Archeointensity Data Research Articles

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.

Archeomagnetic intensity investigations of French medieval ceramic workshops: Contribution to regional field modeling and archeointensity-based dating

Seven new archeointensity data are obtained through the analysis of groups of pottery and kiln fragments from ceramic workshops unearthed in France, precisely dated from the High Middle Ages. The measurements are carried out using the Triaxe magnetometer, following a dedicated experimental protocol that takes into account the effects of anisotropy and cooling rate (CR) on thermoremanent magnetization acquisition. The new data are consistent with the evolution of intensity variations described by our previous data obtained in France and Northern Italy, which display between the 5th and 10th c. a pronounced camel-back shape. In particular, they provide supporting evidence of an intensity minimum that occurred around the transition between the 7th and 8th century. These data, combined with a selection of previously published results within a 700 km radius of Beaune and re-examined based on CR correction, formed the basis of new regional mean intensity variation curves based on two independent modeling approaches. The first algorithm developed by Thébault and Gallet (2010) based on bootstrapping and now irregularly spaced knots according to the data distribution gives rather smooth intensity variations, while the second approach proposed by Livermore et al. (2018) based on a transdimensional Bayesian technique shows more abrupt variations with sometimes stronger amplitudes. We explore the dating potential of these two variations curves, which have an unprecedented resolution, by studying two medieval pottery workshops. Six fragment groups (three per workshop) are analyzed using the Triaxe protocol, providing mean archeointensity values for each of the two sites. Two different procedures are used for their dating, either by comparing the intensity value to be dated with the reference intensity variation curves obtained from the two modeling techniques or by analyzing the marginal posterior probability distribution of the age values derived from the method of Livermore et al. (2018). For France, the two techniques yield very similar results. The archeointensity dating results combined with archeological arguments and radiocarbon data, make it possible to better constrain the age of the end of activity of the two workshops. Archeointensity investigation of displaced materials thus appears as an effective means to obtain original chronological constraints on the age of their production, paving the way for a wide range of complementary research on Medieval pottery.

Geomagnetic field intensity changes in the Central Mediterranean between 1500 BCE and 150 CE: Implications for the Levantine Iron Age Anomaly evolution

The magnitude and origin of the Levantine Iron Age geomagnetic Anomaly (LIAA), which spanned the first half of the first millennium before the common era, are not yet well understood. Recent archeomagnetic studies from the Levant and Western Europe suggest a western drift of this feature, stressing the importance of investigating the temporal and spatial behaviour of this event over the Central Mediterranean area. To analyse this issue, we here present 37 new archeointensity data obtained from the archeomagnetic study of 118 ceramics and brick fragments collected in 8 archeological sites in Greece and Italy with ages ranging between 1500 BCE and 150 CE. The samples were analysed using the classical Thellier and Thellier method for paleointensity determination, including the correction for the anisotropy effect of the thermoremanent magnetization (TRM) and for the cooling rate dependence upon TRM acquisition. The results reveal the first evidence of a high-intensity peak in Greece between 1070 and 1040 BCE associated to high virtual axial dipole moment (VADM) values of around 140 ZAm2. A global analysis of available paleointensities suggests that the origin of these high values is the same to the one which produced the maximum VADM of the LIAA in the Levantine region. Our results suggest that the source of the LIAA is located in the Levantine region vanishing to the north, to the west and to the east where lower VADMs are observed. In addition, another high intensity maximum, less pronounced than the one of the LIAA, seems to be present around 500 BCE all over Europe, from the Canary Islands to Turkey showing similar VADM values (around 150 ZAm2) in the different regions. Both events seem to span over a large region at the Earth's surface covering more than 60° of longitude, verifying an Earth's outer core origin for these intensity features.

