Palaeomagnetic results from Upper Miocene and Pliocene rocks from the Internal Zone of the eastern Betic Cordilleras (southern Spain)

Palaeomagnetic and rock-magnetic studies were carried out on samples from thirteen volcanic and nine sedimentary sites of Late Miocene to Pliocene age from the Internal Zone of the eastern Betic Cordilleras. After comparing palaeomagnetic results with the expected Pliocene/Miocene direction, rotated and unrotated areas can be recognized, rotations being clockwise and counter-clockwise. Some rotations are of great magnitude. Unlike deformation in the External Betics, Late Miocene to Present block rotations in the Internal Betic Zone are non-systematic, and related to the movement of faults as local responses to the Late Miocene-Present regional stress field, due to the N140 convergence of Africa and the Iberian Peninsula

Propiedades magnéticas de sedimentos del Triásico Medio- Jurásico Inferior de la Cordillera Ibérica Occidental

A rock magnetic and palaeomagnetic study on two Middle-Late Triassic sections in the westernmost margin of the Iberian Ranges -that mostly consist of a thick sequence of interbedded mudstones and sandstones- and their Lower Jurassic top -dolomitized limestones- has been performed. Anisotropy of magnetic susceptibility thermomagnetic curves and hysteresis properties have been measured and the palaeomagnetic directions interpreted according to these analyses. Whereas the magnetic signal of the dolomitized Lower Jurassic limestone and some of the Triassic beds is unstable, multicomponent magnetisation behaviour has been observed in theTriassic red beds. The characteristic component presents high coercitivity and distributed unblocking temperatures up to 700-C (carried by hematite), showing both polarities and defining a consistent magnetic zonation. The NRM also contains lower unblocking temperature components with normal polarities. These are interpreted as result of the overlapping of a present day field component and an older overprint, probably related to the extensional phases of the Iberian Basin during the Cretaceous time. Primary remanence is better preserved in the fine-grained beds, showing a lower overlapping degree between the characteristic component and the high temperature overprint

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

Palaeomagnetism of Late Miocene to Quaternary volcanics from the eastern segment of the Trans-Mexican Volcanic Belt

A systematic palaeomagnetic study in the eastern part of the Trans-Mexican Volcanic Belt includes 39 Miocene, Pliocene and Quaternary volcanic rocks in the southeastern Mexico Basin (Sierra Nevada and Sierra de Rı´o Frı´o), the Altiplano area, and the Palma Sola Massif. A total of 430 samples have been selectively demagnetized using mostly alternating field demagnetizing methods, supplemented by thermal analyses. Most characteristic remanences are carried by low-Ti titanomagnetites, with occasional titanohematites or slightly maghemitized low-Ti titanomagnetites, of similar direction. Seven sites were discarded because they presented intermediate directions, hydrothermal alteration or were remagnetized by lightning strikes. The mean directions of 32 sites, together with 24 sites from Sierra de las Cruces in the western Mexico basin, indicate rocks older than 2 Ma are rotated some 10° counterclockwise with respect to Quaternary rocks, whereas there is no rotational difference between Miocene and Pliocene rocks. Statistical analyses between different regrouped populations confirm that the rotational pattern is due to the age of the volcanics rocks but not to their spatial distribution. The Quaternary mean direction from the three Mexico Basin ranges is consistent with the geographical reference pole. In contrast, the Pliocene mean direction from volcanic rocks of the Altiplano area and the Sierra de Las Cruces is slightly rotated some 10° westwards with respect to the reference direction from North America. No significant rotations have been observed in the eastern TMVB (from the western Mexico Basin to the border of the Altiplano), between late Miocene and late Pliocene times. It suggests that a very small, counterclockwise vertical-axis rotation may have been taken place in this segment of the TMVB between late Pliocene and Quaternary times. Comparisons of these results with a summary of the available palaeomagnetic data in the area indicate that the previously reported Quaternary rotations are of questionable reliability, and that the large counterclockwise rotations, reported in Cretaceous to Miocene rocks, probably took place before the late Miocene. These new palaeomagnetic data support the idea that the eastern TMVB since the late Miocene, has been a zone of extension with a little, left-lateral shear component

Epsilon iron oxide: Origin of the high coercivity stable low Curie temperature magnetic phase found in heated archeological materials

The identification of epsilon iron oxide (-Fe2O3) as the low Curie temperature high coercivity stable phase (HCSLT) carrying the remanence in heated archeological samples has been achieved in samples from two archeological sites that exhibited the clearest evidence of the presence of the HCSLT. This uncommon iron oxide has been detected by Confocal Raman Spectroscopy (CRS) and characterized by rock magnetic measurements. Large numbers of -Fe2O3 microaggregates (in CO) or isolated clusters (in HEL) could be recognized, distributed over the whole sample, and embedded within the ceramic matrix, along with hematite and pseudobrookite and with minor amounts of anatase, rutile, and maghemite. Curie temperature estimates of around 170 degrees C for CO and 190 degrees C for HEL are lower than for pure, synthetic -Fe2O3 (227 degrees C). This, together with structural differences between the Raman spectra of the archeologically derived and synthetic samples, is likely due to Ti substitution in the -Fe2O3 crystal lattice. The -Fe2O3--Fe2O3--Fe2O3 transformation series has been recognized in heated archeological samples, which may have implications in terms of their thermal history and in the factors that govern the formation of -Fe2O3

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

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

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