Evidence of widespread Cretaceous remagnetisation in the Iberian Range and its relation with the rotation of Iberia

A palaeomagnetic investigation has been carried out at 13 sites of Jurassic age in the Iberian Range (northern Spain). Two components of remanent magnetisation have been found at each site. A primary high-temperature component shows an average counterclockwise rotation with respect to the north of 33 2º clockwise about a vertical axis corresponding to the absolute rotation of the Iberian plate since the Jurassic. A secondary low-temperature component shows a systematic declination difference of 16 4º with respect to the primary component. This indicates that a rotation of Iberia must have occurred between the two acquisition times. Comparison of the magnetisation directions with previous palaeomagnetic data and with sea-floor spreading data, constrains the age of the remagnetisation between 95 and 125 Ma. The remagnetisation may be associated with the extensional phases in the Iberian Basin in the Early Cretaceous (Barremianearly Albian) or Late Cretaceous (Cenomanian). A principal characteristic of the remagnetisation is its widespread character in the Iberian Range. Ó 1998 Elsevier Science B.V. All rights reserved

Estudio paleomagnético del volcanismo de Campos de Calatrava

A first systematic paleomagnetic study has been carried out in the upper Miocene-Pliocene volcanísm of the Campos de Calatrava region. 78 localities have been sampled and 74 oi them yíelded useiul paleomagnetíc results. A Pliocene paleomagnetic pole has been obtaíned for the {be rían Plate (Lat.= 82.7°N, Long.= 771.6°E, A95= 6.9). On the basis of the magnetíc polarity results a zoníng oi the voJcanism may be recognízed which could be explaíned by a NE-wardmigration of the volcanism in thís regíon

Rapid Intensity Decrease During the Second Half of the First Millennium BCE in Central Asia and Global Implications

Recent paleomagnetic studies have shown that important short-lived intensity fluctuations occurred during the first millennium BCE. However, the knowledge of the spatial and temporal extension of these features is still limited by the scarce availability of robust data. In this study we focus on the study of the intensity decrease that took place in Central Asia during the second half of the 1st millennium BCE after the high intensities that characterized the Levantine Iron Age Anomaly. Since previous archeointensities available for this period and region were obtained without accomplishing modern standards of quality, we present here new archeointensities that are derived from classical Thellier and Thellier experiments, including partial thermoremanent magnetization (pTRM) checks, thermoremanent magnetization (TRM) anisotropy and cooling rate corrections at the specimen level. The new 51 archeointensities, together with previous archeointensities, have been used to present a new local paleosecular variation curve for Central Asia. The results confirm the existence of an important geomagnetic field intensity decrease in South Uzbekistan from the 4th century BCE to the end of the 1st century BCE associated with rates of changes up to −15 μT/century. A critical analysis of the archeointensity global database indicates that this feature was present at continental scale, from Western Europe to Central Asia. However, this trend is not identified in other regions such as Japan or Mexico. Finally, the comparison with the dipole moment derived from recent global geomagnetic field reconstructions suggests a strong influence of non-dipolar sources upon this continental intensity feature

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

Oxiordian magnetostratigraphy of the Aguilón and Tosos sections (Iberian Range, Spain) and evidence of a pre-Oligocene overprint

A composite magnetic polarity sequence has been constructed for the middle and late Oxfordian (late Jurassic) from four overlapping sections situated in both limbs of an anticline. Two stable magnetisation components could be isolated in every sample analyzed. Both components pass the fold test: a low-temperature secondary component, with Dec. = 340.9°and Inc. = 44.9° (a 95 = 1.70), of pre-Oligocene age, showing always normal polarity, and a high-temperature primary component, with Dec. = 324.1°and Inc. = 40.6°(a95 = 2.9°).The latter shows both normal and reversed polarities and provides the geomagnetic record for the late Jurassic. The magnetostratigraphy of the four overlapping sections has given consistent results and indicates that a high frequency of reversals characterises the pattern of the geomagnetic field during the middle to upper Oxfordian. The corresponding Oxfordian paleopole is slat = 251.2°,P1ong = 55•90 (a95 = 3.1)

