Archaeomagnetism: its application to the study the Earth’s magnetic field and the characterization of archaeological sites

El arqueomagnetismo generalmente se define como el estudio e interpretación de las propiedades magnéticas de materiales arqueológicos calentados a alta temperatura. Esta técnica proporciona una importante herramienta aplicable al ámbito de las Ciencias de la Tierra y especialmente, a la Arqueología. El arqueomagnetismo se basa en dos fenómenos físicos: la existencia de variaciones espacio-temporales del campo magnético terrestre y la capacidad de ciertos materiales arqueológicos que han sufrido procesos de calentamiento a alta temperatura de adquirir una magnetización termorremanente. Esta magnetización es estable, paralela y proporcional al campo magnético terrestre presente en el momento de enfriamiento del material arqueológico. Por ello, a partir del estudio de la termorremanencia adquirida por materiales arqueológicos de distintas edades se pueden establecer curvas de la variación secular del campo magnético terrestre. Una vez establecidas, estas curvas regionales constituyen una nueva herramienta para la datación de yacimientos arqueológicos. Además, el arqueomagnetismo puede aportar información muy valiosa en el ámbito de la Arqueología ya que permite identificar y caracterizar objetos arqueológicos quemados y fuegos paleolíticos. En este trabajo se explican de forma didáctica los principios básicos de esta técnica y se presenta un caso práctico en el que se procede a la datación arqueomagnética de un horno arqueológico.Archaeomagnetism is generally defined as the study and interpretation of the magnetic properties of archaeological baked materials. This technique provides an important tool for Earth Sciences research and particularly for Archaeology. It relies on two physical phenomena: spatial and temporal changes of the geomagnetic field and the ability of certain burned archaeological materials to acquire a thermoremanent magnetization. This magnetization is stable, parallel with and proportional to the ambient field in which the heated materials have cooled. For this reason, for any particular region secular variation curves can be constructed by studying well-dated archeological burned materials. Once established, these regional curves can be used as a dating tool for archaeological features from the same region. In addition, the study of the magnetic properties of archaeological materials can provide a wealth of information including insight into their composition, provenance and paleofiring conditions. In this contribution, the principal physical bases and applications of this technique are reviewed from a non-exhaustive didactical perspective.Junta de Castilla y León, proyecto BU066U1

First paleomagnetic results of mid-to late Holocene sediments from Lake Issyk-Kul (Kyrgyzstan): Implications for paleosecular variation in central Asia

We present new paleomagnetic and rock magnetic data from mid and late Holocene sediments recovered in two gravity cores (C087 and C142a) from Lake Issyk-Kul (central Asia), for which independent radiocarbon-based age models are available. Our results indicate that sediments from core C087 are characterized by fine (pseudo single domain) magnetite grains and are reliable recorders of Holocene geomagnetic paleosecular variation (PSV) in central Asia, which is a region with poor data coverage. Similarity is found between the core C087 PSV record, which spans the last 5700 years, and the Lake Baikal PSV record, which is the only published Holocene central Asian PSV record of comparable length with an independent radiocarbon-based chronology. Our new results represent a step forward in constructing a reference PSV curve for central Asia that can be used to date sedimentary sequences. These results can also be used to improve the reliability and accuracy of global geomagnetic field models. We have not been able to disentangle past environmental changes in the Lake Issyk-Kul region based on rock magnetic properties, probably because the magnetite and hematite in the studied sediments have a mixed (fluvial and eolian) terrigenous origin. Copyright 2012 by the American Geophysical Union.Financial support for this research was provided through a CSIC JAE-Doc post-doctoral research contract (MGP), the GRACCIE (Spanish Consolider-Ingenio CSD2007-00067), APELIK (EU ICA2-CT-2000-10003), CGL2008-02203/BTE, and PALEONAO (CGL2010-15767/BTE) research projects.Peer Reviewe

