| The estuary is a major hub for the river basin and ocean.It is the most active region by the interaction of ocean and land and is sensitive zone of the natural variability and human activities.Therefore,the estuary is one of the hot research areas for the land-ocean interaction research in coastal area.The Liao River is the principal river in southern Northeast China,and one of the seven main river systems in mainland China.The river is also popularly known as the "mother river"in Northeast China.Coursing 1390 kilometers long,the Liao River system drains a catchment basin of over 2.15×105 square kilometers,but its mean discharge is quite small at only about 500 cubic meters per second.Along with the increasing activities of human beings,the sedimentary environments have been changing all the time,posing serious problems at present in the aspects of ecology and economy in the Liao River estuary.In the present thesis,the137 Cs and Pu isotopes measured in 17 sediment cores collected from the Liao River coastal zone were subjected to analyze 137Cs and Pu isotopes activities,240Pu/239Pu atom ratio.We investigated the distribution characteristics and the geochemistry behavior of the 137Cs and Pu in sediment cores.Additionally,we applied a theoretical model based on the 137Cs deposition flux to Calculate the distribution of 137Cs in sediments of Liao River estuary and to compare differences between the calculated and measured values.Lastly,we calculated the 137Cs and 239+240Pu inventories and based on the 240Pu/239Pu to trace Pu source.These results were useful to understand the geo-chemical processes of 137Cs and 239+240Pu in the region,the interaction process between the land and the ocean and the response of the regional environment to the natural and human activities.The main conclusions are summarized as follows:(1)Based on grain-size and characteristics of the sediment composition analysis,silt is dominant,with the next being clay,and sand content is minimal,whilst no gravel has been found on both sides in the Liao River estuary.But,the sediment types of the sampling stations(LT-1,LT-2)mainly belong to sand-silt-clay in the tidal flat.The ranges of activities of 137Cs in the surface sediments(0?5 cm deep)were 1.03?15.68 Bq/kg and had large spatial variation in space.In general,the activities of 137Cs in the surface sediments increased from tidal flat to land and east to west in the Liao River estuary.In the profile of sediment cores,the vertical distributions of 137Cs activities were identified,which the distribution type can be sectioned to a one-peak curve,two-peak curve and irregularly distributed curve.For the two-peak curves of 137Cs,there was a sub-peak value of 137Cs in the sediment profiles except the maximum value of 137Cs.Through the analysis of the sub-peak value of 137Cs,it was probably impacted by the Chernobyl Accident in 1986.As for the irregularly distributed curves of 137Cs(LH-15,LH-18),they showed the deposition processes were discontinuous due to the faults generation.(2)Sedimentation rates calculated from 137Cs method ranged from 0.51 to 1.77 cm/a during 1954?2012(2015),with a mean of 0.80 cm/a in the Liao River estuary.However,sedimentation rates derived from 137Cs maximum value range from 0.48 to 1.63 cm/a from 1963 to 2012(2015),with a mean of 0.70 cm/a.The sedimentation rates based on the profiles of 137Cs in cores increased from north to south and land to sea in the Liao River estuary.The 137Cs inventories vary from 980 ±46 to 6094 ± 92 Bq/m2,with average of 2278 ± 42 Bq/m2.Furthermore,it is also evidenced that the sediment rates based on 137Cs show an increase tendency with the 137Cs inventory.The 137Cs inventories were higher than the 137Cs global fallout flux 1259 Bq/m2(137Cs activity was decay corrected to 2015).This result clearly indicates that the the 137Cs atmospheric deposition is the main source of 137Cs in the Liao River estuary.(3)In this theoretical model,we investigate and quantify the 137Cs atmospheric deposition in the Liao River estuary based on the records of 137Cs atmospheric deposition from 1957 to 2008 in Akita in Japan.Then,we calculated the maximum peak and sub-peak of 137Cs activity in the sediment cores which depended on the deposition rate and the sub-sampling interval.We compared the calculated values with the measured ones.All 137Cs maximas in sediment cores calculated for different sedimentation rates and sample intervals are at the position of the year 1963.The 137Cs maximum decreases with the increasing of the sedimentation rate and the sub-sampling interval.Within the range of the errors permitting,the theoretical value is essentially in agreement with the experimental results.The result provides useful information on the reliability of the measured 137Cs maxima and sub-peak value in sediment profiles frequently used for dating of sediments in coastal regions as well as in terrestrial(lake)environment.