【作者】 雷国良；

【导师】 张虎才；

【作者基本信息】 兰州大学 ， 自然地理学， 2009， 博士

【摘要】 额济纳盆地处于欧亚大陆中心,巴丹吉林沙漠西侧,气候干燥,生态脆弱,其环境变化直接受大气环流的影响,对气候干湿变化反应敏感。盆地内广泛分布的晚更新世以来的湖相沉积地层,揭示出当时湖泊发育,气候环境优于当代,前人的研究也表明晚更新世晚期（深海氧同位素第三阶段（MIS3））西北地区湖泊普遍发育,环境湿润,这一结果也得到气候模拟的支持。但是由于此阶段气候的特殊性,其湖泊形成原因及机制目前尚未有突破性结论。本论文选取额济纳盆地小狐山剖面的湖相沉积物为研究对象,在AMS¹⁴C测年基础上建立年代模型,通过对沉积物粒度、碳酸盐含量、有机质含量、总氮、碳氧同位素以及沉积物矿物组成的测试与分析,重建了晚更新世中晚期以来额济纳古湖的演化过程和额济纳盆地此阶段气候环境变化的历史。本研究不仅为MIS3阶段“大湖期”的研究补充了详细可靠的古环境资料,有助于加深对全球晚更新世中晚期气候变化机制和过程的理解。小狐山湖相沉积物的粒度分析表明,沉积物粒度的变化对湖泊的演化反映敏感。在额济纳古湖发育期,沉积物颗粒较细,而在古湖退缩阶段,沉积物粒度较粗,以风成砂为主,表明沉积物粒径的大小主要受湖泊水动力的影响。通过计算标准偏差的方法,得到了环境指标的敏感组分,并恢复了额济纳古湖水位的变化历史。对沉积物有机碳和总氮含量进行分析发现两者呈同向变化趋势:TOC/TN比值总体波动较小,平均值为9.98。进一步的分析表明,沉积物中有机质主要受湖泊自身生产力控制,外源维管束植物也有一定的贡献。有机碳含量变化与粒度结果有较好的相关性,表明在水位较高时,湖泊生产力也较高。沉积物中碳酸盐含量相对较低,其变化较为复杂,主要反映了湖泊有效湿度的变化,其高频波动主要与短时间尺度气候事件有关。沉积物稳定同位素的分析结果显示,碳酸盐δ¹³C与沉积物的粒度、碳酸盐含量和TOC等指标均具有一定的相关性,表明碳酸盐δ¹³C是诸多影响因素的综合指标,因此推测碳酸盐δ¹³C的变化应主要受湖水盐度控制,与湖区的有效湿度密切相关。碳酸盐δ¹⁸O结果显示,δ¹⁸O值的变化不仅受湖泊水体δ¹⁸O变化的影响,还受到湿度、温度、碳酸盐矿物分馏作用的影响,δ¹⁸O值指示的古环境信息较为复杂,其中碳酸盐δ¹⁸O在湖相沉积物中偏正的特征指示出蒸发与降水比值总体上呈增加趋势,表明自晚更新世晚期以来额济纳盆地的大气湿度呈现出不断减小的趋势。对沉积物XRD的分析发现,沉积物中含有石英、方解石、白云石,文石、长石、云母和粘土矿物等多种矿物。对通过半定量的方法计算得到的主要矿物含量结果的分析表明,在古湖泊发育阶段,沉积物中的石英含量低,而长石等与风化作用有关的矿物明显增加,推测长石/石英主要反映了盆地内部的风化作用强度。沉积物中碳酸盐主要由方解石、文石与白云石组成,三种矿物总含量的变化与利用气体方法得到的碳酸盐含量变化相一致。在整个剖面上文石含量较低且波动相对较小,但是方解石与白云石波动较大,碳酸盐矿物组成上的变化可能指示了湖泊盐度和温度的变化。磁学分析表明,沉积物磁化率值总体较低,这与XRD分析结果所指示的磁性矿物含量相对较低相一致。通过XRD方法对磁性矿物富集后表土样品的研究表明,沉积物中的磁性矿物主要由磁铁矿和赤铁矿组成。沉积物样品的磁信号对环境响应较为复杂,受到湖泊氧化还原作用和磁性矿物来源等多因素的影响。通过对各代用指标的综合分析,可将额济纳古湖的演化过程划分为以下几个阶段:43.5～40.7ka BP,剖面点处为风成砂沉积,盆地内气候干燥,风沙盛行;40.7～39.3ka BP,古湖泊开始扩张,湖泊水位达到剖面点处,并形成稳定的水下沉积环境,盆地内气候开始转湿;39.3～26.4ka BP,盆地内大面积湖泊形成,湖泊生产力较高,指示盆地内相对湿润,各代用指标的变化也均显示出此阶段是晚更新以来气候环境相对湿润、环境最为适宜的时期,但此阶段内的气候变化并不稳定,其间有3次湖泊退缩的过程;26.4～20.9ka,代用指标显示此阶段湖泊逐渐退缩,湖泊生产力逐渐降低,气候开始转型,由湿润向干燥过渡;20.9～17.1ka BP,沉积物由湖相沉积转变为风成砂沉积,代用指标也指示了此阶段气候干燥,大面积湖泊消失,与区域内早期环境相类似;17.1～4.6ka BP,大面积湖泊形成,沉积环境较为稳定,指示了在末次冰盛期之后和全新世早期盆地内气候相对湿润,其中代用指标显示在8.9 ka BP左右气候开始变干,湖泊水位开始波动下降,在4.6ka BP左右湖泊快速退缩,退出剖面点位置。小狐山湖相沉积物记录与邻区腾格里断头梁剖面和柴达木盆地贝壳堤剖面可以很好的对比,三个剖面的记录均表明在40～20 ka BP左右研究区存在大面积的湖泊,与现在西北干旱区干旱的气候格局完全不同,表明此阶段湿润的气候特征并不是小范围局部区域现象。与石笋和冰芯记录的对比也表明,具有全球性的H事件和D-O旋回在沉积序列记录中均有反映,当H事件发生时,额济纳盆地内湖泊退缩,气候变干,区域内的风化强度减弱,表明额济纳盆地湖泊的演化受全球气候变化,至少是北半球气候变化的影响,这也为探讨40～20ka BP西北干旱区湖泊形成的原因与气候变化机制提供了重要的记录资料。更多还原

