Modern Sedimentation Rate And Its Implications For Environmental Evolutions In The Changjiang Estuary In China

The 18" core collected in Yangtze River Estuary was analyzed for 137Cs,210Pb radioisotope measurement and particle size measurement. Regional changes in the deposition process of erosion can be revealed by analyzing the profile characteristics and different geochemical properties of 210Pb and 137Cs.And also,the main reasons for regional deposition rate decrease were discussed.The core sediments consist mainly of yellow-gray and gray-black clayer silt by visual inspection. Based on grain-size analysis, silt is dominant, with the next being caly,and sand content is minimal, whilst no gravel has been found. Moreover,Median diameter have ranged from 6.158-7.775cp,and mean contents of clay have ranged from 16.75～36.86%,while clayer silt ranged from 58.71～79.71% in profile.There was a higher correlation (R2=0.717)between 137Cs activities and clay content than 210Pb and clay content,which showed that 137Cs was more particle reactive than 210Pb. Sedimentation rate in recent 50 years was large and could be divided into some distinct stages:sedimentation rate calculated from 137Cs method was 5.9 cm/yr during 1954-1964,while reduced to 3.36 cm/yr during 1964-2006.Two parts of the graph showed a good exponential decay in the 210Pb profile of the 18# core.Sedimentation rate is 5.47 cm/yr during 120～225 cm,and corresponds to 4.58cm/yr during 18～100 cm.It can be found that erosion occurs in this area through analyzing the results of two methods maybe,and sedimentation rate decreases from 1968～1972. The time when sedimentation rate changed corresponds to the variation of sediment fluxes from changjiang river. Sedimentation rate trend calculated from 210Pbex profile isn't exactly the same as the variation sediment fluxes,which shows that sediment fluxes isn't the only factor for the variation of sedimentation rate in the Yangtze Estuary mud area.Comparing radionuclide inventories in sediment cores with these references of East China Sea,these radionuclides in sediment not only contain direct fallout from atmosphere,as well as mainly derive from the Changjiang drainage basin due to erosion,to a great degree.The conclusion is consistent with previous study.It also illustrates that although sediment fluxes have distinctly decreased in the past two decades,radionuclides in subaqueous delta still epresent the process of net input and accumulation.