Archaeomagnetic secular variation in the UK during the past 4000 years and its application to archaeomagnetic dating

This paper examines the limitations and deficiencies of the current British archaeomagnetic calibration curve and applies several mathematical approaches in an attempt to produce an improved secular variation curve for the UK for use in archaeomagnetic dating. The dataset compiled is the most complete available in the UK, incorporating published results, PhD theses and unpublished laboratory reports. It comprises 620 archaeomagnetic (directional) data and 238 direct observations of the geomagnetic field, and includes all relevant information available about the site, the archaeomagnetic direction and the archaeological age. A thorough examination of the data was performed to assess their quality and reliability. Various techniques were employed in order to use the data to construct a secular variation (SV) record: moving window with averaging and median, as well as Bayesian statistical modelling. The SV reference curve obtained for the past 4000 years is very similar to that from France, most differences occurring during the early medieval period (or Dark Ages). Two examples of dating of archaeological structures, medieval and pre-Roman, are presented based on the new SV curve for the UK and the implications for archaeomagnetic dating are discussed.     

Archaeomagnetic studies on some archaeological sites in Tamil Nadu, India

A new secular variation curve for the geomagnetic field intensity for the Tamil Nadu region, India, is obtained using bricks, potteries and tiles of known archaeological ages. The data were obtained with the aid of a large volume zero-field space and an improved version of the astatic magnetometer.

The secular variation curve can be used with advantage for archaeological dating. The ages of the recent excavations in Tamil Nadu — Kodumanal, Darasuram, Erukkur and Auroville are established in this way.     

An updated catalogue of Greek archaeomagnetic data for the last 4500 years and a directional secular variation curve

We present an updated compilation of Greek directional archaeomagnetic data for the last 4.5 millennia. The data set comprises 89 directions from archaeological artefacts and volcanic rocks. Most of the data come from the Late Bronze Age (1700-1400 BC) that is the flourishing period of the Minoan civilization in Crete, while parts of the classical (480-323 BC), Hellenistic (323-31 BC) and Roman (146 BC-330 AD) periods are also well covered. The dataset has been analysed using the Bayesian approach for curve building and a directional secular variation (SV) curve for Greece is proposed. Comparisons with regional and global model predictions show a general agreement even though some discrepancies are observed for some time intervals. The new curves together with the previously published intensity SV curve for Greece, also using the Bayesian approach, form a homogeneous set and enrich our knowledge of the full geomagnetic field vector variation in Greece during the last millennia.     

Archaeomagnetic study of ceramics from the El Molon archaeological monument (Spain

Data on the geomagnetic field intensity in the first millennium BC and the 8th–10th centuries AD are obtained from the study of materials from the El Molon archaeological monument (Spain). A 75-yr averaged curve of the variation in the geomagnetic field intensity in the area of the zero meridian over the last three millennia is constructed from a set of new and previously obtained data on the geomagnetic intensity variations in Spain and France. The field intensity smoothly decreases in the time interval studied, and variations a few centuries long are superimposed on this trend. Unlike the present-day relationship, the average level of the geomagnetic field intensity in Spain in the first millennium BC was higher than in the Caucasus.     

Constraints of archaeomagnetic dating and field intensity determinations in three ancient tile kilns in Belgium

The aim of this study is to date by the archaeomagnetic method the last heatingcooling cycle of one Roman and two Medieval tile kilns, discovered in Belgium. The investigation demonstrates the limitations when well-documented local directional secular variation curves of the geomagnetic field in the past are used for dating and the difficulties when trying to determine field intensities from “in situ” baked clays from the kilns. The three kilns yielded very well defined ancient field directions but two possible dating solutions for each of them when no a priori time constraints are taken into account, due to field direction recurrence. As an increase of the dating accuracy and reduction of the number of dating solutions can be expected using the full field vector information, also field intensity determinations on burnt clays from the kilns were attempted. Field intensities from samples of the Roman and of one of the Medieval kilns are quite scattered. On the other hand, results obtained applying the Thellier-Thellier method and the modified method developed by Dekkers and Böhnel on sister samples from the Roman kiln agree fairly well. Rock magnetic properties reveal high variance in the kilns that point to varying spatial heating and cooling conditions in the kilns. Even well burnt material from the kilns shows irreversible changes when heated in air in the laboratory. Reliable field intensities on “in situ” baked materials from kilns themselves can therefore only be obtained when measuring sufficient number of samples from different parts of the kiln, taking into account the spatial-temporal conditions during kiln operation and cooling history. More reference intensity data is needed in our regions in order to improve dating based on directional reference data only.     