Synchronizing Geomagnetic Field Intensity Records in the Levant Between the 23rd and 15th Centuries BCE: Chronological and Methodological Implications

AbstractArcheomagnetic records are an important source of information on the past behavior of the geomagnetic field. Frequently, however, coeval archeomagnetic intensity (archeointensity) datasets from nearby locations display significant discrepancies, hampering precise reconstruction of high‐resolution secular variation curve. This is the case for the time interval between the later phase of the Early Bronze and the early phase of the Late Bronze Ages (23rd–15th centuries BCE) in the Levant and Mesopotamia. We address the problem by cross‐correlating archeointensity datasets from four major multilayered archeological sites in the southern Levant (Hazor and Megiddo), northern Levant (Ebla), and western Upper Mesopotamia (Mari). We report new archeointensity data, obtained using the Thellier‐IZZI‐MagIC and the Triaxe methods, from six strata at Hazor and four radiocarbon‐dated strata at Megiddo. From 39 pottery fragments, 199 specimens passed our selection criteria, from which we calculated the mean archeointensity for each stratum. To strengthen the comparison of these data with previously published data from Mari and Ebla, obtained using the Triaxe method, we conducted a blind test of the methods that resulted in indistinguishable results or a difference of less than 1 μT. The synchronized compilation, constrained by radiocarbon data from Megiddo, displays a V‐shaped pattern with a prominent minimum of at least 200 years centered around the 18th century BCE. The study highlights the importance of stacking archeomagnetic data obtained by different archeointensity methods only after cross‐testing the methods and ensuring that archeological samples were dated in a consistent manner.

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.

New archeointensity data from South Brazil and the influence of the South Atlantic Anomaly in South America

We obtained six new high-quality archeointensity results for the Pelotas city region, in South Brazil with ages ranging from 1790 to 1943 CE. Archeointensity measurements were performed with the double heating technique, including partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks. Measurements were corrected from anisotropy of TRM and cooling-rate effects. Archeological sites providing successful results comprised: Charqueada Santa Bárbara (1790–1838 CE; 36.4 ± 0.6 μT), Chácara da Brigada Militar (1830–1851 CE; 32.6 ± 2.9 μT), Casa Número 08 (1878–1879 CE; 32.1 ± 3.1 μT), Colégio da Família de Eliseu Maciel (1881–1883 CE; 28.7 ± 2.7 μT), Fundação Simon Bolívar (1884–1889 CE; 30.1 ± 2.0 μT), Campus Porto UFPel (1942–1943 CE; 27.8 ± 2.0 μT). Two other sites did not provide meaningful results. The new data complement previous results obtained in South and Southeast Brazil, Argentina and Chile and provide additional evidence for the rapid decay of the field in the region from about 1500 to 2000 CE. In addition, they show a gradual increase in the contribution of non-dipolar components in the geomagnetic field of South America since 1800 CE, likely indicating a strengthening of the South Atlantic Anomaly over the continent from this time onward.

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.

New archeointensity data from Novgorod (North-Western Russia) between c. 1100 and 1700 AD. Implications for the European intensity secular variation

Reconstructing the secular variation of Europe’s geomagnetic field over the past millennium is challenging because of the lack of recently acquired archeomagnetic data from Western Russia. In this paper, we report on nine new archeointensity values obtained from groups of brick fragments sampled in Novgorod (North-Western Russia) and its vicinities. These fragments were collected from churches whose precise ages range from the beginning of the 12th century to the end of the 17th century AD. All the archeointensity measurements were carried out using the Triaxe experimental protocol, which takes into account the thermoremanent magnetization (TRM) anisotropy effect. Intensity determinations were performed using fast and slow cooling rates for laboratory-TRM acquisition. The results confirm that the Triaxe protocol overcomes the TRM cooling rate dependence. The new data shows that geomagnetic field intensities in North-Western Russia have decreased in the past millennium. Comparisons were made with other data previously obtained in Western Europe, the Balkans and Russia, as well as with intensity values expected in Novgorod from global geomagnetic field models. These comparisons yielded three main results: 1) The new archeointensity data do not show the occurrence of large intensity variations in North-Western Russia, as those observed in the Balkan dataset. Conversely, they appear more compatible with Western European results, which suggests a limited non-dipole field effect across Europe during the past millennium; 2) Our data are weaker than the intensity values expected in Novgorod from the available global geomagnetic field models. This suggests that the field models are inaccurate for the Novgorod area; 3) A constant linear decrease of the geocentric axial dipole moment since 1600 AD does not appear compatible with our younger data.

