VARVECHRONOLOGY AND RADIOMETRIC DATING (137Cs, 210Pb) FROM THE SIHAILONGWAN MAAR,NORTHEASTERN CHINA
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摘要: 对东北四海龙湾玛珥湖SHLF6孔纹层沉积物的137Cs放射性测量表明:137Cs比活度的最大值出现在55cm处,对应于1963年世界原子弹试爆高峰期。纹层计年表明0～6cm共有35个纹层层偶。从75cm到65cm,137Cs比活度从256±009dpm/g急剧增加到1868±017dpm/g,可能65cm对应于1954年。通过测量226Ra子核214Pb和214Bi(能量为295keV,352keV和609keV)放射的光子数获得226Ra比活度数据,然后求得过剩210Pb比活度(210Pbuns)。210Pbuns比活度随深度增加而呈指数衰减,其异常波动可能与人类活动以及沉积速率变化有关,例如55cm处210Pb比活度较高,与137Cs的峰值对应,这可能与1963年前后人工核实验的高峰有关,因为核试验不仅产生137Cs,而且可以产生208Pb和210Pb;45cm处210Pb比活度较低,而226Ra较高,可能与人类活动加剧,导致沉积速率增加有关。根据210PbunsCRS模式,SHLF6孔0～19cm的平均沉积速率为20mg/cm2·a,或约为011cm/a。210Pb测年数据与137Cs时标及纹层计年均有很好的一致性。四海龙湾玛珥湖发育的纹层为年纹层,可以建立高分辨率时间序列。
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关键词:
- 四海龙湾玛珥湖 /
- 年纹层 /
- 137Cs、210Pb测年
Abstract: Sediments from maar lakes in the Longgang volcanic filed have been demonstrated to contain high-resolution nature archives for paleoclimatic and paleoenvironmental research. In 1998, freezing cores were recovered using a freeze-coring system by scientists from our institute and Geo-science Center in Germany. A frozen core (SHL-F6) was sawed into sediment slabs, freezing-dried, and then impregnated with epoxy resin and manufactured as thin sections. Based on the study of thin sections from core SHL-F6 under microscope, varve couple appears as dark-coloured layer and light-coloured layer. The dark-coloured layer mainly consists of valves of diatoms and organic matter, and with less variation in thickness. The light-coloured layer consists of clastic and organic matter, and with large variation in thickness. The dark-coloured layer is an important mark to identify annually interface. Radiometric dating shows that the highest 137Cs activity appears at 5.5cm depth of core SHL-F6, assumed to correlate to the 1963 maximum of emission due to the nuclear bomb testing. Base on varve counting, 35 couples were found in the upper part of the core (0～6cm). It well corresponds with the highest 137Cs activity at 5.5cm. 137Cs activities increase quickly from 2.56±0.09dpm/g at 7.5cm to 18.68±0.17dpm/g at 6.5cm, and may correlate to the 1954. The short-lived 226Ra activity was measured from daughter nuclides 137Cs and 214Bi (295 keV, 352keV and 609keV). Unsupported 210Pb activity exponentially attenuated as the increasing of the core depth. The abnormal variations of 210Pb activity can be found in the profile. The relative high value of 210Pb at the depth of 5.5cm may be also linked with the 1963 maximum due to the nuclear bomb testing for nuclear bomb testing producing both 137Cs and 208Pb and 210Pb. The relative low value of 210Pb and relative high value of 226Ra at the depth of 4.5cm may due to human activity that induced high sediment input. 210Pb (CRS) and 137Cs dating result show a good agreement with the varve-chronology. Varved sediments in the Sihailongwan maar are annually laminated and high-resolution records for paleoclimatic and paleoenvironmental research.-
Key words:
- the Sihailongwan maar /
- varve /
- 137Cs and 210Pb dating
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