Ron Shaar

On improving the selection of Thellier‐type paleointensity data

The selection of paleointensity data is a challenging, but essential step for establishing data reliability. There is, however, no consensus as to how best to quantify paleointensity data and which data selection processes are most effective. To address these issues, we begin to lay the foundations for a more unified and theoretically justified approach to the selection of paleointensity data. We present a new compilation of standard definitions for paleointensity statistics to help remove ambiguities in their calculation. We also compile the largest-to-date data set of raw paleointensity data from historical locations and laboratory control experiments with which to test the effectiveness of commonly used sets of selection criteria. Although most currently used criteria are capable of increasing the proportion of accurate results accepted, criteria that are better at excluding inaccurate results tend to perform poorly at including accurate results and vice versa. In the extreme case, one widely used set of criteria, which is used by default in the ThellierTool software (v4.22), excludes so many accurate results that it is often statistically indistinguishable from randomly selecting data. We demonstrate that, when modified according to recent single domain paleointensity predictions, criteria sets that are no better than a random selector can produce statistically significant increases in the acceptance of accurate results and represent effective selection criteria. The use of such theoretically derived modifications places the selection of paleointensity data on a more justifiable theoretical foundation and we encourage the use of the modified criteria over their original forms.

PmagPy: Software package for paleomagnetic data analysis and a bridge to the Magnetics Information Consortium (MagIC) Database

Author(s): Tauxe, L; Shaar, R; Jonestrask, L; Swanson-Hysell, NL; Minnett, R; Koppers, AAP; Constable, CG; Jarboe, N; Gaastra, K; Fairchild, L | Abstract:

A New Chronological Framework for Iron Age Copper Production at Timna (Israel)

This paper presents the results of the 2009 excavations at Site 30 in the Timna Valley, Israel. The results, coupled with a suite of 11 new radiocarbon dates, fix the chronology of the site between the 11th and 9th centuries B.C.E. and challenge the previous chronological framework of the copper production activities in the southern Arabah Valley. The paper also presents a striking correlation between Site 30 and the recently reported archaeological record of Iron Age Faynan, indicating technological and social unity between the two regions. In light of the new results and reexamination of previously published materials, we suggest that the peak in copper production in the southern Arabah occurred after the Egyptians had left their small outpost at Timna; this activity was an offshoot of the more elaborate enterprise at Faynan. The well-organized Iron Age copper production in the Arabah Valley was based on local initiatives and conducted by local seminomadic tribes, probably belonging to the Edomite polity.

Decadal-scale variations in geomagnetic field intensity from ancient Cypriot slag mounds

Geomagnetic models based on direct observations since the 1830s show that the averaged relative change in field intensity on Earths surface over the past 170 years is less than 4.8% per decade. It is unknown if these rates represent the typical behavior of secular variations due to insufficient temporal resolution of archaeomagnetic records from earlier periods. To address this question, we investigate two ancient slag mounds in Cyprus—Skouriotissa Vouppes (SU1, fourth to fifth centuries CE, 21 m in height), and Mitsero Kokkinoyia (MK1, seventh to fifth centuries BCE, 8 m in height). The mounds are multilayered sequences of slag and charcoals that accumulated near ancient copper production sites. We modeled the age-height relation of the mounds using radiocarbon dates, and estimated paleointensities using Thellier-type IZZI experiments with additional anisotropy, cooling rate, and nonlinear TRM assessments. To screen out ambiguous paleointensity interpretations, we applied strict selection criteria at the specimen/sample levels. To ensure objectivity, consistency, and robust error estimation, we employed an automatic interpretation technique and put the data available in the MagIC database. The analyses yielded two independent subcentury-scale paleointensity time series. The MK1 data indicate relatively stable field at the time the mound accumulated. In contrast, the SU1 data demonstrate changes that are comparable in magnitude to the fastest changes inferred from geomagnetic models. We suggest that fast changes observed in the published archaeomagnetic data from the Levant are driven by two longitudinally paired regions, the Middle East and South Africa, that show unusual activity in geomagnetic models.

Six centuries of geomagnetic intensity variations recorded by royal Judean stamped jar handles

Significance Understanding the geomagnetic field behavior in the past, and, in particular, its intensity component, has implications for various (and disparate) fields of research, including the physics of Earth’s interior, atmospheric and cosmologic sciences, biology, and archaeology. This study provides substantial data on variations in geomagnetic field intensity during the eighth to second centuries BCE Levant, thus significantly improving the existing record for this region. In addition, the study provides further evidence of extremely strong field in the late eighth century BCE (“geomagnetic spike”), and of rapid rates of change (>20% over three decades). The improved Levantine record is an important basis for geophysical models (core−mantle interactions, cosmogenic processes, and more) as well as a reference for archaeomagnetic dating. Earth’s magnetic field, one of the most enigmatic physical phenomena of the planet, is constantly changing on various time scales, from decades to millennia and longer. The reconstruction of geomagnetic field behavior in periods predating direct observations with modern instrumentation is based on geological and archaeological materials and has the twin challenges of (i) the accuracy of ancient paleomagnetic estimates and (ii) the dating of the archaeological material. Here we address the latter by using a set of storage jar handles (fired clay) stamped by royal seals as part of the ancient administrative system in Judah (Jerusalem and its vicinity). The typology of the stamp impressions, which corresponds to changes in the political entities ruling this area, provides excellent age constraints for the firing event of these artifacts. Together with rigorous paleomagnetic experimental procedures, this study yielded an unparalleled record of the geomagnetic field intensity during the eighth to second centuries BCE. The new record constitutes a substantial advance in our knowledge of past geomagnetic field variations in the southern Levant. Although it demonstrates a relatively stable and gradually declining field during the sixth to second centuries BCE, the new record provides further support for a short interval of extreme high values during the late eighth century BCE. The rate of change during this “geomagnetic spike” [defined as virtual axial dipole moment > 160 ZAm2 (1021 Am2)] is further constrained by the new data, which indicate an extremely rapid weakening of the field (losing ∼27% of its strength over ca. 30 y).