An integrated magnetic, geochemical and archeointensity investigation of casting debris from ancient metallurgical sites of Michoacán, Western Mesoamerica

The ability of baked clay-elaborated objects to record the temporal variations of both direction and intensity of the geomagnetic field at the time of their elaboration or last use has been widely used during the last decades to fill the gaps left by available time-discrete volcanic-rock logs for a better knowledge of the evolution of the geomagnetic field. Further refinement of the secular variation curves has motivated the search for alternative non-conventional materials as attractive targets for archeomagnetic studies. Both clay and copper have been used since pre-Hispanic times for the elaboration of a wide variety of ornamental and utilitarian objects in Mesoamerica. Previous studies carried out in Europe and Israel have shown the usefulness of copper slags as one of these non-conventional materials. We present the results of an integrated study carried out on ten copper slags from seven ancient metallurgical sites of Michoacan. The analysis included a series of rock magnetism experiments, scanning electron microscope analysis, archeointensity determinations following the Thellier-Coe method and their corresponding dating. Based on the results obtained the slag-set analyzed can be divided in two main groups: low Fe concentrations (associated with high Cu concentrations) for slags comprising the first group, and high Fe concentrations (associated with low Cu concentrations) for slags from second group. Slags from the first group are characterized by almost reversible thermomagnetic curves, while those corresponding to the second group show a quite irreversible behavior. A crystalline (vitreous) structure within the massive surface of the slags is observed for those coming from the second (first) group. These striking features could reflect significant differences in the metallurgical process followed at these sites, which according to archeological and historical evidences correspond to Colonial and pre-Hispanic usage metallurgical sites; first and second group, respectively. The good experimental behavior exhibited by these materials demonstrates the suitability of copper slag to obtaining absolute archeointensity data which can be used to improve the existing Mesoamerican archeomagnetic record. This in turn, can be used both for geomagnetic field modelling purposes and archeomagnetic dating.

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.

A detailed rock-magnetic and archaeomagnetic investigation on wattle and daub building (Bajareque) remains from Teuchitlán tradition (nw Mesoamerica)

We carried out a detailed mineral magnetic and archaeomagnetic investigation on wattle and daub (locally known as Bajareque) building remains recovered at Teuchitlán archeological site (Jalisco, Mexico) in order to estimate the suitability of such material for Thellier double heating absolute geomagnetic intensity determinations and to contribute to the still incipient archaeomagnetic database for Mesoamerica. The single-domain Ti-poor titanomagnetite and hematite seem to be main magnetic carriers in most of samples. Although the presence of hematite is confirmed for all samples, its contribution to the remanent magnetization seems negligible and the most part of remanent magnetization is carried by almost magnetite phase. All 42 analyzed Bajareque samples yielded technically acceptable archaeointensity determination, which is probably a unique case since a 100% of success rate is reached. Present study underlines the exceptional magnetic properties of Bajareque samples and their high suitability for absolute geomagnetic intensity determinations. Selected mean archeointensity data currently available for Mesoamerica show unusually high dispersion, which makes it difficult to make any firm conclusion about the evolution of Earth's magnetic field through archeological time.

Holocene archeointensities from mid European ceramics, slags, burned sediments and cherts

The Earth’s geomagnetic field intensity in the past can be determined from archeological artifacts. These archeointensity data are important inputs for geomagnetic field models and local reference curves of Earth’s magnetic field. Although archeointensities have been measured on materials for more than half a century ago, data are still scarce before 1000 BC and for the Alpine area in general. This investigation presents new absolute archeointensity data from a time period of 5000–700 BC from Italy and Switzerland. The archeological materials that were studied are ceramics, copper slag, and burned sediments from fireplaces. In addition, we investigated archeointensities from burned cherts, in order to uncover if they are a suitable material for paleomagnetism. Rock magnetic properties of all samples indicate magnetite, and small amounts of maghemite and hematite in the pseudosingle domain range as the ferromagnetic carriers. The IZZI protocol was used for 96 specimens to obtain absolute intensities; 23 ceramics, slags and burned cherts passed the threshold criteria, which we applied. The choice of the threshold values allowed us to obtain the linear part in the Arai diagram, which corresponds to the characteristic remanent magnetization. Burned sediments did not pass the threshold criteria, most probably because they acquired a thermochemical remanent magnetization during their formation. We demonstrate that magnetic susceptibility and saturation isothermal remanent magnetization can be used to select cherts that are suitable for paleointensity determinations. After applying anisotropy and cooling rate corrections, the new archeointensity values are lower for some samples, but fit well with available models and other archeomagnetic data.

