| The naturally-occurring radionuclide pairs,210Po/210Pb and 234Th/238U,are ideal tracers for evaluating the export flux of POC from the euphotic zone in the ocean.However,the 210Po-210Pb disequilibrium method is a newly-developed technique in recent years,and very few studies have been conducted to apply this method in Chinese seas.As one of sources of 210Po and 210Pb in the ocean,atmospheric deposition has always been neglected in evaluating POC export flux.In this dissertation,variation and influencing factors of atmospheric deposition of 210Po,210Bi,210Pb and 7Be have been investigated by two years observation for every single rainfall event at the coastal city,Shanghai.Based on the expeditions to the Yellow Sea?YS?,the Changjiang River Estuary?CRE?and its adjacent East China Sea?ECS?,and the Northwest Pacific Ocean?NPO?,the source-sink patterns and geochemical behaviors of 210Po and 210Pb in Chinese seas were evaluated.Moreover,the export fluxes of POC in the Northern South China Sea?SCS?were evaluated by the 234Th-238U and 210Po-210Pb radionuclide pairs,which can gain understanding on the biological carbon pump of the SCS.According to the profiles of 210Po and 210Pb at three time-series stations in the SCS,it was found that the abnormal increase of the particulate 210Po was related to the diel vertical migration?DVM?of mesopelagic organisms.The main results and conclusions of this thesis are as follows:?1?Atmospheric deposition is one of important sources for 210Po and 210Pb in the marginal seas.By setting up an observation station at a coastal city of the ECS,Shanghai,during July 2016 to August 2019,the activities and deposition fluxes of 210Po,210Bi,210Pb and 7Be were measured in each rainfall event.The specific activities of210Po,210Bi,210Pb and 7Be in rainwater decreased sharply with the increase of precipitation,indicating that the wet scavenging processes of these four radionuclides were related to the size of raindrops and occurred rain-out and wash-out process.The wet deposition fluxes of four nuclides showed similar seasonal variation trends with high values in summer and low values in winter.In addition,the strong positive correlations between 210Po and 210Pb,210Bi and 210Pb,7Be and 210Pb wet deposition fluxes indicated that all of these four radionuclides were largely regulated by the same scavenging and removal processes.However,the correlations between wet deposition fluxes of these four nuclides and rainfall were weak,indicating that the wet deposition fluxes of the nuclides are not controlled by rainfall only,and other weather factors and weather processes?typhoon,plum rain,cold wave events,etc.?are also impact factors.By observing the activity ratios of 210Po/210Pb,210Bi/210Pb and 7Be/210Pb in four extreme rainfall events with high time resolution,the old air masses intrusion processes were clearly captured.Hence,we concluded that old air masses intrusion would be interspersed within some extreme rainfall events and further affect the atmospheric deposition fluxes of these radionuclides.Based on the reported data in literatures,the average annual atmospheric deposition fluxes?Bq/m2/yr?of 210Po,210Pb and 7Be for the East Asia and its adjacent seas were 18.6±4.2,309±182 and?1.10±0.45?×103,respectively.In the East Asia seas,the estimated POC fluxes by 210Po-210Pb disequilibrium will be underestimated around 7-44%,if the contribution of the 210Po atmospheric deposition flux was not taken into account.?2?Based on four cruises in the YS,the CRE and its adjacent ECS and the Northwestern Pacific Ocean,the activity concentrations of dissolved and particulate210Po and 210Pb in seawater were measured.Combined with the literature data,the distribution of 210Po and 210Pb in the NPO and the eastern China Seas were mapped for the first time from a large spatial scale.The sequence of 210Po and 210Pb activity concentration in different waters is the Changjiang River water>Kuroshio and the NPO seawater>the ECS seawater.Based on a mass balance model,it can be found that atmospheric deposition is the most important source of 210Pb,which accounts for 53.3%of the total 210Pb sources in the ECS.However,210Po budget implied that atmospheric deposition of 210Po accounts for only 3.4%of the total 210Po sources.In-situ decay of210Pb and open ocean mixing are two dominant sources,which account for 48.4%and42.7%,respectively.The riverine inputs of 210Pb and 210Po account for 8.2%and 5.5%of all the 210Pb and 210Po sources,respectively.As above mentioned,it can be concluded in this dissertation that the contribution of the atmospheric deposition of 210Po and 210Pb should not be ignored in open oceanic areas.Moreover,it can be found that the suspended particles in the open sea have much higher specific activities?Bq/kg-particles?of 210Po and 210Pb than those in nearshore seawaters,which indicates that biogenic particles in pelagic seas have stronger accumulation for 210Po and 210Pb.In addition,the distribution coefficients?Kd?of 210Po and 210Pb are?104 m L/g,which proves that 210Po and 210Pb have strong particle affinity to particulate matters in marine environment.The relationship between Kd and POC/TSM ratio shows that Kd values for 210Po and 210Pb increased with the increase of organic carbon content in particles,and Kd value of 210Po increased to a higher value than that of 210Pb with the increase of POC/TSM ratio.The phenomena suggested that 210Po consistently exhibited a higher affinity for biogenic marine particles and POC than 210Pb,especially in the open ocean away from the margins.?3?Vertical distributions of 210Po,210Pb and 234Th activity concentrations were measured in the Northern SCS in autumn,summer and spring.Deficits of 234Th relative to 238U and deficits of 210Po relative to 210Pb were observed in the euphotic zone of water column.There was a significant difference between the estimated POC fluxes based on 234Th?spring,10.7±11.1 mmol/m2/d,n=7?and 210Po?spring,4.6±1.9mmol/m2/d,n=8?tracers,which could be due to the different geochemical behaviors between 234Th and 210Po and their representation processes of different time scale.By210Po-210Pb disequilibrium,the export fluxes of POC estimated by Eppley method?annual average,6.5±2.7 mmol/m2/d,n=16?were relatively higher than those results estimated by Buesseler method?annual average,4.0±2.4 mmol/m2/d,n=17?.The export flux of POC?mmol/m2/d?in the northern slope of the SCS showed significant seasonal variations,and the order of POC export flux?mmol/m2/d?was autumn>spring>summer?Eppley method:8.8±2.7>6.3±2.2>4.9±2.0;Buesseler method:4.8±3.5>4.6±1.9>2.4±1.0?,reflecting the seasonal variability of the intensity of the biological carbon pump in the SCS.Using the disequilibria of 234Th-238U and 210Po-210Pb,the estimated POC fluxes ranged from 1.0 to 31.2 mmol/m2/d,which is within the range of those values reported by other various techniques?eg.sediment traps?.Combined with the reported primary production data from literatures,the export efficiency?f value?of the biological carbon pump would be 12.5-17.7%for the SCS.In addition,it is firstly found that the abnormal increase of particulate 210Po activity in 450 m layer at noon was related to the DVM of mesopelagic fish,indicating that particulate 210Po could be one of potential tracer to study the carbon flux brought by the DVM of mesopelagic fish. |
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