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| 中国北方中新世风尘物质来源及古气候的地球化学指示 | |
| 其他题名 | Geochemical characteristics of the Miocene eolian deposits in Northern China: their provenance and climate implications |
| 梁美艳 | |
| 出版年 | 2009 |
| 学位类型 | 博士 |
| 导师 | 郭正堂 |
| 学位授予单位 | 中国科学院地质与地球物理研究所 |
| 中文摘要 | 新生代亚洲地区最显著的地质－环境事件是青藏高原的阶段性隆升、亚洲内陆干旱化、季风环境的形成演化以及由此引起的中国南方湿润区和西北干旱区的环境分异。我国北方中新世以来的风尘堆积是探讨上述环境事件的重要载体。目前为止上新世和第四纪风尘物质的地球化学特征已有较深入的研究，但中新世的研究较少。\n 本文对秦安（22.0～6.2 Ma）和西峰剖面（3.5 Ma以来）风尘堆积古土壤样品进行了常量、微量、稀土、Sr-Nd同位素分析以及元素的粒度效应分析，并结合以前的黄土分析结果，对22 Ma以来风尘堆积的地球化学特征、物质来源及其指示的古环境进行了讨论。主要获得了如下认识：\n1.\tSi、Na、Zr、Sr等元素含量随粒度的增大而增大；Ti、Nb元素含量随粒级增大先增加后减少（在2-8 μm粒级中富集）；Al、Fe、Mg、K、Mn、Rb、Cu、Ga、Zn、V、Cr、Ni等元素和LOI含量随粒级增大而减少；元素Y与粒级变化相关关系不明显。在此基础上发现：已有的风尘物质地球化学替代指标如CIA、Fe2O3/Al2O3等多数受粒度分异影响，因此我们引入了新的地球化学替代指标：K2O/Al2O3比值为不受粒级分异影响的指示风尘物质物源变化的指标；SiO2/Al2O3分子比值是目前为止最为理想的反映风尘原始粒度的替代指标，与前人的研究结果一致，可以作为指示冬季风变化的指标应用于中新世风尘堆积的研究中；对处于大陆风化早期去Ca、Na阶段的沉积物，VR值(Vogt ratio=(Al2O3+K2O)/(MgO+CaO+Na2O)，分子比)能较好地消除风力分选作用的影响，相较于前人建立的同类指标（如(CaO+MgO+Na2O)/TiO2），可应用于更大的时间范围。\n2.\t通过对比已有沙漠数据及L1以来风尘物质Sr、Nd同位素和本次结果，辅以稳定微量元素La、Th、Zr、Sc等的构造背景判别，发现塔克拉玛干沙漠不是黄土高原粉尘物质的重要贡献区。腾格里、巴丹吉林和柴达木沙漠有可能是黄土高原L1以来风尘物质的主要物源区。中新世以来风尘物质的常量、微量、稀土、Sr、Nd同位素等地球化学组成接近且与L1相似，表明22 Ma以来的风尘物质物源特征相似，物质组成未发生明显变化。\n3.\t中新世时期较上新世和第四纪时的古土壤风化更为强烈，指示当时的气候条件相对于第四纪更为温暖、湿润，但其处在脱Ca去Na的初级风化阶段，指示了秦安地区中新世时已经处于半干旱-半湿润的气候条件下，与前人古地理学的研究结果一致。 |
| 英文摘要 | The most prominent tectonic and environmental events during the Cenozoic in Asia are the uplift of the Himalaya-Tibetan plateau, aridification in the Asian interior, and onset of the Asian monsoons. These caused more humid conditions in southeastern China and the formation of inland deserts in northwestern China. The 22 Ma eolian deposits in northern China provide an excellent terrestrial record relative to the above environmental events. Up to date, many studies have focused on the geochemical characters of the late Mio-Pleistocene eolian deposits, however, the geochemical characteristics of the Miocene loess and soils is still much less known.\nIn this study, the elemental and Sr-Nd isotopic compositions of the eolian deposits from the Qinan (from 22.0 to 6.2 Ma) and the Xifeng (from 3.5 Ma until now) loess-soil sections were analyzed to examine the grain size effects on the element concentrations and the implications about the dust origin and climate. The main results are as follows: \n1. The contents of Si, Na, Zr and Sr are higher in the coarser fractions while Ti and Nb have the highest contents in the 2-8 μm fractions. Al, Fe, Mg, K, Mn, Rb, Cu, Ga, Zn, V, Cr, Ni, LOI have clear relationships with grain-size, more abundant in the fine fraction while non significant relationship is observed for Y. Based on these features, we suggest that K2O/Al2O3 ratio can be used to address the dust provenance, and that VR (Vogt ratio = (Al2O3+K2O)/(MgO+CaO+Na2O)) can be used as a chemical weathering proxy for the Miocene eolian deposits because of their relative independence on the grain size. Meanwhile, SiO2/Al2O3 molar ratio is a best geochemical indicator of original eolian grain size, as suggested in earlier studies.\n2. Analyses on the Sr and Nd isotope composition of the last glacial loess samples (L1) and comparison with the data from the deserts in northern China suggest that that Taklimakan desert is unlikely to be the main source region of the eolian dust. In contrast, these data suggest greater contributions of the Tengger, Badain Jaran and Qaidam deserts to the eolian dust during the last glacial cycle. Since the geochemical compositions (major, trace, REE and Sr, Nd isotope) of loess samples for the past 22 Ma are broadly similar with the samples from L1, these data trend to suggest relatively stable and insignificant changes of dust sources over the past 22 Ma. \n3. Chemical weathering is stronger for Miocene paleosol samples than for the Plio-Pleistocene ones, showing warmer/more humid climatic conditions with a stronger summer monsoon in the Miocene. However, chemical weathering is typical of Ca-Na removal stage, suggesting a climate range from semiarid to subhumid conditions. These support the notion about the formation of a semi-arid to semi-humid monsoonal regime by the early Miocene, as is consistent with earlier studies. |
| 中文关键词 | 风尘堆积 ; 地球化学 ; 粉尘源区 ; 古气候 ; 季风 |
| 英文关键词 | eolian deposits geochemistry dust provenance paleoclimate monsoon |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 第四纪地质学 |
| 来源机构 | 中国科学院地质与地球物理研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/286746 |
| 推荐引用方式 GB/T 7714 | 梁美艳. 中国北方中新世风尘物质来源及古气候的地球化学指示[D]. 中国科学院地质与地球物理研究所,2009. |
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