Archaeomagnetic dating of archaeological sites from Switzerland and Bulgaria

Baked materials from three archaeological sites in Switzerland and one in Bulgaria have been examined and dated archaeomagnetically. The detailed rock-magnetic properties of the burnt clay collected have already been studied therefore and have been published elsewhere (Geophysical Journal International 153 (2003) 146). The main aim of this paper is to determine the terminus ante quem for the last firing of the clay collected from pottery kilns or burnt layers. The directional and intensity results of the ancient geomagnetic field corresponding to the time of the structure's last firing are given for each site. These geomagnetic parameters are then compared with reference curves to provide archaeomagnetic dates for the last use of the kilns. Application of archaeomagnetism as a dating tool is only possible for those regions that already have reference curves. Bulgaria is fortunate in having established reference curves covering almost the last 8000 years. Two Swiss sites were dated using only directional results on the basis of existing palaeodirectional results for Switzerland as well as the French directional reference curves. The archaeomagnetic dates of three of the sites were then compared to archaeological evidence and available radiocarbon dates. The palaeointensity data obtained from this study will enrich the existing archaeomagnetic database for western Europe.     

TOWARDS A NEW APPROACH TO ARCHAEOMAGNETIC DATING IN EUROPE USING GEOMAGNETIC FIELD MODELLING*

We introduce a new approach to producing secular variation curves for archaeomagnetic dating, using global geomagnetic field modelling techniques. Using published palaeosecular variation curves for five European locations as input, and a previous global model for regularization, we produce a model for the period ad 0–1900, which reproduces the input curves within their 95% confidence intervals, and successfully fits data from other regions in Europe. Our global model performs as well as a previous regional model, but additionally ensures consistency with physical constraints. We show that the curves currently used for dating are not mutually consistent, and therefore that in the future this new approach will provide more reliable curves for archaeomagnetic dating.     

Combined archaeomagnetic and thermoluminescence study of a brick kiln excavated at Fontanetto Po (Vercelli,Northern Italy)

A combined archaeomagnetic and thermoluminescence study was carried out as part of a rescue archaeological excavation on a kiln discovered during the installation of methane gas pipelines beneath a rice field, along the southern border of Fontanetto Po village (Vercelli province, Italy). A total of 23 independent brick samples have been collected, oriented in situ with an inclinometer; the use of magnetic and sun compass was not possible due to the existence of metallic tubes beneath the kiln and a plastic cover above it. Standard archaeomagnetic procedures have been used for the determination of the archaeomagnetic inclination and absolute geomagnetic intensity. Stepwise thermal demagnetization shows a very stable characteristic remanent magnetization and the calculated mean inclination of the 23 samples is I = 65.3° with α₉₅ = 2.4° and k = 156. Archaeointensity experiments have been performed using the classical Thellier method as modified by Coe, with regular partial thermoremanent magnetization (pTRM) checks. The cooling rate and remanence anisotropy effects upon thermoremanent magnetization (TRM) have been investigated in all the specimens. A total of 19 archaeointensity determinations (at specimen level) that correspond to linear NRM–TRM plots were used for the calculation of the site mean archaeointensity that is 46.4 ± 2.9 μT. Archaeomagnetic dating results show two possible dating intervals for the last 1000 years, calculated at 95% confidence interval: a first one from 1511 to 1614 AD, and a second one from 1768 to 1872 AD. Thermoluminescence (TL) study has been also performed on two brick samples from the kiln's internal wall, using conventional laboratory procedures. According to the thermoluminescence results the kiln's last usage lies between 1796 and 1914 AD. This age is in good agreement with the second dating interval obtained by the archaeomagnetic analysis. The combined archaeomagnetic and thermoluminescence results suggest that the last firing of the kiln could have occurred between the end of the 18th century and the beginning of 20th century.     

