| The record of coastal sediments is very important to the coastal environmental evolution research.The sedimentary environment of mangrove coast is stable and can storage large amount of information,so it become an important area of sedimentary environment research. Collecting the three cores along the mangrove coast in Dongzhaigang Harbor (BD Core, DX07 Core, DX09 Core) and sampling the cores at 2 to 5 cm intervals, the environmental proxies including the grain size, major and trace elements, analysis of the structure of quartz sand, loss on ignition, 137Cs and 14C dating were analyzed. And the environmental evolution of mangrove tidal flat in the research area was analyzed.The components on mangrove tidal flat sediment of Dongzhaigang harbor showed a certain regularity, the main component was silt, the average content was above 70 percent, There is acert-ain content of sand and clay.The sorting coefficient data is betweenl.337-3.239, and the sorting is poor.The data range of skewness is-0.091-0.539, the the overall state of peak value is moderate. The changes of the sediment grain size were influenced by the sedimentary environment and the storm surges.The data statistics on major and trace clement of mangrove tidal flat sediment in Dongzhaigang harbor showed, The major element of BD Core, the average concentrations of K, Mg, Na, P, Ti, Mn were below 3 percent, SiO2 was up to 67.05 percent; The average concentration of trace element Ba was the highest to 335.14μg/g. The concentrations of Rb, Sr, Zn, Cr, V ranged from 50μg/g to 100μg/g, the concentrations of Pb, Cu, Ni, As were less than 30μg/g. The major element of DX core, the average concentrations of A12O3 and Fe2O3 were 15.96 and 10.17 percent. The average concentrations of trace elements Cr, V, Cu, Ni were high. The distribution on the major and trace elements of sediment in mangrove tidal flat was impacted by the organic matter of the mangrove sediment, water medium, the seawater medium and the components of sediment grain size etc.The surface textures of quartz sand of the DX09 Core sediments showed the grain shape and surface markings of quartz sand is different at different layers.The quartz sand of the lower layer had surface textures of continental deposits shape:conchoidal fractures, mechanical impact pits, cleavage plane etal. The quartz sand of the upper layer has surface textures of marine sediments like:smooth shape,mechanical V marks etal. The evolution of the sediment environment experienced a transition from terrestrial sedimentary environment to marine sedimentary environment.According to the granularity, geochemical elements of DX07 Core, and structure of the surface of quartz sand changes in DX09 Core and other indicators, combined with age data, the sharp changes of the environment from terrestrial environments to marine sedimentary environment was distinguished because of the earthquake subsidence in the year of 1605 in the study area. According to the analysis results of the BD Core, there are three stages of the environmental changes since the middle Holocene.. The first phase was from the depth of 567-380cm, the age was 7652-7472aBP, sedimentary facies was lagoon deposition; the second stage from the depth of 380-185cm with the age of 7472-6155aBP, sedimentary facies was transitional facies deposition, which was the subject of early human activity influenced the ancient deposits of soil erosion. The third stage with the depth from 185cm to 0cm, the age was 6155-OaBP indicated the coastal plain environment few influenced by marine dynamics.
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