黄土高分辨率元素地球化学研究

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	黄土高分辨率元素地球化学研究
	梁莲姬
学位类型	博士
导师	孙有斌
	2013-05
学位授予单位	中国科学院研究生院
学位授予地点	北京
学位专业	第四纪地质
关键词	黄土高原东亚季风古浪黄土元素地球化学轨道-千年尺度
其他摘要	随着过去全球变化研究的深入开展，高分辨率的元素地球化学研究在海洋和湖泊沉积物中取得了重要进展，尤其是应用新型的XRF 岩芯扫描方法（Scanning XRF）可以更有效、更精确地反映气候快速变化。中国黄土记录了 2.6Ma 以来亚洲内陆气候的干湿变化过程和东亚季风演变历史，在重建构造-千年时间尺度季风变化特征发挥了不可替代的作用，其元素组成及比值包含着黄土堆积-成土过程中的重要古气候变化信息。本研究选择位于黄土高原西北部的古浪剖面，同时采集粉末和U-channel 样品开展了系统的元素地球化学分析，探讨高分辨率地球化学方法在黄土研究里的适用性并确定Scanning XRF 方法可测准的元素。通过开展系统的黄土地球化学分析，寻找能够反应风力风选相关的冬季风指标和风化成壤相关的夏季风指标，进而将高分辨率的元素地球化学记录同多个古气候地质记录对比研究，探讨黄土地球化学记录在轨道和亚轨道时间尺度上的变化特征。获得的初步认识如下： 1．中国黄土能够开展高分辨率元素地球化学工作。黄土加工块和从钻孔和探井中采集的U-channel 样品Scanning XRF 元素测量结果表明，Al，Si，K，Ca， Ti，Fe，Zn，Rb，Sr 和Zr 十个元素可在Scanning XRF 谱图中清晰检测出。黄土加工块样品的裂隙，断裂，松动和含水量的条件实验结果指示，Scanning XRF 的Si，K，Ca，Ti 和Fe 五个元素受到样品质量影响相对较小，而Al，Zn，Rb， Sr，Zr 元素受到样品质量影响较大。通过对比传统XRF 测量获得的元素含量和 Scanning XRF 的信号强度，发现黄土加工块样品中Al，Si，K，Ca，Ti，Fe，Rb， Sr 和Zr 九个元素具有高相关性，钻孔样品中Si，K，Ca，Ti，Fe，Sr 和Zr 七个元素具有很好的对比性，探井样品中Si，Ca，Fe，Sr 和Zr 五个元素相关性较高，说明黄土U-channel 样的质量对高分辨率元素测量结果有很大影响。 2．选择3 个典型的黄土样品（S0，L1 和L1SS1），分为五个粒级（﹤4μm， 4-8μm，8-16μm，16-32μm 和32-63μm）测量不同粒径元素含量。结果表明：Al， Fe，Mg，K，Mn，Zn，Rb，Cr 和V 元素含量随着粒级的增大而逐渐减少，Si和Na 元素含量随着粒级的增大而逐渐增大，Ti，Zr，P，Ba，Ca 和Sr 含量与粒度变化没有明显对应关系。结合常量和微量元素含量在不同样品中的活动特征和剖面中的变化表明，富集在偏细颗粒的稳定元素有Al，Fe，K，Ti，Rb，Zn，V 和Cr（A 组元素），富集在粗颗粒中富集的稳定元素有Si 和Zr 和Na（B 组元素），和风化成壤过程中活动的元素有Mg，Mn，P，Ca 和Sr（C 组元素）。A 组和B 组元素比值可指示与风力分选相关的粒度变化，但各指标的灵敏度将随着空间变化存在异同。C 组和B 组或A 元素比值，不同程度地反应风化成壤相关的元素淋溶或富集变化，这些元素比值在黄土-古土壤序列中的变化存在差别。考虑到黄土地球化学指标在Scanning XRF 的应用性，Ti/Si 和Ca/Si 比值可分别视为与粒度分选和风化成壤密切相关的东亚冬、夏季风的代用指标。 3．高分辨率的元素地球化学结果清晰地记录最后两个冰期旋回轨道-千年尺度的季风变化特征。26 万年以来黄土高分辨率地球化学记录的频谱结果表示，黄土 Ti/Si 记录主要包含100kyr，41kyr 和23kyr 的地球轨道特征周期，并以100kyr 周期为主要周期成分，而Ca/Si 记录包含100kyr，41kyr，23kyr，19kyr 和13kyr 多周期成分。东亚冬季风和夏季风所反应的周期特征不同，暗示了其驱动机制上存在异同。尽管北半球冰量和高纬太阳辐射能量变化都对冬季风和夏季风有影响，频谱分析结果表明冬季风可能更多受到全球冰量变化的影响，而夏季风则更多与太阳辐射变化相关。13 万年以来古浪黄土，格陵兰冰芯和董哥/葫芦/三宝洞石笋记录的对比表明，古浪黄土的元素比值清晰地记录千年尺度的冷暖气候事件 (DO 旋回)。值得注意的是，千年尺度气候波动在冰期和间冰期时段的周期和振幅有明显区别，表现为Ti/Si 指标反映在间冰期气候波动振幅和频率小于冰期，而Ca/Si 指标显示冰期振幅明显小于间冰期，造成这些差异的机理仍不清楚。; X-ray fluorescence core scanner (Scanning XRF) technique is increasingly accepted as an effective method to obtain high-resolution elemental compositions of marine and lake sediments because of its effective detection, non-destructive and nearly continues scanning on the split cores. The loess-paleosol sequence developed in northern China is a unique continental archive of Quaternary climate change, and its elemental components or ratios can provide significant insights into variations in the East Asian palaeomonsoon at tectonic to millennial timescales. However, high-resolution elemental analysis of loess deposits has seldom been reported. In this study, we collected U-channel and powder loess samples from a loess section in northwestern Chinese Loess Plateau for the systematic geochemical analysis. Our objectives are (1) to test whether Scanning XRF technique can be applied to loess geochemistry, (2) to extract appropriate geochemical indices for better evaluating the winter monsoon- induced grain size sorting and summer monsoon-related weathering and pedogenisis, (3) to address the broad implications of high-resolution loess geochemistry on the East Asian monsoon variability by comparison with other paleoclimate geological records at glacial-interglacial and millennial timescales. The primary conclusions are drawn as follows: 1. This study for the first time adopted the Scanning XRF technique to conduct a high-resolution elemental analysis of U-channel loess samples. The results show that ten elements (Al, Si, K, Ca, Ti, Fe, Zn, Rb, Sr and Zr) can be detected robustly in the Scanning XRF spectrogram of U-channel loess samples. The elemental results