陇西盆地第四纪黄土的风化成壤特征及其古环境意义

In this study, two parallel loess-paleosol sequences from the Longxi Basin will be studied using magnetostratigraphic, pedological and geochemical methods. The main objectives are: 1) to analysis the spatial contact of the loess deposits in the Longxi Basin, and further to investigate the stratigraphic difference of the loess-paleosol sequences in the Longxi and Longdong Basins; 2) to study the difference of pedological characteristics of the typical paleosol layers in the Longxi and Longdong Basins and to interpret their signification for climatic changes; and 3) to reconstruct the paleoenvironmental evolutionary history since the Quaternary in the Longxi Basin through detailed analyses of weathering and pedological characteristics of the loess-paleosol sequence in this region. Based on the results above, to identify a series of paleoenvironmental events occurring in the Longxi Basin and to discuss the relationship between these events and those with global or regional significance. The Longxi Basin is located in the northeastern margin of the Tibetan Plateau, The climate in this region is controlled by the east Asian monsoon, the westerly circulation, the India southwest monsoon and the Tibetan Plateau monsoon. On the other hand, the Longxi Basin is also in the transitional belt of the semi-arid and semi-humid zones, the environmental response to climatic change in this region is very sensitive. Therefore, detailed studies on weathering and pedological characteristics of the loess-paleosol sequence in the Longxi Basin is valuable for better understanding paleoenvironmental change and mechanism in the eco-environmental fragile area, as well as paleoenvironmental response to uplift of the Tibetan Plateau in this region.

对陇西盆地的两个典型黄土剖面利用地层学、土壤学、地球化学等方法进行系统研究。分析陇西黄土在空间上的地层学联系及其与陇东黄土的地层对比关系；研究陇西和陇东黄土-古土壤序列中的古土壤在成壤过程、特征上的异同及其所反映的古环境变化的区域差异；在陇西地区建立以风化成壤事件为代表的古环境演化的时间序列。在上述研究的基础上，识别在此期间发生的重大环境事件，从理论上探讨该区环境演化与全球/区域性重大地质环境事件在时代和行为特征上的联系。陇西盆地位于青藏高原东北缘，气候受东亚季风、印度洋西南季风、西风环流及高原季风等多个大气环流交互作用的影响，同时该区处在半干旱、半湿润区的过渡地带，是我国重要的生态环境脆弱区。因此，本项目有望在青藏高原东北缘第四纪以来高原隆升的环境响应以及我国生态环境脆弱区环境演化过程与机制等研究方面从风化成壤的角度获得新认识。

在青藏高原东北缘选择会宁、民和两个黄土剖面开展了系统的年代地层学、风尘物源、风化成壤等方面的研究工作，主要取得了以下几个方面的进展：（1）通过磁性地层及ESR测年，结合分析磁化率的对比分析，建立了会宁、民和两个黄土剖面的时间标尺，其底界年龄分别为2 Ma 和0.8-0.9Ma。在此基础上，通过与东黄土高原典型黄土剖面的对比，建立了东、西黄土高原第四纪黄土-古土壤序列的地层学联系。（2）通过对会宁剖面系统的碎屑锆石年龄谱分析，研究了青藏高原东北缘2 Ma以来的风尘物源及变化特征，结果表明：青藏高原和中亚造山带-戈壁阿勒泰的物源贡献呈现此消彼长的特征，第四纪时期绝大多数时段陇西黄土主要来自青藏高原，作为第二大潜在源区的中亚造山带-戈壁阿勒泰仅在1.5-1.4Ma有限时段成为会宁黄土的主要源区之一，并且祁连山是西黄土高原主要碎屑物质提供者，除东亚冬季风外，高原季风及西风环流可能也是主要的粉尘搬运动力，地质历史时期的物源变化与青藏高原东北缘的构造地貌演化及气候变化密切相关。(3)对会宁、民和两个剖面开展了高分辨率的磁化率及色度的测试分析，建立了青藏高原东北缘2 Ma以来风化成壤的时间序列。研究结果显示：在青藏高原东北缘，磁化率和红度相关性较差，磁化率可能受物源影响较大，色度是反映风化强度的更好指标，在构造时间尺度上，2 Ma以来风化成壤强度没有显著变化。