可可西里盆地古近纪化学风化过程及其与青藏高原隆升和全球变化的关联

Collision between the Indian and Eurasian plates produced concomitant uplift of the Tibetan Plateau and its basin-ridge geomorphological systems. As the world’s largest and highest plateau, Tibetan Plateau surface relief has significant dynamic and thermal effects on atmospheric circulation and on regional and global climate. It has been considered as one of the key drivers for the formation of the Asian monsoon, enhanced erosion and weathering, global decreased CO2 during the Cenozoic. Finally, this uplift caused global cooling in the Cenozoic. However, at present, the driving mechanisms of these processes still remain controversies and have not been clearly confirmed by records of erosion, chemical weathering, paleoenvironment changes from mountain regions in western China, especially from the Tibetan Plateau. Meanwhile, continental chemical weathering records deduced from marine sediments are inconsistent with global cooling and records of terrestrial chemical weathering. The famous hypothesis termed “uplift of the Tibetan Plateau--enhanced chemical weathering--global cooling” is being challenged by recent geological studies. Large numbers of Cenozoic sedimentary basins that developed concomitantly in and around the Tibetan Plateau are attributed to have formed in association with intracontinental deformation and Tibetan Plateau uplift due to India-Eurasia collision, such as the Hoh Xil Basin which is located in the hinterland of the Tibetan Plateau. Therefore, in this project, we will focus on studying of the Paleogene deposits in the Hoh Xil Basin, and aim to expolre the Paleogene paleoclimate and chemical weathering history of the studied area. Based on lithology and sedimentary feature analysis, we will firstly subdivide the sedimentary facies of the deposits and then construct a high resolution time scale combined with magnetostratigraphy, biostratigraphy, and radioactive isotope geochronology of potential tuff interbedded in the stratigraphic units. Then, the integrated studies of multiple climatic proxy parameters of sediments, such as environmental magnetism, clay manerials, geochemical proxies, will be performed to construct the placlimate and chemical weahtering sequences. We will constrain the trends of these sequences and determine ages of characterstic climatic enents. Next, we will analysis the internal links between the intensity of chemical weathering and local paleoclimate evolution. Finally, in term of the Earth’s systematic science, the dynamic mechanisms and links among the Paleogene chemical weathering in the Hoh Xil Basin, uplift of the Tibetan Plateau, and global change will be discussed.

新生代青藏高原的隆起被认为是亚洲季风形成、全球风化作用增强、大气二氧化碳浓度下降并导致全球变冷的核心驱动力之一，然而其驱动过程和机制存在争议，更未得到青藏高原自身化学风化过程及环境变化记录的清晰印证。同时，来自海洋沉积的大陆风化记录又与全球变冷和陆地化学风化记录相矛盾，著名的“青藏高原隆升－风化增强－全球变冷”假说面临当代地质研究的挑战。本项目将从圈层相互作用出发，立足青藏高原，以其腹地独具特色的可可西里盆地古近纪陆相沉积物为研究对象，选择高原长尺度化学风化过程为主线。首先通过磁性地层学构建沉积序列高分辨率的年代学标尺，其次通过环境磁学、粘土矿物学、地球化学等替代性指标的综合研究，重建研究区古近纪的古气候、化学风化序列，厘定其演变趋势及特征事件时限，分析化学风化强度与区域气候、构造变化的耦合关系，从地球系统科学角度探讨青藏高原大陆化学风化过程与高原隆升和全球变化的内在动力学联系。

新生代青藏高原的隆起被认为是亚洲季风形成、全球风化作用增强、大气二氧化碳浓度下降并导致全球变冷的核心驱动力之一，然而其驱动过程和机制存在争议，更未得到青藏高原自身化学风化过程及环境变化记录的清晰印证。本项目从圈层相互作用出发，以高原腹地及周缘盆地古近纪陆相沉积物为研究对象，选择典型剖面开展了年代学、化学风化及古气候学研究，获得了以下认识：.首先，通过磁性地层学构建了可可西里、四川、尼玛盆地典型剖面的年代学。将可可西里盆地风火山群（厚6761米）的最终年代约束为72-43.5 Ma，将四川盆地雅安剖面（厚3221米）的年代约束为128-28 Ma，将尼玛盆地达则措剖面（厚1018米）的年代约束为27.3-23.2 Ma。.其次，通过环境磁学、地球化学等替代性指标的综合研究，重建了可可西里及尼玛盆地的古气候、化学风化序列。发现可可西里盆地化学风化强度变化与全球温度变化基本一致，即热室期化学风化强，冷期风化弱。环境磁学结果显示盆地干旱时风化强，变湿时风化弱，干湿变化不匹配于传统上认为的化学风化强度变化。因此，我们认为可可西里盆地的化学风化强度变化主要受控于全球温度。我们在沱沱河盆地开展的37-19 Ma、尼玛盆地开展的27.3-23.2 Ma期间的化学风化重建也进一步支持了这一结论。.再者，通过青藏高原伦坡拉、尼玛两个盆地晚渐新世-早中新世的河湖相沉积序列，利用磁性地层学、环境磁学及旋回地层学手段约束了研究区构造及轨道时间尺度的干湿环境变化历史，揭示晚渐新世青藏隆升和天文轨道共同驱动的南亚季风降水于~26 Ma时强化并北进至青藏腹地。.最后总结，通过本项目的开展，构建了可可西里、尼玛、四川盆地的年代学标尺，解译了盆地的沉积及构造历史，这些是古气候研究的基础。相关风化及古气候重建，揭示了高原化学风化与全球温度的关系，揭示了南亚季风北上高原的历程，为深入认知“高原隆升与化学风化、季风演化的关系”这些重要的科学问题提供了新的来自高原腹地的关键数据支撑。