Characteristics And Its Record Of Coastal Environment Change 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. The specific geographic location and the complexity of regional morphological dynamics determined the sensitive response of the Yuantuojiao Point silty-muddy tidal flat to environmental changes. The coastlines along the Yuantuojiao Point have shifted eastward about6km in the past50years by the rapid seaward reclamation. Sediments on the tidal flat recorded the information about environmental changes. The surface sediments and three cores (the YT,YY and YYZ core) were collected near the Yuantuojiao Point coast. Basing on the field investigation, and the analyses of sediments on grain size, elements, magnetic susceptibility, quartz sand scanning electron microscopy, and137Cs dating methods, combined with50years reclamation history near the Yuantuojiao Point, sedimentary characteristics and its records on coastal environment changes under the backgroud of human-nature interaction along the Yuantuojiao Point coast were discussed in this paper.The grain size analysis results of the surface and cores sediments indicated that silt, sandy silt and clay silt were the main sedimentary type, and this show the typical sedimentary feature of silt-muddy tidal flat under accumulation background at the Yuantuojiao Point. Affected by the levee and storm surges, the coarser sediments were deposited, and the coast at the front of the levee reclaimed by Hengda Group presented the characteristics of sandy shore. The cores sediments became generally finer and finer from the bottom to the top, corresponding to the natural silt-up process of the tidal flat.The mutations recorded by the cores were often associated with environment transition of the estuary-coast, storm surges, and the seaward reclamation near the Yuantuojiao Point.The average sedimentation rates calculated by the137Cs method were about2-3cm/a in the study area. Both the YT Core and YY Core indicated the average sediment rates has declined since1960s, influenced by the shrink of the North Branch of the Yangtze River corresponding with the decline of water and sediments entering the North Branch as well as the reclamaition along the coast of Yuantuojiao Point. According to the field observation recently, the storm surges not only caused the retreat of the cliff between the silty-muddy tidal flat and salt marsh region, but also brought a large of sediments and accelerating the sedimentation in the study area. During the period of2007-2009, the4cm/a sedimentatary rate was observed. The rapid sedimentation on the salt marsh and silty-muddy tidal flat caused the the extinction of the cliff. The recent morphological changes of the tidal flat at the Yuangtuojiao Point were the results of the impact of the human reclaimation and the tidal dynamics especially the storm surges.The quartz sands of six coarser samples from the YYZ core were analyzed by scanning electron microscope, the analysis results indicated that grinding roundness of the quartz sands were relatively poor. There were dish-shape impact pits or crescent impact craters in quartz sands surface, corresponding to the typical aeolian-environment, and there are also underwater impact v-shaped pit and straight-bent hit ditch in surface of the quartz grains, corresponding to the typical beach environment. At the same time, more siliceous flake and siliceous ball, dissolution ditch and dissolution pit appeared also in quartz sand particles, and they existed in the form of superposition or accompanying. According to the above characteristics, the sedimentary environment of the YYZ core may experience the evolution process of shoal-tidal flat-supratidal zone sand. The magnetic susceptibility of the upper part of the YYZ core was higher, and the under was lower. The magnetic susceptibility of the YYZ core were related to the grain size of sediments, the mean grain size of the YYZ core, which are greater than4Φ(<63um), were positively correlated with the magnetic susceptibility, but less than4Φ(>63um) of that were the inverse correlation. The grain size curves of the YYZ Core showed the fine trend from the bottom to the surface, the mutation at the depth of160cm of the YYZ core indicated estuary environmental changes companying with the changes of the sediment source. The results of the analysis of quartz sand scanning electron microscopy indicated the quartz sands transported by more complex sedimentary dynamics duiring the evolution of the tidal flat at the Yuantuojiao Point.The content of most macroelements and microelements were lower in the clam culture zone and mudflat, but higher on the slat marshes. The most peak of most elements existed at the depth of the132cm of the YY core, and the change trends of most elements were more similar. The mean grain size of the YY core had a good relationship with most elements, indicated that most elements at the Yuantuojiao Point followed the regularity of "grain size control effect". The grain size and the contents of most elements from the YY core had greater difference before and after1958. Based on the comprehensive analysis of the coastal environment, the elemens changes may indicate the main sediment sources changes in the tidal flat at the Yuantuojiao Point, estuary of the North Branch. Because the channel shrink of the North Branch of the Yangtze River and the decline of the water and sediments entering the North Branch since the late of1950’s. The main source changed from the Yangtze River before late-1950s to southern part of the submarine radial sand ridges of South Yellow Sea after late-1950s.The geo-accumulation index and potential ecological risk index were used to assess the pollution degree of heavy metals at the Yuantuojiao Point, the analysis results suggested that V, Ti and Cr of the surface sediments were pollution-free, Zn, Ni, Cu, Co and Mn existed light pollution, and the pollution level of the salt marshes is higher than that of the mudflat and clam culture zone. Co, Cu, Mn and Zn of the YY core sediments had light pollution only at the depth of the132cm, the other depth had no pollution. The potential ecological risk factors for single heavy metal were below40, indicating the risk of single heavy metal was minor. The ecological risk of eight heavy metals of surface sediments was in the order:Co>Cu>Ni>Cr>V>Mn>Zn>Ti, and that of the YY core was in the order:Co>Ni>Cu>Cr>V>Mn>Ti>Zn. Ni, Cu and Zn had obvious enrichment, and the geo-accumulation index and potential ecological risk index of Co and Cu were higher in the surface sediments at the Yuantuojiao Point tidal flat.