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Instability of Thermoremanence and the Problem of Estimating the Ancient Geomagnetic Field Strength from Non-Single-Domain Recorders

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Instability of Thermoremanence and the Problem of Estimating the Ancient Geomagnetic Field Strength from Non-Single-Domain Recorders Instability of thermoremanence and the problem of estimating the ancient geomagnetic field strength from non-single-domain recorders Ron Shaara,b,1 and Lisa Tauxeb aThe Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel; and bScripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0220 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved July 27, 2015 (received for review April 23, 2015) Data on the past intensity of Earth’s magnetic field (paleointensity) elevated temperatures through which the ancient TRM (TRManc) are essential for understanding Earth’s deep interior, climatic model- is gradually replaced by a laboratory TRM (TRMlaboratory)acquired ing, and geochronology applications, among other items. Here we in a controlled field. Each double heating step includes one de- demonstrate the possibility that much of available paleointensity magnetization procedure in zero field and one remagnetization data could be biased by instability of thermoremanent magnetiza- procedure in the presence of a fixed laboratory field (Blaboratory). tion (TRM) associated with non-single-domain (SD) particles. Paleo- After each procedure, the magnetization vector is measured and intensity data are derived from experiments in which an ancient the portion of NRM lost and the partial TRM (pTRM) gained are TRM, acquired in an unknown field, is replaced by a laboratory- calculated and plotted on an Arai diagram (18) (Fig. 1). Ideal Arai controlled TRM. This procedure is built on the assumption that the plots yield data points scattered along a perfectly straight line with a process of ancient TRM acquisition is entirely reproducible in the slope of TRManc=TRMlab, from which the paleointensity can be laboratory. Here we show experimental results violating this as- calculated by Banc = Blaboratory · slope. sumption in a manner not expected from standard theory. We show In this work, we inspect the most fundamental assumption of all − that the demagnetization remagnetization relationship of non-SD paleointensity methods, taken as true a priori: that TRM (not specimens that were kept in a controlled field for only 2 y show overprinted by viscous remanent magnetization, VRM) is stable a small but systematic bias relative to sister specimens that through time unless chemical or physical alteration occurs. If this were given a fresh TRM. This effect, likely caused by irreversible assumption is true, then the process of TRM acquisition can be changes in micromagnetic structures, leads to a bias in paleointen- sity estimates. reproduced in the laboratory, and TRMlaboratory can be compared with TRManc. A violation of this assumption calls the reliability of paleomagnetism | paleointensity | thermoremanent magnetization | all absolute paleointensity procedures into question. Putting this multidomain assumption more simply, if paleointensity experiments were car- ried out on two identical specimens that acquired their TRMs in the presence of the same field but at different times, one holding a he aim of paleointensity research is to reconstruct variations Tin the absolute intensity of the ancient geomagnetic field fresh TRM and another holding an aged TRM, the results of the throughout Earth history. Paleointensity data are essential for paleointensity experiments should be exactly the same. To test this constraining the conditions in the core (1–4), for studying the role assumption, we examine two groups of sister specimens, one with “ ” “ ” that the geomagnetic field plays in controlling Earth’s atmosphere a fresh TRM and another with 2-y-old aged TRM. Surpris- (5–7), and as a geochronological tool (8, 9). Despite the necessity ingly, we find a small but systematic change in the shape of the for a large amount of reliable paleointensity data, there are still remanence decay curves, which affect the Arai plots and lead to a systematic bias in the interpretation. significant ambiguities in the available paleointensity information EARTH, ATMOSPHERIC, (10–12); these call for a reevaluation of the existing database (13). AND PLANETARY SCIENCES One fundamental problem in paleointensity research arises Significance from the difficulty in locating dateable ancient materials that fulfill the most basic requirement of the absolute paleointensity The evolution of Earth’s magnetic field is one of the greatest method. This requirement states that the natural remanent riddles of Earth’s past. Despite decades of paleomagnetic re- magnetization (NRM) should be a pure thermoremanent mag- search, some fundamental properties