独流减河盐沼^(210)Pb_(exc)和^(137)Cs剖面记录的现代洪水事件沉积

Deposition of modern flooding events indicated by ^(210)Pb_(exc) and ^(137)Cs records from salt marsh of the Duliujian River,Tianjin

在独流减河入海口附近河道之间的盐沼采用人工探坑侧壁连续取样,获得2个站位的沉积物样品。通过^(210)Pb和^(137)Cs定年法建立年代框架,结合粒度分析、有孔虫鉴定和水文数据,探讨了该区的现代洪水事件沉积。结果表明,该区沉积物的^(210)Pb_(exc)比活度-深度剖面中存在2处明显的低值沉积层,分别在3~6cm和10~14cm深度,这2个沉积层分别对应^(137) Cs曲线的次峰和主峰。这两层与上下相邻层位相比呈现粒径较粗、分选较差,有孔虫丰度较低、破损百分比较高的特征。^(210) Pb测年结果显示,^(137)Cs曲线下部的主峰对应的年龄为1963年,与全球性的最大峰值形成时间相对应,而上部的次峰对应的并非1986年切尔诺贝利核泄漏事故形成的次峰。区内水文数据显示,在1963和1996年分别发生了2次较大的洪水事件。因此,推断S4和S5站位剖面中2个特殊的沉积层是由洪水事件造成的。本研究说明高分辨率沉积特征的多指标研究可以较为准确地揭示洪水等灾害性事件发生的年代和频率,这有利于系统地揭露海陆过渡带地区地质历史时期灾害和气候事件的频率和强度,为灾害预防和未来规划提供科学依据。

Sediment samples were collected from two sites in the artificial channel salt marsh of the Duliujian River,through continuous sampling along the side wall of artificial dig pits.The chronology was developed using ^210 Pb and ^137 Cs dating methods,and grain size,foraminifera and hydrologic data were used to study the deposition of modern flooding events.The results show that ^210 Pbexc activities are obviously lower in 3～6cm and 10～14cm layers,corresponding to the main peak and minor peak of ^137 Cs.In addition,comparing with the adjacent layers,these two layers show coarse-grained,poorly sorted,low foraminiferal abundance and high foraminiferal brokenness percentage.The results of ^210 Pb dating show that the main peaks of ^137 Cs was1963,corresponding to the global ^137 Cs fallout well;however,the minor peaks was not 1986,so it was not caused by the accident of the Chernobyl Reactor.The hydrological data show that there were two flood events in 1963 and 1996in this area.Therefore,it can be concluded that these two special layers of S4 and S5sites were caused by the flood events.It is indicated that multi indexes can help to reconstruct flood deposition,which also can help to find disasters and climate events in geological periods,and to provide scientific data for disaster prevention and future plan.