南沙海域沉积物^(239+240)Pu和^(137)Cs的分布特征及环境意义

Distribution characteristics and environmental significance of^(239+240)Pu and^(137)Cs in sediments in Nansha Sea area

放射性核素^(239)Pu、^(240)Pu和^(137)Cs具有沉积物定年和互相印证的优势,已被广泛运用于沉积速率及放射性物质来源的评估研究中.本文测量和分析了从南沙海域采集的6个沉积物岩芯中的^(137)Cs、239+240Pu活度和240Pu/239Pu原子比.结果表明,南沙海域沉积物中^(137)Cs、^(239+240)Pu活度的变化范围分别为(0.758±0.078)—(1.687±0.145)Bq·kg^(-1),(0.256±0.019)—(0.752±0.078)Bq·kg^(-1).^(240)Pu/^(239)Pu原子比变化范围为(0.184±0.020)—(0.201±0.028),介于全球沉降和太平洋核试验场(PPG)240Pu/239Pu原子比范围之间,通过两端元法计算出PPG对南沙海域钚(Pu)的相对贡献为7.15%—15.89%(平均为11.29%).^(137)Cs和^(239+240)Pu活度的垂直分布特征表明,除NS-3岩芯外,其它5个岩芯的^(137)Cs和^(239+240)Pu均为典型的单峰.^(239+240)Pu法与^(137)Cs法计算的沉积速率基本一致,说明在采样区域,沉积物混合和外界扰动过程对颗粒物沉积影响不大.在NS-2站位,210Pbex法测得的沉积速率大于其它两种方法的沉积速率,可能是由于NS-2站位离岸较近,人为干扰、沉积物混合作用等其他因素致使210Pbex法测定的结果受到了影响.

Radioisotopes 239Pu,240Pu and^(137)Cs have the advantages of sediment dating and mutual verification,and have been widely used in the evaluation of deposition rates and sources of radioactive materials.In this paper,the activities of^(137)Cs,^(239+240)Pu and the atomic ratio of 240Pu/239Pu in six sediment cores collected from Nansha sea area were measured and analyzed.The results show that the variation range of^(137)Cs,^(239+240)Pu activity in the sediments of Nansha sea area range from(0.758±0.078)—(1.687±0.145)Bq·kg-1 and(0.256±0.019)—(0.752±0.078)Bq·kg-1.240Pu/239Pu atomic ratio is(0.184±0.020)—(0.201±0.028),which are between the global subsidence and^(240)Pu/^(239)PuPu atomic ratio of PPG(Pacific Nuclear Test site).Through the two-terminal method,the relative contribution of PPG to plutonium(Pu)in Nansha sea area are calculated to be 7.15%—15.89%(average 11.29%).The vertical distribution characteristics of^(137)Cs and^(239+240)Pu activity in sediments show that^(137)Cs and^(239+240)Pu are typical single peaks.The sedimentation rates calculated by^(239+240)Pu method and^(137)Cs method are basically the same,which indicate that sediment mixing and external disturbance process have little influence on the sedimentation of particles in the sampling area.At NS-2 site,the sedimentation rates measured by 210Pbex method is higher than those by the other two methods,which may be due to the fact that the NS-2 site is close to the shore,so the results of 210Pbex method are affected by human interference,sediment mixing and other factors.