Characteristic periods of the paleosecular variation of the Earth's magnetic field during the Holocene from global paleoreconstructions

The knowledge of the secular variation of the geomagnetic field at different time scales is important to determine the mechanisms that maintain the geomagnetic field and can help to establish constraints in dynamo theories. We have focused our study on the secular variation at millennial and centennial time scale searching for characteristic periods during the last 10 kyr. The frequency study was performed using four recent updated global paleomagnetic field reconstructions (SHA.DIF.14k, CALS10k.2, BIGMUDI4k and SHAWQ2k) by applying three techniques commonly used in signal analysis: the Fourier transform, the Empirical Mode Decomposition, and the wavelet analysis. Short-term variability of the geomagnetic field energy shows recurrent periods of around 2000, 1000–1400, and 600–800 and 250–400 years. The characteristic time around 600–800 years is well determined in all paleomagnetic reconstructions and it is mostly related to the axial dipole and axial octupole terms, but also observable in the equatorial dipole. In addition to this period, longer characteristic times of around 1000–1400 years are found particularly in the equatorial dipole and quadrupole terms in SHA.DIF.14k, CALS10k.2 and BIGMUDI4k while the 2000 year period is only well determined in the total geomagnetic field energy of SHA. DIF.14k and CALS10k.2. The most detailed paleoreconstructions for younger times also detect shortest characteristic times of around 250–400 years. The long-term variation of the geomagnetic energy is only observable in the axial dipole. A characteristic period of around 7000 years in both SHA.DIF.14k and CALS10k.2 has been found. This long period is related to two decays in the dipole field and a period of increasing intensity. The oldest decay took place between 7000 BCE and 4500 BCE and the present decay that started around 100 BCE. We have modeled the 4500 BCE up to present variation as a combination of a continuous decay, representing the diffusion term of the geomagnetic field, and one pulse that reinforces the strength of the field. Results show a characteristic diffusion time of around 11,000–15,000 years, which is compatible with the diffusion times of the dipole field used in geodynamo theories

Further progress in the study of epsilon iron oxide in archaeological baked clays

[EN] The occurrence of ε-FeO in archaeological samples that have been subjected to high temperatures is gradually being detected by the use of micrometric structural characterization techniques. This work provides new information by revealing that the ε-FeO is formed as a response to temperature, the aggregation state and the position within the baked clay with respect to the nearest heat source. In addition, depending mainly on the atmospheric environment, the temperature reached by the combustion structure, the distance from the heating source and the particle aggregation, other iron oxide magnetic phases are produced. In the baked clay studied here, hematite is found over the whole range of samples studied but its magnetic contribution is negligible. Magnetite is observed at the sample surface, probably due to local atmospheric environment closest to the combustion source. Maghemite is found at all depths up to 6 cm below the sample surface. ε-FeO has a limited distribution, found within 2–3 cm of the sample surface. Furthermore, the viability of this compound as a palaeofield marker has been evaluated in both archaeological and synthetic samples. The results indicate that ε-FeO is able to register the direction of the magnetic field. Linear palaeointensity plots have been obtained in synthetic samples, although the value of the palaeofield could be, sometimes, overestimated.The authors also acknowledge the financial support from the Spanish Ministry of Science, Innovation and Universities under the projects RTI2018-095856-B-C21, CGL2017-87015-P, CGL2017-92285-EXP, CGL2017-92285-EXP/BTE, MAT2017-86540-C4-1-R, MAT2017-87072-C4-2-P and RTI2018-095303-A-C52, from Comunidad de Madrid NANOFRONTMAG S2013/MIT-2850 and NANOMAGCOST S2018/NMT-4321, and from the European Commission under H2020 frame by AMPHIBIAN Project ID: 720853. APO thanks the Ministry of Economy, Industry and Competitiveness (PTA Contract).Peer reviewe

Late ron Age occupation at Povoado de Crestelos (Meirinhos, Mogadouro, Portugal): combustion structures and reoccupation of ditch 1 for food processing activities

The archaeological excavation undertaken at Quinta de Crestelos (Merinhos, Mogadouro, Portugal), revealed a large occupation site dated between Late Iron Age and Early Roman periods. On the ridge top a small fortified site with elaborated defensive structures, consisting of large stone walls and ditches, showed several different phases of construction and large modifications. The different phases of the site evidenced the ‘Romanization’ Process of this area and the large changes occurring in the Late Iron Age sites, with defensive structures, such as the original ditches, reused as functional and production areas. Paleomagnetism studies and analysis of the combustion structures of the later phases of the site intended to clarify nature, main components and changes during the time when they were used

Actualización de la base de datos arqueomagnética de Iberia y nuevas curvas de variación paleosecular para los últimos 3000 años