Geomagnetic field variations in southeastern Europe between 6500 and 100 years B.C.

Results from joint work between the Geophysical Institute (Sofia, Bulgaria), and the Geomagnetic Institute (Grocka, Yugoslavia) on the geomagnetic field variation in the prehistoric past are presented. Preliminary curves of variation of the three geomagnetic elements: declination, inclination and intensity are presented. The movement of the virtual pole position for the 6500 years time period B.C. is derived. The curves and the virtual pole positions provide a dating tool for archaeological purposes and determination of the periodicities in the geomagnetic secular variations in southeastern Europe.     

Archaeomagnetic studies of materials from the Gorelyi Les and Ust-Khaita monuments (Eastern Siberia)

Archaeomagnetic studies of materials from archaeological monuments of the Irkutsk and Baikal regions provided new constraints on the geomagnetic intensity variation in Eastern Siberia for the interval from the 5th millennium BC through the 1st millennium AD. Material from the Gorelyi Les and Ust-Khaita multilayer archaeological monuments of the Irkutsk region is dated according to layer depths of deposits. Data on the geomagnetic intensity obtained from archaeomagnetic studies corroborate the validity of this method of dating. It is established that the average level of the geomagnetic intensity smoothly rose in the interval from the 5th millennium BC through the 1st millennium AD, with faster variations being superimposed on this trend. The pattern of the geomagnetic intensity variation is similar to those in other Eurasian regions.     

Archaeomagnetic study and rehydroxylation dating of fired-clay ceramics

Archaeomagnetic study and rehydroxylation dating of fired-clay ceramics from Great Britain, Spain, and the Black Sea region is carried out in order to refine the dating of the material from the archaeological monuments used in the archaeomagnetic research for determination of the elements of the main magnetic field during the past few millennia. The archaeomagnetic analysis revealed the factors responsible for deviations of the rehydroxylation dating from the true values. They include the processes of weathering (magnetite transformation into hydroxides) and secondary magnetization (e.g., magnetization in a fire). In order to bring the dating closer to the true values, corrections for the influence of the distorting factors are suggested. The data on the geomagnetic field intensity derived from the magnetization of the studied material are used as independent criteria to validate the dating of the field. When applied to the fired clay material with reliable dating, the rehydroxylation method provides the estimates of the temperature in the region of archaeological monuments during the past.     

Rock-magnetic and archeomagnetic survey from some classical settlements at Chapultepec archeological site (western Mesoamerica)

Many archaeological artifacts contain magnetic minerals that may record the direction and strength of the Earth’s magnetic field. The geomagnetic field parameters (declination, inclination and intensity) change through time and then may be used as a dating tool. Over the last three decades, the archeomagnetic method was successfully applied in Europe. Still, no systematic studies are devoted to the American sites. We report here, a detailed rock-magnetic and archeomagnetic investigation of some pre-Columbian settlements at Chapultepec archeological site (western Mesoamerica). Continuous low-field susceptibility vs. temperature curves performed in air point to Ti-poor titanomagnetites as remanence carriers. Hysteresis ratios fall essentially in the pseudo-single-domain region. The twelve oriented samples taken from the furnace located in the habitation area were demagnetized applying pick alternative fields. In most cases, a stable uni-vectorial remanent magnetization was found yielding a reasonably well defined mean characteristic direction: inclination I = 32.8°, declination D = 353.4°, and parameters of Fisherian statistics α₉₅ = 10.4° and k = 14. Still poorly defined directional master curves for Mesoamerica, together with relatively dispersed mean directions obtained in this study, make dating targets difficult. Archaeomagnetic dating was carried out on the basis of Bayessian statistics. This established a most probable time interval from 512 to 634 A.D. with an average of 573 ± 61 A.D.     

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 Opeño archeological site which has the earliest funeral architecture known in western Mesoamerica. The El Opeño chronology is supported by six C¹⁴ 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 × 10²² Am². 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 while two smaller peaks are observed on the ARCH3K curve for the same time interval. The intensity value obtained at about 200 B.C. is a clear outlier and thus its geomagnetic significance should be confirmed by further investigations.     

