Archaeointensity Data Research Articles

Archaeomagnetism at Ebla (Tell Mardikh, Syria). New data on geomagnetic field intensity variations in the Near East during the Bronze Age

Thanks to systematic excavations conducted at Tell Mardikh/Ebla (Syria) during more than 40 years, we collected eleven groups of Bronze Age ceramic fragments defining a series of seven time intervals dated to between ∼2300 BC and ∼1400 BC. Archaeointensity experiments were performed using the Triaxe protocol that takes into account both anisotropy thermoremanent magnetization and cooling rate effects. The results, complemented by three other data previously obtained from Ebla, allow the recovery of geomagnetic field intensity variations over nearly 1000 years characterized by a V-shape, with a distinct relative intensity minimum around the 18th century BC. They also permit to constrain the occurrence of an intensity maximum between ∼2300 and ∼2000 BC. Together with other archaeointensity data obtained from Syrian, Levantine and Anatolian regions, the results from Ebla help to make emerging a coherent pattern of geomagnetic field intensity variations in the Near East over the entire Bronze period. This evolution was marked by distinct intensity maxima at ∼2600–2500 BC, ∼2300–2000 BC, ∼1550–1350 BC and at the very beginning of the first millennium BC (Iron Age), the latter showing a much higher magnitude than the three older ones. We discuss the fact that the detected geomagnetic field intensity maxima could be associated with the occurrence of archaeomagnetic jerks that appear synchronous, within age uncertainties, with significant regional climatic fluctuations.

New archaeointensity data from Italy and geomagnetic field intensity variation in the Italian Peninsula

We present new archaeointensity results from three Italian kilns situated at Ascoli Satriano, Vagnari and Fontanetto Po obtained with the Thellier modified by Coe double heating method. These data complement the directional results previously published. All sites are dated on the basis of archaeological information and/or thermoluminescence dating. The results are corrected for the anisotropy of the thermoremanent magnetization and the cooling rate effects. The new data are compared with previously published archaeointensity data from Italy and nearby countries within 900 km radius from Viterbo. An initial data set including archaeointesity data mainly coming from Italy, France, Switzerland, Czech Republic, Slovakia, Hungary, Greece and Bulgaria has been compiled. After the application of strict selection criteria, the most reliable data have been used for the calculation of a preliminary Italian intensity secular variation (SV) curve for the last 3000 yr. The new curve covers the 300 BC–400 AD and 1200–1900 AD periods. It is established by means of sliding windows of 200 yr shifted by 100 yr. The lack of reliable data for the 1000–200 BC and 400–1200 AD time intervals does not permit the calculation of a continuous curve. Clearly, more high-quality archaeointensity data from Italy and Europe are still needed to draw a robust intensity SV curve for the Italian Peninsula that could be used for archaeomagnetic dating in combination with the directional data.

Intensity of the Earth’s magnetic field in Greece during the last five millennia: New data from Greek pottery

New archaeointensity results have been obtained from the study of four ceramic collections coming from four different archaeological sites in Greece. The ages of the ceramic fragments, based on archaeological constrains and radiocarbon analysis, range from 2200 BC to 565 AD. Low-field magnetic susceptibility versus temperature reveals a good thermal stability for most of the samples. However, for some samples the thermomagnetic curves are not reversible indicating mineralogical changes during heating. Isothermal remanent magnetisation (IRM) acquisition curves and thermal demagnetisation of three orthogonal IRM components have also been performed. The rock magnetic results identify magnetite and/ or Ti-magnetite as the main magnetic carriers in the studied samples. Classical Thellier experiments with regular partial thermoremanent magnetisation (pTRM) checks have been conducted on 125 specimens belonging to 34 independent ceramic fragments. Only 61 archaeointensity determinations (at specimen level) that correspond to linear NRM–TRM plots were used for the calculation of the site mean archaeointensities. The effect of the anisotropy of the thermoremanent magnetisation (TRM) and cooling rate upon TRM intensity acquisition have been investigated in all specimens. The maximum difference between the TRM anisotropy corrected and uncorrected intensities is around 30% at specimen level confirming that the TRM effect can be very important in ceramic samples. Cooling rate correction factors determined per specimen are up to 10% with only one exception that reaches 35%. Despite the moderate success rate of archaeointensity determination (around 50%) reliable mean site intensities have been obtained, with in situ intensities ranging from 53.6±4.1 to 69.3±3.9μT, corresponding to virtual axial dipole moments from 9.2±0.7 to 11.9±0.7×1022Am2. The new data are reasonably consistent with other available data for the studied region as well as with the SV reference curves for Greece and the South Balkan Peninsula, and the regional and global geomagnetic field models. Combined with previously published data from the area, they confirm that important changes of the Earth’s magnetic field intensity occurred in Greece during the last five millennia. For some periods, the available archaeointensity data for the Balkan area show a large dispersion, even for data corresponding to high quality intensity standards, whereas for other periods their limited number prevents an accurate description of geomagnetic field intensity changes. This evidences the need of new reliable and well dated archaeointensity data in order to obtain a robust description of geomagnetic field intensity changes during the last five millennia in this area.