【Abstract】 Ejina （Gaxun Nur） Basin is one of the largest inland basins in arid NW China, where has fragile ecosystem and unstable climate. Meanwhile the environment is controlled by atmospheric circulation and it is sensitivity to global climate changes in this region. There is a widespread lake sediment deposited since late Pleistocene , indicating the environment is wet. It was consistent with other stratigraphic evidence in NW China which was proved by the result of model. Therefore, a continue lake sediment collected from XHS, which was located in the north of Ejina Basin, was studied here. Through a variety of proxies, such as grain size, magnetic susceptibility, carbonate content, total organic carbon, organic carbon isotopes, inorganic carbon isotopes and mineral composition, the history of lake evolution since the late Pleistocene was reconstructed. It is important to study the history of climate change at this area since the late Pleistocene, which will help us to understand the process and mechanism of the climate change and provide a detailed record for the model.Grain size of sediment showed that it is sensitive to the history of lake evolution. The sediment was fine when the lake was extending, Grain size increased when the area of lake was decreasing. The distribution of Grain size was mainly controlled by the dynamic change of lake. The sensitive particles were calculated by the method of calculating standard deviation and can be used as an indictor of lake level.TOC and TN showed a similar variation through the profile. TOC/TN ratios were constant through the profile with an average value of 9.98, indicating that organic carbon was original from lake-internal input and detritus input. A high value of TOC occurred when the lake was during a high level period, indicating that TOC can be used as a proxy for productivity under warm climate conditions. The content of authigenic carbonate, which was mainly controlled by water temperature and biomass, can be used as a proxy for the variation of effective humidity in the study area. It is complex to interpret the variation ofd¹³C extracted form carbonate, which show high correlation coefficient with Grain size the content of carbonate and TOC. the variation ofd¹³C was controlled by the change of salinity of lake,The identified minerals were Quartz, calcite, aragonite, dolomite, feldspar and clay minerals with the method of Powder X-ray Diffraction （XRD）. The concentration of major minerals was calculated as the semi-quantitative index. During the extending period of lake, quartz has a low concentration and feldspar which original from weathering has a high concentration. So the ratios of feldspar/quartz can be used as a proxy for weathering in the basin. The carbonate was made up of by calcite and dolomite at the bottom of the profile. The concentration of calcite decreased which was instead by the increasing of dolomite at the centre of profile. Dolomite became the primary composition at the top the profile. Aragonite showed a constant through the profile.According to the TOC, CaCO₃ contents, d¹³C and grain size variations of the sediments, and the lithofacies of the studied section, the history of climate change and paleolake evolution history of the studied section can be described by the following: 43.5～40.7ka BP, climate was dry and the sediment was made up of by sand; 40.7～39.3ka BP, the area of lake increased and the site of profile was at a stable under-water condition, indicating that the climate became to wet; 39.3～26.4ka BP, a large lake had been there and the productivity increased. All proxies showed that the climate was wet and it maybe the opportunity period since the late Pleistocene; 26.4～20.9ka BP, the climate fluctuated significantly and the water level was dropping, but it was wet and the paleolake was in the high level overall; 20.9～17.1ka BP, The climate was colder and dryer than before and the water level was dropping down, the effective humidity of the study area became smaller, and the paleolake retreated beyond the site of the study section.; 17.1～4.6ka BP, the lake level raised and the environment turned to be better at beginning, but the climate changed in the end, therefore, the environment became cold and dry again and the paleolake started to retreat until it disappeared.The records of lacustrine sediment at XHS were consistent with the record at DTL profile of Tengger Desert and BQD profile of Qaidam Basin. All the three records provide strong evidences for the existence of large area during 40-20ka BP, which showed a different environmental condition. A series of global climate change events which recorded by ice cores and stalagmites had been found at the records of XHS. During the period of H events, the area of lake decreased and weathering effect was weakly, indicating a dry condition. So the evolution of the lake was controlled by the global climate change.更多还原