Study On The Depositional Processes Of The Atmospheric Fallout Radionuclides And Their Application On Tracing Modern Sedimentation Processes At The East China Sea

Radionuclides exist extensively in the environment and participate in various biogeochemical cycles,due to their different sources and half lifes,radioisotopes are widely used as tracers to study different biogeochemical processes at different time scales.The natural radioisotopes(²¹⁰Pb,²¹⁰Bi,²¹⁰Po and ⁷Be),with different half lifes,are all particle-reactive and served as natural tracers to study the source and transportation of sediments,sedimentation rates,sediment chronology,atmospheric depositional fluxes and residence time of atmospheric aerosols?RTAA?.With the wide application and rapid development of isotope tracer technique in various biogechemical processes,there has been a lot of studies focus on the atmospheric fallout processes of²¹⁰Pb,²¹⁰Bi,²¹⁰Po and ⁷Be,however,the long term observations on the bulk depositional fluxes of above radioisotopes along China's coastal area were rare,and studies focused on single rainout event were more limited.Monthly atmospheric bulk depositional fluxes of ⁷Be and ²¹⁰Pb were measured over 11 years?December 2005 to December 2016?at the costal city of the ECS-Shanghai,China.The average annual ⁷Be depositional flux at Shanghai was 1603±62Bq m^-2 yr^-1,and the value was comparable with the word average,the enhanced stratosphere-troposphere exchange of air masses in the midlatitudes area and the high production of ⁷Be in the stratosphere led to the higher ⁷Be depositional fluxes in late winter and early spring.The average annual ²¹⁰Pb depositional flux at Shanghai was424±30 Bq m^-2 yr^-1,which was the second highest reported in literature.Affected by the subtropical climate,the prevailing wind direction in summer was southeast from the East China Sea and the Pacific Ocean,and it carried ²¹⁰Pb-depleted maritime air masses to Shanghai,resulting in decreased ²¹⁰Pb depositional fluxes,while in winter,highly ²¹⁰Pb-enriched continental air masses with much less vertical tropospheric mixing and the formation of inversion result in higher concentrations of ²¹⁰Pb in air masses closer to the ground,leading to elevated depositional fluxes of ²¹⁰Pb in Shanghai.The results from the synthesis of global fallout data indicate that the ²¹⁰Pb depositional fluxes increase with the distance from the coast in inland regions?distance from t he coast>50 km?and increase with the amount of precipitation in onshore region?distance from the coast?50km?.The particle-reactive radionuclides are predominantly removed by wet precipitation from the atmosphere.From the first time-series measurements of radionuclides during one thunderstorm and two typhoons in Shanghai,large temporal variations of the depositional fluxes and specific activities of ⁷Be,²¹⁰Pb and ²¹⁰Po as well as the activity ratios of ⁷Be/²¹⁰Pb within one rainout event were observed,indicating varying sources of air masses contributing to the rainout and events as well varying fraction of continental and maritime air mass intruding from the surrounding area.Besides,three pulse rainout events contributed to about 18.6%of annual precipitation,but also contributed 28%and 11%of the total annual bulk depositional fluxes of ⁷Be and ²¹⁰Pb,respectively.From the analysis of radionuclides in aerosol samples,it could be found that Shanghai,one of the typical coastal cities,was deeply affected by the ²¹⁰Pb-depleted maritime air masses,but it also received amounts of ²¹⁰Pb-enriched continental air masses in winter companied with local combustion source,motor vehicle source and biomass combustion source,leading to the higher concentration of ²¹⁰Pb,²¹⁰Bi and ²¹⁰Po in the atmosphere at Shanghai than the typical inland city-Detroit,U.S.²¹⁰Pb,²¹⁰Bi,²¹⁰Po and ⁷Be can be rapidly absorbed on the atmopspheric aerosol when it has been produced,so that the acitivities ratios between different rasioisotopes can be used to evaluate the RTAA.The RTAA_B calculated by ²¹⁰Bi/²¹⁰Pb activity ratios at Shanghai range 0.66±0.08-16.9±5.4 d with the average of 6.39±1.46 d,while RTAA_P calculated by ²¹⁰Po/²¹⁰Pb activity ratios at Shanghai range 45.3±5.2-196±27 d with the average of125±18 d.Meanwhile,The RTAA_B and RTAA_P at Detroit range 2.40±0.38-10.4±3.4 d and 5.68±0.29-21.8±2.7 d,with the average of 6.11±2.10 d and 13.0±2.9 d,respectively.Due to the large amount of additional input of ²¹⁰Po,the RTAA_B was more approximate to the actual value.After reaching the surface waters from atmospheric deposition,²¹⁰Pb was quickly removed by particulate matter and eventually removed from the water column and transferred to the sediments.In recent decades,using ²¹⁰Pb to trace the modern sedimentation processes has greatly developed studies like sedimentology,biogeochemistry,etc.The published literature usually directly ignored the nonideal sediment cores(the sedimentation rates of these can not be calculated by ²¹⁰Pb,indicating turbulence or erosion condition),so there has always been a dispute on the study of modern sedimentation flux.In the present study,based on the vertical distribution of ²¹⁰Pb in the sediment cores,sedimentation rates of nonideal sediment cores were assumed to re-evaluate the modern sedimentation process at the Changjiang Estuary and adjacent ECS.For example,compared with the interpolation method which ignored the nonideal sediment cores directly,the sedimentation flux of the PN section?with the area of 2.07×10⁴ km²?would decrese about 15%,17%,18%,20%and 22%,respectively,if the sedimentation rates of the nonideal sediment cores were assumed as 0.2 g cm^-2 yr^-1,0.1 g cm^-2 yr^-1,0 g cm^-2 yr^-1,-0.1 g cm^-2 yr^-11 and-0.2 g cm^-2 yr^-1,respectively.So that future study like sedimentation flux and buried pollutants fluxes should fully consider the impact of those nonideal sediment cores and reasonably assummed the sediment rates.In summary,the 11-year continuous observation on ²¹⁰Pb and ⁷Be depositional fluxes at Shanghai in the present dissertation improved the studies on temporal variations of these two radionuclides,and also proved the basic hypothesis of ²¹⁰Pb chronology to some extent.The analyses of global distributions of ²¹⁰Pb and ⁷Be depositional fluxes would help to provide the basic sicientific data on studies about the transport pathways and deposition processes of particles/sediments at the coastal area.Moreover,the consideration of nonideal sediment cores in the study of modern sedimentation processes at the ECS provided new ideas for the future studies on the sedimentation processes of particles and its-derived pollutants.