Archaeomagnetic and rock magnetic study of six kilns from North Africa (Tunisia and Morocco)

New full-vector archaeomagnetic data for North Africa recovered from the study of six kilns, five from Tunisia and one from Morocco, are presented. Archaeological and historical considerations, along with three radiocarbon dates, indicate that the age of the kilns ranges between the 9th and 15th centuries AD. Rock magnetic analyses showed that the principal magnetic carriers are magnetite and low Ti titanomagnetite, along with variable contributions of thermally stable maghemite and a high coercivity phase with low unblocking temperatures. The magnetic mineralogy of the studied material is thermally stable and behaves ideally during archaeointensity experiments. Stepwise alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions at both specimen and structure levels. Mean archaeointensities have been obtained from successful classical Thellier experiments conducted on between five and eight independent samples per kiln. Thermoremanent magnetization (TRM) anisotropy and cooling rate effects upon TRM intensity have been investigated. The results showed that these effects are low for four of the six studied kilns, with differences between the uncorrected and corrected means of less than 3 per cent. For the other two structures differences between the uncorrected and corrected mean site intensities are 4.4 per cent and 5.8 per cent. These results highlight the necessity for TRM anisotropy and cooling rate corrections in archaeomagnetic studies if accurate archaeointensities are to be obtained. The new results suggest that high intensities occurred in Northwest Africa during the 9th century. Although more data are clearly needed to define this period of high intensity, the results are in agreement with the available European archaeointensity data. Acomparison between the newdata, other available archaeomagnetic determinations in nearby locations, and palaeosecular variation (PSV) curves derived from the regional SCHA.DIF.3k and global ARCH3K.1 geomagnetic field models shows good agreement between the new data and directional results derived from the models. However, some differences are observed between geomagnetic field models intensity results and available archaeointensity data for the studied regions. This highlights the need for new data for unexplored regions such as North Africa. The new data presented here better constrains the evolution of the geomagnetic field during historical times in this region. They represent a new step towards the construction of a reference PSV curve for Northwest Africa. Once established, this curve will represent a new dating method for this region.

Consistent treatment of errors in archaeointensity implies rapid decay of the dipole prior to 1840

We present a meta-analysis of 223 archaeo and palaeointensities spanning the period 1840–1990 and compare these data with the geomagnetic field model gufm1 ( Jackson et al., 2000 Philos. Trans. R. Soc. Lond. A 358, 957–990.). We find that the principal source of uncertainty in all the data, regardless of experimental method or number of samples, is due to systematic error. This error is quantified and it is shown that the data is capable of estimating the magnitude of the geomagnetic field for the period with reasonable accuracy, but only if errors are estimated on the basis of the field strength, rather than the measured intensity. The findings are then applied to archaeointensity data from the period 1590–1840 and it is shown that the most likely behaviour of the field was a decay of the axial dipole at a similar rate to that observed since 1840. We show that underestimation of the field's magnitude can be an artefact of inconsistent error estimation.