Archeomagnetic reference curves for the Iberian Peninsula

1 p

Archeomagnetic results from Spain

1 p.International audienceA first secular variation (SV) curve for the Iberian Peninsula was computed by hierarchical Bayesian method using a total of 134 archaeomagnetic directions with ages ranging from 775 BC to 1959 A.D. A general agreement is observed between the Iberian curve and the French and German SV curves, excepted between the 11th and 14th centuries. The analysis of these three reference curves indicates that SV in western Europe is characterized by three major directional changes at 125, 200, and 1350 A.D. It is suggested that these cusps are regional features of the geomagnetic field. Archeointensity studies were also conducted on 24 Spanish archeomagnetic sites (220 AD to 1959 AD). Intensity data were obtained using the original Thellier method with anisotropy and cooling rates corrections. Our new 24 data, together with 62 previously published results, were used to recover, by Bayesian modelling, the geomagnetic field intensity evolution over the past two millennia for western Europe. Our results indicate that the geomagnetic intensity remains more or less constant between the 1st and 4th centuries, and between the 13th and 15th centuries, whereas an important decrease occurs during the last 5 centuries. The evolution of the Earth's magnetic field intensity during High Middle Ages remains uncertain. The available geomagnetic global models predict reasonably well our data

On the reliability of archaeomagnetic dating in Iberia: two case studies from Portugal and Zamora

Archaeomagnetic investigations have been carried out on 22 combustion structures from Portugal and Spain, along the Duero River Valley at the archaeological sites of Crestelos and Olival Poço da Barca (NE Portugal) and at El Castillón (NW Spain). The age of most of the investigated structures at Portuguese sites ranges from the 3rd century BC up to Roman times according to archaeological information. At El Castillón the investigated kilns are considered to cover the 5th – 7th centuries AD. Stepwise thermal and alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions. Mean site directions were well grouped in most sites. Only the two most poorly preserved structures showed a relatively high directional dispersion. Classical Thellier paleointensity experiments were conducted on pilot specimens. Successful results were obtained in specimens from el Castillón and from Olival Poço da Barca. In contrast, alterations occurred during thermal treatment at Crestelos kilns and hearths. Archaeomagnetic dating was conducted in all sites by using six reference Palaeosecular Variation (PSV) curves provided by local studies (from Iberia and France) and by regional (SCHA.DIF.3k) and global models (ARCH3k.1 and SHA.DIF.14k). The best PSV master curves for dating purposes on the investigated structures are the French curve for the old structures (older than 200 AD) and the regional SCHA.DIF.3k model for the younger structures. The new archaeomagnetic dating results suggest that the Crestelos site was active since 265±87 BC up to 252±138 AD, with a mean occupation period between 198 BC-43 AD. There could have been a previous occupation of the site but it could not be precisely dated. Distributions of the directions from Crestelos describe a PSV trend consistent with the French curve, suggesting that the archaeomagnetic dating tool could be improved in the near future by dating selected structures by independent methods. Archaeomagnetic dating at El Castillón site provide two phases of occupation of the site, at 441±12 AD and 488±53 AD, consistent with recent radiocarbon ages from the site. The effect of TRM anisotropy on directions has been evaluated, with the results highlighting the necessity for TRM anisotropy corrections if accurate dating is to be obtained

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

Quality control of archaeomagnetic determination using a modern kiln with a complex NRM

International audienceA modern (1959) brick kiln from western Spain has been studied in order to conduct a quality control test of the archaeomagnetic method in a partially heated structure. The kiln exhibits two stable natural remanent magnetisation components: a low-temperature component(150­620 C) acquired during kiln use and a randomly oriented, high temperature component (500­680 C) acquired during the original firing of the bricks. A detailed rock magnetic study revealed a magnetic mineralogy dominated by non-stoichiometric magnetite and hematite, both of which contribute to the characteristic remanent magnetisation. Both the direction and intensity of the characteristic magnetisation have been compared with the known values for the geomagnetic field at the site location. Irrespective of the method used to determine the mean archaeomagnetic direction (principal component analysis, great circle analysis, with or without hierarchy) the results are statistically indistinguishable from each other and from the known field direction. In all cases the direction is within 5 ofthe expected value. Thellier-style archaeointensity determinations have been carried out on a smaller specimen set, with the mean result consistent with the known field value. The results demonstrate the reliability of the archaeomagnetic method in determining the featuresof the geomagnetic field in the past, even in cases with a complicated magnetic history. Furthermore, they highlight the importance of a uniform sample distribution in order to obtain truly representative values for the direction and intensity of the ancient geomagnetic field