Kiaman负极性超静磁期间地磁场古强度及长期变研究

The Earth's magnetic field is the only directly observable property of Earth's deep interior and has existed for over 3 billion years. Despite being one of the oldest known features of Earth, there remains many aspects of how the field is generated and how it has evolved that are poorly understood. Paleomagnetism (the study of the ancient geomagnetic field) allows us to investigate magnetic field variations as recorded by geological material and develop long-term records of geomagnetic field behaviour over millions of years. The history of geomagnetic field behaviour is marked by distinct period of extremely stable behaviour when the polarity of the field did not reverse for tens of millions of years. The exact cause of these enigmatic periods, known as superchrons, is still unclear. New computer models suggest that superchrons are the results of changes in the heat flow of Earth's liquid outer core, which is responsible for generating the geomagnetic field. The realism of these models needs to be constrained and tested with paleomagnetic data, however, the number of data that we have during superchrons is largely limited to only the most recent superchron (the Creataceous Normal Superchron). This makes it difficult to assess what are the general features of all superchrons. The purpose of the proposed research is to obtain detailed information on the direction and the strength of the ancient geomagnetic field (paleointensity) during the longest known superchron, the Kiaman Reverse Superchron (KRS), which ended ~265 million years ago. Geological samples will be collected from Xinjiang Province in NW China, where it is possible to collect samples that cover the entire superchron (over 50 million years) from a single region. This will allow a detailed record to be obtained. These new data will be used to test previously proposed ideas about superchron magnetic field behaviour. Namely, that the small-scale directional changes (termed paleosecular variation) were low and that the paleointensity was high. Our new data will be combined with archived data sets to understand geomagnetic field variability during the Kiaman Reverse Superchron on a global scale and over a time scale of ~50 million years. When combined with similar analyses from different time periods our new paleosecular variation assessment will provide a long-term record of geomagnetic field behaviour that spans over 320 million years.

地磁场已有30多亿年，但对其演化仍知之甚少。古地磁学通过获得地质样品记录的地磁场信息可建立地磁场长期变特征，为探索地磁场演化规律提供重要途径。超静磁期是地磁场演化过程中一个重要特征，其起因尚不明确。新的模拟结果指示超静磁期由地球液态外核的热流变化引起，其真实性需要古地磁数据来验证，通常认为超静磁期地磁场长期变较小、强度高，但已有数据多集中在最近一次超静磁期，致使对超静磁期的普遍特征认识不足。Kiaman负极性超静磁期（KRS: 318-265Ma）是目前已知持续时间最长的超静磁期，位于中国西北部的新疆地区火山岩分布涵盖整个KRS。本项目拟选择上述区域作为主要研究区域，以获得KRS详细的地磁场方向和强度，对上述观点进行检验。新增数据将与数据库已有数据结合共同探讨KRS期地磁场全球空间尺度和~50Myr时间尺度上的演化性质，并结合不同时期的研究结果，从而建立~320Myr以来地磁场长期变特征。

古代地磁场的强度（古强度）是认识地球演化规律的重要途径，但古强度数据的获得和分析仍然是制约其广泛应用的主要挑战。该项目将实验观测数据和强度实验数值模拟相结合，从而推进对强度结果的认识。这些新的观点和模型在认识古地磁场的演化和早期地磁发电机等方面有重要应用。在该项目资助下，共发表论文12篇（第一作者5篇），部分论文发表在Nature、PNAS等高水平国际期刊上。.该项目提出关于控制古地磁场记录基本过程以及实验室如何测量的新观点。这些新观点成为数值模拟预测磁性矿物粒径大或测量过程中容易发生化学变化的非理想样品强度实验行为的基础理论。我们提出了一套新的数据分析标准来判断这些模型模拟结果的可靠性并判断其是否适合用来讨论地磁发电机的长期演化问题。.这些数值模拟结果得到了由该项目资助的详细的实验观测结果的支持。其中我们发表在PNAS上的一项研究提出磁性矿物基本磁学性质和其强度实验行为及结果准确性之间存在必然联系。一直以来学者们都在探索这种联系却一直没有实质性的认识，而我们的研究结果在认识古强度数据方面是一个重大突破。.这些进展在认识中国地磁场近几千年变化规律及探索地磁场前寒武纪演化方面有重要应用。我们关于地磁场早期演化的研究揭示出地磁场在~1.3 Ga发生了显著变化，推测可能与地球内核的形成有关，在认识地球的早期演化方面具有重要贡献。这些关于早期地磁发电机行为的研究对认识趋磁细菌（依靠地磁场帮助觅食的微生物）的演化历史也有重要的指示意义。该项目资助的一项研究揭示这些趋磁细菌起源于太古代的某个时期，且其地质时期的演化规律可能与地磁场的演化密切相关。