Establishing a first archaeointensity record for the SW Pacific

The dearth of archaeomagnetic intensity data from the southern hemisphere is a limiting factor in evaluating models of global geomagnetic field evolution during the Holocene. Here we present high quality microwave archaeointensity data obtained from 34 ceramic fragments (21 archaeological contexts) from the Duke of York Islands, Fiji and Vanuatu, SW Pacific. Complementary Thellier-type experiments, corrected for anisotropy give good agreement with the microwave results. The majority of the new data prior to 250 AD exhibit significantly lower intensity than predicted by current global field models (CALS3k.3 and ARCH3k) for the region, with an apparent intensity minimum at 250 BC reaching as low as 50% of the present-day field strength. Between 400 AD and 1500 AD, the data are broadly consistent with the global field models but with a 20% higher field between 1200 and 1400 AD. These new results therefore imply that the geomagnetic field has a greater range of variability than predicted and that further data from the region are essential to better constrain the global field models. The results also demonstrate that establishing an archaeomagnetic reference curve for dating SW Pacific ceramic artefacts is feasible, which has potentially significant implications for Pacific archaeological research.

Historical variation of the geomagnetic axial dipole

The geomagnetic axial dipole (hereinafter denoted g 1 0 ) is the largest component of our planet’s magnetic field. Its magnitude determines the morphology of solar-terrestrial electrical current systems and it is the most fundamental diagnostic property of the core-generated geodynamo. Elucidating past and future variations of g 1 0 ( t ) is consequently of central importance in geomagnetism. Previous historical geomagnetic field models, such as gufm1 of Jackson et al. [Jackson, A., Jonkers, A.R.T., Walker, M.R., 2000. Four centuries of geomagnetic secular variation from historical records. Philos. Trans. R. Soc. Lond. A 358, 957–990], used direct observations to constrain g 1 0 ( t ) only after 1840 A.D.; before this time a crude linear extrapolation of the post-1840 A.D. rate of change (15 nT/year) was employed. In this contribution I construct historical field models with g 1 0 ( t ) instead constrained from 1590 A.D. to 1840 A.D. by an archaeointensity dataset compiled by Korte et al. [Korte, M., Genevey, A., Constable, C.G., Frank, U., Schnepp, E., 2005. Continuous geomagnetic field models for the past 7 millennia. 1. A new global data compilation. Geochem. Geophys. Geosyst. 6, doi:10.1029/2004GC000800]. A range of possible linear models of the form g 1 0 ( t ) = g 1 0 ( 1840 ) + β ( t − 1840 ) are first explored; β = 2.74 ± 42.32 nT/year is found to explain the archaeointensity dataset with maximum likelihood, consistent with the recent findings of Gubbins et al. [Gubbins, D., Jones, A.L., Finlay, C.C., 2006. Fall in Earth’s magnetic field is erratic. Science 312, 900–902]. Relaxing the linear constraint in an effort to find more physically plausible models, I find it is necessary to artificially increase the weight given to the archaeointensity data in order to obtain acceptable models. Despite satisfactorily explaining both the historical and archaeointensity data, and possessing reasonable spatial and temporal complexity, such free evolution models perform worse than the simpler linearly constrained models when tested against the independent dataset of Gallet et al. [Gallet, Y., Genevey, A., Fluteau, F., 2005. Does Earth’s magnetic field secular variation control centennial climate change? Earth Plan. Sci. Lett. 236, 159–173]. Bayesian model comparison techniques indicate that a model ( gufm1-g10c) involving no change in g 1 0 ( t ) between 1590 A.D. and 1840 A.D. is most probable given the presently available data and current modelling techniques. I propose that this new, empirically derived, constraint on the evolution of the geomagnetic axial dipole be incorporated into the next generation of historical field models.

