New archaeomagnetic directions from Portugal and evolution of the geomagnetic field in Iberia from Late Bronze Age to Roman Times

This study presents new archaeomagnetic results from 33 combustion structures (kilns and hearths) from the archaeological sites of Castelinho, Crestelos, Olival Poço da Barca and Fonte do Milho in NE Portugal. The age of the investigated structures ranges from 1210 BC to 200 AD according to calibrated radiocarbon dating, thermoluminescence dating and archaeological constraints. Stepwise thermal and alternating field demagnetization isolate a single, stable, characteristic remanence component with very well defined directions. Rock magnetic analyses suggest low-Ti titanomagnetite/maghemite as the main magnetic carrier of the remanence. Mean directions are well grouped in most structures. The effect of thermoremanent anisotropy on mean directions has been evaluated and was found to be important. Inclination increases of between 2° and 13° after applying the anisotropy correction at specimen level. This highlights the requirement of evaluating this effect on the directions of small and flattened thin kilns and hearths. The 31 new directional data improve both the temporal and spatial distribution of the Iberian archaeomagnetic dataset from Late Bronze Age to Roman Times. Finally, a new directional palaeosecular variation curve for Iberia for the last twelve centuries BC is proposed. The curve has been computed using the bootstrap method and includes data coming from sites within 900 km of Madrid. The new palaeodirectional secular variation curve for Iberia is consistent with the Western European palaeosecular variation curve and with the prediction of regional European models

Dating the breccia of the base of the Jurassic sequence by means of the analysis of widespread remagnetization in the Iberian Ranges

Se ha utilizado el estudio de la reimanación regional cretácica de la Cordillera Ibérica para acotar la edad de formación de un afloramiento de brechas de la base del Jurásico (Fm. Cortes de Tajuña) del norte de la Cordillera Ibérica. Los experimentos de magnetismo de las rocas y el análisis de la NRM demuestran que los cantos de la brecha han registrado la reimanación pero muestran direcciones aleatorias, lo que implica que la formación de la brecha es post-reimanación y por tanto posterior al Cretácico Inferior. La polaridad inversa registrada por la matriz confirma que la brechificación es posterior al Supercrón Normal del Cretácico, es decir post-Santoniense.From the study of remagnetization of Cretaceous age in an outcrop of the widespread breccia unit of the Lower Jurassic sequence (Cortes de Tajuña Fm.) in the northern Iberian Range, the age of formation of this unit can be inferred. Rock magnetism experiments and NRM analyses indicate that breccia clasts are remagnetized and show paleomagnetic vectors with random orientation, what demonstrates that breccia formation post-dates the Early Cretaceous remagnetization stage. The breccia matrix records a reverse paleomagnetic vector thus confirming that brecciation must post-date the Cretaceous Normal Superchron (i.e. they must be post-Santonian in age).Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEMinisterio de Educación y Cienciapu

Estudio paleomagnético del dique de Messejana-Plasencia

A paleomagnetic investigation of 39 sites (591 samples) across the 530 km ot the Messejana-Plasenciadike has been carried out. Rock magnetic experiments indicate PSD low Ti titanomagnetite and magnetite as the minerals carrying the NRM. The samples where mostly demagnetised by thermal demagnetisation. Most sites exhibit a characteristic remanent component of normal polarity with the exception of two sites, where samples with reversed polarities have been observed. The paleomagnetic pole derived from the sites is well defined, with values ot Plat=70.5, Plong=238.0, K= 47.8 and ags=3.5. Paleomagnetic data indicates: (i) the dike had a brief emplacement time, (ii) the age of intrusion can be constrained between 180-200 Ma, (Hi) the high grouping of the VGPs directions suggest no important tectonic perturbations of the whole structure of the dike since its intrusion to the present

Datación paleomagnética de la parte sur del dique de Messejana- Plasencia

A Paleomagnetic investigation of 11 sites across the southern part of Messejana-Plasencia dike has been carried out. The magnetic behaviour o f pilot samples indicates that thermal demagnetisation is more effective than alternating field cleaning (AF) in isolating the Characteristic Remanent Magnetisation (ChRM). Therefore thermal demagnetisation was systematically used with the remaining samples. The ChRM is well grouped. Most sites exhibit normal polarities with the exception of two sites, where samples with reversed polarities have been observed. The paleomagnetic pole derived from the sites is well defined, with values of Plat=72.5, Plong=243.7 and Al);=6.0. This coincides with the pole calculated in a previous investigation (Schott et at. 1981) o f the dike. It is also in agreement with Toarcian poles from the Iberian Range (Osete et al. 2000) and statistically different from the Oxfordian Pole obtained in the same region. Previous radiometric studies suggested that the age o f dike intrusion is 160-200Ma. Paleomagnetic data indicates that the age o f intrusion can be constrained between 180-200Ma

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.Financial support was given by the Spanish Ministry of Science and Innovation thorough the projects CERAC (HAR2016-75133-C3-1-P), CONCERAC (HAR2016-75133-C3-3-P), SPYKE (CGL2017-87015-P), SÚMATE (PID2020-113316GB-I00) and PULSES 5.K (PID2020-117105RB-I00). R. Bonilla-Alba thanks the FPI fellowship (PRE2018-085949) associated with the SPYKE project. This study has been also funded by the Ramón y Cajal program (contracts of M. Gómez-Paccard and V. Martínez-Ferreras, refs. RYC-2013-14405 and RYC-2014-15789). The authors wish to acknowledge the professional support of the CSIC Interdisciplinary Thematic Platform Open Heritage: Research and Society (PTI-PAIS). E. Beamud thanks the Geomodels Research Institute (UB) and the Grup de Geodinàmica i Anàlisi de Conques-2017SGR596 (Generalitat de Catalunya). A. Palencia-Ortas thanks the PTA contract of the Spanish Ministry of Science and Innovation

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

[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