living sources;Cu and As in core XYK were mainly controlled by local agricultural sources and industrial sources.?7?The industrial development of Sheyang had a good response with the element Cr in core XYG,and the human contribution of Cr has increased significantly since 1992.The development of urbanization,animal husbandry and coal industry in Dafeng has a good response relationship with the human contribution of Zn,Pb and Cu in core CDG samples,and the human contribution has been increasing since 1992.After 1986,contribution of Cu and As increased significantly in core XYK,which indicating the economic development of Rudong,such as agricultural development,reclamation engineering and chemical park construction.At that same time,the establishment of the nature reserves have also play an important role in protecting the tidal flat in XYG and CDG.