Comparison Research On The Sedimentation Rate In The Changjiang Estuary And Its Adjacent Area

The annual sediment supply from the Changjiang River is 4.14×10⁹ tons,in which almost 60%was deposited from the Changjiang River mouth area to 123°E area.The research on the sedimentation rate of the Changjiang Estuay and its adjacent area is of great help to realize the progradation of the tidal flat and the characteristic of the deposition in the adjacent area.The measurements of deposition rate were carried out in Chongming tidal flat and adjacent area by using multi-radioisotopes.Moreover,the geochemical characteristic and the source of surface sediment of the tidal flat were analyzed by multi-radioisotope.The main results are as follows:(1)Different sedimentation rates revealed different sampling conditions, associated with the input of terrestrial sediment and transportation across the shelf,the sedimentation of E4,S5-2,E6 were 3.7cm/a,1.1cm/a and 1.2cm/a,respectively.(2)From the high flat to the low flat of the Chongming tidal flat,the sedimentation rates were decreased.The values were obtained as 4.6±0.3cm/a, 1.2±0.1cm/a and 1.0±0.1cm/a by ²¹⁰Pb_ex,respectively;5.8±0.3cm/a,2.5±0.1cm/a and 2.3±0.1cm/a by ²²⁸Th_ex,respectively.The sedimentation rates obtained by ⁷Be presented seasonal variations.In high flat,the sedimentation rate of sediment cores(collected in June and November,2007, respectively)were 10.0 cm/a and 2.8 cm/a,respectively.In the middle flat,only one sediment core's(collected in June,2007)sedimentation rate could be obtained,that is 8.4cm/a.But in the low flat,it is hard to estimate the sedimentation rate for both of the two cores by ⁷Be.(3)Under the influence of the vegetation and hydrodynamics,the sediment grain size in tidal wetland shows temporal and spatial varations.Compared to mean size of surface sediement in autumn and winter,the mean size is much smaller in spring. Sediment grain size exhibited a landward decresing trend,so did the activity of surface sediment.(4)The ratios of ⁷Be/²¹⁰Pb_exand ¹³⁷Cs/²¹⁰Pb_exin Chongming tidal flat all exhibited close to those of Xuliujing,which revealed that the sediment of the Chongming tidal flat was mainly controlled by the supply of the Changjiang River. Owing to the hydrodynamics,the influence of terrial materials to tidal flat of Chongming exhibited an offshore decreasing trend from high flat to low flat.