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

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

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.