A new approach for geomagnetic archaeointensity research: insights on ancient metallurgy in the Southern Levant

We present results from an archaeointensity investigation based on a relatively unexploited recording medium, copper slag deposits. Together with a recently improved experimental design for the archaeointensity experiment, we demonstrate the applicability of this medium, as well as other archaeometallurgical artifacts, for the study of the ancient geomagnetic field intensity. In addition to archaeointensity data from well-dated archaeological contexts, we obtained reliable archaeointensity results from poorly dated or contentious archaeometallurgical sites in the Southern Levant. These results shed new light on the dating of these sites, among them the copper smelting installation of Timna 39b – a site that has important implications for the beginning of metallurgy during the fifth millennium BCE. The paper also aims to introduce archaeointensity research to the archaeologist scholar, and to encourage further collaboration between the disciplines in future research.

New archaeointensity results from archaeological sites and variation of the geomagnetic field intensity for the last 7 millennia in Greece

In this study six new intensity determinations are presented, obtained from five well dated archaeological sites, located in northern Greece and in Paros, Cyclades Islands. The fired structures consisted of ceramic and pottery kilns belonging to the Hellenistic, Roman and Byzantine periods. Between 8 and 21 samples of highly fired baked clays, tiles and bricks were taken, homogeneously distributed over the structures. The samples were analysed using the classical Thellier method, providing the past intensities and directions of the geomagnetic field recorded at each site. The intensity values have been corrected for anisotropy of thermal remanent magnetisation and cooling rate effects. Differences in the mean archaeointensities per site ranging from 1% to 11%, before and after TRM anisotropy and cooling rate corrections, were obtained. The new results indicate a decrease of 20% of the geomagnetic field strength in Greece, during the last four centuries BC. In order to compare our results with previously published data, a catalogue of archaeo- and palaeointensity results for the Aegean area has been established, covering the last 7 millennia. It consists of 336 data from Greece, western Turkey and Former Yugoslavia, collected from various authors. Weighting factors have been applied to these data, that then have been treated with a hierarchical Bayesian modelling, and a geomagnetic field intensity variation curve for Greece was constructed. A good agreement is observed when comparing the curve for Greece with the Bulgarian secular variation curve (SVC) for intensity. Satisfactory coincidence is also found with the archaeointensity data from Mesopotamia. Despite the presence of some time gaps, a more precise secular variation intensity curve has been constructed for Greece which, combined with a forthcoming directional SVC, will help for dating purposes.

An archaeomagnetic investigation of a Roman amphorae workshop in Albinia (Italy)

SummaryAn intensive archaeomagnetic investigation of an Italian Roman amphorae workshop has been carried out in order to produce high quality data to enhance the European archaeomagnetic database. Additionally, and importantly, this study also investigates within and between structure variations and, the influence of anisotropy and cooling rate corrections. Eighty-six oriented samples were taken from five kilns for full geomagnetic vector (directions and intensity) determination. Additionally, cores from 39 amphorae found at the site were drilled for archaeointensity analysis. The site is archaeologically dated as being between 2nd century BC and 1st century AD, and the amphorae as being 1st century BC. A full suite of rock magnetic experiments were carried out which indicate the samples' suitability for archaeointensity experiments. The classical Thellier method with correction for anisotropy of thermal remanence (TRM) was used to determine the direction of the characteristic remanence and the archaeointensity. Differences between fast and slow cooling during remanence acquisition were investigated and a cooling rate correction applied to the archaeointensity estimates. After correction for anisotropy of TRM, the scatter about the kiln (amphorae) mean value is reduced and the scatter between kilns is also reduced for both directions and archaeointensity, demonstrating the necessity of carrying out the anisotropy of TRM correction for these samples. Application of the cooling rate correction results in a decrease in archaeointensity as expected on theoretical grounds for single domain grains. The correction, whilst not reducing scatter in the mean archaeointensity results, does result in a reduction in the scatter found between the kilns. The directional results are compared to the French, and a preliminary Italian, secular variation (SV) curve and suggest that the kilns may be towards the older limit of the archaeologically given age however the master curves are not well constrained in this time interval. Instead, the five new directional data should be used to help constrain future curves. The Albinia archaeointensity data are consistent with the broad trends seen in the limited high quality Western European and Mesopotamian data sets and with the newly constructed archaeointensity SV curve for Greece. Similar to other studies whilst the archaeointensity results for each kiln (the amphorae) are well constrained (4–7 per cent scatter about the mean) variations are seen between the kilns (mean archaeointensity 62–70 μT). This further supports the suggestion that it is necessary to obtain a number of archaeointensity data for each time interval in order to reliably record variations of the Earth's magnetic field from archaeological material. The archaeointensity result for the set of amphorae which has the better constrained age falls within the archaeointensity values from the kilns. Combining the results from the five kilns and the amphorae gives a mean archaeointensity of 64 ± 3 μT at Albinia (69 ± 3 μT relocated to Paris) during 200 BC–100 AD.

