Grain Size, Geochemical Element Characters And Its Environmental Significance Recorded By The Core Sediments In The Tidal Flat At Yuantuojiao Point,Estuary Of The North Branch Of The Yangtze River

The Yuantuojiao Point located at the intersecting point of the coastal lines of the North Branch of the Yangtze River and Jiangsu Province. The complex and variable coastal dynamics at the Yuantuojiao Point determined the complexity of sedimentation environment and the sensitive response of silty-muddy tidal flat to environmental changes. This dissertation has focused on the complex dynamic environment of the Yuantuojiao Point based on the field observations, the core sediment analyses of grain size,137Cs dating, and geochemical metal elements, combined with reclamation history in the past50years near the Yuantuojiao Point, sedimentary characteristics and its records on coastal environment changes under the background of human-nature interaction along the coast of Yuantuojiao Point were discussed.The results of sediment grain size analysis show that the tidal flat of study area mainly composed of sandy silt and silt, indicating that the study area is mainly fine-grained sediment. The grain size of YY Core sediments from the bottom to the up was thinner first then thicker. The grain size of core sediments was varied obviously at the depth of116cm. The lower part of the YY Core sedimentation represented of the natural silting up of the tidal flat, and the upper part of the YY Core with coarser sediment upward, that was related to the large-scale reclamation in the areas near to Yuantuojiao Point. Since the fifties and the sixties of the20th century, there have been a large-scale reclamations and embankment projects in Yuantuojiao Point area. Obviously changes in the core grain size records were often associated with the environment changes induced by the variation of sedimentary environment, storm surge and human reclamation. The grain size of LP Core sediments became fine gradually from the bottom to the up, reflecting the tidal flats silted up continuously. At the depth of150-155cm, the sand increased suddenly, sorted behavior became poorer, kurtosis was very narrow, indicating that sediment source is relatively concentrated, and this may be caused by the extreme storm surge induced by typhoon in the Yangtze River.The137Cs dating results of the YY Core revealed that the sedimentation rates at the Yuantuojiao Point changed rapidly with the silting up of the tidal flats, which caused probably by human activities such as costal reclamation and embankment projects around the Yuantuojiao Point. Human activities resulted in the decline of water and sediments discharge in the North branch of the Yangtze River. The sedimentation rates changes were mainly controlled by the local sedimental environment.Elements analysis showed that the geochemical characteristics in the research areas are related to the grain size of sediments. The element Mn, Fe, Ni, Zn, Cu, Cr, Co had a good relationship of mean particle size and clay content, which suggested that these elements came from the consistent source and the terrigenous fine particulate matter adsorption was the main controlling factor of element variation. The element Sr, V, Ti showed low correlation with Al suggested that these elements may be in addition to other factors outside of terrigenous input and fine particulate matter adsorption. Combined analysis with dating results showed that since the1950s the content of elements in the sediment changed obviously, influenced possiblely by the South Yellow Sea and the human activities, especially since the1990s. China’s social economy has achieved rapid development, but the efforts on the environment protection still on the way, resulting in emissions of pollutants in the increasing state.. The industrial impacts from the river catchment and the estuary region, caused the sediment pollution increasing in the past decades.