西宁黄土的高精度磁极性序列和磁性气候记录

Most high-level fluvial terraces occurring along the Huangshui River are directly overlain by thick eolian loess covers, which contain a wealth of paleoclimatic and paleoenvironmental information related to late tectonic uplift of the Northern Tibetan Plateau. However, systematic magnetostratigraphic and paleoclimatic studies for the Xining loess are scarce due to uncertain reasons. Relying on high-sampling-resolution magnetostratigraphic and environmental magnetic study for the Xining loess-paleosol sequences, in conjunction with redness, grain size and geochemical analyses as well as Zircon U-Pb dating technique, the main purposes of this proposal are to (1) provide robust age constraints for the Xining loess and the uppermost terrace of the Huangshui River; (2) to explore reliable magnetoclimatologic proxies for faithfully resolving high-resolution fluctuations of the paleomonsoon since the Quaternary in the northern Tibetan Plateau; (3) and to discern possible linkages and driving mechanism(s) among tectonic uplift of the Northern Tibetan Plateau, the formation and evolutionary history of the Huangshui River, aridification in western China, and global cooling during the Quaternary. We also carry out detailed comparisons of accumulation rate, the characteristics of geochemistry and anisotropy of magnetic susceptibility, provenance, and paleomagnetic recording fidelity between the loess developed in the hinterland of the Chinese Loess Plateau, the Lanzhou and Xining areas.

流经青藏高原东北缘的湟水高阶地上堆积的厚层风成黄土是认识高原第四纪环境演化和气候变化的理想载体。然而，由于多种因素的限制，多年来针对西宁黄土较系统的磁性年代学和第四纪相关研究成果一直比较匮乏。本项目拟选择该地区代表性黄土地层剖面开展高分辨磁极性地层学研究，在精确厘定西宁黄土磁性年代的基础上，开展精细的多参数环境磁学研究，并结合粒度和地球化学指标分析，提取出可靠的磁性气候学指标来构建第四纪以来青藏高原东北缘高分辨率长尺度气候环境变化序列，加强西宁黄土与兰州地区及陇东黄土在碎屑锆石年龄谱和磁组构特征所指示的物源、磁气候记录等方面的对比分析，深入探讨青藏高东北缘晚期可能的隆升过程、湟水和西宁黄土的形成发育等区域构造-地貌事件的耦合关系及其与全球变冷、西部干旱化过程的某种动力学联系。

西宁黄土作为青藏高原东北缘的西宁盆地内发育的典型第四纪沉积记录，将该地区黄土从物源-沉积-地层年代-古气候记录-环境效应等方面的系统研究，对于深入理解青藏高原东北缘与陇西黄土、沙漠交接带的古环境变迁与西部干旱化过程、湟水和西宁黄土的形成发育等区域构造-地貌事件的耦合关系均具有重要科学意义。通过本项目的实施，取得了以下几方面重要成果：1）建立了西宁黄土-古土壤序列高精度年龄框架，发现该地区第四纪黄土地层的地质年代老于1.33 Ma，表明黄河一级支流—湟水至少在约1.3 Ma前已形成；2）查明了青藏高原东北缘黄土的物源及其时空变化特征，发现西宁黄土中锆石的主要年龄分布与黄土高原黄土相似，表明其在物源与黄土高原主体间的高度一致性，进一步证实了中国黄土多源、充分混合的特点；3）揭示了西宁盆地第四纪以来气候是全球冰量和太阳辐射变化共同驱动的结果，发现冬、夏季风替代性指标不仅记录了轨道尺度上的10万年偏心率和4万年地轴斜度周期，同时在中更新世以来的2万年岁差周期也明显强于黄土高原典型剖面；4）证实了青藏高原的隆升加剧了亚洲内陆干旱化过程，发现西宁黄土记录的东亚冬季风呈长周期增强而化学风化强度逐渐减弱的特点与与黄土高原主体间相比更为显著。