| 柴达木盆地察尔汗贝壳堤剖面年代学研究* |
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| 引用本文： | 张虎才,雷国良,常凤琴,樊红芳,杨明生,张文翔.柴达木盆地察尔汗贝壳堤剖面年代学研究*[J].第四纪研究,2007,27(4):511-521. |
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| 作者姓名： | 张虎才 雷国良 常凤琴 樊红芳 杨明生 张文翔 |
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| 作者单位： | 1. 中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室,南京,210008;兰州大学资源环境学院西部环境教育部重点实验室,兰州,730000
2. 兰州大学资源环境学院西部环境教育部重点实验室,兰州,730000 |
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| 基金项目： | 中国科学院"百人计划"
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国家自然科学基金 |
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| 摘 要： | 利用常规14 C、加速器(AMS)和230 Th等测年方法,对位于青藏高原东北部柴达木盆地察尔汗古湖贝壳堤剖面化石贝壳、湖泊沉积(包括现代沉积)样品有机质(包括碱性残留和胡敏酸组分)、碳酸盐(CaCO3)和剖面顶部石盐(NaCl)晶体在不同实验室进行了系统的实验研究,探讨了不同样品测年结果的可靠性和精确性,并通过与已有研究结果的对比,对柴达木盆地贝壳堤剖面记录的察尔汗古湖高湖面演化历史进行了探讨,得出察尔汗古湖高湖面形成于约39.7~17.5 14 C kaB.P. ; 研究证明柴达木盆地沉积物中有机质含量很低,且主要来源于菌藻类等低等藻类和微生物,受老碳和溶解物质影响强烈,因此必须经过多种方法综合测年才能比较可靠地确定所测样品的形成年代; 测年结果的不确定性不仅来源于样品自身元素(同位素)的特性和沉积过程中再沉积作用的影响,也可能源自于放射性14 C产生过程的不稳定性; 对比发现贝壳化石老于同层位有机质14 C年龄15~18ka,这可能与柴达木盆地所处的特殊地理位置和巨大的古湖泊面积及水体有关; 同时,不平衡铀系测年在确定开放体系矿物晶体年代应用中的可靠性尚须进一步深入分析,其机理也还需要探讨和实验研究。
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| 关 键 词： | 柴达木盆地 贝壳堤 常规及AMS测年 |
| 文章编号： | 1001-7410（2007）04-511-11 |
| 收稿时间： | 2007-04-20 |
| 修稿时间： | 2007-04-202007-05-16 |
AGE DETERMINATION OF SHELL BAR SECTION IN SALT LAKE QARHAN, QAIDAM BASIN |
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| Zhang Hucai,Lei Guoliang,Chang Fengqin,Fan Hongfang,Yang Mingsheng,Zhang Wenxiang.AGE DETERMINATION OF SHELL BAR SECTION IN SALT LAKE QARHAN, QAIDAM BASIN[J].Quaternary Sciences,2007,27(4):511-521. |
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| Authors: | Zhang Hucai Lei Guoliang Chang Fengqin Fan Hongfang Yang Mingsheng Zhang Wenxiang |
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| Institution: | Key Laboratory of Lake Sedimentation of Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing210008|Key Laboratory of Western Chinas Environments Systems (Ministry of Education), College of Earth and Environments Sciences, Lanzhou University, Lanzhou730000 |
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| Abstract: | Shell Bar is one of the most prominent features in the Qaidam Basin. It is composed of abundant fossil shells of Corbicula fluminea Müller and Corbicula largillierti Philippi and located at the southeastern end (36°30'N,96°12'E) of the paleolake Qarhan. This bar is the highest site that fossil shells have been reported in the Late Pleistocene age.A 2.6-m-thick fresh profile, plus a 10-cm-thick overlying crystallized salt (95 ％ NaCl) layer, is manually excavated to address its forming ages and scope of the high paleolake levels. Accelerator mass spectrometry (AMS), conventional radiocarbon dating, and sector inductively coupled plasma mass spectroscopy 230 Th methods were used to investigate the reliability and accuracy of dating results in different laboratories, such as Lanzhou University in China, Hannover, Kiel in Germany and University of Minnesota in USA. Ages of various components (e.g. acid residual and humid acid of the organic matter) from the same sample were determined. Age differences of a variety of materials (e.g. organic matter, fossil shells, and salt crystals), and age differences at the same sampling position were evaluated.As the content of the organic matter, which was originated from one or two kinds of thermophilous bacteria and/or algae is low, and in influenced by the reworked organic matters from the old deposits and younger ones from the movable under ground waters, multi-methods dating on various materials can provide reliable and accurate ages. It seems that the AMS ages given by alkali residues and humic acids vary separately, e.g. , from 124.5cm upwards; the dating results by humic acids were normal. But moving downward, the dating results of humic acids showed a reversed pattern and those of alkali residues, especially in the lower part of the section, showed an unstable pattern. These phenomena imply that organic matter had been influenced by two separate processes. Possible explanations are that for the alkali residual fraction, it most likely had been contaminated by dead carbon-bearing reworked material because the study section was located near the edge of the paleolake and could be easily influenced by eroded old deposits. Nevertheless, the humic acid fraction could have been contaminated by upward-flowing groundwater moving along with transported soluble organic matter. The amount of this soluble organic material should be very small because the strongest age reversal was in the lower part, where the TOC content remained low, indicating that the change of soluble organic fraction alters neither TOC content nor the δ13 C of it dramatically.It was concluded that a uniform mega-paleolake was developed in the Qaidam Basin, Northeastern Tibetan Plateau between 39.7 and 17.5 14 C kaB.P. During the period high paleolake level of Qarhan was formed, with dramatic lake level fluctuations and the huge palaolake covered a vast area. It was found out that the ages determined may be influenced by either radioactive 14 C various of the reworks of the deposition processes or the content variations in the atmosphere. The ages given by fossil shells are 15~18ka older than that given by organic matter, this large difference might be resulted from the large water area and huge water volume, and the special location of the high-elevated Qaidam Basin. This study also shows that the reliability and accuracy of the 230 Th dating method on the salt crystals is doubtable. Whether it could be applied to the study area needs further confirmation. |
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| Keywords: | Qaidam Basin Shell Bar section traditional and AMS datings |
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