Modern Environment Evolution Recorded In The Sediment Of Estuary By Multi-radionuclides

Particle-reactive radionuclides (210Pb、137Cs、7Be and228Th) combined with particle sizes; were used in this paper to reveal the temporal and spatial variation of grain-size and activities of surface sediment in east Chongming tidal flat; the impact factors for these variations were discussed in details.Besides that, the sedimentation rates of east Chongming tidal flat in different time scales, development trends and influencing factors were also discussed, Some other techniques such as GIS, RTK, numerical simulation were also used to verify the results.We summarized the types of the curves of excess210Pb(210Pbex), some main reasons which caused these abnormals also discussed. Based on these discussions, we estimated the sedimentation rates of the offshore continental shelf areas of the East China sea and Yellow sea. The main results are showed below:(1) The grain-size’s temporal and spatial distribution in surface sediment of east Chongming tidal flat showed that the mean size of surface sediment increased from high flat to low flat and increased from north to south. The grain size of surface sediment in spring and winter is much small compared to summer and autumn.(2) Affected by the Changjiang river input and tidal action, different regions had different sedimentation rates in east Chongming tidal flat, and vegetation is a key factor, and generally speaking, the south section had a low rate.East Chongming tidal flat is growing to the northeast direction.(3) Sedimentation rates by using7Be in flood seasons showed that east Chongming tidal flat had a high rate in flood seasons, the highest rate can reach2.3cm/month.We slso found that the south section was eroded even in flood seasons. And the results from GIS, RTK and numerical simulation were similar with our conclusion.(4)52sediment cores collected in the continental shelf regions of the East China sea and Yellow sea areas combined with the dates from documents showed that:although the flux of sediment and water in the Changjiang estuary is high, and the hydrodynamic is strong, the sedimentation rates are still high(more than0.5cm/cm), the rate at river mouth area is more than4cm/a; the offshore continental areas affected by Zhejiang and Fujian coastal current had a relatively stable deposition environment, the sedimentation rates are between1-3cm/a; while in off-sea continental areas, because of the deep water depth and lighter disturbance, the sedimentation rates are relatively lower, located in0-1cm/a; Generally speaking, the sedimentation rate decreases from offshore to open seas, while the mean grain size of the surface sediment has a completely different trend.(5) Events like storms, floods, ecological damage, digging channels and building dams would change the depositional environment and may be recorded in sediment cores. So different events showed different curves of the plots of210Pbex activity vs depth in sediment cores: storms, digging channels and ecological damage can lead to a lower activity of210Pbex in sediment cores, in this case, we can ignore these low points and gain our results. Flood in some specific areas can cause an abnormally high value of210Pbex, we can set this layer as a time marker to gain the rate.Besides that, soil erosion caused by ecological damage in upstream will change the flux of210Pbex and bring about the curve of210Pbex, the nodes here can also be time markers. From the characteristic of the curves, we can get more accurate results.