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Shaar, R, Tauxe L.  2013.  Thellier GUI: An integrated tool for analyzing paleointensity data from Thellier-type experiments. Geochemistry Geophysics Geosystems. 14:677-692.   10.1002/ggge.20062   AbstractWebsite

Thellier-type experiments are a method used to estimate the intensity of the ancient geomagnetic field from samples carrying thermoremanent magnetization. The analysis of Thellier-type experimental data is conventionally done by manually interpreting data from each specimen individually. The main limitations of this approach are: (1) manual interpretation is highly subjective and can be biased by misleading concepts, (2) the procedure is time consuming, and (3) unless the measurement data are published, the final results cannot be reproduced by readers. These issues compound when trying to combine together paleointensity data from a collection of studies. Here, we address these problems by introducing the Thellier GUI: a comprehensive tool for interpreting Thellier-type experimental data. The tool presents a graphical user interface, which allows manual interpretation of the data, but also includes two new interpretation tools: (1) Thellier Auto Interpreter: an automatic interpretation procedure based on a given set of experimental requirements, and 2) Consistency Test: a self-test for the consistency of the results assuming groups of samples that should have the same paleointensity values. We apply the new tools to data from two case studies. These demonstrate that interpretation of non-ideal Arai plots is nonunique and different selection criteria can lead to significantly different conclusions. Hence, we recommend adopting the automatic interpretation approach, as it allows a more objective interpretation, which can be easily repeated or revised by others. When the analysis is combined with a Consistency Test, the credibility of the interpretations is enhanced. We also make the case that published paleointensity studies should include the measurement data (as supplementary files or as a contributions to the MagIC database) so that results based on a particular data set can be reproduced and assessed by others.

Shaar, R, Ron N, Tauxe L, Kessel R, Agnon A.  2011.  Paleomagnetic field intensity derived from non-SD: Testing the Thellier IZZI technique on MD slag and a new bootstrap procedure. Earth and Planetary Science Letters. 310:213-224.   10.1016/j.epsl.2011.08.024   AbstractWebsite

Experimental techniques to determine paleomagnetic field intensity are based on a theoretical framework that is valid only for single-domain (SD) ferromagnetic particles. Yet, most of the available materials exhibit distinctly non-SD properties. Designing the optimal paleointensity methodology for non-SD is, therefore, a fundamental challenge in paleomagnetism. The objective of this study is to experimentally test the IZZI Thellier absolute paleointensity method on small MD recorders. The test has two purposes: 1) to describe the characteristic non-SD patterns occurring in Arai plots, and 2) to identify the optimal approach in interpreting non-SD behavior. We carried out paleointensity experiments on 40 specimens from 4 synthetic re-melted slag samples with identical magnetic properties (mineralogy, texture, and non-SD state) produced under different field intensities. We ran three batches of IZZI experiments using different conditions that allow for a detailed characterization of the non-SD behavior. We find that the curvature of the Arai plot is systematically dependent on the angle and the proportion between the field used in the paleointensity experiment (B(TRM)) and the field in which the NRM was acquired (B(NRM)). Straight-line Arai plot occur when the two fields are parallel and equal, and seems to always give the 'true' slope. Convex curves occur when B(TRM) is parallel and significantly stronger than B(NRM). Concave curves occur in all the other cases and yield two end-case slopes that are always different than the 'true' slope. In addition, zigzagged patterns increase with the angle the proportion between B(TRM) and B(NRM). We test the accuracy of the 'best fitting' line approach and conclude that 'best fitting' line in curved plots cannot provide robust paleointensity estimates. Yet, the two 'end-case' slopes in concave curves provide adequate constraints for the true value. We introduce a new procedure to calculate a 95% confidence interval of the paleointensity from curved plots using bootstrap statistics. We substantiate the new procedure by conducting two independent tests. The first uses synthetic re-melted slag produced under known field intensities - 3 SD samples and 4 non-SD samples. The second compares paleointensity determinations from archeological slag samples of the same age - 34 SD samples and 10 non-SD samples. The two tests demonstrate that the bootstrap technique may be the optimal approach for non-ideal dataset. (C) 2011 Elsevier BM. All rights reserved.