of block loess samples with cracks, gaps, looses and high water content indicate that the quality of U-channel loess samples has less effect on Si, K, Ca, Ti and Fe elements compared to the Al, Zn, Rb, Sr and Zr elements. Comparison of Scanning XRF data with elemental concentrations measured by conventional XRF suggests good correlation of nine elements (Al, Si, K, Ca, Ti, Fe, Rb, Sr and Zr) in block loess samples, seven elements (Si, K, Ca, Ti, Fe, Sr and Zr) in drilling samples, and only five elements (Si, Ca, Fe, Sr and Zr) in the digging samples. Therefore, the reliability of the high-resolution Scanning XRF results is highly dependent on the quality of the U-channel loess samples. 2. Three representative samples (S0, L1 and L1SS1) were selected to be separated into five size fractions (﹤4μm, 4-8μm, 8-16μm, 16-32μm and 32-63μm) for elemental analysis. The results show that Al, Fe, Mg, K, Mn, Zn, Rb, Cr and V are mainly enriched in fine size fractions, Si and Na display higher contents with increased particle sizes, and Ti, P, Ba, Zr, Ca and Sr show irregular variations among different size fractions. Based on the geochemical behavior features of major and trace elements in different loess samples and down-section variations in Gulang section, these elements were classified into three types: (A) Al, Fe, K, Ti, Rb, Zn, V and Cr are stable during the weathering process and enriched in fine grain size (namely A-group), (B) Si, Zr and Na are immobile in weathering condition and have high content in coarse grain size (namely B-group), and (C) Mg, Mn, P, Ca and Sr are easily mobile during weathering pedogenesis (namely C-group). Elemental ratios of A-group to B-group are closely connected with grain size sorting, while elemental ratios of C-group to A-group or B-group can reflect the leaching or reprecipitation of these elements in C-group during the weathering and pedogenic processes. Considering the reliability of high-resolution elemental results measured by Scanning XRF method, the Ti/Si and Ca/Si ratios (sensitive to grain size sorting and weathering/pedogenesis intensity, respectively) were considered tentatively as good indicators of the winter and summer monsoon variability. 3. High-resolution elemental results display distinct monsoon variability at orbital-to-millennial timescales over the past two glacial cycles. Spectral results of high-resolution geochemical records show that Ti/Si and Ca/Si ratios exhibit dominant peaks around 100-, 41-, and 23-kyr, but with different contributions of three dominant periodicities between these two elemental ratios. Different power densities of 100- and 23-kyr periods indicate that East Asian winter monsoon was influenced more by ice volume while summer monsoon are more closely linked with solar insolation. Comparison of loess geochemical records with NGRIP ice-core and Dongge/Hulu/Sanbao speleothem δ18O records shows that the loess could well document millennial-scale monsoon oscillations comparable to the Dansgaard-Oescheger events recorded in the Greenland ice core during the past 130ka. Notably, millennial monsoon fluctuations recorded in Chinese loess display different frequency and amplitude under interglacial and glacial conditions, e.g., Ti/Si exhibits relatively lower amplitude and frequency oscillations during last interglacial, whereas Ca/Si displays relatively higher amplitude and frequency during last interglacial. The behind mechanism for such amplitude and frequency differences of millennial-scale climate variability remains unclear.
学科领域	地质学 ; 地球化学
语种	中文
文献类型	学位论文
条目标识符	http://ir.ieecas.cn/handle/361006/2603
专题	博士研究生论文
推荐引用方式 GB/T 7714	梁莲姬. 黄土高分辨率元素地球化学研究[D]. 北京. 中国科学院研究生院,2013.