Archaeomagnetic research in the Andean highlands

Eleven archaeomagnetic samples were collected from several sites near Porco, Bolivia as part of the Proyecto Arqueológico Porco-Potosí. These samples were collected in order to 1) explore the availability of appropriate sample materials in the Bolivian Andes, and 2) assess the feasibility of pursuing reference curve research for this region. In addition, this research explored the archaeomagnetic dating application of reference curves constructed from the global geomagnetic field model gufm1 (Jackson et al., 2000), which covers the period from AD 1590–1990. This model is particularly applicable to our project, since many of the project sites are late prehispanic (pre-AD 1532) and early historic (ca. AD 1532–1825) in age. Of the 11 samples, three appeared to contain an insufficient amount of ferromagnetic minerals to provide reliable results, while eight samples produced acceptable-to-good results. The data from the eight good samples were compared to a reference curve for the period of AD 1600–2000 that was calculated for this project from the global field model, gufm1. The archaeomagnetic dates obtained for two of the samples agreed with archaeological expectations, while those for four other samples marginally or greatly postdated the archaeological age expectations. Two additional samples produced significantly different directions and could not be dated against the reference curve. It is likely that the age discrepancies for four of the samples stem from inaccuracies in the archaeomagnetic data, the archaeological age expectations, the calculated reference curve, or some combination of these factors.     

Improvements in Archaeomagnetic Dating in Western Europe from the Late Bronze to the Late Iron Ages: An Alternative to the Problem of the Hallstattian Radiocarbon Plateau

We present a new curve of the directional secular variation of the geomagnetic field in Western Europe between 1500 bce and 200 ce . Its computation relies on a Bayesian framework. The fast secular variation during the Late Bronze and Early Iron Ages makes archaeomagnetic dating efficient with a respective precision of 150–200 and 60–100 years during these periods. The Bayesian method also provides posterior date distributions that refine the dating of reference data, especially during the period of the Hallstattian radiocarbon plateau. Archaeomagnetism becomes a valuable alternative to radiocarbon and will help to improve the archaeological chronologies.     

Combining an archaeomagnetic and radiocarbon study: dating of medieval fireplaces at the Mühlegasse,Zürich

During the restoration of the Radium cinema located in the old Town of Zürich, a stratigraphic sequence of medieval fireplaces came to light. Based on ceramic finds, a preliminary age of about 1000 AD was assigned. In this paper we show the results from archaeomagnetic and radiocarbon dating, which combined with the stratigraphic information yield a better constrain of the period of use of these fireplaces. Whilst the radiocarbon technique continues to be the most widespread methodology for dating young archeological artifacts, archaeomagnetic dating is becoming increasingly more established in Europe thanks to the increased number of available measurements. A Bayesian model on the period of use of these fireplaces was produced with OxCal, by a combination of magnetic and radiocarbon ages, together with the stratigraphic information. The results show that the fireplaces were probably used during 1 to 2 generations (e.g. 20–40 years each), and are in good agreement with the archaeological context of this site. To make use of the magnetic measurements for future reconstructions of the field, we carried out an additional age model using only the radiocarbon and the stratigraphic information. Although some of the structures show large directional scatter due to poor baking in antiquity, the results confirm the trend of the geomagnetic field around 1000 AD.     

Archaeomagnetic versus luminescence methods: the case of an Early Byzantine ceramic workshop in Thessaloniki,Greece

In this contribution, we present a combined archaeomagnetic and luminescence study of archaeologically dated structures. The investigated area is a ceramic workshop comprising several Early Byzantine kilns. Based on (a) the archaeological–anthropogenic stratigraphy of the site, (b) the structural characteristics of the kilns and (c) the few ceramic findings revealed within their context, the operation of this brick and tile factory is approximately dated between the middle of the fifth century until the first decades of the sixth century AD. Three of the well-preserved workshop kilns have been studied archaeomagnetically. The full vector of the geomagnetic field, accompanied by rock magnetic analyses of the studied material, has been defined. The archaeomagnetic study revealed similar directions among the three kilns indicating and confirming their contemporary use. Additionally, several luminescence measurements were obtained on material from the same kilns. The dating of the site was performed with both methods. The archaeomagnetic dating is convergent with the archaeological estimation only when its upper limit is considered. Concerning the luminescence dating, the calculated ages (corrected for anomalous fading and for the ⁴⁰K content) with their standard deviations are convergent with the archaeological estimations for the first kiln, while for the other two, the results seem to be incompatible. The possible factors that provoked this divergence are thoroughly discussed.     

Magnetic properties of ancient coins

The almost ubiquitous presence of small amounts of iron in ancient coins gives them remanent magnetic properties which can be measured easily on commercial magnetometers. The stability of the remanence suggests that ancient coins might retain information about the geomagnetic field at the time and location of manufacture and so be valuable in setting up archaeomagnetic dating curves. However, measurements show that the remanence directions tend to be too scattered for this purpose, although it is quite possible to distinguish magnetically the obverse and reverse faces of struck coins.Saturation remanence studies of coins and coin analogues indicate that the main carrier of the remanence in struck coins is precipitated fine-grained iron (rather than the oxides of iron) which is produced during the minting process. The intensity of the remanence generated depends upon the thermo-mechanical history of the flan prior to striking, which suggests that the magnetic properties of such coins are not simply related to the provenance of the ores used.     

