| Tidal flat sedimentation is the result of land-ocean interactions,which affected by diverse factors,such as the sediment sources,depositional hydrodynamic conditions,topographical features,biological succession influence,and human reclamation activity.The tidal flat around Qidong Foreland located on the junction between Yangtze Estuary and Jiangsu coast,the sedimentary system of tidal flat is highly sensitive to environmental changes.Meanwhile,it is also important area for national economic construction.With the rapid development of economy and increase sharply of population,human reclamation activities have more and more strong influence on tidal flat.Sediment records the environment change information.The core samples are collected fromtidal flat around Qidong Foreland.Based on experimental analyses among sediments on grain size,radionuclide 137Cs dating,microelement,rare earth elements,and clay mineral;further,connection with field investigation.The sedimentary characteristics and sand source of tidal flat around Qidong Foreland are discussed.The result as follows:Bade on analysis of the grain size distribution data,the sediment constituents comprise,the core of YTJ-1,YTJ-2 and YTJ-3,mainly of clayey silt,silt,and sandy silt.The core of YTJ-4 is almost composed of sand.These cores reflected the sedimentary process of tidal flat under influence of natural environment,included under 32.5 cm depth of YTJ-1,under 37.5 cm depth of YTJ-2,and the entire core of YTJ-3.The low tide beach gradually turn to high tide beach in the course of tidal flat continuous accretion,which showed with a clear tendency of upward fining.Furthermore,these are some layers become suddenly coarse or fine at the middle and lower ofprofiles,especially,at the depth of 147.5cm,187.5cm,242.5cm of YTJ-1 core;at the depth of 142.5cm,162.5cm of YTJ-2 core;at the depth of 172.5cm of YTJ-3 core,and the reason is that sediments the reason is that under the influence of extreme environmental changes such as storm surge.Human reclamation activities play a more and more important role to tidal flat development.The hydrodynamic condition surrounding waters is changed by reclamation activities,and than sediment transport is changed,finally a new tendency of erosion and deposition could be formed.The grain-size profiles showed that a clear tendency of upward coarsening,at upper 32.5 cm depth of YTJ-1,at upper 37.5 cm depth of YTJ-2 and the entire core of YTJ-4.These cores in different sites.The core YTJ-1 is located on farming area between coast defended road(constructed in 1950s)and reclamation dike(constructed in 1970).The sediment showed coarsening trend at upper section of core profile.The reason is that fine-grained fraction is more than coarse-grained fraction leached loss by rainwater under influence of the rotation of agricultural activities.The core YTJ-2 is located on site between reclamation dikes(constructed in 1992 and 2006).The core YTJ-4 is located on outside of reclamation dikes(constructed in 2006).As well as core YTJ-1,grain-size coarsening at upper section of core profile,but for different reasons.Since reclamation dikes constructed,subsequent hydrodynamic environment becoming more strong than before,coarse-grained fraction is more than fine-grained fraction adapt to high-energy environment and deposit.And nowadays beach in the region shows characteristics of sandy coast.According to the 137Cs dating analysis in Core,sedimentation rates could be divided into some distinct stages:1963 to 1986 was 2.61cm/a,1963 to 2011 was 1.82cm/a,1986 to 2011 was 1.10cm/a.Furthermore,salt marsh,such as Spartina Alternifloras is introduced from 1970s,plant play a important role to tidal flat development.The tied flat has a high sedimentary rate during this time.With the tidal flat continuous accretion,low tide beach turn to high tide beach,sedimentary rate is gradually reduced,and comply with the general rule of tidal flat sedimentation.The sand source is analyze by environmental index include microelement,rare earth elements,and clay mineral.Bade on analysis of the microelement data,nine elements are most close with the south Yellow Sea sediment on the element abundance,accounted for 56.3%.The second is Yangtze sediment,there are six elements for 37.5%.The result is show that sediment is affected by the material in the south Yellow Sea is the most significant.According to the element ratios such Cr/T and Ti/Nb,and the scatter plot was made on the axis,conclusion reached show that the relationship is very close with the Yangtze sediment,however,the influence more and