Chongming Dongtan Sedimentary Record In The Past 40 Years, And Environmental Processes

The research object of this article is Chongming east tidal flat .The means and methods including grain-size analysis, magnetic measurement and other chemical analysis are used in the article to discuss process of sedimentary environment and characteristic of changing about magnetism, organic carbon and phosphorus etc in Chongming east tidal flat in the past 40 years. It also analyzed the factors which influenced the changing of magnetism , organic carbon and phosphorus etc. and discuss their meaning in environment. The main conclusions are as follows: (1)The surface sediment with fine sand as the main ingredient has two types of fine-sand with clay and fine sand. Most of grain size frequency curves are unimodal and right leaning. The space distribution characteristics of mean parameter of surface sediment indicated that the upper tidal is more than the middle tidal and the lower tidal and lower tidal flat is most fine. From Section North to Section South, the mean size of sediment increases gradually. The percentage of organic carbon in the surface sediments is between 0.37-0.87%, the range of total phosphorus is from 541.0 to 633.7 g/g, the average is 594.2 g/g. The range of inorganic phosphorus is from 497.3 to 590.7 g/g, the average is 524.8 g/g and is 89.3% of total phosphorus; the range of organic phosphorus is from 37.6 to 109.9 g/g and is 6.5 to 17.7% of total phosphorus, the average is 11.7%. All these nutrients elements have obvious space distribution characteristics and the quality of organic phosphorus and organic carbon in Route North is more than that in Route south, which may be have some relations with the exuberant vegetable and a large amount living beings in North route.(2) According to the grain-size change characteristic of surface sediment and the vertical distribution curves of three section of sediment, the sections were divided in sampling depth by layers. Section on Route middle (CDM ) has the lower tidal' sediment (below 105cm ) , the median tidal' sediment (49-104cm ) and the upper tidal' sediment (1-48cm ); It is the upper tidal' sediment from 1 to 40cm in the section on Route North (CDN ) and depth below 40cm is the sediment of the middle tidal sediments. The upper tidal flat is from surface to 20cm and below 20cm is thesediment of middle tidal flat in the Section south (CDS).(3) The vertical variation curve of Fe2 + is differ from that of Fe3 +, the content of Fe2 + increase from top to bottom, but the content of Fe3 + is opposite. The variation of nutrients elements has the decreasing tendency on the direction and has a increasing tendency from the section on the Route north to the section on the South section. The vertical variation curve of Inorganic phosphorus is waviness, but the content of inorganic phosphorus in South section sediments is more than that of North section and Middle section, which may be relevant with a large amount of agricultural phosphorus fertilizer used in the Yangtze River valley.(4) In the Median and North section: There is an oxidation-deoxidization interface between 40 and 50cm, the upside above 40cm is oxidizing environment, especially the zone of strong oxidize environment between 20cm to 40cm, below 50cm is deoxidization environment. In the section CDS: oxidation-deoxidize interface at the depth is above 20cm and below 20cm is the deoxidization environment. Oxidation-deoxidize interfaces of sections are basically accord with the transition between the upper tidal flat and the middle tidal flat. So in all, environment of sediments of the upper tidal flat in the section is oxidation and below it, the middle-lower tidal flat, is deoxidization.(5) Basing on the vertical variation of organic carbon, Section middle and Section north At the depth of 40-50cm and Section south at the depth of 20cm can be divided two layers, top layer and low layer, by a interface which is accord with the interface between upper tidal flat and middle tidal flat and between oxidation and deoxidization. Results above tell that the content of organic carbon in the ox...