| Radionuclides have been widely used for determining the modern sedimentation rate due to their advantages on quantification,accuracy and mutual identification among different radionuclides.210Pb and 137Cs are widely applied in recent decades,while the application of 239+240Pu was relatively scare,besides,the mutual validation of modern sedimentation rate by these three dating methods were also rarely mentioned.Due to the different geo-chemical properties and different sources of these radionuclides,the sedimentary behaviors among 210Pb,137Cs and 239+240Pu are different in sediment core.Therefore,the comparison of the three methods under different aqueous environment will help to understand the discrepancy of each other,and help to choose the priority of the three methods.However,the previous study cases mainly focused on the vertical profiles of radionuclides in sediment core under a single environment?freshwater or seawater system?,and the comparative study of 210Pb,137Cs and 239+240Pu among saltwater lake,freshwater lake and marine systems was still rare.Meanwhile,the laboratory simulation experiment of these radionuclides diffusion in sediment cores were rarely reported,as well as the study of diffusion process on 210Pb method.In order to observe the applicable of these three dating methods in saltwater lake,freshwater lake and high-latitude fjord?sea water?,this paper carried out a comparative analysis of 210Pb,137Cs and 239+240Pu dating methods based on the sediment cores from two linked lakes?Tuosu lake:Qinghai Province,Saltwater;Keluke Lake:Qinghai Province,Freshwater?,and two Arctic fjords?Kongsfjorden and Krossfjorden,Seawater?.Furthermore,combined with the field observations and reported data in the literatures,a simulated diffusion experiment was designed to understand the influence of organic matter content in sediment?450?ignition loss?and dissolved oxygen content on the diffusion of 210Pb and 137Cs in sediment core,under seawater condition.The mainly results and findings are listed as follows:1)Although the 210Pbex profile of K4 core?Keluke Lake:Qinghai Province,Freshwater?showed a near exponential decay trend,the 210Pb dating method was not suitable to K4 sediment core.It means that a misleading chronology may occur by using dating methods were necessary.Through the comparative analysis of 210Pb,137Cs and239+240Pu dating methods,the mass accumulation rates of K2 station?Keluke Lake:Qinghai Province,Freshwater?,TS2 station?Tuosu lake,Qinghai Province,Saltwater?,B3 station?Kongsfjorden,Arctic,Seawater?,and B4 and B5 station?Krossfjorden:Arctic,Seawater?are 0.11±0.01 g cm-2 y-1,0.10-0.37 g cm-2 y-1,0.17±0.04?0.28±0.06 dating results for core K2,B4 and B5 agreed well,indicating based on the results of the2)At core B5,due to the molecular diffusion of 137Cs 1986 peak,the 137Cs 1963peak?lower specific activity?was migrated,which indicate that the molecular diffusion of the 137Cs peak with high specific activity would lead to the migration of the lower137Cs peak,under seawater environment.There was also significant migration of 137Cs and 239+240Pu in TS2 sample column?Tuosu Lake,saltwater?in which there was significant organic matter degradation,and the 1952-1954 timemark of 137Cs and239+240Pu were not suitable for the determination of sediment chronology.The same situation also occured in B4 and B5 cores?Krossfjorden,seawater?.However,the?Keluke Lake,freshwater?.The situation of K2,TS2,B4 and B5 cores indicated that the 1952-1954 timemark of 137Cs and 239+240Pu can be used for sediment dating in the freshwater environment,but for the environment of saltwater?saline lakes and seawater?and the degradation of sediment organic matter,the 1952-1954 timemark of 137Cs and239+240Pu were not suitable for the determination of sediment chronology.On the other side,the dating results of B5 and B4 core indicated that it is suitable for the application of 210Pb dating method in the high latitude fjords even though there is a complicated source of 210Pbex and strong land and sea interaction.3)According to the diffusion experiments of 210Pb and 137Cs,the mobility of 137Cs is significantly stronger than that of 210Pb,especially in the case of low organic matter content?ignition loss?,which also indicates that the nuclides with higher particle reactive have poor molecular diffusion ability.In addition,the profiles of 210Pb and137Cs shown that the molecular diffusion of radionuclides would less affect the position of nuclide peak,but the molecular diffusion can result in a peak platformization,and the ability of diffusion to both sides of the peak was not uniform.Therefore,the migration of peak is possible for radionuclides under the influence of molecular diffusion,which further suggests that the mutual validation between 210Pb,137Cs and239+240Pu chronology is important.The diffusion experiment also indicated that high dissolved oxygen in water and low organic matter content of sediment would constrain the diffusion of radionuclides.Therefore,the molecular diffusion has little influence on210Pb chronology method when high water oxygen content and low sediment organic matter existed. |
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