Geomagnetic Field Intensity Variations Research Articles

An archaeomagnetic intensity-based search for order in the chaos of the destruction of Hama (Syria) dated to 720 BCE

Destruction layers of ancient settlements are extremely valuable for archaeomagnetic studies as they provide a particular archaeological context allowing for the analysis of ceramic fragments strongly linked to the date of destruction. However, when examining pottery for archaeomagnetic intensity, instead of considering a snapshot in time in the geomagnetic field record, we must also consider the various dates of their initial production, which can span several decades. This introduces an unknown time interval, which is typically shorter than the temporal resolution offered by pottery typology. In this study, we obtained new archaeomagnetic intensity data using the Triaxe protocol from 16 fragments (77 specimens) of different ceramics recovered from the destruction layer of Hama (Syria) dated to 720 BCE, caused by the troops of the Assyrian King Sargon II. The selected pottery consists of serving vessels, which are generally thought to have a short lifespan, all of which were found in royal or other important buildings. Our results reveal a significant scatter, ranging from ∼65 µT to ∼81 µT (average 74.2 ± 4.2 µT). We attribute this dispersion mainly to the time interval for the production of the pottery during the 8th century BCE, a period marked by rapid intensity fluctuations according to the currently available reference geomagnetic field intensity variation curve for the Near East. Using this curve, we show that the ceramics studied had been in use for at least 30 years at the time of the city’s destruction, a surprisingly long use-life for ceramics intended for everyday use. While we acknowledge a 30-years use-life as possible, we suggest that this minimum time interval could be a result of inaccuracies in the reference intensity variation curve during the 8th century BCE. Nevertheless, our study illustrates and confirms the ability of pottery from a destruction layer to trace rapid variations in geomagnetic field intensity. Furthermore, it establishes a chronological order of production date in ceramic assemblages found in an otherwise chaotic context.

Applicability of meteoric 10Be in dating marine sediment cores

Quaternary paleoclimatic and paleomagnetic reconstructions using marine sediment cores require appropriate dating techniques in order to build the chronology. Due to the widespread use of the radiocarbon dating method, the majority of studies focusing on paleo reconstructions are restricted to the last 50 ka. There are very few methods those can be used to obtain chronology beyond the radiocarbon dating limit. Marine sediment cores as old as 10–12 Ma can be dated using 10Be because of its comparatively long half-life of 1.39 Ma. However, there are various intricacies which restrict this method to date young marine sediment cores (<1 Ma). In this study, we provide the findings of measurements obtained for beryllium isotopes (10Be and 9Be) in a sediment core from the central Indian Ocean. It was observed that the changes associated with the decay of 10Be are much smaller than the Be isotopic fluctuations related to variations in geomagnetic field intensity. While ignoring the points showing high anomalous 10Be/9Be ratios, a continuous decrease in the 10Be/9Be ratio with depth was observed as a function of the decay of 10Be. Based on the decay of 10Be, the sediment core was dated to be ∼350 ka (at 570 cm depth) with an average sedimentation rate of 1.6 cm/ka.Radiocarbon dating in conjunction with beryllium isotope data were employed to decipher the past 10Be production changes due to variations in geomagnetic field intensity over the last 45 ka. A drastic increase in the 10Be/9Be ratio at 41.4 ka was attributed to the Laschamp geomagnetic excursion event. About 40% increase in the 10Be/9Be ratio (hence, 10Be production) was observed during this event. Another anomalous increase in the 10Be/9Be ratio was observed at ∼300 cm depth, possibly representing the Iceland Basin event. This study highlights the importance of 10Be as a chronological tool and the limitations associated due to various anomalous excursions associated with 10Be production and/or environmental modulations.

The variation of geomagnetic field intensity in Central Anatolia during the Neogene-Quaternary period