Geomagnetic Field Variations as Determined from Bulgarian Archaeomagnetic Data. Part II: The Last 8000 Years

The knowledge about past secular variations of the geomagnetic field is achieved on the basis of archaeomagnetic researches of which the Bulgarian studies form an extended data set. In Part I (Kovacheva and Toshkov, 1994), the methodology used in the Sofia palaeomagnetic laboratory was described and the secular variation curves for the last 2000 years were shown. In Part II (this paper), the basic characteristics of the prehistoric materials used in the archaeomagnetic studies are emphasised, particularly in the context of the rock magnetic studies used in connection with palaeointensity determinations. The results of magnetic anisotropy studies of the prehistoric ovens and other fired structures are summarised, including the anisotropy correction of the palaeointensity results for prehistoric materials, different from bricks and pottery. Curves of the direction and intensity of the geomagnetic field during the last 8000 years in Bulgaria are given. The available directional and intensity values have been used to calculate the variation curve of the virtual dipole moment (VDM) for the last 8000 years based on different time interval averages. The path of virtual geomagnetic pole (VGP) positions is discussed.     

Historical measurements of the Earth's magnetic field compared with remanence directions from lava flows in Italy over the last four centuries

Direct measurements of the Earth's magnetic field in Italy since 1640 a.d. have been used to check the remanence directions derived from historically dated volcanic rocks of Etna and Vesuvius. Direct measurements consist of the records of L’Aquila and Pola geomagnetic observatories, the repeat stations of the Italian Magnetic Network and the data base of the Historical Italian Geomagnetic Data Catalogue. All have been relocated to the same reference site (Viterbo — lat. 42.45°N, long. 12.03°E) in order to draw a reference secular variation (SV) curve. The direction of the Earth's field at Viterbo has also been calculated from the historical records (2000-1600) of ref. [Jackson, A., Jonkers, A.R.T., Walker, M.R., 2000. Four centuries of geomagnetic secular variation from historical records. Phil. Trans. R. Soc. London, Ser. A 358, 957-990] database. The remanence directions from Etna show a general agreement with the trend of the SV curve, although their inclination is usually lower than that from the direct measurement. The directions from Vesuvius are more scattered. Large discrepancies occur at both volcanoes and in some cases have been ascribed in the literature to poor geographic information, making it difficult to identify the flows actually emplaced during the eruptions reported in the chronicles. Closer examination shows that the great majority of the best-defined remanence directions (semi-angle of confidence α₉₅ < 2.5°) deviate significantly from the geomagnetic direction measured at the time of the emplacement, the angle between the two directions being larger than the α₉₅ value. The value of 2.5-3.0° can thus be regarded as a conservative evaluation of the error when dealing with dating Etna and Vesuvius lava flows older than 17th century, even when the accuracy attained in remanence measurements is higher. In default of a SV curve for Italy derived from archaeological artefacts, a further error in dating is introduced when reference is made to SV curves of other countries, even if well-established, as these are from regions too far from Italy (>600 km) to confidently relocate magnetic directions.     

用曲面Spline方法表示1900～1936年中国(部分地区)地磁场及其长期变化的分布

根据19360年426个地磁测点和28个IGRF计算的地磁数据,计算地磁场和地磁异常场各个分量的曲面Spline模型,并绘制相应的地磁图和地磁异常图.依据我国部分地区的1909～1915,1915～1920,1920～1930,1930～1936年间地磁偏角长期变化图,1908～1917,1917～1922,1922～1936年间水平强度长期变化图和1908～1922,1922～1936年间垂直强度长期变化图,使用曲面Spline方法,分别计算上述9个时间段的磁偏角（D）、水平强度（H）和垂直强度（Z）长期变化的曲面Spline模型,并绘制相应的长期变化图.根据这些长期变化模型,将19360年426个点的三分量绝对值数据归算至1940,1930,1920,1910年和1900年,从而为计算这5个年代的地磁场模型奠定了坚实的基础.     

Determination of the ancient geomagnetic field elements from thermal remanent magnetization corrected for magnetic anisotropy

A method is proposed for correcting the magnetic anisotropy of baked magnetic material from archaeological objects and rocks intended for the study of its magnetization and for gaining data on the angular elements of the ancient geomagnetic field. The application of this correction decreases the spread of individual determinations obtained in studying the sample magnetization from an object and increases the determination accuracy of the average value of the ancient geomagnetic field elements. This, in turn, makes it possible to increase the accuracy of its dating.     

