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

NSFGEO-NERC：转变对古地磁记录的理解：实验观察和数值预测的见解

• 批准号: 1827263
• 负责人: Lisa Tauxe
• 金额: $ 45万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-07-01 至 2022-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1827263&HistoricalAwards=false
• 关键词: NSFGEO; NERC; Transforming; understanding; paleomagnetic

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.

我们对岩石记录方式的方式的理解是基于一个分析理论，该理论假设颗粒被均匀磁化。乍一看，磁性恢复的理论很简单，并提出了研究磁场向量的过去变化的方法。当磁性矿物质从高于高温的地球磁场中冷却时，它们将获得热隔离磁化（TRM）。在居里温度以下的温度下，固有的原子和晶体磁能可以通过热波动很容易克服，并且晶体的磁矩可以在局部能量最小值之间自由移动，与外部磁场平衡。冷却至环境温度时，在某种温度下，热能不足以克服局部能量最小值和磁化强度之间的障碍，以特定的构型冻结。磁化被“阻塞”。在阻塞过程中存在弱磁场（例如地球）的存在，在晶粒组合的净磁矩中会略有偏差，从而引起与施加场平行并成比例的TRM（假设易于轴的各向同性分布）。 从简单的理论来看，热起源的自然延迟几乎与它们在地球一样低的田地获得的田地几乎是线性相关的，并且原则上可以估计古代磁场的强度。 但是，实际上，恢复古代田野的力量很复杂。 简单的理论仅与均匀磁化的材料有关。数值建模的最新突破已经开辟了新的途径，以理解较大的磁性晶粒的性质，从而有可能估计其时间和热稳定性。现在，现在有机会结合实验和数值方法，以对古磁记录产生重大的新理解，并有可能改变了古显着研究的方式，这是该项目的目标。该项目的更广泛影响包括国际合作，由NERC在英国资助，培训命名的文档和研究生的建模和实验室方法研究生，扩大了代表性不足的团体参与STEM的参与，为现场古磁性的基础工具的开发，以及与魔术社区通过魔术社区共享的数据库。研究人员希望解决的具体问题是：1）现有理论预测，热延迟取决于样品的冷却速率，但文献中有一些声称，许多具有较大谷物的样品没有冷却速率依赖性。 目前尚不清楚为什么没有冷却速率依赖性，因此不清楚如何处理此类数据。 2）现有理论还预测，在温度t上获得的磁化应通过重新加热为T的磁化。 3）最近的实验表明，与小晶粒随着时间的推移在小晶粒中的稳定性相反，较大的晶粒的remance不稳定，在仅几年内表现出“衰老”，这是理论上出乎意料的。导致这种衰老的原因是一个谜。 解决这些谜团是破解古显着估计问题的关键。该提案利用了新的计算资源来检查re漫不经意的获取过程，以改变古磁性和尤其是古显着实验的方式。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的知识分子优点和更广泛影响的审查标准来通过评估来通过评估来支持的。

项目成果

Investigating the Accuracy, Precision, and Cooling Rate Dependence of Laboratory‐Acquired Thermal Remanences During Paleointensity Experiments (Dataset)

研究实验室在古强度实验期间获得的剩磁的准确性、精密度和冷却速率依赖性（数据集）

• DOI: 10.7288/v4/magic/16897
• 发表时间: 2019
• 期刊: Geosystems
• 作者: N., C. Santos;Tauxe, L.
• 通讯作者: Tauxe, L.

Earth’s magnetic field strength and the Cretaceous Normal Superchron: New data from Costa Rica (Dataset)

地球磁场强度和白垩纪正常超纪元：来自哥斯达黎加的新数据（数据集）

• DOI: 10.7288/v4/magic/16869
• 发表时间: 2021
• 期刊: Geosystems
• 作者: Di, A. Chiara;Tauxe, L.;Staudigel, H.;Florindo, F.;Protti, M.;Yu, Y.;Wartho, J‐A.;van, Paul den;Hoernle, K.
• 通讯作者: Hoernle, K.

Bias Corrected Estimation of Paleointensity (BiCEP): An Improved Methodology for Obtaining Paleointensity Estimates

• DOI: 10.1029/2021gc009755
• 发表时间: 2021-08-01
• 期刊: GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS
• 影响因子: 3.5
• 作者: Cych, Brendan;Morzfeld, Matthias;Tauxe, Lisa
• 通讯作者: Tauxe, Lisa

Paleointensity estimates from the Pleistocene of Northern Israel: Implications for hemispheric asymmetry in the time averaged field

以色列北部更新世的古强度估计：对时间平均场半球不对称性的影响

• DOI: 10.1002/essoar.10511070.2
• 发表时间: 2022
• 期刊: Geochemistry geophysics geosystems
• 影响因子: 3.5
• 作者: Tauxe, L.;Asefaw, H.;Behar, N.;Koppers, A.A.P.;Shaar, R.
• 通讯作者: Shaar, R.

Estimating the Effect of Cooling Rate on the Acquisition of Magnetic Remanence

估计冷却速率对获取剩磁的影响

• DOI: 10.1029/2021gl095284
• 发表时间: 2021
• 期刊: Geophysical Research Letters
• 影响因子: 5.2
• 作者: Nagy L