Instability of thermoremanence and the problem of estimating the ancient geomagnetic field strength from non-single-domain recorders

Significance The evolution of Earth’s magnetic field is one of the greatest riddles of Earth’s past. Despite decades of paleomagnetic research, some fundamental properties of the geomagnetic field, such as the nature of its intensity fluctuation (paleointensity), are still elusive. Paleointensity is recovered from ancient materials that were thermally magnetized in the presence of the ancient field. The paleointensity procedure is based on the assumption that the ancient magnetization is stable with time. Here we show that this assumption is violated for many of the widely used materials, such as, for example, crystalline volcanic rocks. Our results put in question the reliability of much of the available paleointensity information, posing new challenges to our understanding of the ancient Earth. Data on the past intensity of Earth’s magnetic field (paleointensity) are essential for understanding Earth’s deep interior, climatic modeling, and geochronology applications, among other items. Here we demonstrate the possibility that much of available paleointensity data could be biased by instability of thermoremanent magnetization (TRM) associated with non-single-domain (SD) particles. Paleointensity data are derived from experiments in which an ancient TRM, acquired in an unknown field, is replaced by a laboratory-controlled TRM. This procedure is built on the assumption that the process of ancient TRM acquisition is entirely reproducible in the laboratory. Here we show experimental results violating this assumption in a manner not expected from standard theory. We show that the demagnetization−remagnetization relationship of non-SD specimens that were kept in a controlled field for only 2 y show a small but systematic bias relative to sister specimens that were given a fresh TRM. This effect, likely caused by irreversible changes in micromagnetic structures, leads to a bias in paleointensity estimates.

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