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沉积物年代及沉积速率的确定是利用沉积记录信息解译河口沉积环境和流域环境变化的基础。百年尺度的测年以往主要是借助历史资料、海图对比或地质学的观测资料推算,不仅费时费力,而且资料不全,实现起来也较为困难。随着210Pb和137Cs测年技术的不断成熟可以快速获取相关数据,在精度和时间尺度上有无可比拟的优越性,使测年及沉积速率工作趋于定量化、精确化。在河口及海洋研究领域,1970年代初期Koide等利用210Pb测定了海洋沉积速率[1],在1970年代末期Delaune等利用137Cs测定了海岸盐沼沉积速率[2]。此后,210Pb和137Cs测年在沿海冲积平原、入海河口、潮间带、浅海区等不同海岸沉积环境中得到了广泛的应用[3-6]。
环境介质中的137Cs完全是人为产生的,在自然环境中没有其他来源。137Cs或来自于大气核实验,或来自于核事故,输入函数简单清楚。沉积物中137Cs的变化能对应大气中137Cs的沉积通量,并记录在相应的沉积层位上,构成了137Cs计年的基础。1952—1958年,美国在太平洋试验场进行了大量的大气核试验,沉积物中137Cs的最大检测深度是1954年,是137Cs的起始计年时标[7]。1961—1962年,前苏联在北冰洋及西伯利亚地区进行了大量的大气核试验,其大气核试验当量占全球大气核试验当量的75%。1963年8月,美、苏、英签订《部分禁止核试验条约》,结束了大规模的大气核试验[8]。沉积物中137Cs的最大沉降量对应1963年,是得到公认和广泛应用的主计年时标。1975年前后中国进行了22次核试验,占全球大气核试验当量的4.7%[9],亚洲部分地区的沉积记录中存在的1975年蓄积峰,可作为辅助计年时标[9]。1986年前苏联切尔诺贝利核电站发生了重大核事故,又向大气环境中释放了42.5 TBq的137Cs[3],也在北半球的很多地区留下了沉积记录[9],成为最后一个计年时标[3]。研究人员对于1954年和1963年的计年时标普遍没有异议,对于中国是否存在1975年和1986年的计年时标存有争议[10]。
河口沉积记录中的137Cs来源有大气沉降和水平搬运2种方式:137Cs大气沉降量世界各地有所不同,但反映出来的137Cs特征峰型基本一致;水平搬运137Cs的沉积过程相对复杂,尤其是河口受到河流供给变化、潮位涨落、风暴潮等极端事件的影响,使实测沉积剖面中的137Cs部分蓄积峰的解译有一定的困难和不确定性。因此,通过分析137Cs的沉积记录,建立河口区域接近大气沉降的(理想)137Cs“参考”剖面十分必要。王福利用渤海湾海岸带地区6个实测的137Cs沉积剖面,建立了渤海湾海岸带地区137Cs的典型(参考)剖面,并对典型(参考)剖面意义、方法及初步结果进行了讨论[11]。然而河口与海岸带沉积环境的差异显著,137Cs“参考”剖面是区域性的。特别需要说明的是,由于137Cs的半衰期很短,随着时间的推移,沉积物中的137Cs信息已经变得越来越弱。
鸭绿江口及其邻近海域,以前不仅没有137Cs“参考”剖面,甚至连137Cs的测年数据都很少。李军等收集了1980年代以来发表的中国东部海域泥质沉积区359组137Cs的测年数据绘制了站位分布图,北黄海区域是一片空白[12]。因此,补充鸭绿江河口及邻近海域的137Cs的沉积记录,探讨鸭绿江口接近大气沉降的137Cs“参考”剖面,不仅对以后实测的137Cs曲线的解译起指导作用,而且对以后鸭绿江口及其邻近海域的各类沉积记录研究也至关重要。这里137Cs沉积记录的研究能够为鸭绿江口及邻近海域,甚至北黄海海域的海洋地质、地球化学、海洋环境的研究提供可信赖的年代框架。
鸭绿江口137Cs的沉积记录及其意义
Deposition records and the significance of 137Cs in Yalu River estuary
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摘要: 根据2013、2014、2018年鸭绿江口16个柱状样的137Cs沉积记录,分析了其垂向分布特征,划分了137Cs 的4种沉积类型。根据柱状样137Cs蓄积量较高、沉积连续、沉积过程清楚、最大蓄积峰对应1963年的特征初步确定了适合河口不同沉积状态的4个“参考”剖面,并与理想状态的大气沉降曲线进行了对比分析,显示鸭绿江口参考剖面的137Cs沉积记录与大气沉降量(输入函数)曲线基本吻合。1963年的主计年时标涵盖大部分柱状样,层位清楚、容易识别、可以信赖。鸭绿江口存在疑似1975年和1986年137Cs的计年时标,但用于计年时需谨慎。1986年以后大多数沉积记录显示了137Cs沉积迅速降低的趋势,其他少部分柱状样的137Cs蓄积峰是动力过程混合或扩散作用的结果,也不排除陆源输送。表层沉积物大多数站位检测不出137Cs,说明近期陆源137Cs的输送量微乎其微,个别站位表层中较多的137Cs来源有待进一步研究。