Sedimentary Sequences Of Column Samples D1631 And D1641 In Yangtze River Estuary Submerged Delta And Their Environmental Significance

The submerged delta in the estuary area of the Yangtze River is located at the interjection of sea and land.Because of the complex hydrodynamic force in the area and the annual variations of sediment runoff in the Yangtze River,great spatial differences exists in the sedimentation rates,grain size and magnetic properties among the column samples.In this paper,the author selects two column samples of the fore slope of the submerged delta in the estuary area of the Yangtze Rivera(near the isobath of 20 m)and uses ²¹⁰Pb and ¹³⁷Cs dating,environmental magnetism,and granulometry to construct sediment chrono sequence in the column samples and discuss the effects of sediments and grain size on the magnetic characteristics of the sediments.By comparing the results with previous studies,the spatial differences of sedimentation rate,grain size and magnetic characteristics in different column samples were obtained,Further evidence for spatial heterogeneity of sediments in Yangtze River Estuary Submerged Delta.The main conclusions are as follows.(1)According to the results of ²¹⁰Pb and ¹³⁷Cs dating of short-period radionuclide,the sedimentation rate of column sample D1631 in 0-61 cm is 0.57 cm/a,and there is a detectable ¹³⁷Cs without obvious high ceiling value in column sample D1631.According to the sedimentation rate of 0-61 cm,the sediments above 61cm are nearly107 a sedimentation but radioactive ²¹⁰Pb is also present at the bottom of D1631,which shows that its age is also between nearly 100-150 a.The sedimentation rate of column sample D1641 from 51 to 101 cm is 1.31 cm/a,and the age of sediments at 101cm is77a.(2)Comparing the sedimentation rates in the column samples,the author finds out that there are great spatial differences of sedimentation rates in the submerged delta.On the whole,the column sample D1631 in the submerged delta in the estuary area of the Yangtze River(near the northern isobath of 20 m)is older,and the column sample D1641(near the southern isobath of 20m)is younger.The southern part(31°N,122°30?)is the recent sedimentation center,so the sedimentation rate of the sample in the southern part of the submerged delta in the estuary area of the Yangtze Rivera is higher.The northern part of the submerged delta is in an erosion environment due to the lack of sediment sources,so the sedimentation rate is lower.(3)The sediments in samples D1631 and D1641 are mainly ochre clayey silt and livid silt,which are soil and interbedded soil with clear bedding structures and no shells.According to the results of the sediment-size test,the median size in this paper and in the neighboring column samples is between 8-16?m,and the sediments are mainly clayey silt.The sediment-size indexes in column samples D1631 and D1641 indicate that the sediments are poorly sorted,which shows a nearly symmetrical skewness and strong positive bias with high values in some layers.Therefore,the coarse-grained materials are enriched in this layer.The sharp changes in grain size parameters show that the region is affected by typhoons,cold waves,and storm surge,and so on.(4)According to the results of environmental magnetics experiments,the magnetic characteristics of the sediments are dominated by ferromagnetic minerals.The magnetic susceptibility?in column samples D1631 and D1641 changes little vertically and there is no sharp decrease on the magnetic susceptibility?,which indicates that the two column samples are slightly influenced by early diagenesis.On the basis of the correlation analysis of the magnetic susceptibility,the magnetic parameter and the grain size,it can be obtained that the magnetic characteristics of column samples D1631 and D1641 are mainly influenced by the ferromagnetic minerals and the magnetic mineral grains are mainly enriched in the grains(<16?m).It shows that the grain size is the dominant factor affecting the magnetic characteristics of the sediments in the two column samples in this paper.(5)The ferruginous to sulfidic boundary(FSB)determined by the magnetic characteristics are well correlated with the age of sediments,which can roughly determine the age of sediments and then deduce the sedimentation rate.The author compares the sedimentation rate with short-period radionuclide dating,which provides a new perspective for the study of early diagenesis.(6)Comparing and analyzing the magnetic parameters in the column samples of sediments,the author concludes that there are spatial differences between the magnetic characteristics of sediments in different columns.The age of sediments and sedimentation rate affect the vertical change of magnetic characteristics,resulting in obvious differences between magnetic characteristics in different column samples.There is no consistent correlation between magnetic characteristics of sediments and grain size parameters,which shows that the grain size is not the only factor that affects the magnetic variation in all column samples.The magnetic variation is also influenced by other factors such as diagenesis and the age of sediments.