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

Geomagnetic Field’s Intensity in Europe for the last 2000 years: the SCHA.DI.00-F geomagnetic model

El modelo direccional europeo de campo geomagnético SCHA.DI.00 válido para los últimos dos mil años ha sido completado mediante la introducción de la paleointensidad. Este modelo, SCHA.DI.00, fue desarrollado a partir de las curvas bayesianas de variación paleosecular existentes en Europa mediante el uso de la técnica de armónicos esféricos en un casquete (SCHA). El modelo regional mejora dentro de la zona de estudio (el continente Europeo) los diferentes modelos globales de campo geomagnético en el pasado. El objetivo de este estudio será usar la reciente base de datos de paleointensidad de Europa de los últimos dos mil años para generar un modelo de campo geomagnético completo (declinación, inclinación e intensidad): el modelo SCHA.DI.00-F.The SCHA.DI.00 directional model for the geomagnetic field in Europe for the last 2000 years has been updated by modelling the palaeointensity. This model, SCHA.DI.00, was developed from available Bayesian European Palaeosecular Variation Curves using the regional Spherical Cap Harmonic Analysis technique. The comparison of the palaeosecular variation curves, given by the regional model, with available archaeomagnetic data not used in its development showed an improvement with respect to the fit obtained by global archaeomagnetic models. In this paper advantage is taken of recently published palaeointensity databases to develop a complete (direction and intensity) regional archaeomagnetic model for the last 2000 years valid for the European region: the SCHA.DI.00– F model.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Educación y Ciencia (MEC)pu

South Atlantic anomaly areal extent as a possible indicator of geomagnetic jerks in the satellite Era

Geomagnetic jerks are sudden changes in the geomagnetic field secular variation related to changes in outer core flow patterns. Finding geophysical phenomena related to geomagnetic jerks provides a vital contribution to better understand the geomagnetic field behavior. Here, we link the geomagnetic jerks occurrence with one of the most relevant features of the geomagnetic field nowadays, the South Atlantic Anomaly (SAA), which is due to the presence of reversed flux patches (RFPs) at the Core-Mantle Boundary (CMB). Our results show that minima of acceleration of the areal extent of SAA calculated using the CHAOS-7 model (CHAOS-7.2 release) coincide with the occurrence of geomagnetic jerks for the last 2 decades. In addition, a new pulse in the secular acceleration of the radial component of the geomagnetic field has been observed at the CMB, with a maximum in 2016.2 and a minimum in 2017.5. This fact, along with the minimum observed in 2017.8 in the acceleration of the areal extent of SAA, could point to a new geomagnetic jerk. We have also analyzed the acceleration of the areal extent of South American and African RFPs at the CMB related to the presence of the SAA at surface and have registered minima in the same periods when they are observed in the SAA at surface. This reinforces the link found and would indicate that physical processes that produce the RFPs, and in turn the SAA evolution, contribute to the core dynamics at the origin of jerks

New perspectives in the study of the Earth's magnetic field and climate connection: the use of transfer entropy

The debated question on the possible relation between the Earth's magnetic field and climate has been usually focused on direct correlations between different time series representing both systems. However, the physical mechanism able to potentially explain this connection is still an open issue. Finding hints about how this connection could work would suppose an important advance in the search of an adequate physical mechanism. Here, we propose an innovative information-theoretic tool, i.e. the transfer entropy, as a good candidate for this scope because is able to determine, not simply the possible existence of a connection, but even the direction in which the link is produced. We have applied this new methodology to two real time series, the South Atlantic Anomaly (SAA) area extent at the Earth's surface (representing the geomagnetic field system) and the Global Sea Level (GSL) rise (for the climate system) for the last 300 years, to measure the possible information flow and sense between them. This connection was previously suggested considering only the long-term trend while now we study this possibility also in shorter scales. The new results seem to support this hypothesis, with more information transferred from the SAA to the GSL time series, with about 90% of confidence level. This result provides new clues on the existence of a link between the geomagnetic field and the Earth's climate in the past and on the physical mechanism involved because, thanks to the application of the transfer entropy, we have determined that the sense of the connection seems to go from the system that produces geomagnetic field to the climate system. Of course, the connection does not mean that the geomagnetic field is fully responsible for the climate changes, rather that it is an important driving component to the variations of the climate

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

Eccentric Dipole Evolution during the Last Reversal, Last Excursions, and Holocene Anomalies. Interpretation Using a 360-Dipole Ring Model

The eccentric dipole (ED) is the next approach of the geomagnetic field after the generally used geocentric dipole. Here, we analyzed the evolution of the ED during extreme events, such as the Matuyama-Brunhes polarity transition (~780 ka), the Laschamp (~41 ka) and Mono Lake (~34 ka) excursions, and during the time of two anomalous features of the geomagnetic field observed during the Holocene: the Levantine Iron Age Anomaly (LIAA, ~1000 BC) and the South Atlantic Anomaly (SAA, analyzed from ~700 AD to present day). The analysis was carried out using the paleoreconstructions that cover the time of the mentioned events (IMMAB4, IMOLEe, LSMOD.2, SHAWQ-Iron Age, and SHAWQ2k). We found that the ED moves around the meridian plane of 0–180◦ during the reversal and the excursions; it moves towards the region of the LIAA; and it moves away from the SAA. To investigate what information can be extracted from its evolution, we designed a simple model based on 360-point dipoles evenly distributed in a ring close to the inner core boundary that can be reversed and their magnitude changed. We tried to reproduce with our simple model the observed evolution of the ED, and the total field energy at the Earth’s surface. We observed that the modeled ED moves away from the region where we set the dipoles to reverse. If we consider that the ring dipoles could be related to convective columns in the outer core of the Earth, our simple model would indicate the potential of the displacement of the ED to give information about the regions in the outer core where changes start for polarity transitions and for the generation of important anomalies of the geomagnetic field. According to our simple model, the regions in which the most important events of the Holocene occur, or in which the last polarity reversal or excursion begin, are related to the regions of the Core Mantle Boundary (CMB), where the heat flux is low