Abstract: Based on the 137Cs deposition records of 16 cores collected in Yalu River estuary, their characteristics of the vertical distribution were analyzed, and 4 sedimentary types of 137Cs were found out. 4 reference sections available for different depositional states in the estuary were preliminarily determined, according to the 137Cs sedimentary characteristics including the high accumulation, the continuous deposition, the definite deposition process and the maximum accumulation peak corresponding to 1963. Then these reference sections were compared with the ideal atmospheric deposition curve. The results indicated that the 137Cs depositional records of the reference section in Yalu River estuary were in almost agreement with the atmospheric settlement (input function) curve. The 1963 key dating markers were reliable, because they were easily found in the most cores with the clear location. The suspected 137Cs dating markers of 1975 and 1986 existed in the deposit records in the Yalu River estuary, but they needed to be used cautiously when dating. The most deposition records after 1986 showed a rapidly decreasing trend of 137Cs specific activity, while the few 137Cs accumulation peaks in the others after 1986 might be the result of the perturbation, mixing or diffusion in the dynamic process, but the land-source transport could not be ruled out. No 137Cs in the surface sediments in Yalu River estuary was detected in most sampling points. It indicated that little 137Cs was transported by the river dynamics. The source of 137Cs in the surface sediments from the particular sampling points needs further studied.
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Key words:
- 137Cs /
- sedimentary record /
- reference section /
- Yalu River estuary
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表 1 鸭绿江口柱状样特征及底部137Cs蓄积峰信息
Table 1. The samples’ characteristics and the 137Cs spikes at the bottom of the cores in the Yalu River estuary
站位
Station采样地貌部位
Platform at
the location采样深度/cm
Depth基底特征
Characteristics
of the bottom137Cs曲线形态
Curve shape of
137Cs底部137Cs
蓄积峰深度/cm
Depth of 137Cs spikes
at the bottom底部137Cs
蓄积峰年份
Age of 137Cs spikes at
the bottom柱底推测年份/a
Inferred age at
the bottomA1 河口上口 140 细砂硬底 不连续 110 1963 1960 C3 绸缎岛西侧边滩 74 细砂硬底 连续 72 1963 1960 C5 绸缎岛西侧边滩 132 未见底 基本连续 98 1954 1935 C4 中水道浅滩 76 细砂硬底 不连续 — — 1960 K5 中水道江心岛边滩 69 细砂硬底 不连续 67 1963 1960 K6 中水道江心岛边滩 110 细砂硬底 不连续 104 1963 1960 C2 中水道江心岛边滩 162 未见底 基本连续 156 1954 1950 K4 中水道江心岛边滩 98 未见底 连续 76 1954 1937 A4 中、东水道分隔滩 160 未见底 基本连续 84 1963 1935 K12 中、东水道分隔滩 108 细砂硬底 不连续 96 1963 1960 K11 中、东水道分隔滩 86 细砂硬底 不连续 82 1963 1960 K10 中、东水道分隔滩 184 未见底 连续 172 1954 1951 K2 三角洲前缘 104 细砂硬底 不连续 98 1963 1960 K1 三角洲前缘 108 细砂硬底 不连续 82 1963 1960 K9 三角洲前缘 152 未见底 不连续 80 1963 1916 K8 三角洲前缘 116 细砂硬底 不连续 104 1963 1960 -
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