Archaeomagnetic intensity results from California and Ecuador: evaluation of regional data

We present new archaeointensity data for southeastern California (∼33°N, ∼115°W, 50–1500 yr BP) and northwestern South America (Ecuador, 2.4°S, 80.7°W, 4000–5000 yr BP). These results represent the only data from California, as well as the oldest archaeointensity data now available in northwestern South America. In comparing our results to previously published data for the southwestern United States and northwestern South America, we note that significant scatter in the existing data makes comparisons and interpretations difficult. We undertake an analysis of the sources of data scatter (including age uncertainty, experimental errors, cooling rate differences, magnetic anisotropy, and field distortion) and evaluate the effects of scatter and error on the smoothed archaeointensity record. By making corrections where possible and eliminating questionable data, scatter is significantly reduced, especially in South America, but is far from eliminated. However, we believe the long-period fluctuations in intensity can be resolved, and differences between the Southwestern and South American records can be identified. The Southwest data are distinguished from the South American data by much higher virtual axial dipole moment values from ∼0–600 yr BP and by a broad low between ∼1000–1500 yr BP. Comparisons to global paleofield models reveal disagreements between the models and the archaeointensity data in these two regions, underscoring the need for additional intensity data to constrain the models in much of the world.

The effects of aliasing and lock-in processes on palaeosecular variation records from sediments

Studies of sedimentary records of palaeointensity variation report periods as long as 50 kyr. Archaeointensity data show geomagnetic periods of 2 kyr with large ampli-tudes. Sampling of the sedimentary records can be as coarse as 8 kyr, so the apparent long periods could be caused by aliasing. The sedimentary lock-in process could smooth the record and remove short periods, thereby preventing aliasing from occurring. We examine possible effects of aliasing by creating a 100-kyr-long synthetic sequence of palaeointensity variation with a similar spectrum to that of archaeomagnetic data from the last 12 kyr and resampling at longer intervals. With no lock-in smoothing,aliasing produces spurious energy in the spectra at long periods. When smoothing by the sedimentation process is applied, the amplitudes of the aliased peaks are reduced but still cause significant, spurious, long-period energy in the spectra for some sedi- mentation rates. We restrict our analysis to palaeointensity data but similar problems may also exist for coarsely sampled directional data. To avoid aliasing we recommend a maximum sampling interval of 2 kyr.

Variations in the geomagnetic dipole moment over the last 12 000 years

SUMMARY An analysis has been made of archaeointensity data for the past 12 000 years. There are 3243 results from diVerent areas of the world covering the past 12 000 years. Of these, 2203 are from the European region and 1040 are from the rest of the world. The archaeointensity data set analysed in the present study is almost three times larger than that used by McElhinny & Senanayake (1982). Although there is no major diVerence between our global data and the earlier data, the data for the non-European region have been improved and we now have a data set for Asia.

Archaeomagnetic database from Bulgaria: the last 8000 years

In 1992 (Kovacheva, 1992) we published the updated archaeomagnetic results from Bulgaria for the last 2000 years where it was mentioned that the compilation of data is a continuous process of accumulation of new data and revision of the previous ones. This is especially true for the old intensity studies considering the newly obtained experimental results of rock magnetic studies of Ozdemir and Dunlop (Ozdemir, O., Dunlop, D., 1993. Chemical remanent magnetisation during gammaFeOOH phase transformations. J. Geophys. Res. 98 (B3), 4191–4198) and new insights about the reliability of the archaeointensity data (reference cited below). Another uncertainty comes from the age estimates of the studied localities. This paper presents a new synthesis of the Bulgarian archaeomagnetic data for the last 2000 years along with newly revised age and an extent of the database to 8000 years before present.