SUMMARY A detailed palaeointensity study was performed using the modified Thellier method on 18 Quaternary and Neogene volcanic units, ages ranging from 0.08 to 5.98 Ma, in Central Anatolia, Turkey. Robust data, which were estimated from 12 lava units in the study area, significantly increase the database of palaeomagnetic data, directions and absolute palaeointensity for the Anatolian region covering the time window back to ∼6 Ma. Previous studies sensitively dated the samples, except for one site dated in this study. The palaeointensity (VDM) value from the upper Miocene site estimated as 48.6 ± 9.2 μT (10.1 × 1022 ± 1.9 Am2) fits well into the Neogene VDM range in the data archives. For Pliocene samples with an age of 4.7 Ma, the palaeointensity was calculated to be 21.0 ± 4.7 μT. For these samples, an inclination of +42.9° was determined, and together with the results of low palaeointensity and normal polarity, this corresponds to the Nunivak or Sidufjall subchrons. These are normal polarity chrons within the Gilbert Chron which predominantly has reversed polarity. Palaeodirectional data and field strength with age between 0.08 and 2.57 Ma clearly showed the Brunhes and Matuyama Chrons in the Pleistocene. Three new data with 25.0 ± 7.9 μT (age 1.84 Ma), 59.7 ± 8.2 μT (age 2.15 Ma) and 79.6 ± 19.3 μT (age 2.57 Ma) from the early Pleistocene period significantly contribute to global data archives, which lack palaeointensity data from similar latitude range. The average VDM values for nine Pleistocene sites were calculated to be 51.5 ± 16.4 μT (10.3 × 1022 ± 3.7 Am2). According to the comparison of our data with the palaeointensity database, field model record and previous studies of the Pleistocene, significantly high field strength obtained from Central Anatolia, located in the Northern Hemisphere, could have occurred due to asymmetry between the Northern and Southern Hemispheres during the Pleistocene.

North Atlantic Oscillation and Variations of Geomagnetic Field

The North Atlantic Oscillation is one of the most influential climatic modes in the Northern Hemisphere. However, the mechanism(s) standing behind its wide spectra of variations is still unknown despite its numerous investigations. This paper presents evidence for a synchronization between secular variations of geomagnetic field intensity and NAO long-term variability. Analysis of the connectivity between geomagnetic secular variations and the sea-level pressure – point by point, in a grid with resolution 10 [deg] in latitude and longitude – reveals that the strength of their relation is unevenly distributed over the Northern Hemisphere. Based on the machine learning analysis over the period 1900–2019, we found that there are two centres of significant geomagnetic-pressure relations – the weaker of them is placed slightly north of Iceland, and the stronger one is in a close proximity to Azores islands. The suggested mechanism for geomagnetic influence on the near surface climatic conditions includes the geomagnetic modulation of energetic particles precipitating in Earth's atmosphere, and their impact on the lower stratospheric ozone. The analysis of ozone-pressure relation shows, in addition, reasonable similarities with the spatial patterns of geomagnetic-pressure relations.

Geomagnetic field intensity variations during the second millennium BCE: new data from the greek middle and late bronze age

The archaeointensity records from Greece present several gaps in the prehistoric period among which the ones in the third and second millennia BCE (Early and Middle Bronze Age) are not justified by the abundance of relevant settlements in the broader Greek area. Their excavations yielded numerous collections of pottery and ceramics, well-studied to a big extent from archaeological and archaeometric point of view. We collected six groups of fragments dated from 2200 BCE to 1500 BCE which were subjected to a classical archaeomagnetic study. The material response to the experiments was mostly satisfactory, and the archaeointensity was calculated both with Thellier-Thellier and multispecimen protocols. These results, complemented by the ones recently published for the period 1500- 900 BCE, and plotted versus the existing secular variation curves for Greece and relevant geomagnetic field models document a slight intensity maximum around 1900 BCE.

Tracing the geomagnetic field intensity variations in Upper Mesopotamia during the Pottery Neolithic to improve ceramic-based chronologies

The transdimensional Bayesian method AH-RJMCMC applied to archeomagnetic intensity data available in the Balkans and the Near East allows us to estimate the variations in intensity of the geomagnetic field in Upper Mesopotamia during the 7th and 6th millennia BCE (Late Neolithic), with adequate treatment of the dating and intensity uncertainties. The results for the 6th millennium BCE appear particularly interesting because there is enough data to trace rapid geomagnetic field intensity variations, with two century-scale peaks around 5800 BCE and 5550 BCE, associated with rates of changes (>0.2 μT/year) higher than the maximum rate observed in the current geomagnetic field. We show that these variations could help decipher the correlations between different archeological sequences or periodizations established from scattered sites in Upper Mesopotamia. So far documented only from the Balkan data, the intensity peak occurring around 5800 BCE may provide accurate chronological constraints for the Early Halaf phase. New insights are also obtained for the Halaf-Ubaid Transitional phase (end of the 6th millennium BCE), which remains poorly defined from an archeological point of view. The AH-RJMCMC results imply that either the archeointensity data currently available from Upper Mesopotamia document only the beginning of this phase, or that this phase occurred between ~5400 and ~5200 BCE, shorter than often considered. Such preliminary archeological inferences will progress and broaden with the addition of new archeointensity data.