太原基准地震台数字化地磁资料质量分析

为了对太原基准地震台数字化仪器及地磁资料作一准确评价,利用太原基准地震台地磁相对和绝对资料为主要研究对象,同时收集兰州、乾陵、红山地磁台的地磁资料作为参考进行对比分析,结果表明,太原基准地震台相对观测仪器M15、GM4运行稳定,能够真实记录场源的变化,绝对观测仪MingeoDIM优于CTM-DI。同时存在受外界环境影响较大和存在背影噪声大的问题。     

Variations in the geomagnetic field strength in the 5th–3rd centuries BC in the eastern Mediterranean (according to narrowly dated ceramics)

The magnetization of ceramics from the eastern Mediterranean dated within a short period (mostly shorter than ±20 years) has been studied, which made it possible to specify the geomagnetic field variations on the time interval 5th–3rd centuries BC. The 11-year time series of the geomagnetic field strength values has been constructed. The field strength changes have been considered, which indicated that the centennial variation with a characteristic time of ～130 years (according to the obtained data) is observed on this time interval as well as during the last two millennia. The ceramic material from the Mayskaya Gora archeological site (Taman), the preparation succession of which was established based on the shape of pottery but the problem of absolute dating was not solved, has been dated.     

Origin of deviations between the remanent magnetisation and inducing geomagnetic field direction in kilns and implications on archaeomagnetic dating

Standard secular variation curves of the geomagnetic field direction for the last few millennia for different countries in Europe, based mainly on baked materials discovered in archaeological sites, are reaching sufficient accuracy and reliability that archaeomagnetic dating within this timespan in these areas is becoming feasible now. Orientation errors during sampling and sample preparation can be kept to a minimum and measurement errors of baked materials are in general very small. Nevertheless, even when the secular variation in the site to be dated is similar to that in the reference site, systematic or regular deviations between the remanence vector and geomagnetic field direction in kilns, due to internal and external factors, may result in biased average magnetisation directions and consequently in diverging archaeomagnetic dates. In more or less circular kilns, important non-random deviations of the remanence direction in the walls, in function of the relative azimuth as seen from the centre of the kilns, have been noticed in spite of precise and highly reliable average kiln magnetisation directions. The effect was revealed in the walls of the combustion chambers of roman pottery kilns and of mediaeval brass melting and working kilns, of different shape, lined with refractory bricks. Several hypotheses for the origin of these deviations were examined and anisotropy and/or magnetic refraction are unlikely to be the principal causes. The regular component of these deviations may be responsible for a strongly biased average magnetisation direction when only part of the kiln wall is preserved or when sampling is limited to certain parts of it and consequently result in unreliable dates when applying the archaeomagnetic dating method.     

Archaeomagnetic determination of the ancient intensity of the geomagnetic field in Tamilnadu,India

A detailed study of the ancient intensity of the Earth's magnetic field in Tamilnadu, India is presented using pottery, bricks, tiles etc., obtained from the excavations that have been carried out at some of the important places of historical and archaeological interest. An attempt is made to reconstruct the secular variation of the geomagnetic field intensity. The results reported here reveal considerable changes in the Earth's magnetic field in this region during the past 2400 y. It is observed that the intensity of the geomagnetic field has been previously 57% greater than at present.     

地磁场Z分量的年变化

利用河北省及邻区共８个台的绝对磁测Ｚ分量的资料,分析了地震磁场Ｚ分量的年变化。结果是,曲线相对长斜线而言,一年中形成两个峰值,时间在４月份和１０月份前后。前年１、７、１２月份为低值,７月份最低。相对长斜线的年变量大幅度各台变化不大,有从南往北逐渐加大迹象,但是各台的年变化幅度又随时间而有较大差异。主伙同时间里各台Ｚ分量的变化差异,主要是各地长期变化速率不同引起的。认识了地磁场Ｚ分量这些变化特征和规     

A Regional Archaeomagnetic Model for the Palaeointensity in Europe for the last 2000 Years and its Implications for Climatic Change

The SCHA.DI.00 directional model for the geomagnetic field in Europe for the last 2000 years (Pavón–Carrasco et al., 2008) 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. Not only does this complete model provide an improvement for example for archaeomagnetic data studies, but it is also shown that this new regional model can be used to study the recently proposed link between the centennial secular variation of the geomagnetic field and climate change. The pattern of the archaeosecular variation of the field intensity obtained by SCHA.DI.00–F seems to verify the hypothesis presented by Gallet et al. (2005) about a possible (causal) connection between changes in the geomagnetic field intensity and in climate parameters, opening the door for more discussions on this challenging subject.