Knowledge Resource Center for Ecological Environment in Arid Area
| 西宁盆地新生代气候变化的沉积地球化学记录 | |
| 其他题名 | Sedimentary geochemical records of the Cenozoic climate change in the Xining Basin |
| 阮笑白 | |
| 出版年 | 2018 |
| 学位类型 | 硕士 |
| 导师 | 方小敏 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 新生代以来，在全球变冷、青藏高原隆升以及副特提斯海西退等全球与区域气候构造背景下，中国的气候环境格局发生了重大转变：从古近纪以行星风系为主导的纬向带状分布格局转变为新近纪季风风系主导的东南季风湿润区与西北干旱区分布格局。重建这一气候环境格局转变的过程，认识东亚季风系统和中亚干旱区的协同演化有助于深入理解地球系统气候环境相互作用机制。位于季风湿润区、西北内陆干旱区和青藏高原高寒区交汇处的西宁盆地保存了连续的新生代沉积地层，提供了亚洲气候环境格局演化研究的理想材料。本研究针对西宁盆地晚始新世至中中新世关家山剖面（43.2~14.1 Ma）和中中新世至早上新世莫家庄剖面（12.7~4.8 Ma）河湖相沉积物开展了详细的沉积地球化学研究，分别获取了沉积物易溶盐、碳酸盐和粘粒组分的地球化学成分变化，建立了西宁盆地从晚始新世至早上新世连续的沉积地球化学记录。研究结果表明：（1）易溶盐记录：关家山剖面的易溶盐元素含量变化反映了蒸发岩矿物的类型与含量。沉积环境在约34 Ma脱离盐湖相后，易溶盐阳离子总量降低，Ca2+、SO42-和Sr2+含量大幅减小，在约22 Ma进入洪泛平原相后，Na+与Cl-离子含量同步增高，这表明沉积相对沉积物易溶盐地球化学的主导作用。对盐湖沉积阶段内的易溶盐元素比值分析揭示了约43.1~37.4 Ma盐湖逐渐扩张，35.1~34.3 Ma盐湖范围逐渐缩小。而莫家庄剖面的易溶盐元素含量总体很低，并且没有表现出明显的变化特征。（2）碳酸盐记录：关家山剖面中，盐湖相碳酸盐具有盐湖自生碳酸盐高Mg/Ca和Sr/Ca的特征，碳酸盐Mn/Ca指示的湖泊水位变化与易溶盐元素揭示的结果一致；而河流相沉积碳酸盐Mg含量的变化与碎屑白云石输入密切相关，30 Ma附近Mg/Ca和Mn/Ca同步升高可能代表了构造隆升事件。剖面560米（约18.7 Ma）以上碳酸盐含量迅速上升，与强古土壤层以及钙板层对应，可能指示了西宁盆地在中中新世气候适宜期较高的降水量。莫家庄剖面碳酸盐Ca-Mg-Sr元素分析和Sr同位素分析表明，至少从12.7 Ma开始，莫家庄剖面的沉积自生碳酸盐在其形成过程可能受到外部粉尘输入影响，并且在8.6 Ma左右粉尘输入增强，指示了同时期亚洲内陆干旱程度加强。结合临夏盆地同时期沉积地层的自生碳酸盐和元素和Sr同位素对比研究，推测8~>12.7 Ma亚洲内陆粉尘影响流域水化学的前缘位置位于两个盆地之间，~8 Ma之后大幅度向东南方向扩张。（3）粘粒组分（<2 μm）元素记录：关家山剖面中，经典的化学风化强度指数CIA和CIW反向变化，其中CIA变化强烈受K/Al变化影响，而CIW强烈受Ca/Al变化控制。由于基于微量元素比值的物源指标（如Cr/Zr、Th/Sc）在剖面中存在显著的变化，化学风化强度指数与元素比值（活动元素/不活动元素）的变化可能叠加了物源、气候和成岩的复杂影响。莫家庄剖面的CIA和CIW同向变化，整体呈现升高趋势，并且与物源指标变化呈现十分相似的趋势与波动，指示了物源的主导控制作用。两个剖面粘粒元素组成的变化特征可能暗示在具有频繁沉积相变的构造活动区山间盆地沉积物中，沉积物元素组成对流域环境气候的响应容易被物源和沉积相变以及成岩过程扭曲甚至掩盖，即使约束在较窄的细颗粒范围可能无法完全避免。 |
| 英文摘要 | Under the background of global cooling, Tibetan Plateau uplift and Paratethys sea retreat, the climate pattern of China reorganized during the Cenozoic, changing from the Paleogene zonal pattern dominated by the planetary wind system to the Neogene monsoonal pattern which was composed of the monsoon-dominated southeastern China and the arid inland northwestern China. Probing into the details about this climate transition and the synergetic evolution of East Asian monsoon development and Central China aridification will greatly facilitate to understand the complex earth surface dynamics. Xining Basin locates on the junction of the monsoon humid region, Asia arid inland and Tibetan Plateau cold and high region, accumulating thick and successive sediments through the Cenozoic, which may provide ideal archive for studying the Cenozoic Asian climate change. Here, based on two sedimentary sections in the Xining Basin including the late Eocene to middle Miocene Guanjiashan (GJS) section (43.2-14.1 Ma) and the middle Miocene to early Pliocene Mojiazhuang (MJZ) section (12.7-4.8 Ma), we investigated the detailed geochemical compositons of the water-soluable salt fraction, carbonate fraction and clay-size fraction in the sediments, and established a continuous sedimentary geochemical record of the Xining Basin area from the late Eocene to early Pliocene. The results are shown below:(1) Water-soluable salt record. The elemental compositions of the water