RESEARCH REPORT: MAGNETIC INVESTIGATIONS AND THE AGE OF A MEDIEVAL KILN AT KUNGAHÄLLA (SOUTH‐WEST SWEDEN)*

The results from a magnetic survey and archaeomagnetic investigation of a medieval brick kiln at Kungahälla in Bohuslän (south‐west Sweden) are reported. Detailed magnetic total field and magnetic gradient measurements over known traces of the kiln showed marked local magnetic anomalies of up to 200 nT, revealing the rectangular shape of the kiln remains. Palaeomagnetic investigations of the remanent magnetization of 12 bricks from the kiln floor gave precise definitions of both the direction of the archaeomagnetic field [(D_m, I_m) = (66.7°, 8.8°), k = 655, α₉₅= 1.7°] and the palaeointensity [B_m= 69.0 ± 3.6 µT]. An archaeomagnetic date is obtained by comparing the direction of the archaeomagnetic field found in the kiln with a geocentral dipole‐transformation of the British master‐curve for secular variation. Of two possible archaeomagnetic dates (ad 1280 ± 50 or ad 1480 ± 50), the ad 1280 ± 50 age is in good accordance with the C‐14 ages and archaeological dates as well as with direct historical evidence, whereas a c. 100 years younger TL date appears to be too young.     

Archaeomagnetic Dating of Pyrotechnological Contexts: a Case Study for Copper Smelting Sites in the Central Timna Valley,Israel

This study is focused on establishing age constraints for several copper slag deposits at the centre of the Timna Valley (Israel) via reconstruction of their ancient geomagnetic intensities as recorded by the individual slag samples at the time of their formation. The results show a correlation between the location of the slag deposits (labelled as individual ‘mounds’ in our survey) and their inferred ages, reflecting varying socio‐economic and political dynamics in the region. While the slag mounds found at the unprotected foothills represent a variety of dates (mostly Early Islamic), the slag mounds on the hilltops are chronologically constrained to the early Iron Age (late 11th to 10th centuries bce ), supporting the idea for a need for protection during this period. Furthermore, in comparing the new data with previous archaeomagnetic studies from Timna, we can assert the existence of simultaneous copper production at the archaeological Sites 30, 30a and 34. This gives further support to the claim of intense smelting in the central Timna Valley during the early Iron Age. Finally, this project demonstrates the potential of archaeomagnetic experiments to provide chronological insights, and their particular advantage in addressing pyrotechnology‐related cases.     

Problems in the interpretation of radiocarbon dates: the Pastoral Neolithic of East Africa

For the Pastoral Neolithic of East Africa radiocarbon dates suggest two apparent anomalies in the archaeological record: pastoralism in the Central Rift at perhaps 7000 bp, and the very long duration of particular pottery traditions. This paper examines the dating evidence closely, in particular the assertion that apatite gives more reliable dates than collagen. Both the assertion and the anomalies appear to be false. It is shown that there is no firm evidence for pastoralism in East Africa before about 4000 bp. Similarly, the Narosura tradition is of shorter duration and more recent than previously supposed. The meagre dating evidence for the Namoratunga sites and its implications are also assessed. Examination of the dates for the Pastoral Iron Age shows that the Turkwel tradition is dated only by a single date and that the appearance of twisted cord rouletted pottery in the present millennium post-dates the end of stone tool manufacture by several centuries. The paper concludes with a plea for more critical assessment of radiocarbon dates and for greater rigour in their presentation.     

NEW ARCHAEOMAGNETIC STUDIES NEAR XI'AN,CHINA*

Archaeomagnetic and mineral magnetic studies were made of known-age samples from fired structures and sediments from archaeological sites near Xi'an, China. The fired materials retained a stable record of the direction of the past geomagnetic field, which compared well with previous investigations and could contribute to an archaeomagnetic calibration curve. However, discrepancies with documented observations of the field were noted.     