Shaar, R, Ben-Yosef E, Ron H, Tauxe L, Agnon A, Kessel R.  2011.  Geomagnetic field intensity: How high can it get? How fast can it change? Constraints from Iron Age copper slag Earth and Planetary Science Letters. 301:297-306.   10.1016/j.epsl.2010.11.013   AbstractWebsite

The intensity of the geomagnetic field varies over different time scales. Yet, constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record. The purpose of this study is to place firm limits on these fundamental properties by constructing a high-resolution archaeointensity record of the Levant from the 11th century to the early 9th century BCE, a period over which the geomagnetic field reached its maximum intensity in Eurasia over the past 50,000 years. We investigate a (14)C-dated sequence of ten layers of slag material, which accumulated within an ancient industrial waste mound of an Iron Age copper-smelting site in southern Israel. Depositional stratigraphy constrains relative ages of samples analyzed for paleointensity, and (14)C dates from different horizons of the mound constrain the age of the whole sequence. The analysis yielded 35 paleointenisty data points with accuracy better than 94% and precision better than 6%, covering a period of less than 350 years, most probably 200 years. We construct a new high-resolution quasi-continuous archaeointensity curve of the Levant that displays two dramatic spikes in geomagnetic intensity, each corresponding to virtual axial dipole moment (VADM) in excess of 200 ZAm(2). The geomagnetic spikes rise and fall over a period of less than 30 years and are associated with VADM fluctuations of at least 70 ZAm2. Thus, the Levantine archaeomagnetic record places new constraints on maximum geomagnetic intensity as well as for its rate of change. Yet, it is not clear whether the geomagnetic spikes are local non-dipolar features or a geomagnetic dipolar phenomenon. (C) 2010 Elsevier B.V. All rights reserved.

Shaar, R, Ron H, Tauxe L, Kessel R, Agnon A, Ben-Yosef E, Feinberg JM.  2010.  Testing the accuracy of absolute intensity estimates of the ancient geomagnetic field using copper slag material. Earth and Planetary Science Letters. 290:201-213.   10.1016/j.epsl.2009.12.022   AbstractWebsite

The Middle-Eastern copper slag is a promising new material for studying intensity variations in the geomagnetic field with high resolution and precision. The purpose of this study is to test the accuracy of archaeointensity estimates determined using copper slag by addressing two questions: 1) "Does slag material display the magnetic properties required for valid Thellier experiments?" and 2) "What is the accuracy of the archaeointensity estimates derived from Thellier-style experiments on optimal samples?" We address the first question through a comprehensive microscopic and magnetic study of representative archaeological slag samples in order to identify the properties responsible for optimal behavior in Thellier experiments. To address the second question, we reproduced slag samples in the laboratory under controlled magnetic fields and analyzed them using the same 1721 paleointensity technique used for the ancient slag. Microscopic analyses of the archaeological slag show that ferromagnetic phases occur as three-dimensional dendritic structures whose branches consist of submicronelongated particles. Magnetic analyses show that these dendrites behave as an assemblage of shape-controlled, single-domain-like particles and that their magnetization is thermoremanent. We conclude that slag material can be magnetically suitable for valid Thellier experiments. The laboratory-produced slag material demonstrated similar magnetic and mineralogical properties as the archaeological slag. IZZI experiments showed that nonlinear TRM acquisition, even at field strengths similar to Earth's, and TRM anisotropy are important factors to monitor during paleointensity studies of slag material. Anisotropy and non-linearity are probably related to the dendritic shape of the oxide grains. Intensity estimates derived from three laboratory-produced slag samples demonstrated accuracy to within similar to 5% after applying the required corrections. (C) 2009 Elsevier B.V. All rights reserved.

Ben-Yosef, E, Tauxe L, Levy TE, Shaar R, Ron H, Najjar M.  2009.  Geomagnetic intensity spike recorded in high resolution slag deposit in Southern Jordan. Earth and Planetary Science Letters. 287:529-539.   10.1016/j.epsl.2009.09.001   AbstractWebsite

In paleomagnetism, periods of high field intensity have been largely ignored in favor of the more spectacular directional changes associated with low field intensity periods of excursions and reversals. Hence, questions such as how strong the field can get and how fast changes occur are still open. In this paper we report on data obtained from an archaeometallurgical excavation in the Middle East, designed specifically for archaeomagnetic sampling. We measured 342 specimens from 72 samples collected from a 6.1 m mound of well stratified copper production debris at the early Iron Age (12th-9th centuries BCE) site of Khirbat en-Nahas in Southern Jordan. Seventeen samples spanning 200 yr yielded excellent archaeointensity results that demonstrate rapid changes in field intensity in a period of overall high field values. The results display a remarkable spike in field strength, with sample mean values of over 120 mu T (compared to the current field strength of 44 mu T). A suite of 13 radiocarbon dates intimately associated with our samples, tight control of sample location and relative stratigraphy provide tight constraints on the rate and magnitude of changes in archaeomagnetic field intensities. (C) 2009 Elsevier B.V. All rights reserved.