Knowledge Resource Center for Ecological Environment in Arid Area
| 西秦岭山间晚中新世-上新世风尘堆积的磁性地层及其古环境意义 | |
| 葛俊逸 | |
| 出版年 | 2010 |
| 学位类型 | 博士 |
| 导师 | 郭正堂 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 秦岭是我国黄土高原的南界，也是我国重要的南北地理、气候界线，其形成、发育与青藏高原隆升关系密切。保存于秦岭山间的风尘堆积同时记录着大气环流的变化和秦岭的构造活动历史，对于研究该区构造和气候的演化具有重要意义。\n本文对西秦岭山间的新近纪风尘沉积进行调查，并选取西和剖面进行了磁性地层、沉积学、地球化学等研究, 本研究得出如下认识：\n1. 西和剖面厚150.2 m，上部0~65.2 m和下部98.2~150.2 m为典型的黄土-古土壤序列，而中部65.2~98.2 m间，受坡面侵蚀的影响，形成含砾石碎屑的黄土/古土壤层。未受改造的部分粒度组成高度均一，中值粒径集中于5~7.5 μm。常量、微量元素组成、稀土元素分布模式及相对稳定的元素比值与第四纪黄土、三趾马红土及秦安中新世红土大致相同，指示了它们是典型的风成沉积。详细的磁性地层研究和小哺乳动物化石确定西和剖面的年代为4.1~10.4 Ma，其中受坡面侵蚀影响的剖面中部的年代为6.9~8.3 Ma。这表明我国北方风尘堆积南界至少在10.4 Ma前后已达到长江流域，进一步证明了中新世风尘堆积广泛分布的特点。\n2. 西和风尘堆积剖面揭示出，西秦岭至少在~10.4 Ma已经隆升并形成类似今天的盆-山结构。8.3~6.9 Ma坡地流水作用的层段指示了该区构造的不稳定性，可能与青藏高原的隆升过程有关。3.5 Ma之后，该区发生明显的侵蚀，可能与这一时期的青藏高原隆升和全球变冷过程均有密切关系。\n西和剖面与董湾、秦安剖面同时期地层的粒度对比显示，新近纪风尘堆积具有南细北粗的特点，进一步揭示出粉尘主要由偏北向环流（冬季风）自西北干旱区搬运而来。磁化率、土壤红度值及游离铁含量变化显示出，东亚冬、夏季风在长时间尺度上基本具有互为消长的演化特征，在10.4~4.1Ma内其演化经历了5个阶段：10.4~9.0 Ma，夏季风强度较低，冬季风相对较强；9.0~7.3 Ma，夏季风发生强化，冬季风减弱；7.3~6.6 Ma，夏季风进一步强化；6.6~5.4 Ma，夏季风减弱，冬季风增强；5.4~4.1 Ma，夏季风再度增强，冬季风减弱。~8 Ma前后冬、夏季风的阶段性加强可能与晚中新世青藏高原的生长有关。 |
| 英文摘要 | The Qinling mountains represents the southern boundary of the Loess Plateau. It is also a key climatic barrier between southern and northern China. Our recent investigations revealed widely spread eolian deposits of Neogene age mantling the piedmont highlands of the intermountain basins within West Qingling mountain.\n In this study, a 150.2-m section in the Xihe basin is studied, based on sedimentology, geochemistry and magnetostratigraphy measurements. Following results and preliminary conclusions were obtained.\n (1) The Xihe section consists of three main parts: the lower (150.2~98.2 m) and upper parts (0~65.2 m) are typical loess-soil sequence while the middle part (65.2~98.2 m) was affected by episodic slope erosions. The median grain-size varying from 5.0 to 7.5 μm, the similarity of geochemical features and the numerous paleosols firmly confirm the wind-blown origin of the upper and lower sections. Detailed magnetostratigraphy measurements and micro-mammalian fossils date the Xihe section for an age spanning from 10.4 Ma to 4.1 Ma. It provides a parallel record to the previously reported late Miocene-Pliocene loess-soil sequence at Dongwan and attests again to the widespread nature of the Neogene eolian deposits in China, of which the southern boundary reached, at least, to the Western Qinling Mountain. \n (2) The Xihe section provides independent sights to the tectonic history of the Western Qinling that the basin-range pattern has been already formed at least by ~10.4 Ma. The slope-erosions at the middle part evidence some kinds of tectonic instability from 8.3 to 6.9 Ma, which may related with the growth of the Tibetan Plateau in the late Miocene. The top age of the sequence suggests an intense erosion phase posterior to 4.1 Ma attributable to either uplift of the Tibetan Plateau or northern hemispheric cooling. \n \n \n \n\n\n(3) Comparison of grain-size changes at Xihe with those at Dongwan and QA-I indicates a strong N-S gradient with coarse textures in the north, finer ones in the south. These imply a northerly dust-carrying wind (i.e. the Asian winter monsoon) and source location (i.e. the inland deserts in Asia). Magnetic susceptibility, Free iron oxides (FeD), and redness of the section show a series of changes in the summer and winter monsoons. From 10.4 to 8.7 Ma, the intensity of summer monsoon was strog while that of the winter monsoon was weak. From 9.0 to 7.3 Ma, the summer monsoon enhanced while the winter monsoon weakened slightly. From 7.3 to 6.6 Ma, further increase of the summer monsoon intensity was observed. The interval from 6.4 to 5.4 Ma was characterized by weakened summer monsoon and strengthened winter monsoon. This interval was followed by a period with strengthened summer monsoon and weakened winter monsoon. The stepwisely intensified monsoon circulations in the late Miocene maybe related with the uplift of the Tibetan Plateau. |
| 中文关键词 | 西秦岭 ; 风尘堆积 ; 晚新近纪 ; 东亚季风 ; 磁性地层 |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 第四纪地质学 |
| 来源机构 | 中国科学院地球环境研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/286818 |
| 推荐引用方式 GB/T 7714 | 葛俊逸. 西秦岭山间晚中新世-上新世风尘堆积的磁性地层及其古环境意义[D]. 中国科学院大学,2010. |
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