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

Abstract

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 laboratorycontrolled 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.paleomagnetism | paleointensity | thermoremanent magnetization | multidomain T he aim of paleointensity research is to reconstruct variations in the absolute intensity of the ancient geomagnetic field throughout Earth history. Paleointensity data are essential for constraining the conditions in the core (1-4), for studying the role that the geomagnetic field plays in controlling Earth's atmosphere (5-7), and as a geochronological tool (8, 9). Despite the necessity for a large amount of reliable paleointensity data, there are still significant ambiguities in the available paleointensity information (10-12); these call for a reevaluation of the existing database (13).One fundamental problem in paleointensity research arises from the difficulty in locating dateable ancient materials that fulfill the most basic requirement of the absolute paleointensity method. This requirement states that the natural remanent magnetization (NRM) should be a pure thermoremanent magnetization (TRM) carried exclusively by noninteracting singledomain (SD) particles (14). As purely SD materials are rare in nature, much of the published data are based on materials that do not entirely fulfill the strict assumption of pure SD, but still demonstrate a reasonably satisfying relationship between the blocking and unblocking temperatures. That is to say, pseudo SD (PSD) or even small multidomains (MD) are frequently assumed to carry stable and reproducible magnetizations.We first outline the principles of the absolute paleointensity method (14, 15), as most of the data in the paleointensity database rely on some variant of this method (10, 16). The basic underlying assumptions of any absolute paleointensity method are that TRM is quasi-linearly proportional to the intensity of the field (B) in which it was acquired (TRM = α · Β), and that in the absence of chemical and physical alteration, the proportion between the TRM and B is an intrinsic...

Understanding Nonideal Paleointensity Recording in Igneous Rocks: Insights From Aging Experiments on Lava Samples and the Causes and Consequences of “Fragile” Curvature in Arai Plots

“… There is a distinct difference between the curvature in Arai plots that results from MD behavior (e.g., Dunlop & Özdemir, 2001; Krása et al., 2003) and what we here call “fragile” curvature, first noticed by Sbarbori et al. (2009) and investigated by Shaar and Tauxe (2015) and Santos and Tauxe (2019). The former is reproducible in the laboratory while the latter disappears when specimens are given a “fresh” TRM Curved Arai plots for MD grains yield lower than expected intensity estimates for known laboratory fields (Krása et al., 2003) and results for specimens with “fragile” curvature are also biased to low values relative to straight Arai plots from the same cooling units Arai plots for specimens with fragile curvature tend to become more curved when given a fresh TRM and allowed to “age” in controlled laboratory fields Experimental protocols and selection criteria that do not test for curvature may yield inaccurate paleointensity results.…”

“… Does fragile curvature develop over time as suggested by Shaar and Tauxe (2015)? Are paleointensity estimates from Arai plots with fragile curvature generally biased (as are results from MD dominated curvature)? Does fragile curvature depend on the strength/direction of the aging field? What causes fragile curvature? …”

“…To address these issues, we subjected sister specimens from the samples investigated by Santos and Tauxe (2019) (see Table 1) to extensive hysteresis experiments and an “aging” experiment, similar to that described by Shaar and Tauxe (2015), but with some modifications. We describe in the following sections the experimental details, summarize the results, and consider the questions raised above concerning the temporal stability of fragile curvature and its effects on our ability to estimate ancient field strength.…”

Direct observation of the thermal demagnetization of magnetic vortex structures in nonideal magnetite recorders

“…It is also apparent that the centers of the profiles become relatively more pronounced with increasing temperature, implying that the vortex core may be less sensitive to thermal effects and better preserve the magnetic intensity. One caveat of this experiment is that it was performed within 1 day and hence does not address the problem of TRM aging [ Shaar and Tauxe , ]. Nevertheless, the particle is observed here to essentially behave like uniaxial SD recorders with a limited choice of direction of magnetic moments, suggesting that vortex states are reliable carriers for recovering ancient directional and intensity information.…”

Models of Maghematization: Observational Evidence in Support of a Magnetic Unstable Zone

“…Moreover, the location and width of the unstable zone depends upon mineralogy and grain morphology (Nagy et al, 2019). Although the implications of these unstable zones for paleomagnetic observations are not yet fully understood, it is possible that they play an important role in degrading paleomagnetic recordings through undesirable effects such as pTRM tails where the blocking and unblocking temperatures are different (Shashkanov & Metallova, 1972;Santos & Tauxe, 2019;Shaar & Tauxe;Yu et al, 2013). Direct observation of such an unstable region, however, is hampered by its narrow grain-size range and its effects are difficult to isolate from the total sample magnetic remanence.…”