| 古湖相碳酸盐沉积和化石贝壳的87Sr/86Sr及其意义 |
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| 关键词:Palaeolake carbonate deposit fossil shell stron-tium isotope |
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| 87Sr/86Sr Ratios of Carbonate and Fossil Shells from Palaeolake Deposits and Their Environmental Impilcations |
| 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. |
| ZHANG Hucai,LEI Guoliang,YANG Mingsheng,FAN Hongfang.2005.87Sr/86Sr Ratios of Carbonate and Fossil Shells from Palaeolake Deposits and Their Environmental Impilcations[J].Acta Geoscientica Sinica,26(z1):237-238. |
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