| 泥河湾盆地有孔虫化石群壳体的地球化学及其古环境意义研究 |
| 贾玉鹤
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| 2001
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| 学位授予单位 | 中国科学院地球化学研究所
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| 学位授予地点 | 中国科学院地球化学研究所
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| 学位名称 | 博士
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| 学位专业 | 环境地球化学
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| 关键词 | 碳酸盐岩
生物壳体
~(87)Sr/~(86)Sr比值
Sr/ca比值
Mg/ca比值
泥河湾盆地小渡口剖面
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| 摘要 | 河北省泥河湾盆地第四纪地层在沉积过程中有无“海侵"，一直是地学界争论的问题之一。争论的原因是这套地层中既存在着典型的陆相化石(如介形虫、腹足类)，又有所谓的“海相性”有孔虫壳体化石。而无论海相成因论还是陆相成因论，都是从纯古生物学角度入手进行研究的。近年来的研究成果表明，纯古生物学方法很难对弱海相及其类似的沉积环境作出准确的判断。鉴于海水与陆表水之间的化学组成、尤其是Sr含量和~(87)Sr／~(86)Sr比值存在较大差异，可以利用生物壳体中的~(87)Sr／~(86)Sr比值来判别海相、过渡相及陆相沉积环境。基于以上认识，本论文选取泥河湾盆地小渡口剖面为研究对象，根据对小渡口剖面22层、27层上部、28层有孔虫、介形虫、腹足类生物壳体的微观观察，对微量元素、~(87)Sr／~(86)Sr比值及其与微量元素特征对比以及对水岩反应~(87)Sr／~(86)Sr比值的混合模式的研究，得出如下几点结论：(1)小渡口剖面的有孔虫、介形虫、腹足类壳体经埋藏后，基本上保持着原生的结构，并没经历过明显的蚀变作用。因~(87)Rb在碳酸盐类矿物中含量相当低，不需考虑~(87)Rb就位衰变对~(87)Sr的贡献。故所测定的小渡口剖面有孔虫、介形虫、腹足类骨骸矿物的~(87)Sr／~(86)Sr比值就代表矿物相形成时宿生水体的~(87)Sr／~(86)Sr比值。(2)海水-陆表水~(87)Sr／~(86)Sr-盐度混合模式计算图示及混合水体~(87)Sr／~(86)Sr-Sr／Ca图表明泥河湾盆地第四系更新世有孔虫宿生水体没有海水的参入，为一纯陆相环境。有孔虫的大量出现仅与当时干旱气候条件下，泥河湾湖水水体达到适合有孔虫大量生存的理化性质有关：从锶同位素特征来看，有孔虫宿生水体与现代桑干河具有较强的可比性。(3)有孔虫壳体~(87)Sr／~(86)Sr比值为0.71190—0.712018，明显高于同时期海水的~(87)Sr／~(86)Sr比值0.709807—0.709147，也略高于现代黄河水的平均值O.7111，说明小渡口剖面有孔虫生活时的水体为内陆湖泊。(4)泥河湾盆地小渡口剖面有孔虫、介形虫、腹足类壳体~(87)Sr／~(86)Sr、Sr／Ca、Mg／Ca比值的变化、可以作为提取古气候、风化作用信息的工具。(5)综合的地质、地球化学、古生物学、古地磁研究表明：泥河湾古湖泊在有孔虫出现期问(大约1.OMa)发生了一次大的环境转变事件。湖泊因强烈蒸发，湖水浓缩，出现了区域性的、与海水性质相似的、咸水或半咸水湖水环境，为有孔虫的生存提供了合适的环境。小渡口剖面有孔虫化石群属于内陆非海相有孔虫。 |
| 其他摘要 | It has been a contestable question in geological field for a long time that whether there had been transgression within depositional process of Quaternary strata in the Nihewan Basin, Hebei province. The reason is that both continental fossils such as ostracods and gasteropod and the so-called marine fossils such as foraminifera existed together in the same strata, and both of the viewpoints for sea-formation or continent-formation based on paleontology only. Recently study shows that it is difficult for paleontological method alone to do precise judgement of weakly marine or similar depositional environment. Because of the significant differences in chemical composition, particularly in Sr contents and ~(87)Sr/~(86)Sr ratios between seawater and lacustrine water, ~(87)Sr/~(86)Sr ratios of biogenic shells may be the way out to judge seawater, lacustrine water and transitional environment. Based on the viewpoint above, as exemplified by Xiaodukou section, Nihewan basin.through the micro-observation to biogenic shells of foraminifera 、 ostracods and gasteropod in 22th bed、 the upper part of 27th bed and 28th bed, and the contrast of Mg/Ca、 Sr/Ca and ~(87)Sr/~(86)Sr ratios and the study of mixed model of ~(87)Sr/~(86)Sr ratios in water-rock reaction, some conclusions can be acquired as following. (1): It is clear that Xiaodukou foraminifera、 ostracods and gasteropod had basically preserved the original structure, suggesting that it did not undergo apparent chemical alteration once it was buried. The contribution of the in-situ decay of ~(87)Rb to the concentration of ~(87)Sr could be neglected due to its low concentration in the calcite minerals. So, ~(87)Sr/~(86)Sr ratios measured for Xiaodukou biogenic shells skeleton mineral represent ~(87)Sr/~(86)Sr ratios of contemporaneous water where it was deposited. (2): The ~(87)Sr/~(86)Sr-salinity model figure of seawater mixture with continental water and the ~(87)Sr/~(86)Sr-Sr/Ca Figure of mixed water body reveal that the inhabitant water body for Quaternary foraminifera in Nihewan basin has nothing to do with seawater, and it is a typical continental environment. If the chemical condition of Nihewan lake evolved to that of the sea under arid weather condition, the appearance and development of foraminifera would be possible. The foraminifera lived water body had considerably comparative property with modern Sanggan river from strontium isotope character. (3): ~(87)Sr/~(86)Sr ratios of foraminiferal shells are with the range of 0.71190-0.712018, significantly higher than the values of contemporary seawater: 0.709087-0.709147,also higher than the average valve of modern Yellow River: 0.7111.This reveals the water body where Xiaodukou foraminifara lived is a interior lake. (4): The variational tendency of ~(87)Sr/~(86)Sr、 Mg/Ca、 Sr/Ca ratios for biogenic shells in Xiaodukou section, Nihewan basin will be the tool for obtaining message of palaeoclimate and weathering. (5): The integrated study of geological, geochemical, paleontological and palaeomagnetic mechods reveal that at the time when foraminifera lived (about 1.0Ma), a large event of environment transformation had occurred. Because of the significant evaporation and concentration of lacustrine water, the regional salt-water or brackish-water lacustrine environment which is similar to sea-water environment had been produced, and such a lacustrine environment is suitable for the inhabitance of foraminifera. So, it is reasonable to conclude that Xiaodukou foraminiferal fossil assemblages belong to non-marine foraminiferal species. |
| 页数 | 65
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| 语种 | 中文
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| 文献类型 | 学位论文
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| 条目标识符 | http://ir.gyig.ac.cn/handle/352002/3588
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| 专题 | 研究生_研究生_学位论文
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推荐引用方式
GB/T 7714 |
贾玉鹤. 泥河湾盆地有孔虫化石群壳体的地球化学及其古环境意义研究[D]. 中国科学院地球化学研究所. 中国科学院地球化学研究所,2001.
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