应用Pu同位素计年法研究长江中下游典型湖泊的现代沉积过程

With the radioactive decay of Cs-137 in the environment, the application of the traditional Cs-137 dating method in the research of recent sedimentation process is becoming difficult. Compared with Cs-137, plutonium isotopes in environment, which have the same atmospheric fallout history as Cs-137, and much longer half-lives (24110 years for Pu-239 and 6561years for Pu-240), as well as the Pu-240/Pu-239 atom ratio for source identification, are more suitable for sediment dating. With the cut-off point of using Pu isotopes for sediment dating, this project intends to select the three typical lakes (Honghu, Guchenghu and Chaohu) in the middle and lower reaches of the Yangtze River as study area and carry out field investigation sampling, aiming 1) to study the distribution characteristics of Pu isotopes and Cs-137 in the lakes by using gamma spectrometric and inductively coupled plasma mass spectrometric measurements; 2) to identify the sources of Pu isotopes and establish the Pu dating time markers based on the atom ratios of Pu-240/Pu-239; 3) to reveal the recent sedimentation process of the lakes and their response to the environmental changes and human activities of lake catchments combing with the analysis of historic references regarding the lake catchments. This study will improve the development of Pu dating method and complete the modern sedimentary chronology, and deepen the understanding on the lake sedimentary evolution in the study area, as well as provide scientific evidence for ecological environment governance of lakes.

随着环境中137Cs的衰变，传统的137Cs计年法在现代沉积过程研究中的应用将会越来越困难。环境中的Pu同位素，其全球沉降历史与137Cs一致，但半衰期更长且具有可以鉴别其来源的240Pu/239Pu比值特征，因此在沉积物计年方面更具优势。本项目拟以运用Pu同位素计年法为切入点，选择长江中下游洪湖、固城湖和巢湖三个典型湖泊为主要研究对象，开展野外调查与取样，采用γ谱仪与质谱仪（ICP-MS）相结合的测定方法，系统研究长江中下游湖泊中Pu同位素与137Cs的分布特征及其影响因素，结合240Pu/239Pu原子比值特征，阐明Pu同位素的来源，确定Pu同位素计年时标，结合湖泊流域历史资料分析，揭示湖泊现代沉积过程及其对流域环境变化及人类活动的响应机制。研究成果有助于推动Pu同位素计年法的发展和促进近代沉积年代学的完善，以及深化对区域湖泊沉积环境演变规律的认识，为湖泊生态环境治理提供科学依据。

Pu同位素计年法被认为是替代137Cs计年法应用于现代沉积计年的理想选择。本项目通过野外调查取样和实验室内分析等研究手段，系统研究了Pu同位素计年法在长江中下游典型湖泊现代沉积过程研究中的适用性。主要获得如下主要研究结果：（1）采用AG1 -x4阴离子交换柱单柱分离法结合Agilent 8900 ICP-MS/MS在NH3-He on-mass模式下测量可实现微量（~1 g）环境样品中超痕量Pu同位素的精准测量并成功应用于沉积物计年研究；（2）长江中下游湖泊沉积物及流域土壤样品中240Pu/239Pu原子比值均位于全球沉降均值范围0.18±0.02内，表明长江中下游区域Pu同位素均主要来自于大气核试验产生的Pu的全球沉降；（3）巢湖、武昌湖、升金湖、滆湖和高邮湖沉积物剖面中均存在明显的最大Pu蓄积峰值，根据Pu同位素计年法估算出其现代沉积速率分别为0.17~0.43 g/cm2·a、0.18 ~0.27 g/cm2·a、0.16 g/cm2·a、0.065~0.246 g/cm2·a和0.16 g/cm2·a，与137Cs计年法和210Pbex计年法估算结果基本一致，表明Pu同位素计年法准确、可靠；（4）1986年切尔诺贝利核事故来源的137Cs对长江中下游湖泊的贡献几乎可以忽略不计，沉积物剖面中出现的小型次级137Cs蓄积峰可能是由于低浓度137Cs测量误差或底泥扰动所导致；（5）巢湖、武昌湖、升金湖、滆湖和高邮湖沉积物中239+240Pu沉积通量分别为33.3~70.3 Bq/m2、65.3~116.8 Bq/m2、71.9 Bq/m2、5.8~22.5 Bq/m2和10.7 Bq/m2，总体与沉积速率的分布特征相一致，反映了各湖泊流域泥沙来源Pu贡献的差异、以及Pu在沉积过程受湖泊底流运动、湖水及沉积物理化性质及湖底地形等因素的影响；（6）Pu同位素在沉积剖面中主要以扩散方式向下迁移，但这种迁移在相当长的时间内并不影响Pu同位素最大蓄积峰作为计年时标的灵敏性。本项目的开展将有助于推动Pu同位素计年法的进一步应用和发展，促进近代沉积年代学的完善。