南海北部始新世-渐新世沉积对东亚季风早期演变的磁学约束（IODP-U1501站）

East Asia is one of the most densely populated regions in the world, where the environment and social-economic activities are deeply affected by monsoon climate. Studying the East Asian monsoon (EAM) and understanding its evolution are of great significance for us to predict climate change and guide social and economic activities in the region. For the long-term (tectonic-scale) evolution history of the EAM, the existing continuous deposition records mainly reveal the development process of the EAM since Miocene, and that it significantly enhanced in the early Miocene when the EAM was also considered to be formed. In recent years, research progress showed that the East Asian continent was already under the influence of monsoon climate in the Eocene. However, due to the lack of continuous Eocene-early Oligocene sedimentary records, the early evolution of the EAM remains unclear. In 2017, continuous Eocene-Oligocene deposits were retrieved at IODP Site U1501 in the northern South China Sea (SCS), providing the possibility to reveal the early evolution of the EAM. This project intends to reconstruct the early development process of the EAM through the magnetic properties of sediments. Previous magnetic studies indicate that the magnetic properties of sediments in the SCS can effectively reflect monsoon climate-driven variations. Rock magnetic results of a few samples from the interested interval of the core show that magnetic properties were probably not affected by non-climate factors and may mainly record climate-controlled variations. Rock magnetic techniques, therefore, are expected to effectively uncover the early development process of East Asian monsoon.

研究东亚季风并了解其演化规律对我们预测该区域的气候变化和指导社会生产活动具有重要意义。对于东亚季风的长期（构造尺度）演变历史，已有的连续沉积记录主要揭示了东亚季风自中新世以来的演变过程，并且在中新世早期显著增强，这也被认为是东亚季风的开启时间。近年来的研究进展显示，东亚大陆在始新世就已受季风气候影响了。然而，由于缺乏始新世至渐新世早期的连续沉积记录，东亚季风的早期演变过程并不清楚。国际大洋发现计划368航次在南海北部的U1501站钻取到连续的始新世-渐新世沉积，为揭示东亚季风早期演变历史提供了可能。本项目拟通过该段沉积物的磁学性质来重建东亚季风的早期演变过程。已有的磁学研究显示，南海沉积物的磁学性质可以有效反映季风气候变化。试验性的岩石磁学结果初步说明研究层位的磁学性质受到非气候因素的影响较小，很可能主要反映了古气候信息。因此，预期本项目通过磁学方法可以有效地揭示东亚季风的早期演变历史。

东亚季风是东亚大陆的主要气候类型，深刻影响着区域环境、经济发展和人类生存，研究季风气候深时的变化规律对预测未来季风气候变化具有重要意义。已有研究主要聚焦于东亚季风于中新世以来的演变历史，而近年来的孢粉学和数值模拟研究显示从始新世开始季风就已经深刻影响东亚大陆的气候和环境了。由于缺少完整连续并且具有良好年代约束的古近纪地层序列，使得我们对东亚季风的早期演变历史及其驱动机制知之甚少。本项目通过对大洋钻探368航次U1501站位钻取的始新世末期至早渐新世沉积物开展详细的磁学调查揭示出沉积物的岩石磁学性质在32-34.5Ma期间对东亚夏季风的变化具有显著的响应。岩石磁学和电子显微镜下观察结果显示沉积物中磁性矿物主要由碎屑成因的（钛）磁铁矿组成。基于磁性矿物组成和已有的Sr-Nd同位素变化，在32-34.5Ma期间，此站位的磁性矿物主要由自中国南方东南沿海的高地河流输入。环境磁学参数序列显示，磁性矿物含量和粒度呈现出同步变化，在EOT之前呈现出显著增加和变粗，而经过EOT后，磁性矿物含量则呈现减少和变细的趋势。另外，漫反射光谱结果得出，赤铁矿相对于针铁矿也呈现出同步变化，在EOT之前增加，而在经过EOT之后则减少。综合磁性矿物的研究结果，U1501站位沉积物磁学性质在32-34.5Ma期间的变化主要受控于夏季风变化，夏季降雨影响地表的侵蚀强弱和径流的搬运能力，进而控制磁性矿物的含量和粒度，同时温度控制着赤铁矿相对于针铁矿的含量变化。磁学参数序列揭示出东亚夏季风在EOT之前增强，经过EOT后减弱，也与北半球的大洋表面温度呈现出一致的变化。本项研究结果对准确认识东亚季风的早期演变提供了环境磁学方面的约束，而且揭示出东亚季风环流的强弱在始新世末期至早渐新世期间主要受同期的全球变化驱动。