soluable salt in the GJS section reflect the types and contents of evaporites in sediments. A concomitant drop in Ca2+, SO42- and Sr2+ concentrations at the boundary of sedimentary facies change at around 34 Ma and an increase in Na+ and Cl- at the boundary of 22 Ma indicate the dominant impact of sedimentary facies on the water soluable salt geochemistry. Elemental ratios of water soluable salt in the saline lake stage indicate that the saline lake expanded during ~43.1-37.4 Ma, and shrinked during 35.1~34.3 Ma. The contents of water soluable salts in the MJZ section were low and changed little.(2) Carbonate record. In the GJS section, saline carbonates are characterized by typically high Mg/Ca and Sr/Ca ratios, and the Mn/Ca ratios indicate a similar history of lacustrine volume change as the soluble salt records. The fluvial carbonate Mg concentrations in the GJS section were deeply influenced by detrital dolomites input. The concurrent high carbonate Mg/Ca and Mn/Ca ratios at around 30 Ma indicated a substantial tectonic uplift event then. Besides, the dramatically accending carbonate contents after 560 m (~18.7 Ma) of the section, corresponding to the strongly developed paleosols and the marked carbonate-rich beds, might indicate heavy moonsonal precipitation during the mid-Miocene Climatic Optimum (MMCO). The analyses for the carbonate elements and 87Sr/86Sr indicate that authigenic carbonates of the MJZ section were likely to be influenced by eolian dust input at least from 12.7 Ma. Since the authigenic carbonate elemental and isotopic compositions of the Linxia Basin revealed evident eolian input since ~8 Ma, we speculated that the hydrochemical boundary of dust influence lied between the Xining Basin and the Linxia Basin at ~8->12.7 Ma, and this boundary expanded towards southeast since ~8 Ma.(3) Clay-size fraction record. In the GJS section, two classical weathering intensity proxies CIA and CIW showed invese variation trends. The CIA was deeply influenced by the K/Al ratios, and the CIW was by the Ca/Al ratios. Because of substantial provenance changes as shown by stable trace elements proxies (e.g, Cr/Zr, Th/Sc), those proxies and elemental ratios (mobile /inmobile) may incorporate complex influences from the provenance, climates and diagenesis. In the MJZ section, the weathering proxies were also strongly dominanted by provenance changes. Our records therefore may indicate that, in such basins with frequent changes of sedimentary facies and provenances, the response of sedimentary geochemical compositions to the regional climatic and environmental changes may be easily distorted or overwhelmed by provenances, sedimentary faices or diagenesis, even for a narry fine particle size fration. |
| 中文关键词 | 西宁盆地 ; 新生代气候变化 ; 沉积地球化学 ; 碳酸盐 ; 粉尘 |
| 英文关键词 | Xining Basin Cenozoic climate change sedimentary geochemistry carbonate dust |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 自然地理学 |
| 来源机构 | 中国科学院青藏高原研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/288106 |
| 推荐引用方式 GB/T 7714 | 阮笑白. 西宁盆地新生代气候变化的沉积地球化学记录[D]. 中国科学院大学,2018. |
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