Influence of Geomagnetic Field on the Long-Range Propagation of Relativistic Electron Beam in the Atmosphere

It is known that Earth’s atmosphere and geomagnetic field affect the propagation of an artificial electron beam; many previous studies have neglected the effect of their variation on the propagation dynamics. In this article, parameters in envelope equations were modified according to the International Geomagnetic Reference Field (IGRF) model and MSIS-90 atmosphere model; then, the model was constructed to study the propagation of artificial electron beam in the atmosphere. Calculation of the beam’s energy loss and radial expansion in propagation showed that the variation of geomagnetic field intensity can be the main reason for the radial pinch in the altitudes above 150 km, and the ambient particle density plays a dominant role in the beam’s radial expansion in altitudes below 150 km. The Monte Carlo model was used to simulate the interactions of a relativistic (1 MeV) electron beam with the atmosphere in the altitude range 100–150 km. Energy deposition density demonstrates that the geomagnetic field can limit the radial expansion of the beam. The relativistic electron beam was emitted horizontally in the atmosphere. The geomagnetic field can limit the beam’s radial expansion and cause the beam to propagate along the field line, irrespective of the direction in which the beam is emitted.

Imprint of magnetic flux expulsion at the core–mantle boundary on geomagnetic field intensity variations

SUMMARY During the last decade, rapid or extreme geomagnetic field intensity variations associated with rates greater than the maximum currently observed have been inferred from archeomagnetic data in the Near-East and in Western Europe. The most extreme events, termed geomagnetic spikes, are defined as intensity peaks occurring over a short time (a few decades), and are characterized by high variation rates, up to several μT yr–1. Magnetic flux expulsion from the Earth’s outer core has been suggested as one possible explanation for these peaks but has not yet been examined in detail. In this study, we develop a 2-D kinematic model for magnetic flux expulsion whose key control parameter is the magnetic Reynolds number Rm, the ratio of magnetic diffusion time to advection time. This model enables the tracking of magnetic field lines which are distorted and folded by a fixed flow pattern. Two processes govern the magnetic evolution of the system. The first is the expulsion of magnetic flux from closed streamlines, whereby flux gradually concentrates near the boundaries of the domain, which leads to an increase of the magnetic energy of the system. If the upper boundary separates the conducting fluid from an insulating medium, a second process then takes place, that of diffusion through this interface, which we can quantify by monitoring the evolution of the vertical component of magnetic induction along this boundary. It is the conjunction of these two processes that defines our model of magnetic flux expulsion through the core–mantle boundary. We analyse several configurations with varying flow patterns and magnetic boundary conditions. We first focus on flux expulsion from a single eddy. Since this specific configuration has been widely studied, we use it to benchmark our implementation against analytic solutions and previously published numerical results. We next turn our attention to a configuration which involves two counter-rotating eddies producing an upwelling at the centre of the domain, and comprises an upper boundary with an insulating medium. We find that the characteristic rise time and maximum instantaneous variation rate of the vertical component of the magnetic field that escapes the domain scale like $\sim R_m^{0.15}$ and $\sim R_m^{0.45}$, respectively. Extrapolation of these scaling laws to the Earth’s régime is compared with various purported archeointensity highs reported in the Near-East and in Western Europe. According to our numerical experiments magnetic flux expulsion is unlikely to produce geomagnetic spikes, while intensity peaks of longer duration (one century and more) and smaller variation rates appear to be compatible with this process.

