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是基于一种分析理论，该理论假设粒子是均匀磁化的。剩磁获取理论乍一看很简单，并提出了研究磁场矢量过去变化的方法。当磁性矿物在地球磁场中从高温以上冷却时，它们获得热残余磁化（TRM）。在略低于居里温度的温度下，原子和晶体的固有磁能可以很容易地被热波动克服，晶体的磁矩可以在局部能量最小值之间自由移动，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和磁化冻结在特定构型之间的障碍；磁化被“阻塞”。在阻塞期间，弱磁场（如地球磁场）的存在会导致颗粒组合的净磁矩出现轻微偏差，从而产生与施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，对于低至地球的磁场，热源的自然剩余物与获得它们的磁场几乎是线性相关的，并且原则上可以估计古代磁场的强度。然而，在实践中，恢复古代的场强是复杂的。这个简单理论只适用于均匀磁化的材料。数值模拟的最新突破为理解较大磁性颗粒的性质开辟了新的途径，使估计其时间和热稳定性成为可能。现在有机会将实验和数值方法结合起来，对古地磁记录有一个全新的理解，有可能改变古地磁强度研究的方式，这是这个项目的目标。该项目的广泛影响包括国际合作，将由英国NERC资助，培训一名博士后和一名研究生，学习建模和实验室方法，扩大STEM中代表性不足群体的参与，开发现场古地磁的基础工具，并创建一个数据库，通过MagIC社区数据库与更广泛的社区共享。研究人员希望解决的具体问题是：1)现有理论预测热残余取决于样品的冷却速率，然而，在文献中有声称，许多具有较大晶粒的样品显示没有冷却速率依赖。目前尚不清楚为什么没有冷却速率依赖，因此也不清楚如何处理这类数据。2)现有理论还预测，在温度T下获得的磁化应通过零场再加热到T来退磁。这种“互易性”要求的失效早已被观察到，但其原因和后果尚不清楚。3)最近的实验表明，与小晶粒中的剩余物随时间的稳定性相反，大晶粒中的剩余物是不稳定的，只在几年内表现出一种“老化”，这是理论上意想不到的；导致这种衰老的原因是一个谜。解决这些谜团是破解古强度估计问题的关键。该建议利用新的计算资源来研究剩余物获取过程，以改变古地磁，特别是古强度实验的完成方式。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.