New Late Neolithic (c. 7000–5000 BC) archeointensity data from Syria. Reconstructing 9000 years of archeomagnetic field intensity variations in the Middle East

We present new archeomagnetic intensity data from two Late Neolithic archeological sites (Tell Halula and Tell Masaïkh) in Syria. These data, from 24 groups of potsherds encompassing 15 different time levels, are obtained using the Triaxe experimental protocol, which takes into account both the thermoremanent magnetization anisotropy and cooling rate effects on intensity determinations. They allow us to recover the geomagnetic intensity variations in the Middle East, between ∼7000 BC and ∼5000 BC, i.e. during the so-called pre-Halaf, proto-Halaf, Halaf and Halaf-Ubaid Transitional cultural phases. The data are compared with previous archeointensity results of similar ages from Northern Iraq (Yarim Tepe II and Tell Sotto) and Bulgaria. We find that previous dating of the Iraqi material was in error. When corrected, all northern Mesopotamian data show a relatively good consistency and also reasonably match with the Bulgarian archeointensity dataset. Using a compilation of available data, we construct a geomagnetic field intensity variation curve for the Middle East encompassing the past 9000years, which makes it presently the longest known regional archeomagnetic intensity record. We further use this compilation to constrain variations in dipole field moment over most of the Holocene. In particular, we discuss the possibility that a significant dipole moment maximum occurred during the third millennium BC, which cannot easily be identified in available time-varying global geomagnetic field reconstructions.

Intensity of the geomagnetic field in Europe for the last 3 ka: Influence of data quality on geomagnetic field modeling

AbstractOne of the main challenges of paleomagnetic research is to obtain high‐resolution geomagnetic field intensity reconstructions. For the last millennia, these reconstructions are mostly based on archeomagnetic data. However, the quality of the intensity data available in the databases is very variable, and the high scatter observed in the records clearly suggests that some of them might not be reliable. In this work we investigate how the geomagnetic field intensity reconstructions and, hence, our present knowledge of the geomagnetic field in the past, are affected by the quality of the data selected for modeling the Earth's magnetic field. For this purpose we rank the European archeointensity data in four quality categories following widely accepted paleomagnetic criteria based on the methodology used during the laboratory treatment of the samples and on the number of specimens retained to calculate the mean intensities. Four geomagnetic field regional models have been implemented by applying the revised spherical cap harmonic analysis to these four groups of input data. Geomagnetic field models strongly depend on the used data set. The model built using all the available data (without any preselection) appears to be the less accurate, indicating some internal inconsistencies of the data set. In addition, some features of this model are clearly dominated by the less reliable archeointensity data, suggesting that such features might not reflect real variations of the past geomagnetic field. On the contrary, the regional model built on selected high‐quality intensity data shows a very consistent intensity pattern at the European scale, confirming that the main intensity changes observed in Europe in the recent history of the geomagnetic field occurred at the continental scale.

Geomagnetic field intensity variations in Western Europe over the past 1100 years