Trabajo presentado en la X Reunión de la Comisión de Paleomagnetismo de la Sociedad Geológica de España, celebrada del 14 al 17 de septiembre de 2017 en el valle del río Grío, Zaragoza (España)En este trabajo se presenta una actualización de la base de datos direccional de Iberia para los últimos 3000 años (Gómez-Paccard et al., 2006) y el primer catálogo de intensidad para este periodo. En direcciones, la base de datos ha aumentado en más de un 80%, incluyendo más de 10 resultados direccionales no publicados hasta la fecha. En intensidades, este estudio representa un aumento de más del 50% en la cantidad de datos de calidad disponibles (si nos referimos al primer milenio a.C., se cuadruplica el número de datos), entendiendo datos de calidad como aquellos que cumplen los siguientes criterios: a) que el número de especímenes empleados para la determinación de la paleointensidad sea igual o superior a 4; b) que el protocolo empleado sea Thellier (Thellier y Thellier, 1959) o derivados; c) que la anisotropía de la imanación termorremanente haya sido investigada y corregida. Toda esta información ha permitido desarrollar nuevas curvas de variación paleosecular de declinación, inclinación e intensidad de Iberia para los tres últimos milenios. Se ha empleado el método bootstrap (Thébault y Gallet, 2010) incluyendo la jerarquía asociada a la información estratigráfica cuando estaba disponible. Las nuevas curvas direccionales no presentan grandes diferencias con las anteriormente publicadas (Fig. 1), aunque se incrementa su precisión. En intensidad las principales diferencias aparecen en el primer milenio a.C. y en el posible doble máximo en torno a 600-800 años d.C, que no es recogido por esta curva. Las nuevas curvas de variación paleosecular suponen una mejora en las posibilidades de la datación arqueomagnética en la península ibéricaPeer reviewe

Further progress in the study of epsilon iron oxide in archaeological baked clays

The occurrence of ε-Fe2O3 in archaeological samples that have been subjected to high temperatures is gradually being detected by the use of micrometric structural characterization techniques. This work provides new information by revealing that the ε-Fe2O3 is formed as a response to temperature, the aggregation state and the position within the baked clay with respect to the nearest heat source. In addition, depending mainly on the atmospheric environment, the temperature reached by the combustion structure, the distance from the heating source and the particle aggregation, other iron oxide magnetic phases are produced. In the baked clay studied here, hematite is found over the whole range of samples studied but its magnetic contribution is negligible. Magnetite is observed at the sample surface, probably due to local atmospheric environment closest to the combustion source. Maghemite is found at all depths up to 6 cm below the sample surface. ε-Fe2O3 has a limited distribution, found within 2–3 cm of the sample surface. Furthermore, the viability of this compound as a palaeofield marker has been evaluated in both archaeological and synthetic samples. The results indicate that ε-Fe2O3 is able to register the direction of the magnetic field. Linear palaeointensity plots have been obtained in synthetic samples, although the value of the palaeofield could be, sometimes, overestimated

Updated Iberian archeomagnetic catalogue: new full vector paleosecular variation curve for the last three millennia

In this work, we present 16 directional and 27 intensity high‐quality values from Iberia. Moreover, we have updated the Iberian archeomagnetic catalogue published more than 10 years ago with a considerable increase in the database. This has led to a notable improvement of both temporal and spatial data distribution. A full vector paleosecular variation curve from 1000 BC to 1900 AD has been developed using high‐quality data within a radius of 900 km from Madrid. A hierarchical bootstrap method has been followed for the computation of the curves. The most remarkable feature of the new curves is a notable intensity maximum of about 80 μT around 600 BC, which has not been previously reported for the Iberian Peninsula. We have also analyzed the evolution of the paleofield in Europe for the last three thousand years and conclude that the high maximum intensity values observed around 600 BC in the Iberian Peninsula could respond to the same feature as the Levantine Iron Age Anomaly, after travelling westward through Europe

Resultados arqueomagnéticos de las estructuras de combustión del yacimiento de El Castillón. Completando la base de datos para la construcción de una curva de variación paleosecular de intensidad de Iberia

Trabajo presentado en el Symposium "El Castillón: un centro de poder a orillas del esla. De la prehistoria reciente a la edad media", celebrado en Salamanca (España), los días 8 y 9 de Junio de 2017Peer reviewe

Resultados arqueomagnéticos de las estructuras de combustión del yacimiento de El Castillón. Completando la base de datos para la construcción de una curva de variación paleosecular de intensidad de Iberia

Trabajo presentado en el Symposium "El Castillón: un centro de poder a orillas del esla. De la prehistoria reciente a la edad media", celebrado en Salamanca (España), los días 8 y 9 de Junio de 2017Peer reviewe

Aportaciones de la arqueología al estudio del campo magnético terrestre durante la segunda edad del hierro

