| Modern sedimentation rate,an important characterization of the sedimentary geomorphologic evolution,could be used to study the sedimentary geochemical processes.Among all the methods to measure the sedimentation rate,radioactive isotope chronology has been rapidly developed due to its quantification,accuracy and mutual calibration between different radioisotopes.The Yellow sea?YS?,a typical semi-closed shelf sea,is divided into the south Yellow sea and the north Yellow sea.The topography,climate,sediment source and the sedimentation processes are different between the south Yellow sea and the north Yellow sea.The Croatia Krka river estuary,a typical karst landform,has the special travertine barrier and the relatively more stable sedimentary environment.In the present study,the modern sedimentation rates of sediment cores in the these much different regions were studied by the 210Pb chronology?Constant Initial Concentration model,CIC and Constant Rate of Supply model,CRS?and the 137Cs/239+240Pu chronology(137Cs and Pu isotopes were firstly recorded in 1952 and the peaks for 1963 and 1986 could also been recorded in the sediment in some regions).And then the different dating models were intercompared to explore the factors that may influence the sedimentation rates.The main results were shown as follows:?1?Firstly,the change characteristics of 210Pb and 137Cs bulk depositional fluxes in the East China Sea?ECS?coastal region were studid.Since the ECS coastal region has a humid subtropical cliamate,it has been deeply impacted by both the continental air masses and the maritime air masses,leading to the high seasonal variation of the210Pb depositional fluxes(0.09-0.82 kBq m-2 y-1)with high value in winter and spring,while the high value were shown in summer and autumn.Besides,a strong correlation between depositional fluxes of 210Pb versus precipitation could be found in spring and winter,indicating that in these three seasons,the amounts of precipitation control the depositional fluxes of 210Pb.However,the contribution of dry deposition to the bulk depositonal fluxes could not been ignored by analyzing the dry deposition samples.137Cs could be detected only in 5 months in the bulk deposition samples,with the depositional fluxes region of 0.33-1.72 Bq m-2 y-1.Due to the decrease of137Cs depositional fluxes and the continuously decay of 137Cs in the sediments following time gose on,new challenges would arise on the 137Cs chronology.?2?The vertical distributiones of 210Pbex activities with depth in the four sediment cores?B25,B07,H07 and H25?in the YS were basically follow the radionuclide's decay,while 137Cs could been only detected in two sediment core?B25 and H07?.The variations of the grain size with depth were small,so that the radionuclides chronology can be used without consideration of granularity effects to study the sedimentary processes in this region.There were well-matched results by the 210Pb chronology of CIC and CRS models.And based on the CIC model,the sedimentation rates of core B25,B07,H07 and H25 were 0.25 cm y-1,1.31 cm y-1,0.37 cm y-1 and0.41 cm y-1,respectively,and the concerned average sedimentation fluxes were 0.19 g cm-22 y-1,0.97 g cm-2 y-1,0.19 g cm-2 y-1 and 0.26 g cm-2 y-1,respectively.?3?For the sediment cores in the YS,there were no obvious 137Cs peaks,or it was difficult to distinguish the accrate position of the peak since the peak has been broadened.However,if the first detection of 137Cs in the sediment core H07?21-25cm?corresponded to 1952,the sedimentation rate of H07 can be estimated as 0.37 cm y-1.Compared this 137Cs chronology results with that the 210Pb chronology,it can be seen that the 137Cs chronology was little younger than the 210Pb chronology for each layer,or it can be said that the sedimentation rate by 137Cs chronology was higher.While the 137Cs chronology results and the 210Pb chronology results were almost the same in sediment core B25.The 137Cs activity of other two sediment cores were lower than the detection limit.?4?The 210Pbex activities decreased exponentially with depth in the sediment cores CC-019 and CC-020 in the Croatia Krka estuary,and the 137Cs and 239+240Pu peaks could be observed in both cores.The variations of the grain size with depth were small,so that the radionuclides chronology can be used without consideration of granularity effects.These results by the two 210Pb chronology models could been well matched with each other.Based on the CIC model,the sedimentation rates of core CC-019 and CC-020 were 0.72 cm y-1 and 0.40 cm y-1,and the concerned sedimentation fluxes were 0.45 g cm-2 y-1 and 0.22 g cm-2 y-1,respectively.Besides,the layer that represents for 1963 by the 210Pb chronology were basically consistent with the pearks in the vertical peaks of 137Cs and 239+240Pu activities.However,the strong diffusion or mobilities of 137Cs and 239+240Pu were observed,making the first detection of 137Cs layer?1952?and the“1986”peak move downward and upward,respectively.So that the 137Cs chronology by the“1952”time marker was little younger than that of the 210Pb chronology for each layer,and the concerned sedimentation rate by this method was higher;while the 137Cs chronology by the“1986”time marker was little older than that the 210Pb chronology for each layer,and the sedimentation rate by this method was lower. |
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