Ten archeointensity results have been obtained from brick and ceramic fragments collected in France and precisely dated to between the tenth and eighteenth centuries. Intensity experiments were performed using the Triaxe protocol taking into account cooling rate and thermoremanent magnetization anisotropy effects. Together with our previous results from France and Belgium, we computed a geomagnetic field intensity variation curve for Western Europe covering the past 1100 years. This curve is characterized by a general decreasing trend at the millennial timescale punctuated by three short intensity peaks, during the twelfth century, around 1350–1400 AD and ∼1600 AD. A similar evolution but with smoother variations due to data scatter is also observed in Western Europe and to a lesser extent in Eastern Europe when all available archeointensity data fulfilling quality criteria are used. Comparison of our archeointensity variation curve with the climatic record derived from fluctuations in length of the Swiss glaciers shows a good temporal concordance between all geomagnetic field intensity maxima detected in Western Europe over the past millennium and colder episodes. A comparison is further discussed between these intensity maxima and episodes of low rates of 14C production. A common pattern of variations between both records is recognized between the middle of the tenth and of the beginning of eighteenth centuries. If significant, such coincidences suggest a dual geomagnetic and solar origin for the century‐scale climate and radionuclide production variations during at least the past millennium.

Rock magnetic and geomagnetic field intensity studies on Megalithic archaeological pottery samples from Tamilnadu, India

Archeointensity (AI) data from India are necessary in order to constraint the global models of the Earth's magnetic field. Unfortunately, no attempts have been made in India for archeointensity determinations using reliable modern scientific methods. In the present study, the rock magnetic and archeointensity investigations were carried out for 40 pottery samples collected from two different archaeological sites of southern India, Marungur (Megalithic burial: 200–300 BC) and Vadaharirajapuram (Megalithic habitation: 200–100 BC). Rock magnetic analyses confirm the suitability of samples for reliable AI measurements. Thermo-magnetic (χ–T) curves for most of the samples indicate the presence of stable magnetite as the main magnetic mineral. AI values were estimated using Zero field/infield (ZI) method, which is a modified Thellier–Thellier technique. All samples were corrected for cooling rate effects. After cooling rate correction, the mean AI values are estimated as 48.22 ± 1.50 μT and 46.08 ± 0.85 μT for the Marungur and Vadaharirajapuram sites respectively. These new AI data from southern India are a little higher compared to the values predicted for the same period from the existing global geomagnetic field models, and would help to improve the global models of the Earth's magnetic field for the past few millennia.

First archeointensity determinations on Maya incense burners from Palenque temples, Mexico: New data to constrain the Mesoamerica secular variation curve

We present archeointensity data carried out on pieces of incense burners from the ancient Maya city of Palenque, Chiapas, Mexico, covering much of the Mesoamerican Classic period, from A.D. 400 to A.D. 850. We worked on pieces from 24 incense burners encompassing the five Classic ceramic phases of Palenque: Motiepa (A.D. 400–500), Cascadas (A.D. 500–600), Otulum (A.D. 600–700), Murcielagos (A.D. 700–770), and Balunté (A.D. 770–850). All the samples come from highly elaborate, flanged pedestal of incense burners that are undoubtedly assigned to a ceramic phase by means of their iconographic, morphological and stylistic analyses. Archeointensity measurements were performed with the Thellier–Thellier's method on pre-selected samples by means of their magnetic properties. We obtained archeointensities of very good technical quality from 19 of 24 pieces, allowing the determination of a precise mean value for each ceramic phase, between 29.1±0.9μT and 32.5±1.2μT. The firing temperatures of ceramics were estimated with Mössbauer spectroscopy between 700°C and 1000°C. These values ensure that a full thermo-remanent magnetization was acquired during the original heating. Our results suggest a relative stability of the field intensity during more than 400 years in this area. The abundance of archeological material in Mesoamerica contrasts with the small amount of archeomagnetic data available that are, in addition, of uneven quality. Thus, it is not possible to establish a trend of intensity variations in Mesoamerica, even using the global databases and secular variation predictions from global models. In this context, our high technical quality data represent a strong constraint for the Mesoamerican secular variation curve during the first millennium AD. The corresponding Virtual Axial Dipole Moments (VADM) are substantially smaller than the ones predicted by the last global geomagnetic models CALS3k.4, suggesting the need for additional data to develop a regional model and a reference curve for Mesoamerica.