Evidence for a highly non-dipolar character of the European 800 AD event

Over the last years new evidences of several short-lived regional maxima of the geomagnetic field intensity at various times and locations have been defined. These features have important implications both for geomagnetic field modeling and for Earth0s dynamo simulations. However, the nature, extent and underlying causes of these variations are still poorly understood. Here we present a detailed analysis of the sharp abrupt intensity change that took place in Western Europe around 800 AD, the most significant geomagnetic field intensity feature observed in Europe over the last two millennia. For this purpose we present an up-to-date regional intensity reconstruction for Western Europe and compare the results with other regional and global geomagnetic field reconstructions. The results indicate that the 800 AD event is mainly controlled by non-dipolar geomagnetic sources.Peer Reviewe

Modelos globales del campo magnético terrestre para los últimos 3000 años: discrepancias según las bases de datos usadas

La modelización global del campo magnético terrestre no es sólo fundamental para comprender cómo funciona y varía en el tiempo, sino también para constreñir los modelos de geodinamo y corregir las tasas de producción de isótopos cosmogénicos. Obtener modelos suficientemente realistas depende en gran medida de las bases de datos empleadas. Los datos arqueomagnéticos y volcánicos proporcionan un registro casi instantáneo del campo, debido al proceso de adquisición de la remanencia magnética (termorremanencia). Este hecho los hace idóneos para estudiar en detalle la evolución del campo magnético terrestre. Sin embargo, su distribución espacial y temporal es muy heterogénea. Además la calidad de algunos registros, especialmente de intensidad, es muy variable, lo que produce ciertas inconsistencias en los modelos. En este trabajo presentamos cuatro modelos globales para los últimos 3000 años generados a partir del análisis en armónicos esféricos (SHA) usando diferentes bases de datos: 1) datos arqueomagnéticos (SHA3k.a), 2) datos arqueomagnéticos con un criterio de selección de calidad (SHAQ3k.a), 3) datos arqueomagnéticos y volcánicos (SHA3k.av) y 4) datos arqueomagnéticos y volcánicos aplicando un criterio de selección de calidad (SHAQ3k.av). Se evalúan las discrepancias entre ellos y se comparan con modelos globales del campo geomagnético ya publicados (SHA.DIF.14k, ARCH3k.1, CALS3k.4b, pfm9k.1a).SAC y MLO dan las gracias al proyecto CGL2014-54112-R del Ministerio Español de Economía y Competitividad y a la beca FPI BES-2012-052991, que ha permitido a SAC llevar a cabo este trabajo. FJPC agradece a su proyecto postdoctoral 659901- CLIMAGNET, programa Europeo Marie Curie.Peer Reviewe

New archaeomagnetic data recovered from the study of Roman and Visigothic remains from central Spain (3rd–7th centuries)

New archaeomagnetic results from four heated/combustion structures recovered from two archaeological sites in central Spain are reported. They have been dated by archaeological evidence and in two cases by radiocarbon dating. Rock magnetic experiments indicate low coercivity magnetic phases, such as magnetite and thermally stable maghaemite, as the main carriers of the remanent magnetization. Haematite has been observed in poorly heated baked clays. Archaeomagnetic directions have been obtained from either alternating field or thermal demagnetization experiments performed on 57 specimens coming from 46 independently oriented samples. The four well-defined archaeomagnetic directions obtained are in good agreement with previous archaeomagnetic data and with recent regional and global field models. They define the beginning of easterly declination drift that was initiated around 350–400 AD and culminated around 800–850 AD, and delineate the maximum in inclination that took place around 600–650 AD. In addition, classical Thellier–Thellier experiments including thermal remanent magnetization anisotropy and cooling rate corrections were conducted on 23 specimens. Only 13 specimens, corresponding to well-defined single component behaviour, gave reliable results. New mean archaeointensities have been obtained for two of the four studied structures (VBK1, 64.2 ± 5.0 μT and VBT1, 62.4 ± 2.6 μT). The new data suggest that two relative intensity maxima occurred in Western Europe around 320 and 630 AD, being of lower magnitude that observed in Eastern Europe.Peer reviewe

