柴达木盆地北部红柳沟剖面粘土矿物记录的新生代古环境演变和化学风化过程

There are many controversies on the relationship between tectonic uplift, climate change, and continental chemical weathering in different temporal and spatial scales. Establishing a chronological age-controlled Cenozoic chemical weathering record in the Tibet Plateau region is one of the effective ways to solve the above controversy. The continuous Cenozoic weathering products in the Qaidam Basin provide ideal materials for exploring the relationship between tectonic uplift, climate change, and chemical weathering. However, the complex and changeable sedimentary facies limit the application of whole rock chemical weathering indicators. As the product of silicate weathering, clay mineral assemble is less affected by sedimentary facies changes and is an ideal indicator for reconstructing long-scale chemical weathering. In this proposal, the clay minerals collected from the Hongliugou section are employed to reconstruct a drainage area chemical weathering sequence in the Cenozoic. The results of polytypes and scanning electron microscopy observations are firstly used to determine the origin of the clay minerals, and then clay assembles are used to reveal chemical weathering history of Qaidam Basin. The stable trace elements ratios and Nd isotopes are employed to constrain the impact of possible provenance change. Paleosalinity derived from clay boron content and the ratios of chlorite Fe/Mg are collectively to reconstruct the Cenozoic paleo-environmental evolution records. Finally, a comparative compare between clay-derived chemical weathering records and regional paleoclimate records as well as regional tectonic activity history is used to reveal the major impact factors, thus providing an important basis for in-depth exploration of the uplift of the Tibetan Plateau and its weathering and climate change effects.

高原隆升、化学风化和气候变化的相互关系在不同时空尺度上存在诸多争议。在青藏高原地区建立有精确年代控制的新生代化学风化记录是解决争议的关键途径之一。柴达木盆地连续的新生代风化产物提供了探究构造隆升、气候变化和化学风化的理想材料。然而复杂多变的沉积相限制了全岩化学风化指标的应用。作为硅酸盐风化产物的粘土矿物，其矿物组合受沉积相变化影响小，是重建长尺度化学风化的理想指标。本项目拟以柴达木盆地红柳沟剖面（54.0-1.8 Ma）粘土矿物为研究对象，通过多型分析和扫描电镜观察确定粘土矿物成因，进而利用粘土矿物组合重建流域新生代化学风化历史；通过Nd同位素和稳定微量元素组合约束可能物源变化的影响；通过粘土矿物硼含量重建的古盐度与绿泥石铁镁比建立流域新生代古环境演化记录；通过化学风化记录与区域、全球气候及构造事件的对比研究，揭示流域化学风化的控制因素，为探讨青藏高原隆升及其风化和气候变化效应提供依据。

本项目针对新生代青藏高原隆升、大陆风化与全球变冷的关系这一重要科学问题，在青藏高原东北部柴达木盆地具有精确年代控制的红柳沟剖面（54-1.8 Ma）开展了详细的矿物学和地球化学方面的研究，通过贝德石含量、粘土矿物硼含量重建的古盐度与绿泥石铁镁比建立流域新生代古环境演化记录；通过粘土矿物及单矿物指标重建了区域综合的化学风化的综合演化历史；最后通过化学风化记录、区域/全球气候及构造事件的对比研究，揭示出流域化学风化的控制因素。结果为：（1）扫描电镜观察、（伊利石和绿泥石）多型分析表明红柳沟剖面的粘土矿物主要是流域化学风化的产物，且受成岩作用影响较小；（2）利用高分辨的物源指标揭示柴达木盆地的物源在14 Ma前变化相对较小，而14-1.8 Ma期间存在多次物源的变化，推测与该时期区域的构造隆升活动相关；（3）贝德石含量、古湖水盐度记录以及绿泥石Fe/Mg比率共同表明柴达木盆地54-1.8 Ma期间的古环境演化主要受全球温度控制；（4）利用粘土矿物组合以及伊利石化学指数等获得的区域综合化学风化记录与区域的古环境演化记录基本相同，表明全球温度通过控制区域气候变化进行控制着区域的化学风化过程，而受同时期青藏高原隆升和特提斯海退的影响相对较小。本项目的成功实施揭示了新生代时期化学风化强度与全球变冷之间的关系，从而为今后进一步验证“高原隆升-风化强化‐CO2下降和全球降温”的碳循环假说提供了重要的证据。