Analysis of geomagnetic field intensity variations in Mesopotamia during the third millennium BC with archeological implications

We present new archeointensity results obtained at two multi-layer archeological sites, Tell Atij and Tell Gudeda (northeastern Syria), dated from the Early Bronze Period in the third millennium BC. The archeointensity data were obtained using the experimental protocol developed for the Triaxe magnetometer. In total, 68 fragments (204 specimens) of 151 fragments analyzed passed our selection criteria, allowing average intensity values to be estimated for 14 archeological layers, nine at Tell Atij and five at Tell Gudeda. Based on the available archeological constraints, the different archeological layers of Tell Atij and Tell Gudeda were dated between ∼2900 BC and ∼2600 BC and between ∼2550 BC and ∼2325 BC, respectively. The Tell Atij data show a significant increase in intensity over the dated period, while the results from Tell Gudeda exhibit a V-shape evolution. Using high-quality data available from Syria, the Levant and Turkey, a regional geomagnetic field intensity variation curve spanning the entire third millennium BC was constructed using a trans-dimensional Bayesian method. It clearly shows two intensity peaks, around 2600 BC and at ∼2300 BC, associated with variation rates of ∼0.1-0.2 μT/yr. This indicates that the occurrence of century-scale intensity peaks with rates of variation comparable to or even slightly higher than the maximum rates observed in the modern geomagnetic field is an ubiquitous feature of the geomagnetic secular variation. From an archeological point of view, the new archeointensity data strengthen the hypothesis that the successive occupation of Tell Atij and Tell Gudeda was synchronous with the two first urban phases of Mari, making possible a sustained trade network between these settlements during the third millennium BC. We further suggest that the end of Mari's first urban phase, contemporaneous with the abandonment of Tell Atij, might have been caused by a regional drought episode around 2600 BC. More generally, the Bayesian approach used to estimate the new reference intensity variation curve offers promising chronological constraints for archeological purposes.

Synchronization of Variations in Geomagnetic Field Intensity with Eustatic Cycles and Bursts of Magmatism Intensification in the Cretaceous–Early Paleogene

The relationship between cyclic processes in the core and lithosphere in the Middle Jurassic–Paleogene (167–22 Ma) was investigated. The dynamics of cyclic processes was associated with paleointensity behavior and sea level. It is shown that variations in paleointensity and ocean level are two broadband oscillatory processes. The characteristic times of variations in geomagnetic and eustatic processes changed in different geologic ages. We discovered synchronization of variations in geomagnetic and eustatic processes that occurred in the Cretaceous–Early Paleogene (125–40 Ma) based on a spectral analysis of paleointensity and sea level data. The characteristic times of coherent variations in paleointensity and global ocean level ranged from 1 to 16 Ma. Possible reasons for the synchronization of geomagnetic and lithospheric processes have been identified. Features of geomagnetic and lithospheric processes in the interval of 125–40 Ma, expressed as sharp increases in the amplitudes of paleointensity variations and activation of magmatic effusive processes, are considered.

Refining the high-fidelity archaeointensity curve for western Europe over the past millennium: analysis of Tuscan architectural bricks (Italy)

AbstractNew archaeointensity results were obtained from 14 groups of baked-brick fragments collected in and around Pisa (Tuscany, Italy). The fragments were assembled from civil and religious buildings whose dating of construction or renovation, over the past millennium, was constrained by documentary sources. This collection, analysed using the Triaxe protocol, was found particularly suitable for intensity experiments, with a success rate of c. 84% corresponding to 276 fruitful specimens associated with 125 independent brick fragments. The Tuscan data clearly show a peak in intensity at the transition between the sixteenth and seventeenth centuries. They are also in very good agreement with, and complementary to, a dense dataset previously obtained in France. Considering the results available within a 700 km radius of Beaune (between Paris and Pisa), all satisfying a set of quality criteria, a mean geomagnetic field intensity variation curve was constructed for the past millennium using a newly developed transdimensional Bayesian technique. This curve, which thus incorporates the new Tuscan results, allows a better recognition of three intensity peaks (during the twelfth century, the fourteenth century and around AD 1600) in western Europe. The detail of this curve is a clear illustration of the centennial-scale resolution that can be achieved using accurate archaeointensity data.

Multidecadal‐ to Centennial‐Scale 10Be Variations in Holocene Sediments of Huguangyan Maar Lake, South China

AbstractWe report a new high‐resolution Holocene 10Be record from Huguangyan Maar Lake in subtropical‐tropical South China, aimed to detect the atmospheric 10Be production signal in low‐latitude regions. After minimizing climatic effects by regression analyses between 10Be concentration and climatic proxies from the same archive, we successfully distinguished variations in geomagnetic field intensity and solar activity using 2,000‐year low‐ and high‐pass filtering, respectively, of the residual 10Be record (a proxy of the atmospheric 10Be production rate). The resulting 10Be‐derived record of geomagnetic field intensity is generally comparable with geomagnetic models, and the solar‐modulated 10Be signal shows significant correlations with solar activity proxies. The preservation of 10Be production signal in the sediments of this low‐latitude maar lake highlights the largely unexplored potential as well as limitations of 10Be as a tool to reconstruct variations in solar activity and geomagnetic field intensity.