Archaeomagnetic and rock magnetic study of six kilns from North Africa (Tunisia and Morocco)

International audienceNew full-vector archaeomagnetic data for North Africa recovered from the study of six kilns, five from Tunisia and one from Morocco, are presented. Archaeological and historical considerations, along with three radiocarbon dates, indicate that the age of the kilns ranges between the 9th and 15th centuries AD. Rock magnetic analyses showed that the principal magnetic carriers are magnetite and low Ti titanomagnetite, along with variable contributions of thermally stable maghemite and a high coercivity phase with low unblocking temperatures. The magnetic mineralogy of the studied material is thermally stable and behaves ideally during archaeointensity experiments. Stepwise alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions at both specimen and structure levels. Mean archaeointensities have been obtained from successful classical Thellier experiments conducted on between five and eight independent samples per kiln. Thermoremanent magnetization (TRM) anisotropy and cooling rate effects upon TRM intensity have been investigated. The results showed that these effects are low for four of the six studied kilns, with differences between the uncorrected and corrected means of less than 3 per cent. For the other two structures differences between the uncorrected and corrected mean site intensities are 4.4 per cent and 5.8 per cent. These results highlight the necessity for TRM anisotropy and cooling rate corrections in archaeomagnetic studies if accurate archaeointensities are to be obtained. The new results suggest that high intensities occurred in Northwest Africa during the 9th century. Although more data are clearly needed to define this period of high intensity, the results are in agreement with the available European archaeointensity data. Acomparison between the newdata, other available archaeomagnetic determinations in nearby locations, and palaeosecular variation (PSV) curves derived from the regional SCHA.DIF.3k and global ARCH3K.1 geomagnetic field models shows good agreement between the new data and directional results derived from the models. However, some differences are observed between geomagnetic field models intensity results and available archaeointensity data for the studied regions. This highlights the need for new data for unexplored regions such as North Africa. The new data presented here better constrains the evolution of the geomagnetic field during historical times in this region. They represent a new step towards the construction of a reference PSV curve for Northwest Africa. Once established, this curve will represent a new dating method for this region

Author correction: multi-centennial fluctuations of radionuclide production rates are modulated by the Earth's magnetic field (vol 8, 9820, 2018)

Tis Article contains errors in the Discussion section

Multi-centennial fluctuations of radionuclide production rates are modulated by the Earth's magnetic field

The production of cosmogenic isotopes offers a unique way to reconstruct solar activity during the Holocene. It is influenced by both the solar and Earth magnetic fields and thus their combined effect needs to be disentangled to infer past solar irradiance. Nowadays, it is assumed that the long-term variations of cosmogenic production are modulated by the geomagnetic field and that the solar field dominates over shorter wavelengths. In this process, the effects of the non-dipolar terms of the geomagnetic field are considered negligible. Here we analyse these assumptions and demonstrate that, for a constant solar modulation potential, the geomagnetic field exerts a strong modulation of multi-centennial to millennial wavelengths (periods of 800 and 2200 yr). Moreover, we demonstrate that the non-dipole terms derived from the harmonic degree 3 and above produce maximum differences of 7% in the global average radiocarbon production rate. The results are supported by the identification, for the first time, of a robust coherence between the production rates independently estimated from geomagnetic reconstructions and that inferred from natural archives. This implies the need to review past solar forcing reconstructions, with important implications both for the assessment of solar-climate relationships as well as for the present and future generation of paleoclimate models

Unidad Técnica de Geodesia, Geofísica y Magnetismo de Rocas: Laboratorio de Paleomagnetismo, Magnetismo de Rocas y Modelado Geomagnético

Trabajo presentado en la I Reunión Científica del Instituto de Geociencias (CSIC-UCM), los días 15 y 16 de septiembre de 2011.Peer reviewe