Further evidence of the Levantine Iron Age geomagnetic anomaly from Georgian pottery

Recent archaeomagnetic data from ancient Israel revealed the existence of a so-called “Levantine Iron Age geomagnetic anomaly” (LIAA) which spanned the first 350 years of the first millennium before the Common Era (B.C.E.) and was characterized by a high averaged geomagnetic field (virtual axial dipole moments, VADM > 140 Z Am2, nearly twice of todays field), short decadal-scale geomagnetic spikes (VADM of 160–185 Z Am2), fast field variations, and substantial deviation from dipole field direction. The geographic constraints of the LIAA have remained elusive due to limited high-quality paleointensity data in surrounding locations. Here we report archaeointensity data from Georgia showing high field values (VADM > 150 Z Am2) in the tenth or ninth century B.C.E., low field values (VADM < 60 Z Am2) in the twelfth century B.C.E., and fast field variation in the fifth and fourth centuries B.C.E. High field values in the time frame of LIAA have been observed so far only in three localities near the Levant: Eastern Anatolia, Turkmenistan, and now Georgia, all located east of longitude 30°E. West of this, in the Balkans, field values in the same time are moderate to low. These constraints put geographic limits on the extent of the LIAA and support the hypothesis of an unusually intense regional geomagnetic anomaly during the beginning of the first half of the first millennium B.C.E., comparable in area and magnitude (but of opposite sign) to the presently active South Atlantic anomaly.

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.

A Rejoinder on the Value of Archaeomagnetic Dating: Integrative Methodology Is the Key to Addressing Levantine Iron Age Chronology

Archaeomagnetic dating is a firmly established dating technique applicable to a wide variety of heat-treated anthropological materials and is advantageous for sites that lack materials suitable for radiocarbon dating. To correct recent misinterpretations of the method, we provide examples of how archaeomagnetic dating curves are calibrated and show how, in some instances, the technique can provide superior results. We emphasize that no single dating technique is capable of resolving the challenging chronology controversies in the Levant, and instead argue that multiple dating methods must be integrated in order to achieve the highest possible temporal resolution.

Quantitative Vectorial Magnetic Imaging of Multi-Domain Rock Forming Minerals Using Nitrogen-Vacancy Centers in Diamond

Magnetization in rock samples is crucial for paleomagnetometry research, as it harbors valuable geological information on long term processes, such as tectonic movements and the formation of oceans and continents. Nevertheless, current techniques are limited in their ability to measure high spatial resolution and high-sensitivity quantitative vectorial magnetic signatures from individual minerals and micrometer scale samples. As a result, our understanding of bulk rock magnetization is limited, specifically for the case of multi-domain minerals. In this work, we use a newly developed nitrogen-vacancy magnetic microscope, capable of quantitative vectorial magnetic imaging with optical resolution. We demonstrate direct imaging of the vectorial magnetic field of a single, multi-domain dendritic magnetite, as well as the measurement and calculation of the weak magnetic moments of an individual grain on the micron scale. These results pave the way for future applications in paleomagnetometry and for the fundam...

The First Catalog of Archaeomagnetic Directions From Israel With 4,000 Years of Geomagnetic Secular Variations

The large and well-studied archaeological record of Israel offers a unique opportunity for collecting high resolution archaeomagnetic data from the past several millennia. Here, we initiate the first catalog of archaeomagnetic directions from Israel, with data covering the past four millennia. The catalog consists of 76 directions, of which 47 fulfill quality selection criteria with Fisher precision parameter (k) ≥ 60, 95% cone of confidence (α95) < 6° and number of specimens per site (n) ≥ 8. The new catalog complements our published paleointensity data from the Levant and enables testing the hypothesis of a regional geomagnetic anomaly in the Levant during the Iron Age proposed by Shaar et al (2016, 2017). Most of the archaeomagnetic directions show <15° angular deviations from an axial dipole field. However, we observe in the 10th and 9th century BCE short intervals with field directions that are 19°-22° different from an axial dipole field and inclinations that are 20°-22° steeper than an axial dipole field. The beginning of the first millennium BCE is also characterized with fast secular variation rates. The new catalog provides additional support to the Levantine Iron Age Anomaly hypothesis.