Geomagnetic field in the Near East at the beginning of the 6th millennium BC: Evidence for alternating weak and strong intensity variations

This study presents new archeointensity results from the multilayered settlement Yarim Tepe I located today in Northern Iraq. Archeological evidences and new radiocarbon dates indicate that this site was occupied for about four centuries, between the end of the 7th and the beginning of the 6th millennium BC, leading to a 6.5 m-thick sequence of archeological deposits. A series of 16 groups of potsherds, with a total of 76 fragments, were collected from superimposed stratigraphic layers, with thicknesses of ∼30 cm on average. Archeointensity measurements were carried out using the Triaxe procedure, which takes into account both anisotropy and cooling rate effects on thermoremanent magnetization acquisition. In this study, 114 specimens from 40 fragments have fulfilled our selection criteria and mean archeointensity results were derived from nine different groups of fragments, with a minimum of three fragments per group. According to an age model constructed using a bootstrap approach, the new archeomagnetic record spans a time interval of ∼220 years between ∼6070 BCE and ∼5850 BCE. No significant intensity variations were observed during this time interval, with an overall mean intensity value of 42.0 ± 1.6 μT. A comparison of the Yarim Tepe I data with other archeointensity results spanning the 7th and 6th millennia BC previously obtained from the Near East, as well as from Eastern and Western Europe was conducted. The Yarim Tepe I results enabled to constrain the short duration, one century at most, of an intensity peak evidenced around 5750 BCE from the Bulgarian database. Intensity variation rates associated with the ascending branch of the peak reached values as high as ∼0.12–0.15 μT/year. Summarizing the geomagnetic field intensity variations in the Near East and Eastern Europe during the entire 6th millennium BC, it appears that they were likely characterized by the occurrence of two short-lasting intensity peaks at ∼5750 BCE and ∼5500 BCE, with intensity variation rates similar or slightly higher than the maximum rates prevailing in the modern field. Finally, it is worth mentioning that the four radiocarbon dates reported in our study provide new constraints for deciphering the temporal correlation between the cultural/historical phases (Halaf and Hassuna) that were independently defined from excavations carried out in Iraq and Syria.

Geomagnetic Field Intensity during the Neolith in the Central East European Plain

The conducted archeomagnetic studies resulted in data on variations in the geomagnetic field intensity in the central East European Plain (Sakhtysh I site area, ϕ = 56°48′ N, λ = 40°33′ E) during the time interval of 5–3 ka BC. The geomagnetic field intensity varied mainly within the range of 30–60 μT. In the first half of the 5th millennium BC, the mean level of geomagnetic field intensity was about 35 μT. In the second half of the 5th–early 4th millennium BC, it rose to about 50 μT and then decreased again to reach a mean value of about 40 μT in the period of 4–3 ka BC. Comparison of the geomagnetic field intensity variation based on the obtained data and the data on the Caucasus region for the same time interval demonstrates a certain similarity.

Ionospheric Trend Over Wuhan During 1947–2017: Comparison Between Simulation and Observation

AbstractSince Roble and Dickinson (1989), who drew the community's attention about the greenhouse gas effect on the ionosphere, huge efforts have been implemented on ionospheric climate study. However, direct comparison between observations and simulations is still rare. Recently, the Wuhan ionosonde observations were digitized and standardized through unified method back to 1947. In this study, the NCAR‐TIEGCM was driven by Mauna Loa Observatory observed CO2 level and International Geomagnetic Reference Field (IGRF) geomagnetic field to simulate their effects on ionospheric long‐term trend over Wuhan. Only March equinox was considered in both data analysis and simulation. Simulation results show that the CO2 and geomagnetic field have comparable effect on hmF2 trend, while geomagnetic field effect is stronger than CO2 on foF2 trend over Wuhan. Both factors result in obvious but different diurnal variations of foF2/hmF2 long‐term trends. The geomagnetic field effect is nonlinear versus years since the long‐term variation of geomagnetic field intensity and orientation is complex. Mean value of foF2 and hmF2 trend is (−0.0021 MHz/yr, −0.106 km/yr) and (−0.0022 MHz/yr, −0.0763 km/yr) for observation and simulation, respectively. Regarding the diurnal variation of the trend, the simulation accords well with that of observation except hmF2 results around 12 UT. Overall, good agreement between observation and simulation illustrates the good quality of Wuhan ionosonde long‐term data and the validity of ancient ionosphere reconstruction based on realistic indices driving simulation.

