| The sea level has rising gradually since the last deglaciation, which has largely affected the living environment of human, especially those living in the coastal regions.The study of sea level variation has become one of the hot topics in global-change research. The Yangtze River Delta has been important area of sea level studies due to its important economic status and unique geographical location. The Yangtze River Delta, located in the tectonic subsidence belt, receives river sediment continuously and becomes a wide coastal plain gradually. However, due to flat landform, it is extremely susceptible to the sea level change. Therefore, we are difficult to acquire a successive and high-resolution sedimentary sequence. To solve these problems, we selected Ningshao Plain in the south of the Yangtze River Delta as our research area and the scientific drilling named HMD1401 as research object. Under the protection of mountains in the north and east, the sedimentary environment of this area is relatively stable and the sedimentary sequence more complete and successive, which is of great significance to study the geomorphic evolution of the Yangtze River Delta.To reveal physiognomy and environmental evolution of Ningshao Plain since the last deglaciation period, we analyzed the analysis of sedimentary facies, AMS 14C-dating, the grain-size characteristics and typical geochemical elements of HMD1401.And we got some results as mentioned below. (1). Before the last deglaciation (>10.7 ka B.P.): Ningshao Plain was in slope wash sedimentary environment. The sedimentary environment changed greatly. The pale soil layer (the uppermost hard clay) was developed on a sand layer with a thinckness of about 80 cm, and it was likely to be formed in weak hydrodynamic environments. (2). 10.7?9.1 ka B.P.: During this period,the sedimentary environment was gradually converted from continental deposit to marine sediment. Ningshao Plain gradually entered the shallow sea stage. During 10.7?9.6 ka B.P.,Ningshao Plain should belong to the shallow water depressions which were affected by seawater sometimes. During 9.6-9.1 ka B.P., the region may be in the coastal environment. (3). 9.1?6.5 ka B.P.: This stage was the heyday of rising sea level,which rose 24 m rapidly, reaching approximately 3 m below present sea level. And the input of terrestrial matter was not stable. During this period, seawater flooded the entire Ningshao Plain, and just left some low mountains and islands, which provided a limited living space for early Humudu culture. (4) since 6.4 ka B.P.: Sea level rose in Ningshao Plain at the same time continental growth during this period. Finally, this area was dominant with continental growth with sea regression, and the sediment was gradually converted from marine depositon to continental deposition. This provided a vast space for the prosperity of Hemudu culture. And we thought since 6.4 ka B.P., it was affected by the invasion of tide many times, which was bound to have a negative impact on the development of Hemudu culture.In order to ensure the accuracy of our study results, we compared the results of elemant scanning of HMD1401 with diatom and sporopollen curves of HMD03. We found that the curves of Ca and Mn reflecting marine environment were consisitent with the dinoflagellates and chenopodiaceae curves. Additionally, we also compared the results of element scanning of HMD1401 with the XRD mineral curves ofTLS13.It was found that the curves of Ca and Mn were similar with that of montmorillonite. |
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