长江口水下三角洲239+240Pu和137Cs的分布特征及环境意义

在长江口水下三角洲采集沉积物柱状样SC07,通过测试沉积物柱样中^239+240Pu和¹³⁷Cs的活度,重点分析^239+240Pu活度的分布特征及其与¹³⁷Cs活度剖面的异同,以提取^239+240Pu活度剖面中的信息。结果表明：^239+240Pu活度剖面中存在与¹³⁷Cs一致的1963年蓄积峰和1958年次蓄积峰,且两者呈较显著的线性相关关系（R²=0.765）,表明在动力环境较强的河口地区,^239+240Pu与¹³⁷Cs仍可以反映相同的沉积过程。^239+240Pu活度起始层位对应于1948年,而¹³⁷Cs对应于1954年,表明^239+240Pu具有更高的测试灵敏度。SC07柱状样中¹³⁷Cs/^239+240Pu的同位素比值比全球均值低,说明^239+240Pu存在区域沉降来源。

Distribution Characteristics of 239+240Pu and 137Cs in Subaqueous Delta at the Changjiang River Estuary and the Environmental Significance

The sediment core SC07 was collected in the subaqueous delta at the Changjiang River estuary. Radioisotopes of ^239+240Pu and ¹³⁷Cs were analyzed focusing on discussing the distribution characteristics of activity profile, then the difference between ^239+240Pu and ¹³⁷Cs profiles were obtained. Results displayed that, there were the same cumulative peaks of 1963 and 1958 both in ^239+240Pu activity profile and in ¹³⁷Cs profile. The ^239+240Pu activity profile showed a good agreement with the ¹³⁷Cs profile (R²=0.765), that is to say, they can convey the same information about sedimentation processes. The start layer of ^239+240Pu activity was corresponding to 1948 while ¹³⁷Cs was corresponding to 1954, which showed the more measurement sensitivity of ^239+240Pu. ¹³⁷Cs/^239+240Pu activity ratio was quite different after moratorium on nuclear testing, which really did good to dating for the sediments. Moreover, the ¹³⁷Cs/^239+240Pu activity ratio played an important role in distinguishing nuclide source in the environment. Meanwhile, it has been seen that ¹³⁷Cs/^239+240Pu activity ratio of the sediments core was lower than the global average, indicating that ^239+240Pu activity concentrations were partly from close-in fallout. ^239+240Pu activity in the sediment core was between 0.072-0.716 mBq/g and ¹³⁷Cs was between 0.390-16.210 mBq/g. ^239+240Pu atmospheric fallout mainly concentrated in the 63-111 cm, while the ^239+240Pu activity between 0-45 cm decreased obviously. There were one peak (71 cm) and two sub-peaks (63 cm and 99 cm) in the vertical profiles of ^239+240Pu. It needs to be mentioned that the peak at 71 cm was corresponding to the year 1963. The cumulative sub-peak at 63 cm may be caused by China's nuclear test that happed during the 1970s-1980s. Also, the cumulative peak (73 cm) in the vertical profiles of ¹³⁷Cs was corresponding to the year 1963, which was nearly the same with vertical profiles of ^239+240Pu. Because of the extremely complicated dynamic action of Changjiang River estuary area, the existence of cumulative peak of 1986 the vertical profiles of ¹³⁷Cs was still needs further research. In a word, it has been suggested that the radionuclide ¹³⁷Cs, ^239+240Pu and the activity ratio were useful tools in studying the material source, evolution of the delta, as well as the deposition rate.