基于210Pb与137Cs分布的近代沉积物定年方法——以巢湖、太白湖为例

对巢湖、太白湖沉积岩心中210Pb、137Cs活度进行了测试分析。结果表明,137Cs活度均低于20 Bq/kg,但具有明显的1963年蓄积峰;210Pbex活度随深度增加呈指数降低,指示了较为稳定的沉积特征。采用CIC、CRS模式计算得到的沉积岩心210Pb年代结果均偏离137Cs时标,反映了源自大气沉降的210Pb的流域输入以及湖泊水体中210Pb河流输出的共同影响。根据137Cs的1963年时标深度,把沉积岩心分为上下两段,依据210Pbex活度变化,分别采用不同公式进行沉积岩心年代计算。由此得到的巢湖与太白湖近百年来的沉积速率具有相似的变化规律,反映了长江中下游地区自然环境及人类活动对湖泊沉积记录影响的一致性。其中20世纪60年代初期,巢湖与太白湖主要入湖河流上游地区水库的建成投入使用,对入湖泥沙起到调蓄作用,使得沉积速率降低;20世纪70年代末期、80年代中期以来,巢湖与太白湖呈现较高的沉积速率,主要反映了流域农业发展等人类活动的影响;1991年与1999年洪水事件在沉积物中也有明显的表现。

137Cs AND 210Pb CHRONOLOGY FOR MODERN LAKE SEDIMENT: A CASE STUDY OF CHAOHU LAKE AND TAIBAI LAKE

The distributions of 210Pb and 137Cs in the core sediments of Chaohu Lake and Taibai Lake were determined.It took on typical peaks in 1963 for 137Cs activity in the profile,and the concentration of 210Pbex decreased exponentially along the mass depth,indicating a stable sedimentary environment.The calculated chronology according to the CIC and CRS model was discrepant with the 137Cs age (1963 AD),denoting the influence of land-derived 210Pb from the atmospheric deposition and its discharge from the lake.Therefore,the composite model should be optimal,i.e.different formulae were used for the chronology calculation according to the concentration of 210Pbex for the upper and lower sections of the sediment cores divided by the 137Cs peak depth.The sedimentation rate calculated by the composite model showed that it varied largely in Chaohu Lake and Taibai Lake in the past one hundred years.Regulation capability of the reservoir built in the upper catchment of the inflow rivers of Chaohu Lake and Taibai Lake from late 1950s to early 1960s facilitated silt deposition and reduced the sedimentation rate of the lakes.The increased sedimentation rates for Chaohu Lake and Taibai Lake since late 1970s and mid 1980s should be mainly influenced by the strengthening human activities in the catchment,mainly agriculture development,and the sedimentation rate peaks were correlated with the influence of flood in 1991 and 1998.