| The Sheyang coast is a typical silty coast in the middle coast of Jiangsu, also one of the fast depositional modern coasts, its vast and growing area of tidal flats is the biggest potential and the most important reserve land resources in the eastern of our country. The development and utilization of tidal flat can relieve the conflict of increasing people and decreasing land source, to keep the dynamic balance of total cultivated, and to meet the requirements of which economic and social development put forward on coastal port resources, land resources and biological resources. At the same time, human activities such as reclamation and embankment have a great impact on the coast in recent years. The coastal ecological environment has deteriorated due to a large amount of pollutants poured into rivers, lakes and coastal waters, which come from industry, agriculture, transportation, people’s life. In view of the complex of hydrodynamic environment and abundant sediment source, through gathering the core sediment of coastal tidal flat, and then analyze the date on the basis of aecurate ehronologies by radionuclide dating(210Pb and 137Cs), to estimate the sedimentary rates. Secondly, through a multi-index analysis to discuss sedimentary source and the changing process after the test of grain size, heavy mineral and heavy mental. Finally, the article finds out the distribution characteristics and pollution history of heavy metals in different sedimentary period, preliminary evaluates pollution levels of heavy metal pollution in the study area by combining the date of sediments with specific pollution event of heavy metal. The results indicated that:(1)The activity of 210Pbex in the cores sediments in Sheyang shows a toothed shape within the mass depth of the sediment, fully considered the process of accumulation influenced by human activities, as well as discovering the changing process of deposition rates since one hundred year. The vertical profile of 137Cs gives obvious 137Cs peaks in the cores corresponding to in 1963 and 1986, since the onset of 137Cs fallout in 1954, the sedimentation rates of Sheyang coast have been figured out from 1954, an accurate and reliable chronostratigraphic framework was created by 210Pbex and 137Cs.(2)According to the results of CRS model, the sedimentary rates of two cores is 1.03 cm/a and 2.04cm/a, and it can be divided into four main evolution stages. A: natural siltation period(1920～1956), a low and stable deposition rate; B:the adjustment stage of sedimentation(1957-1990), the sedimentation rates appeared rising slowly in the process. C:fast siltation period(1991～2000), the largest increases in sedimentary rates, affected by human activities intensely; D:fluctuations period(2001～2013), the largest average deposition rate, shows a toothed shape because of the combination of natural factors and human activities.(3)According to the results of the137 Cs dating column, we calculated respectively that the average deposition rate of SY-1 was 0.88 cm/a,0.87 cm/a and 1.69 cm/a since 1954, s ince 1963, and the values for SY-2 were 1.72cm/a,1.13cm/a and 2.22cm/a, which was con sistent with previous studies.(4) The study area are mainly composed of fine silt and coarse silt form the analysis of sediment grain size parameter, which represent Sheyang coast is a accumulation area of fine grain sediment. The vertical profile of cores consistently show a trend of getting thinner firstly and then becoming coarsen again,which is closely related to human activities, such as sea reclamation on a large scale form the late 1980s to the early 1990s.(5) There are main 17 kinds of heavy minerals like hematite-limonite, magnetite, epidote, garnet, zircon, rutile, tourmaline and so on in Sheyang coast, with a low content generally, less than 1%. The cores changed a little along with their depth. The heavy mineral gradually reduced from top to bottom, and in shape of columnar mostly except slice state. With stable mineral as the main component, the coast of Sheyang had a strong hydrodynamic and sustained source. The instable amphibole appeared in the fourth layers, and it indicated a bit weak hydrodynamic condition which was consistent with rapid deposition of sediment in the same period.(6) On the whole, all the heavy metals of sediments had a similar tendency. But for Cr and Mn, other heavy metals showed a vertical change of increasing firstly and then decreasing, or a fluctuations later. Heavy metal enrichment appeared around 2005 and 1991 was synchronous with that of the sediment grain size refinement and hydrodynamic weakness. The elements of Al、Fe、Zn、Mg、Mn、Ni、Cu and Pb mainly came from the natural weathering process, but the accumulation of Cr,Hg and As mainly reflected the influence of human activity. At last it could be concluded that Hg appeared light polluted, the other elements are in good condition form the evaluation index method of all the heavy metals. |
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