(4)The 239+240Pu activities in surface sediments of the investigated area ranged from 0.103±0.008 to 0.978±0.035 mBq/g,with an average of 0.294 ± 0.053 mBq/g(n=7).The activities of 239+240Pu in the surface sediments increased from tidal flat to land and east to west in the Liao River estuary,which the distribution tendencies of 239+240Pu were similar to 137Cs.The 240Pu/239Pu atom ratios in surface sediments ranged from 0.173±0.047 to 0.215±0.061,with an average of 0.188±0.039(n=7),which is consistent with global fallout.Additionally,the linear correlativity between 239+240Pu and 137Cs in the surface sediment is statistically significant at the 1%significance level.The 239+240Pu maximum peaks are clearly identified in profiles,and taken as the time horizon of 1963.The sedimentation rates calculated from 239+240Pu initial appearance ranged from 0.46 to 1.62 cm/a,with a mean of 0.79 cm/a in the Liao River estuary.However,sedimentation rates derived from 239+240Pu maximum value range from 0.25 to 1.51 cm/a during 1963 to 2012(2015),with a mean of 0.62 cm/a.Sediment rates deduced from 239+240Pu maximum peak seem to be lower than ones from level of 239+240Pu initial appearance.The sedimentation rates based on the profiles of radionuclide 239+240Pu in cores increased from north to south and land to sea in the Liao River estuary.Furthermore,the difference between the gained results from the two radionuclides(239+240Pu and 137Cs)has been assessed using a statistical t-test,and suggested that the sedimentation rates estimated by the two radionuclides were in good agreement and without any statistically significant difference.These results further demonstrated that using 239+240Pu to estimate the sedimentation rates is a practicable way and further improve tracer technique to assess the sedimentation rates.Through comparing Pu and 137Cs activities in sediment profiles,the layer depth,which presented the 137Cs sub-peak value,did not appear the 239+240Pu sub-peak value in the sediment cores.Combining with the existing work,this was further evidence that the Chernobyl Accident in 1986 might affect the 137Cs sub-peak value.(5)Except for the sediment core LT-2,the 240Pu/239Pu atom ratios in other sediment cores ranged from 0.180 to 0.199,which was close to the global fallout value of 0.18±0.02.This indicates that global fallout is the source of Pu in sediments from the Liao River estuary.As for the sediment core LT-2,the mean of 240Pu/239Pu atom ratios,slightly higher than that of the global fallout value of 0.18±0.02,was 0.217±0.050.Such pattern of Pu isotopic compositions indicated that Pu on the tidal flat in the Liao River estuary is sourced from a combination of global fallout and close-in fallout from the Pacific Proving Grounds(PPG)in the Marshall Islands.Using a simple two end-member mixing model,we estimated the relative proportions of global fallout and PPG close-in fallout presented in the tidal flat sediment for the Liao River estuary.The calculated results indicate the relative contribution of the PPG close-in fallout to core LT-2 is 26.57%.The remaining 73.43%can be attributed to global fallout and drainage input.The total inventory of 239+240Pu in the Core LT-2 is determined above as 109.9±3.76 Bq/m2.The contribution of PPG close-in fallout was therefore around 29.2 Bq/m2 and the remaining approximately 80.7 Bq/m2 derived from global fallout and basin input.At 30?40°N region,contribution of the direct global fallout to Pu inventory is about 42 Bq/m2.Assuming all the direct global fallout was transferred to the sediment,we estimate the contribution from the land-origin global fallout transported by the Liao River basin to be 38 Bq/m2.These results clearly indicate that the direct global fallout is the main source of Pu in the Liao River estuary.Therefore,comparing radionuclide inventories in sediment cores with the global fallout,these radionuclides in sediment not only contain direct fallout from atmosphere,as well as mainly derive from the Liao River basin due to erosion,to a great degree. |
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