| 摘要: |
| 137C湖泊沉积年代学方法是测定现代湖泊沉积物沉积年代和沉积速率的重要同位素年代学方法之一。对加拿大Crawford湖采集的沉积孔柱内137Cs垂直分布的研究发现,该方法给出时的标有明显偏差。比较210pb和纹层等年代方法的结果,其1963年时标蓄积峰值所在位置明显移向表层,而作为1954年时标的该核素出现蓄积的层位则远早于该年沉积物蓄积层位。研究还表明,孔柱中较高的间隙水含量、缺少足以吸附沉降137Cs的粘土矿物和较高有机质含量是导致137Cs沉积后迁移并引起其时标年代偏差的主要原因,而同时这类湖泊中210Pb方法给出的结果则较为可靠。 |
| 关键词: 137Cs时标 湖泊沉积年代 Crawford湖 |
| DOI：10.18307/1995.0403 |
| 分类号: |
| 基金项目:国家自然科学基金资助项目(编号:49401011) |
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| Limitations of the application of 137Cs limnochronology:a case study of 137Cs profile in crawford lake sediment |
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Xiang Liang
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Lake Sediment and Baviroament Lab., Nanjing Institute of Geography & Limnology, CAS, Nanjing 210008
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| Abstract: |
| Over the last three decades, artificial radionuclide, 137Cs, has been introduced into the paleolimnologicai study for interpreting the geochronology of recent sedimentary deposits. The main source of 137Cs was fallout from the atmospheric testing of nuclear weapons. Where this isotope is strongly adsorbed onto sediments i the variation of its activity with depth in a core should reflect the fallout history, in which the onset of fallout in 1954 and peak fallout in 1963 provide two distinct chronological markers. However, in the lake sediment core which was studied in this paper, the two time markers deviate greatly from expectations based on 210Pb and varve chronology. 137Cs is distributed more deeply in the sediment that has been dated decades older than the beginning of 137Cs fallout and the profile of it has its maxima nearer to the sediment surface than expectations. In the lake there is no evidence of physical mixing, disturbance and time lag in the sediment of catchment material. We argue that the high interstitial water content of these sediments provides a possibility of 137Cs redistribution in cores by molecular diffusion and could transport it downward. So, the occurrence of 137Cs at pre-fallout sediment depths is due mostly to net downward molecular diffusion and scarcity of clay minerals for adsorption. On the other hand, the presence of high organic matter content suggests some redistribution of the isotope within the sediments and will tend to enrich the upper sediment layers. Because more of this nuclide is accumulated in organic matter than in inorganic mineralsi the mobility of this nuclide in the sediment may begin with release of it into interstitial water by decomposition of organic matter. Thus more 137Cs which has been released by organic decomposition would tend to be re-adsorbed by upper part sediments which have high organic matter content than lower part sediments and the peak of the 137Cs profile would remove from lower sediment layer to the upper one. The 137Cs profile in the lake core is believed to be the result of a combination of diffusion, biological concentration from sediment. Under these circumstances, 137Cs dating is not useful, but 210Pb dating can be valid because it differs from 137Cs in physical and chemical characteristics. |
| Key words: 137Cs dating Limnochronology Crawford Lake |