Rapid geomagnetic field intensity variations in the Near East during the 6th millennium BC: New archeointensity data from Halafian site Yarim Tepe II (Northern Iraq)

We present new archeointensity results from a series of groups of pottery fragments that were collected from the multi-layered archeological site Yarim Tepe II in Northern Iraq (Northern Mesopotamia) dated to the 6th millennium BC. This site comprises a 7-m-thick sequence of archeological deposits encompassing the Middle Halaf, Late Halaf and the Halaf-Ubaid Transitional (HUT), between ∼5750 and ∼5000 BC according to the chronology currently considered for the Halafian archeological period. Three new radiocarbon dates obtained from bone fragments confirm that Yarim Tepe II was likely not occupied before the Middle Halaf, as was independently established from archeological constraints. Archeointensity determinations were carried out using the protocol developed for the Triaxe magnetometer. This procedure takes into account thermoremanent magnetization anisotropy and cooling rate effects. 114 fragments fulfilled our set of archeointensity selection criteria, with intensity data obtained from at least two but most often three specimens per fragment. Mean archeointensity values were estimated for 23 groups of fragments well distributed across the entire stratigraphic sequence from the averaging of the data obtained from a minimum of three fragments per group. These values were dated using a bootstrap procedure relying on the stratigraphic position of the different groups of fragments and on the different age constraints available inside the Yarim Tepe II sequence. The new data show a significant decrease in geomagnetic field intensity by ∼12 μT between the Middle Halaf and the Late Halaf–HUT time interval. This decrease was accompanied by a short intensity peak, which may have lasted only a few decades, around the Middle Halaf–Late Halaf boundary, at ∼5500 BC. This evolution is quite similar to that observed from Syrian and Bulgarian archeointensity data, even though the precise duration of the intensity peak is presently questionable. The Bulgarian data set further suggests that the intensity secular variation in the Near East and in Eastern Europe during the 6th millennium BC was in fact principally punctuated by two successive short-lasting intensity peaks, the first around 5800 BC and the second around 5500 BC. The scarcity of the intensity data available worldwide, however, prevents us constraining the geomagnetic dipole or non-dipole origin of these features. The variation rates associated with the rapid intensity fluctuations observed in Yarim Tepe II are of ∼0.15–0.25 μT/yr. This range of values appears similar to that of rapid intensity variations that sporadically occurred in more recent times, such as in Western Europe around 700 BC and 1000 AD. In contrast, it is lower than the variation rates that were proposed for geomagnetic spikes. Our results also have interesting implications on Halafian archeology; in particular, they suggest that the Late Halaf–HUT boundary was older by ∼ one century than previously considered.

Fast geomagnetic field intensity variations between 1400 and 400 BCE: New archaeointensity data from Germany

Thirty-five mean archaeointensity data were obtained on ceramic sherds dated between 1400 and 400 BCE from sites located near Munich, Germany. The 453 sherds were collected from 52 graves, pits and wells dated by archaeological correlation, radiocarbon and/or dendrochronology. Rock magnetic analyses indicate that the remanent magnetization was mainly carried by magnetite. Data from Thellier-Thellier experiments were corrected for anisotropy and cooling rate effects. Triaxe and multispecimen (MSP-DSC) protocols were also measured on a subset of specimens. Around 60% of the samples provide reliable results when using stringent criteria selection. The 35 average archaeointensity values based on 154 pots are consistent with previous data and triple the Western Europe database between 1400 and 400 BCE. A secular variation curve for central-western Europe, built using a Bayesian approach, shows a double oscillation in geomagnetic field strength with intensity maxima of ∼70µT around 1000-900 BCE and another up to ∼90µT around 600-500 BCE. The maximum rate of variation was ∼0.25µT/yr circa 700 BCE. The secular variation trend in Western Europe is similar to that observed in the Middle East and the Caucasus except that we find no evidence for hyper-rapid field variations (i.e. geomagnetic spikes). Virtual Axial Dipole Moments from Western Europe, the Middle East and central Asia differ by more than 2·1022A·m2 prior to 600 BCE, which signifies a departure from an axial dipole field especially between 1000 and 600 BCE. Our observations suggest that the regional Levantine Iron Age anomaly has been accompanied by an increase of the axial dipole moment together with a tilt of the dipole.