more intensity by south Yellow Sea.Bade on analysis of the rare earth elements data,average content of REE is 178.57?g/g,and very close with south Yellow Sea sediment(170.22?g/g),more than Yellow river sediment(131.56?g/g),less than Yangtze sediment(211.10?g/g).The patterns of REE Chondrite normalized is similar to the Yangtze sediment and the south Yellow Sea sediment,indicating negative gradient,weak Ce negative anomaly and Eu obvious negative anomaly.According to the element ratios such REE and LREE/HREE,REE and ?Eu,and the scatter plot was made on the axis,conclusion reached show that the relationship is very close with the south Yellow Sea sediment.Further,the provenance discriminate function FD value is calculated,The result is show that Yellow sediment,Yangtze sediment and the south Yellow Sea sediment respectively is 0.36,0.16,0.07.this show that sediments mainly comes from the south Yellow Sea material.Bade on analysis of the rare earth elements data,the content of Illite in the core sediment is the highest,average up to 54.11%;the second is Chlorite and Smectite,respectively is 21.30%and 16.21%;Kaolinite is the lowest for 8.37%.There are many kinds of type of the clay minerals assemblage,the Illite—Chlorite—Smectite—Kaolinite type and Illite—Chlorite—Kaolinite—Smectite type are the main type in the core sediment,both are 37.21%of all type.followed by Illite—Smectite—Chlorite_Kaolinite type,is 20.93%of all type.The ratio value of Illite/Smectite more than 8 for 30.23%of all type,less than 6 for 65.12%,between 6-8 is the lowest for 4.65%.According to the triangular map with Smectite—Illite—Kaolinite + chlorite,the projection data point distribution obviously around near the south Yellow Sea sediment,which showed that sediments is affected mainly by the south Yellow Sea sediment.Recent hundred years,sedimentary dynamic of the North branch of Yangtze river have taken place great changes,and could be divided into three stages:the before 1931 is first stage,the runoff is the main hydrodynamic force,and the evolution of North Branch Channel is affected by natural reasons.During from 1931—1970 is second stage,under the influence of Flood,such in 1931,1949,1954,and reclamation activity,main hydrodynamic force transform from runoff to tide.The after 1970 is third stage,tide is the main hydrodynamic force,the division angle close to 90°right-angle,flow diversion ratio decreased year-by-year,and both the velocity and duration of the flood tide are larger than those of the ebb tide,the net sediment transport upwards,the northern branch has no sediment into sea.In the end,the Northern branch became narrower and narrower,tends to decline.The seas circulation system is complicated in Yangtze estuary,and has important influence on the suspended sediment transport,include Taiwan warm current,Zhejiang-Fujian coastal current,Subei coastal current,and the Yangtze river diluted water.According to the relative position and flow direction of those hydrodynamic force,the influence from Subei coastal current is largest on the near-shore sediment transport and diffusion.The influence of other could be ignored.When sedimentary dynamic of the North branch of Yangtze river from runoff to tide,and there is no significant change for Subei coastal current changes.The net suspended sediment fluxes mainly to the south by Subei coastal current,which from abandoned delta of Yellow river and radial sand ridges in the south Yellow sea.Sediment provenance of was quantitatively estimated based on element geochemical data.The sediments from the tidal flat around Qidong Foreland are characterized end-members.On he basis of evolution of the North branch and profile characteristics of geochemical element,the sediment source is divided into three sections to changes of estimated contribution rate.The result showed that sediment came mainly from the Yangtze river sediment into the sea before 1930,and contribution rate is 68.1%.Subsequently,the contribution rate gradually decreases with the decline of the North Branch,reduce to 38.5%during the period of 1930-1972,and to reduce to 17.5%after 1972.Suspended sediments from south Yellow sea have being moved mainly to the south by strong tide and Subei coastal current,and the contribution rate is 27.1%before 1930.Subsequently,the contribution rate gradually increases,reduce to 55.6%during the period of 1930-1972,and to reduce to 75.9%after 1972.The sediment source of the tide flat around Qidong foreland mainly from the Yangtze sediment to the south Yellow Sea sediment. |
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