of the geomagnetic field, netization (TRM) carried exclusively by noninteracting single- such as the nature of its intensity fluctuation (paleointensity), domain (SD) particles (14). As purely SD materials are rare in are still elusive. Paleointensity is recovered from ancient ma- nature, much of the published data are based on materials that terials that were thermally magnetized in the presence of the do not entirely fulfill the strict assumption of pure SD, but still demonstrate a reasonably satisfying relationship between the ancient field. The paleointensity procedure is based on the blocking and unblocking temperatures. That is to say, pseudo SD assumption that the ancient magnetization is stable with time. (PSD) or even small multidomains (MD) are frequently assumed Here we show that this assumption is violated for many of the to carry stable and reproducible magnetizations. widely used materials, such as, for example, crystalline volcanic We first outline the principles of the absolute paleointensity rocks. Our results put in question the reliability of much of the method (14, 15), as most of the data in the paleointensity database available paleointensity information, posing new challenges to rely on some variant of this method (10, 16). The basic underlying our understanding of the ancient Earth. assumptions of any absolute paleointensity method are that TRM is quasi-linearly proportional to the intensity of the field (B)inwhich Author contributions: R.S. and L.T. designed research, performed research, analyzed data, it was acquired (TRM = α · Β), and that in the absence of chemical and wrote the paper. and physical alteration, the proportion between the TRM and B is The authors declare no conflict of interest. an intrinsic property of the sample that does not change with time. This article is a PNAS Direct Submission. The laboratory procedure in the Coe variant of the Thellier ap- 1To whom correspondence should be addressed. Email: [email protected]. proach (17), widely considered one of the most robust of the many This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. methods, involves a series of double heating steps at progressively 1073/pnas.1507986112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1507986112 PNAS | September 8, 2015 | vol. 112 | no. 36 | 11187–11192 Downloaded by guest on October 2, 2021 A B C Fig. 1. Paleointensity experiments on fresh and aged TRM. (A) Experimental workflow. (B) A representative case showing the typical difference between the aged (green) and the fresh (red) TRM, where the aged TRM specimen has a slightly more curved and zigzagged Arai plot. (C) An example of two sister specimens yielding identical behaviors, demonstrating that B is not a result of experimental noise. All measurements are normalized to NRM0, the initial measurement after TRM acquisition, before TRM aging. (B and C)(Left) Arai plots of the mother sample with the original ancient NRM. (Middle and Right) Arai plots (Middle) and demagnetization/remagnetization plots (Right) of the aged and fresh TRM. Methods built paleointensity oven at 580 °C (Fig. 1A). After the magnetizations were measured, one group, consisting of 181 specimens, was given TRM in a 70-μT In our experiments (shown schematically in Fig. 1A), we used samples that field by cooling from 580 °C. Specimens from this group were kept in a fixed had already undergone Thellier-type experiments in the paleomagnetic field of 70 μT parallel to the direction of the NRM, and were allowed to age for laboratory at the Scripps Institution of Oceanography (SIO), showed no al- 2 y at room temperature. According to Néel theory (27), uniaxial SD particles teration in the original experiments, and demonstrated behaviors attributed should not experience any change in their net magnetization, as there is in- to small MD or PSD (i.e., curved and/or zigzagged Arai plots, Fig. 1 B and C, sufficient energy to switch between the two energetically stable magnetiza- “ ” Left). From a collection of mother samples, we assembled two groups of tion states. However, it is not clear if non-SD particles that have multiple stable “ ” nearly identical sister specimens by breaking each sample into smaller magnetization states can experience change in their NRM in these conditions. pieces. The samples were mostly volcanic rocks from different locations: For this reason, the NRMs of the specimens from this group were routinely mea- McMurdo (19), the Island of Socorro (20), the Snake River Plain (21), Jan sured. As shown below, we observed that some specimens did gain some VRM. Mayen (22), Costa Rica (23), Spitsbergen (22), and Hawaii (24); some speci- Two years after putting the first group in a fixed field, we gave the second set a mens are archaeological and synthetic slag samples (25, 26). fresh TRM in the same field, i.e., 70 μT, by cooling from
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