Archaeomagnetic dating of Dogmersfield Park brick kiln (Southern England)

Archaeomagnetic dating of samples from a brick kiln discovered at Dogmersfield Park has been achieved using both field direction and intensity. Detailed rock-magnetic properties and microwave archaeointensities have already been studied and published elsewhere [Ll. Casas, J. Shaw, M. Gich, J.A. Share, High-quality microwave archaeointensity determinations from an early 18th century brick kiln, Geophys. J. Int. 161 (2005) 653–661]. The archaeomagnetic measurements were assigned time-probability distributions by comparing with predictions from a global model at the sampling site, this procedure suppresses errors arising from relocation to a common central reference location. All three probability distributions consistently indicate the same age (∼AD 1700). Once the probability distributions are combined a narrower probability distribution is obtained, stressing the importance of pursuing combined (directional and intensity) archaeomagnetic analyses. The inferred age is also highly consistent with available historical evidence.     

Archaeological and archaeomagnetic dating at a site from the ager Tarraconensis (Tarragona,Spain): El Vila-sec Roman pottery

A very accurate archaeological dating of a Roman site in NE Spain (El Vila-sec) was made based on the typology of pottery artifacts. Three different phases were identified with activity ranging from the mid-1st century BC to the early-3rd century AD. Analyses of bricks from kilns at El Vila-sec produced data on their stored archaeomagnetic vector. These data were compared with the secular variation curve for the Iberian Peninsula and the SCHA.DIF.3K regional archaeomagnetic model. Both, the reference curve and the model, produced probability distributions for the final period of use for two kilns from the second archaeological phase that were not used during the third phase. At a 95% confidence level, both time distributions cover a wide chronological range including the presumed archaeological age. Both the Iberian secular variation curve and the SCHA.DIF.3K regional model proved to be suitable models for dating the site, although on their own they do not produce a single unambiguous solution. This archaeomagnetic approach could also be applied to neighbouring archaeological sites that have an imprecise archaeological age.     

ARCHAEOMAGNETISM IN ARCHAEOMETRY— A SEMI‐CENTENNIAL REVIEW*

The journal Archaeometry and its first co‐editor Martin Aitken played key roles in the development of archaeomagnetism. This paper briefly reviews all papers published in Archaeometry, and a few others, dealing with archaeomagnetic secular variation, archaeomagnetic dating, archaeointensity studies and related applications of magnetic studies to archaeology, to indicate some of the major developments over the past 50 years.     

A Stamford Ware Pottery Kiln in Pontefract: A Geographical Enigma and a Dating Dilemma

Abstract

THIS REPORT PRESENTS the results of analyses carried out following the discovery and excavation of a medieval pottery kiln in Pontefract, a discovery which featured in the ‘Fieldwork Highlights’ section of Medieval Archaeology 53 (2009). The kiln is remarkable for having produced vessels previously only associated with the Stamford ware industry of the late 11th and 12th centuries and demonstrates the existence of at least one northern production site, the likely existence of which was first mooted in the 1950s. As both radiocarbon and archaeomagnetic dating methods indicate a last firing of the Pontefract kiln in the late pre-Conquest period, the discovery also poses questions about the chronology of Stamford ware production.     

Unpicking the Chronology of Carrowmore

This paper reports 25 dates on pin fragments from two monuments within the Carrowmore passage tomb complex, Co. Sligo, Ireland. This is the first occasion that a core artefact from the Irish passage tomb assemblage has been systematically dated. Methodological issues with previous dating programmes at Carrowmore, including the general problem of using charcoal to date megalithic monuments, are considered. The implications of the results are discussed, with a focus on their impact on former interpretations of the chronology of Carrowmore and the beginnings of the passage tomb tradition in north‐west Ireland.     

Salt production in pre-Funan Vietnam: archaeomagnetic reorientation of briquetage fragments

Bronze to Iron Age briquetage found in the northern Mekong Delta has an appearance similar to analogous material from Europe and Asia; however, the orientation in which the briquetage was employed during the production of salt is still under debate. As a consequence of the heating and subsequent cooling of the briquetage during the evaporative recovery of salt, the magnetic mineral particles within the ceramic formed a stable thermoremanent magnetization in alignment with the Earth's magnetic field. It thus becomes possible to find the orientation in which the ceramics were last fired by aligning their recorded archaeomagnetic signal with estimates of the Earth's ancient field direction in Vietnam. The archaeomagnetic directions obtained from 22 samples taken from five different briquetage artefacts are somewhat scattered, but they reveal a consistent orientation and thus the mode in which the briquetage was employed can be reconstructed.