【作者】 昝金波；

【导师】 方小敏；

【作者基本信息】 兰州大学 ， 自然地理， 2010， 博士

【摘要】 亚洲内陆干旱化被认为是晚新生代以来全球气候恶化的一个重要标志。研究表明,干旱化过程所产生的粉尘被季风和西风环流带到沙漠外围,沉降在黄土高原、北太平洋乃至格陵兰地区,通过“阳伞效应”、“冰核效应”和“铁肥料效应”等一系列物理化学过程对中亚干旱区乃至全球气候产生巨大的影响,是全球海陆气耦合变化中关键的连接纽带。我国西北内陆地区尤其是塔克拉玛干沙漠是亚洲粉尘最重要的源区之一,目前我们对这一地区的演化历史及其对东亚环境的影响了解还相当粗略,对于亚洲内陆干旱化的驱动机制的理解也有待深入。西昆仑山北坡发育有中国极端干旱区厚度最大的黄土,紧邻我国重要粉尘源地——塔克拉玛干沙漠,是沙漠发生尘暴将粉尘扬起并沉降在昆仑山北坡堆积而成,直接记录内陆地区干旱演化历史。我们于2006～2007年在西昆仑山北坡开展钻探工作,钻穿黄土地层,获取了深达671m的黄土岩芯,为揭示塔克拉玛干沙漠和中亚干旱区的形成演化以及其与青藏高原隆升和全球变化的关系提供了难得的宝贵材料。本论文通过对前207m黄土岩芯进行的地层学、古地磁与岩石磁学和古气候记录研究,得出了以下主要结论：(1)前207m的西昆仑山黄土岩芯时代约为0.95Ma,根据沉积速率推算,671m的黄土岩芯底界年代可达上新世,是中亚地区已知的时代最老、厚度最大的风成黄土地层。(2)西昆仑山黄土的磁化率和碳酸钙古气候意义与黄土高原黄土有着明显的不同。岩石磁学分析表明,西昆仑山黄土的主要载磁矿物为磁铁矿和磁赤铁矿,同时还含有少量的针铁矿、赤铁矿。该地区磁化率的变化主要受源区粗颗粒的软磁性矿物含量的影响,成壤作用形成的细颗粒磁性矿物对磁化率的贡献极小,这一结论也得到了表土样品岩石磁学实验的支持,而磁化率在0.5Ma左右急剧升高,也主要与该时期干旱化加剧导致的源区扩大有关。此外,西昆仑山黄土碳酸钙含量随着降水量的增加呈逐渐增多的趋势,也与黄土高原地区碳酸钙含量随降水量的增加而减少有着本质的区别。对于西昆仑山黄土碳酸钙含量的这种变化,我们初步推测可能主要与极端干旱地区特殊的地表过程有关。(3)各气候代用指标的综合分析表明,近1Ma以来中国西北内陆极端干旱区气候总体上呈持续变干的趋势,其干旱化过程主要经历了五个演化阶段：0.95-0.87 Ma(207～191m),相对湿润阶段；0.87-0.52Ma(191～111m),干旱化发展阶段；0.52-0.33Ma(111～67m),干旱化强烈发展阶段；0.33-0.13Ma(67～30m),现代干旱环境格局的调整过渡阶段；0.13-0Ma(30～0m),现代干旱化格局的形成阶段。并存在有四次重大的干旱化事件。其中,0.87Ma左右的干旱化事件可能是全球气候变化以及构造活动双重作用的结果；推测0.52Ma和0.13Ma左右的干旱化事件主要受控于青藏高原的隆升；0.35Ma左右的干旱化事件的触发机制目前不是十分明确,还需要更多的证据支持。更多还原

【Abstract】 The aridification of Asian inland is generally regarded as one of the most important mark of global climatic deterioration since the Late Cenozoic. Many previous researches showed dust due to drying and desertification of Asian inland was carried away by the monsoon and westerlies to desert adjacent areas (such as the Loess Plateau, northern Pacific Ocean and Arctic region), and had an important impact on global substance circulation and climate change by its sun umbrella, ice core and iron fertilization effects, becoming a key link of land-sea-air coupling change. The inland of Northwest China, especially the Taklimakan Desert, is one of the most important source areas of Asia dust. So far, our understanding about the drying process of this region and its environmental impact on East Asia is still quite rough, and the driving mechanism of Asian interior aridity has yet to be thoroughly understood.Loess on the west Kunlun Mountains is so far the thickest loess found in the extreme arid region of China inland, which is a roughly synchronous accompanied product of desert process. During 2006-2007, a 671-meter-long loess deposit core had been retrieved from the northern slope of the West Kunlun Mountains, which provides a good opportunity to study the formation and evolution of drying climate of Central Asia and its possible links with the Tibetan Plateau uplift and global cooling. Based on the detailed analysis about magnetostratigraphy and several climatic proxies for the upper 207m loess deposit core, the following conclusions and new findings are revealed:(1) Detailed plaeomagnetic dating of the upper 207m loess deposit core has been performed and the B/M boundary is found to be located at 170 m of the core, assigning an age of 3-4 Ma for the whole 671m loess deposit, which is the oldest aeolian sediments in the central Asia.(2) The paleoclimatic implication of susceptibility and carbonates content is significantly different between the loess on west Kunlun Mountains and Loess Plateau. Our results suggest that magnetic properties of the Kunlun Mountains loess are dominated by low coercivity ferrimagnetic minerals, i.e. magnetite and maghemite. Hard magnetic minerals, including goethite and hematite, are also found but make a minor contribution.Rock magnetic and grain size studies suggest that magnetic susceptibility of loess from the northern slope of West Kunlun Mountains are not controlled by changes in the concentration of ultrafine pedogenic magnetite/maghemite, but instead controlled by changes in the concentration of aeolian multidomian magnetite/maghemite, which can be further confirmed by the perfect correlation between grain size and magnetic susceptibility of the surface sediments. In addition, the carbonates content increasing with the precipitation for the west Kunlun Mountains loess was also different from loess on Loess Plateau, which was possibly induced by the special surface processes in the extreme arid region of Asian inland.(3) According to the characteristics of multiple climatic proxy records, the drying process of Central Asia since-1Ma can be roughly divided into 5 stages: 0.95-0.87 Ma(207-191m), relative wetting period; 0.87-0.52Ma(191-lllm), development of aridity period; 0.52-0.33Ma(111-67m), significant arid intensification period; 0.33-0.13Ma(67-30m), modulating and transition period; 0.13-0 Ma(30-0m), modern dry climate period. Further research shows that the development of aridity in Asian inland since-1Ma were mainly caused by the tectonic uplift and/or global cooling:the drying event at 0.52 Ma and 0.13 Ma were controlled by the Tibetan Plateau Uplift; the drying event at 0.87 Ma were caused by combined action between the Tibetan Plateau uplift and the increase of the global ice volume.更多还原