Archaeointensity Variations for the Past 7,500 Years Evaluated from Ancient Chinese Ceramics.

Archaeointensity data have been obtained for the past 7, 500 years by analyzing ancient Chinese ceramics using a new version of the Shaw techniques (Shaw, 1974) and the Coe (1967) version of the Thellier techniques (Thellier and Thellier, 1959). Several rock magnetic parameters and measurements, such as Curie temperature, low temperature susceptibility, hysteresis and magnetic anisotropy, have been made on sub-samples. These measurements have enabled us to preselect the most suitable samples for archaeomagnitude analysis. There is acceptable agreement between the results obtained from separated areas of China. Major features of the geomagnetic field in China determined from this study are generally consistent with those of the global average field model of McElhinny and Senanayake (1982) but with detailed differences which we may be due either to local non-dipole field effects or lack of resolution in the global model. Comparison of the present results with contemporaneous results from Greece (Aitken et al., 1989) shows significant differences that may be due to the movement of a large non-dipole anomaly. Samples younger than 4, 000 years ago were usually dated by archaeological context and/or artefact type; radiocarbon dating was used to date the older samples.

Greek archaeomagnetic intensities: Some aspects of reliability and geophysical implications

Archaeointensity results from Greece for the last 2000 yr have been obtained using thermal and alternating field (AF) demagnetisation techniques. Most of the samples consisted of tiles and bricks derived from Byzantine churches which were epigraphically dated. The accuracy of the dating of the samples (to within a few years) and the comparison of the two techniques for measuring geomagnetic intensity combine to provide a good basis from which to derive meaningful conclusions — i.e. (i) About 70% of the archaeointensity measurements, made by the two methods, were highly compatible, differing less than ±7% from their mean. In 60% of the cases, the AF technique yielded higher palaeointensity results than that for thermal demagnetization. The results for the thermal and AF techniques are discussed in accordance with reliability criteria and attention is drawn to the reliability of archaeointensity data in general. (ii) An overall similarity of the intensity trend was observed for the last 400 years between the AF, the thermal demagnetization and a modelled intensity curve derived from the spherical harmonic coefficients since AD 1600. (iii) A decrease of intensity amounting to 40% is noted in the new data for the period since AD 400, and (iv) a non-dipole disturbance was noted at around AD 1300 in southern Greece, where the intensity changed by about 30% within a period of 30 years or so.

Revised Archaeointensity Data from Bulgaria

This paper describes the process of accumulation of the new archaeointensity results from Bulgaria after 1978. The methods of laboratory treatments are discussed. Beside the difficulties connected with the experimental determination of the ancient intensity, the main obstacle for a better elucidation of its variations is inaccurate dating, especially in the prehistoric past. The conclusion is drawn that the relative chronology of prehistoric cultures has to be preserved.The revised archaeointensity results are systematized as site averages and a weight is given to each individual result. Data are given in terms of the ratio FA/FD, where FD is the geomagnetic field intensity at the site due to an axial geocentric dipole with the present-day moment.A first attempt for computer processing of the experimental data is shown. A second order polynomial is taken as a local model in a running window of 5 points. In the Appendix the theoretical basis is described and the weights given to experimental site mean values are explained. To obtain the continuous curve of smooth values of the intensity a spline cubic interpolation with a step of 10 years is used.

Eastern Australian geomagnetic field intensity over the past 14000 yr

Two north-eastern Australian volcanic crater lake cores have been used to obtain relative intensity estimates for the geomagnetic field. ARM imparted in a low dc bias field has been used as a normalizing parameter. The intensity fluctuations in the two lakes are in excellent agreement with each other and with south-eastern Australian archaeointensity data over their coeval time spans. This strongly suggests that the same sources are influencing the geomagnetic secular variation throughout eastern Australia at this time. The relative intensity records go back to about 14000 yr bp thereby extending currently available recent Australian intensity records by some 7000 yr.