| 古湖相碳酸盐沉积和化石贝壳的87Sr/86Sr及其意义 |
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| 引用本文： | 张虎才,雷国良,杨明生,樊红芳.古湖相碳酸盐沉积和化石贝壳的87Sr/86Sr及其意义[J].地球学报,2005,26(Z1):237-238. |
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| 作者姓名： | 张虎才 雷国良 杨明生 樊红芳 |
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| 作者单位： | 中国科学院南京地理与湖泊研究所，江苏 南京，210008；中国西部环境教育部重点实验室 兰州大学资源环境学院，甘肃 兰州，730000;中国西部环境教育部重点实验室 兰州大学资源环境学院，甘肃 兰州，730000;中国西部环境教育部重点实验室 兰州大学资源环境学院，甘肃 兰州，730000;中国西部环境教育部重点实验室 兰州大学资源环境学院，甘肃 兰州，730000 |
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| 基金项目： | 国家自然科学基金，中国科学院"百人计划"，国家重点实验室基金 |
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| 摘 要： | 位于极端干旱区的额济纳盆地，是我国内陆地区代表性的盆地之一。这里不仅具有典型的大陆性气候、脆弱的生态系统，而且在平缓的戈壁滩下面，沉积有厚达200-400余米的古湖相地层。盆地东南与最高达近500m沙丘的巴丹吉林沙漠比邻，构成了特征性的盆地-沙漠系统，是世界少有的自然景观，也是目前研究热点之一。
考察研究工作揭示，巴丹吉林沙漠在蹑今30000a左右存在一湿润时期（Wunnemann等，1998；杨小平，2002；杨小平等，2003），并与临区腾格里沙漠相应时期发育的大湖期可以对比（Zhang等，2002，2004）。
新近的考察工作发现，在盆地东南部古日乃与巴丹吉林沙漠过渡区，存在有多级古沙丘被水夷平的、高度不等的平台，在这些平台之上，保存有水体快速蒸发环境条件下沙粒物质被钙质胶结的坚硬、类似砂岩的顶盖，其高度在大范围内可以对比，同时伴有湖相碳酸盐沉积、根管等。这些地貌、沉积与盆地北部沙砾石层中大量出现的贝壳化石和湖相地层，构成了古湖泊存在的地质证据。
14C测年结果表明，分布于第二级平台上的碳酸盐结核其形成年代为距今25000a前（25630±270a、25530±230a、25640±220a，半衰期5568a，下同），掰鳃类贝壳化石的年代为距今28000a(28530±430a、28630±320a、28780±340a、28560±280a)。
对这两组样品进行87Sr/86Sr分析结果表明（表1），碳酸盐沉积的87Sr/86Sr比值变化较为显著而贝壳化石的87Sr/86Sr比值变化相对较小，且贝壳化石的87Sr/86Sr比值的平均值小于碳酸盐沉积的87Sr/86Sr比值。
由于贝壳化石和碳酸盐沉积的87Sr/86Sr比值在一定意义上代表了古湖水的锶同位素比值，而87Sr/86Sr比值又与湖泊水体盐度相关（刘秀明等，2000），因此可以推测，贝壳化石所代表的古湖泊发育时期的水体较碳酸盐结核形成时期的盐度要低，也就是说古湖泊发育早期的为淡水，后期水体盐度有所增高。这是与贝壳化石本身代表的淡水种属相一致。由于后期湖泊水拉的快速下降和水体的快速蒸发，使得碳酸盐结核沉积的87Sr/86Sr比值增高，而这种古湖泊水位的快速下降和结核的形成是直接与气候变化相关联的。反映了该时期额济纳盆地古湖泊高湖面的存在和气候快速变化的历史。
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| 关 键 词： | 古湖泊 碳酸盐沉积 贝壳化石 年代学 |
87Sr/86Sr Ratios of Carbonate and Fossil Shells from Palaeolake Deposits and Their Environmental Impilcations |
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| ZHANG Hucai,LEI Guoliang,YANG Mingsheng and FAN Hongfang.87Sr/86Sr Ratios of Carbonate and Fossil Shells from Palaeolake Deposits and Their Environmental Impilcations[J].Acta Geoscientia Sinica,2005,26(Z1):237-238. |
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| Authors: | ZHANG Hucai LEI Guoliang YANG Mingsheng and FAN Hongfang |
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| Institution: | Nanjing Institute of Geography and Limnology, Nanjing,Jiangsu,210008; Key Laboratory of Wetern China's Enviromments, MOE, College of Earth sciences and Enviromments, Lanzhou University, Lanzhou, Gansu,730000;Key Laboratory of Wetern China's Enviromments, MOE, College of Earth sciences and Enviromments, Lanzhou University, Lanzhou, Gansu,730000;Key Laboratory of Wetern China's Enviromments, MOE, College of Earth sciences and Enviromments, Lanzhou University, Lanzhou, Gansu,730000;Key Laboratory of Wetern China's Enviromments, MOE, College of Earth sciences and Enviromments, Lanzhou University, Lanzhou, Gansu,730000 |
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| Abstract: | Located in a hyperarid area, Ejina basin is one of the typical inland basins in China. This area not only possesses a typical continental climate but also has very fragile ecosystem. Under the flat Gobi desert the lacustrine deposits are 200-400 m in thick-ness. Ejina basin connects with Badain Jaran desert where the megadunes are more than 500 m high, forming a typical basin-desert system, which not only constitutes a rarely-seen landscape in the world but also is one of the areas which arouse scientists'ateention both in china and abroad.
Field investigation reveals that there existed a humid period at about 30 ka B P, which can be correlated with the megalake deve-lopment history in neighboring Tengger desert.
Recent field work has found that there are several terraces formed by water erosion of the dune sand in the transitional area bet-ween southeastem Ejina basin and Badain Jaran desert. The upper parts of these terraces were covered by the carbonate-solidified sandy crusts. They possess nearly the same elevation and are extensively correlatable. Lake carbonate and root cast are widely dis-tributed in the area. Geomorphologic features, lake deposits and follil shells in sand-gravel layers indicate the existence of palaeolakes.
14-C ages indicate that the carbonate sampled from the second high platform was formed at ca,25 ka B P(samples were dated at 25630±270,25530±230 and 25640±220,with a radiocarbon degradation half-life of 5568 years), while the follil shells taken from the northem part of the basin were formed at ca. 28 ka B P (samples were dated at 28530±430,28630±320,28780±340 and 28560±280 yr B P).
The strontium isotope analyese (see the following table) demonstrate that 87Sr/86Sr ratios of the carbonate are much variable than those of the follil shells, and that the average value of the 87Sr/86Sr ratios of the shells in smaller than that of carbonate.
The 87Sr/86Sr ratio of the fossil shell and lake carbonate represents the 87Sr/86Sr ratio of the palaeolake water, and the 87Sr/86Sr ratio is related to the salinity of the lake water. It can therefore be deduced that the salinity of the palaeolake water woexisting with the shells is lower than that during the deposition of the carbon-ate,which implies that the palaeolake at the early stage had fresh water and the salinity increased at the late stage. This is consistent with the linving conditions of the shells that are fresh water species. The 87Sr/86Sr ratio of the carbonate became higher along with strong evaporation and quick lowering of the water levd,and these processes were directly related to the cli-mate change. Evidence shows that there existed megaplaeo-lakes and high water lake levels in Ejina basin which had experi-enced abrupt climate changes. |
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| Keywords: | Palaeolake carbonate deposit fossil shell stron-tium isotope |
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