The spatial and temporal evolution of the Earth’s magnetic fi eld in the past is a crucial point to understand its generation in the external core. But it is also required to investigate the solar activity, the 14C production and the debated relation between the geomagnetic fi eld and the past climate of the Earth. The geomagnetic field reconstructions may be obtained from studies on heated archaeological structures whose age is well-known, such as ovens, ceramic fragments, bricks, etc. From this information Palaeosecular Variation Curves (PSVC) are defi ned, describing the evolution of the geomagnetic field elements (declination, inclination and intensity) during a time interval in a specific region. The current curve for Iberia spans the last 3000 years, and it is defi ned from a sparse database, which is not even distributed, either spatial or temporally. The less defined periods (with less data) are the times previous to the Roman Age and the period between VI – IX A.D. The Palaeomagnetism Group of the Complutense University of Madrid has developed the archaeomagnetism in Spain. We are now focused on trying to fill the present gaps of the curve. Two different sites in the north of Portugal have been investigated, Castelinho and Crestelos, from the Second Iron Age. The collaboration between archaeologist and archaeomagnetist is the key point to improve the archaeomagnetic dating technique and to defi ne the past evolution of the Earth’s magnetic fi eld.El estudio de la evolución espacial y temporal del campo magnético de la Tierra en el pasado es crucial para entender su generación en el núcleo externo e investigar la actividad solar, la producción de 14C y la posible relación entre el campo geomagnético y el clima. Las reconstrucciones del campo geomagnético pueden obtenerse mediante el estudio de estructuras arqueológicas calentadas y bien datadas (hornos, hogares, cerámicas, ladrillos, etc.). Con esta información se definen las curvas de Variación Paleosecular (PSVC en sus siglas en inglés), es decir, curvas que describen la evolución de los elementos del campo geomagnético (declinación, inclinación e intensidad) a lo largo del tiempo en una cierta región. La curva actual de Iberia abarca los últimos 3000 años, y está defi nida a partir de una base de datos escasa, distribuida además de forma inhomogénea tanto a nivel espacial como temporal, siendo los periodos más críticos (con menor cantidad de datos) el anterior a la época romana y el comprendido entre los siglos VI – IX d.C. El Grupo de Paleomagnetismo de la Universidad Complutense de Madrid ha desarrollado el arqueomagnetismo en España. Actualmente, nuestros esfuerzos se centran en intentar completar las zonas con mayor falta de datos en la curva de Iberia. Con este fi n, se han investigados dos yacimientos arqueológicos al norte de Portugal, Castelinho y Crestelos, cuyas estructuras datan de la Segunda Edad del Hierro. El potencial de la colaboración entre arqueólogos y arqueomagnetas es la clave para mejorar la técnica de datación arqueomagnética y definir la evolución del campo magnético en el pasado

Estudio arqueomagnético en el yacimiento de el Castillón (Zamora)

An archaeomagnetic study in the archaeological site of El Castillón in Santa Eulalia de Tábara, Zamora (Spain) has been carried out studying samples from four ovens, two bricks and a tile. These structures have been dated from between 5th and 7th centuries A.D. New information for that period is really valuable because of the scarcity of data for Iberian Peninsula between 5th and 10th centuries. The study consisted of thermal demagnetization of 51 specimens, including anisotropy corrections and rock magnetism measures. Two directional and three intensity values were obtained: at the end of 5th century the intensity was 53.7 ± 5.6 μT; at the beginning of 6th century the intensity was 52.0 ± 6.94 μT and the direction was D = 2.7º, I = 59.5º (k=209.2; α95=6.4º); fi nally, in the 7th century the intensity was higher, 61.1 ± 3.1 μT, and the direction D = –7.2º, I = 63.5º (k=1926.2; α95=5.7º). These results have been compared with previous databases and Palaeosecular Variation Curves for Iberian Peninsula and Western Europe. For Iberian Peninsula, the new data are consistent, but for Western Europe the secondary relative maximum around 600 A.D. is not observed in the present study. This fact may mean that there is an error in the dates for the structures analysed in this study or in the French study which shows that maximum.Se ha llevado a cabo un estudio arqueomagnético en muestras de cuatro hogares, dos ladrillos y una teja recogidos en el yacimiento arqueológico de El Castillón, en Santa Eulalia de Tábara, Zamora (España). Estas estructuras están datadas mediante criterios arqueológicos entre los siglos V y VII. La obtención de nueva información para ese periodo resulta valiosa porque la disponibilidad de datos entre los siglos V y X es especialmente escasa en la península ibérica. El estudio consistió en la desimanación térmica de 51 especímenes, incluyendo corrección por anisotropía y estudio de magnetismo de rocas. El análisis de esas mediciones permitió la obtención de dos datos direccionales (declinación e inclinación) y tres de intensidad del campo magnético terrestre. Para el fi nal del siglo V se ha obtenido una intensidad de 53.7 ± 5.6 μT, para el comienzo del s.VI, una intensidad de 52.0 ± 6.94 μT y una dirección con D = 2.7º e I = 59.5º (k=209.2; α95=6.4º), mientras que para el s.VII la intensidad es mayor, 61.1 ± 3.1 μT, y la dirección resulta D = –7.2º, I = 63.5º (k=1926.2; α95=5.7º). Los resultados obtenidos han sido comparados con las bases de datos previas y curvas de variación paleosecular (evolución de la dirección y la intensidad con el tiempo) para la península ibérica y para todo el oeste de Europa. En el primer caso, los nuevos datos resultan consistentes, mientras que en el segundo el máximo de intensidad en torno al año 600 d.C. no es respaldado por lo observado en este estudio. Esto plantea la posibilidad de que exista algún error en las dataciones, bien de las estructuras de este estudio o bien de la del estudio francés que registra ese máximo