NSFGEO-NERC: Transforming understanding of paleomagnetic recording: Insights from experimental observations and numerical predictions

我们对岩石记录地磁场的方式的理解建立在一个分析理论的基础上，该理论假设粒子被均匀磁化。磁剩磁获取理论乍看起来很简单，并提出了研究磁场矢量过去变化的方法。当磁性矿物在高于高温的地球磁场中冷却时，它们获得了热剩余磁化(TRM)。在略低于居里温度的温度下，原子和晶体的本征磁能很容易被热涨落所克服，晶体的磁矩可以在局部能量极小值之间自由移动，与外部磁场达到平衡。在冷却到环境温度时，存在一个温度，在该温度下，热能不再足以克服局部能量最小值之间的障碍，并且磁化被冻结在特定的配置中；磁化被“阻挡”。在阻塞过程中，弱磁场(如地球磁场)的存在会导致一组颗粒的净磁矩略有偏差，从而产生与外加磁场平行并与之成比例的TRM(假设易轴的各向同性分布)。从简单的理论来看，热源的自然剩磁与获得它们的磁场几乎是线性相关的，对于低至地球的磁场，原则上可以估计古代磁场的强度。然而，在实践中，恢复古老的场强是复杂的。这个简单的理论只适用于均匀磁化的材料。最近在数值模拟方面的突破为理解较大磁性颗粒的性质开辟了新的途径，使估计它们的时间和热稳定性成为可能。现在有机会将实验方法和数值方法结合起来，对古地磁记录有一个彻底的新理解，并有可能改变古强度研究的方式，这是本项目的目标。该项目的更广泛影响包括将由英国国家地磁研究中心资助的国际合作，对一名指定博士后和一名研究生进行建模和实验室方法方面的培训，扩大未被充分代表的群体在STEM中的参与，开发用于该领域古地磁学的基本工具，以及创建一个通过MAGIC社区数据库与更广泛社区共享的数据库。研究人员希望解决的具体问题是：1)现有理论预测热剩磁取决于样品的冷却速度，但文献中有说法称，许多颗粒较大的样品没有表现出冷却速度的依赖关系。目前尚不清楚为什么不会有降温速度依赖，因此也不清楚如何处理这些数据。2)现有的理论还预测，在温度T时获得的磁化应该通过零场再加热到T来退磁。这种“互易性”要求的失败早已被观察到，但其原因和后果尚不清楚。3)最近的实验表明，与小颗粒中剩磁随时间变化的稳定性相反，大颗粒中的剩磁是不稳定的，只在几年内表现出理论上意想不到的“老化”；导致这种老化的原因是一个谜。解开这些谜团是破解古强度估算问题的关键。这项建议利用新的计算资源来检查剩磁获取的过程，以改变进行古地磁，特别是古强度实验的方式。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Our understanding of the way in which rocks record the geomagnetic field is based on an analytical theory which makes the assumption that particles are uniformly magnetized. The theory of magnetic remanence acquisition is at first glance simple and suggests ways of studying past variations in magnetic field vectors. When magnetic minerals cool in the Earth's magnetic field from above high temperature, they acquire a thermal remanent magnetization (TRM). At temperatures just below the Curie Temperature, the intrinsic atomic and crystalline magnetic energies can be easily overcome by thermal fluctuations, and the magnetic moment of a crystal is free to move between local energy minima, coming into equilibrium with the external magnetic field. On cooling to ambient temperature, there is a temperature at which the thermal energy is no longer sufficient to overcome the barriers between local energy minima and the magnetization is frozen in a particular configuration; the magnetization is "blocked". The presence of weak magnetic fields (such as the Earth's) during blocking, causes a slight bias in the net magnetic moment of an assemblage of grains giving rise to a TRM that is parallel to and proportional in magnitude to the applied field (assuming isotropic distribution of easy axes). From the simple theory, natural remanence of thermal origin are nearly linearly related to the field in which they are acquired for fields as low as the Earth's, and it is in principle possible to estimate the strength of ancient magnetic fields. In practice, however, recovering the ancient field strength is complicated. The simple theory only pertains to uniformly magnetized material. Recent breakthroughs in numerical modeling have opened new avenues for understanding the nature of larger magnetic grains making it possible to estimate their temporal and thermal stability. The opportunity now exists to combine experimental and numerical approaches for a radical new understanding of paleomagnetic recording, with the potential to transform how paleointensity research is done, which is the goal of this project. The broader impacts of this project include international collaboration that will be funded by NERC in the UK, training of a named post doc and a graduate student in both modeling and laboratory methods, broadening of participation of underrepresented groups in STEM, development of foundational tools for the field paleomagnetism, and creation of a database to be shared with the broader community via the MagIC community database. The specific problems the investigators wish to address are: 1) Existing theory predicts that thermal remanence depends on a sample's cooling rate, yet there are claims in the literature that many samples with larger grains show no cooling rate dependence. It is unclear why there would be no cooling rate dependence, and therefore unclear how to treat such data. 2) Existing theory also predicts that a magnetization acquired at a temperature T should be demagnetized by zero-field reheating to T. Failure of this "reciprocity" requirement has long been observed but the causes and consequences are unknown. 3) Recent experiments have demonstrated that, in contrast to the stability of remanences in small grains over time, remanences in larger grains are unstable, exhibiting an "aging" over only a few years that is unexpected from theory; what causes this aging is a mystery. Solving these mysteries is key to cracking the problem of paleointensity estimation. This proposal exploits new computational resources to examine the process of remanence acquisition in order to transform how paleomagnetic, and paleointensity experiments in particular, are done.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.