New historical archeointensity data from Brazil: Evidence for a large regional non-dipole field contribution over the past few centuries

We report new archeointensity data obtained from the analyses of baked clay elements (architectural and kiln brick fragments) sampled in Southeast Brazil and historically and/or archeologically dated between the end of the XVIth century and the beginning of the XXth century AD. The results were determined using the classical Thellier and Thellier protocol as modified by Coe, including partial thermoremanent magnetization (pTRM) and pTRM-tail checks, and the Triaxe protocol, which involves continuous high-temperature magnetization measurements. In both protocols, TRM anisotropy and cooling rate TRM dependence effects were taken into account for intensity determinations which were successfully performed for 150 specimens from 43 fragments, with a good agreement between intensity results obtained from the two procedures. Nine site-mean intensity values were derived from three to eight fragments and defined with standard deviations of less than 8%. The site-mean values vary from ~ 25 μT to ~ 42 μT and describe in Southeast Brazil a continuous decreasing trend by ~ 5 μT per century between ~ 1600 AD and ~ 1900 AD. Their comparison with recent archeointensity results obtained from Northeast Brazil and reduced at a same latitude shows that: (1) the geocentric axial dipole approximation is not valid between these southeastern and northeastern regions of Brazil, whose latitudes differ by ~ 10°, and (2) the available global geomagnetic field models (gufm1 models, their recalibrated versions and the CALSK3 models) are not sufficiently precise to reliably reproduce the non-dipole field effects which prevailed in Brazil for at least the 1600–1750 period. The large non-dipole contribution thus highlighted is most probably linked to the evolution of the South Atlantic Magnetic Anomaly (SAMA) during that period. Furthermore, although our dataset is limited, the Brazilian archeointensity data appear to support the view of a rather oscillatory behavior of the axial dipole moment during the past three centuries that would have been marked in particular by a moderate increase between the end of the XVIIIth century and the middle of the XIXth century followed by the well-known decrease from 1840 AD attested by direct measurements.

Magnetic properties and Archeointensity of Earth’s magnetic field recovered from El Opeño, earliest funeral architecture known in Western Mesoamerica

Despite of the impressive cultural heritage and abundant archaeological sites, absolute geomagnetic intensity data from Mesoamerica are still sparse. Archeointensity determinations using the Coe variant of the Thellier and Thellier method have been carried out on some selected pottery fragments from the El Openo archeological site which has the earliest funeral architecture known in western Mesoamerica. The El Openo chronology is supported by six C 14 datings performed on carbon-bearing materials. Detailed rock magnetic experiments including susceptibility vs. temperature curves, hysteresis cycles and thermal demagnetization procedures were carried out in order to estimate the magnetic carriers and their stability. Cooling rate and anisotropy remanence corrected intensity values range from 25.0 ± 2.3 to 40.2 ± 3.0 µT and corresponding virtual axial dipole moments (VADM) range from 5.6 ± 0.5 to 8.9 ± 0.7  10 22 Am 2 . In addition, we present here a new compilation and analysis of existing absolute intensity data in order to try to estimate the variation of the Earth’s magnetic field over the past three millennia. The mean archeointensity obtained in the present study agrees reasonably well with the predicted absolute intensities retrieved from the CALS7K main field model. Other available Mesoamerican data, however, differ from this model. Most of available archeointensity data from Mesoamerica agree reasonably well with the ARCH3K main field model prediction. The broad peak defined at about 50 A.D. by our data is also predicted by the CALS3k.3 main field model but slightly displaced to the right

On the use of archeology in geomagnetism, and vice-versa: Recent developments in archeomagnetism

Abstract Archeomagnetism allows one to track the temporal evolution in direction and intensity of the Earth's magnetic field over the past few millennia, mainly from archeological material baked during their manufacturing or use. Its applications are of interest to both geomagnetism, which investigates the behavior and origin of the geomagnetic field, and archeology, which relies on curves of the magnetic field direction and/or intensity secular variation for dating purposes. Recent archeomagnetic results also suggest a connection between the geomagnetic field and climatic changes during the Holocene. In this article, we also discuss the acquisition of new archeointensity data obtained from potsherds found in the Etruscan site of La Castellina (Italy) and in Vanves (France), respectively dated to the VIIth century BC and to the IXth century AD. To cite this article: Y. Gallet et al., C. R. Physique 10 (2009).