New constraints on geomagnetic field intensity variations in the Balkans during the Early Byzantine period from ceramics unearthed at Thasos and Delphi, Greece

We report on five new archeomagnetic field intensity data obtained in Greece from groups of pottery fragments precisely dated to between the middle of the fourth century and the beginning of the seventh century CE. These potsherds were unearthed on the islands of Thasos (Northern Greece) and Delphi (Central Greece). Their dating is primarily ensured by typo-morphological arguments, combined with archeological and historical constraints. Archeointensity measurements were performed using the Triaxe protocol, which involves continuous magnetization measurements at high temperatures and which allows us to overcome the thermoremanent magnetization anisotropy and cooling rate effects. Magnetic mineralogy measurements such as low-field magnetic susceptibility versus temperature and thermal demagnetization of three orthogonal IRM components have identified magnetite with possible impurities as the main carrier of the magnetization. The new data range from 52.0μT to 61.5μT after reduction to Thessaloniki and show an increase in geomagnetic field intensity in Greece during the Early Byzantine period. They appear in good agreement with previous intensity results satisfying a set of quality criteria and obtained in a region of 700km around Thessaloniki, therefore incorporating data from Bulgaria, Greece and South Italy. This study is part of an ongoing effort to better constrain the evolution in geomagnetic field intensity in the Balkans over the past few millennia, with potential use for dating in archeology. The rapid intensity variations documented here during the Early Byzantine period are clearly of interest in this respect.

Variations of the main geomagnetic field intensity in the region of the Taman Peninsula during the last 13 centuries

The archaeomagnetic studies of ceramics from the Hermonassa multilayer archaeological monument in the Taman Peninsula provided the data on the intensity of the main geomagnetic field in the past. The data for the interval from VIII to XX centuries A.D. demonstrate a well pronounced decreasing trend in the geomagnetic field intensity during this time. Three stages, each lasting for a few centuries, are distinguished in the variations of the centennial average field which slightly varies within each stage and generally decreases from 70 to ~45 μT during the entire period from VIII to XX centuries A.D. The variations of the geomagnetic field in the interval from XII to XVII centuries A.D. have a form of quasi-harmonic oscillations with a characteristic time of about 300 years.

New archeointensity data from French Early Medieval pottery production (6th–10th century AD). Tracing 1500 years of geomagnetic field intensity variations in Western Europe

Nineteen new archeointensity results were obtained from the analysis of groups of French pottery fragments dated to the Early Middle Ages (6th to 10th centuries AD). They are from several medieval ceramic production sites, excavated mainly in Saran (Central France), and their precise dating was established based on typo-chronological characteristics. Intensity measurements were performed using the Triaxe protocol, which takes into account the effects on the intensity determinations of both thermoremanent magnetization anisotropy and cooling rate. Intensity analyses were also carried out on modern pottery produced at Saran during an experimental firing. The results show very good agreement with the geomagnetic field intensity directly measured inside and around the kiln, thus reasserting the reliability of the Triaxe protocol and the relevance of the quality criteria used. They further demonstrate the potential of the Saran pottery production for archeomagnetism. The new archeointensity results allow a precise and coherent description of the geomagnetic field intensity variations in Western Europe during the Early Medieval period, which was until now poorly documented. They show a significant increase in intensity during the 6th century AD, high intensity values from the 7th to the 9th century, with a minimum of small amplitude at the transition between the 7th and the 8th centuries and finally an important decrease until the beginning of the 11th century. Together with published intensity results available within a radius of 700km around Paris, the new data were used to compute a master curve of the Western European geomagnetic intensity variations over the past 1500years. This curve clearly exhibits five intensity maxima: at the transition between the 6th and 7th century AD, at the middle of the 9th century, during the 12th century, in the second part of the 14th century and at the very beginning of the 17th century AD. Some of these peaks are smoothed, or nearly absent when the selection of the data is extended to a 1250km radius around Paris. The apparent regularity in the occurrence of intensity maxima, with a recurrence of ∼250years, is particularly intriguing and might reflect a new characteristic of the secular variation, at least in Western Europe. It clearly requires further investigation and in particular the acquisition of new data from older periods.