Extended and revised archaeomagnetic database and secular variation curves from Bulgaria for the last eight millennia

International audienceThe efforts of geophysicists to describe geomagnetic field behaviour in the past lead to creation of different geomagnetic field models. On the other hand, the established regional palaeosecular variations of geomagnetic elements are increasingly used for dating purposes in archaeology. Both of these goals can be achieved if sufficient amounts of long archaeomagnetic data sets exist for different geographical regions. The accumulation of archaeomagnetic determinations began at the middle of the last century, parallel with the progressive development of experimental methodology and acceptance criteria. The presence of great number of old determinations requires their critical assessment. The important question about the reliability of the associated dating intervals should be also re-assessed. All this requires the continuous refinement and extension of the accumulated databases. This paper presents the last synthesis of Bulgarian archaeomagnetic database and the local palaeosecular variation curves obtained using a statistical treatment based on Bayesian approach (RenCurve software). The rock-magnetic characteristics of the newly included, non-published results are summarized

Further progress in the study of epsilon iron oxide in archaeological baked clays

The occurrence of ε-Fe2O3 in archaeological samples that have been subjected to high temperatures is gradually being detected by the use of micrometric structural characterization techniques. This work provides new information by revealing that the ε-Fe2O3 is formed as a response to temperature, the aggregation state and the position within the baked clay with respect to the nearest heat source. In addition, depending mainly on the atmospheric environment, the temperature reached by the combustion structure, the distance from the heating source and the particle aggregation, other iron oxide magnetic phases are produced. In the baked clay studied here, hematite is found over the whole range of samples studied but its magnetic contribution is negligible. Magnetite is observed at the sample surface, probably due to local atmospheric environment closest to the combustion source. Maghemite is found at all depths up to 6 cm below the sample surface. ε-Fe2O3 has a limited distribution, found within 2–3 cm of the sample surface. Furthermore, the viability of this compound as a palaeofield marker has been evaluated in both archaeological and synthetic samples. The results indicate that ε-Fe2O3 is able to register the direction of the magnetic field. Linear palaeointensity plots have been obtained in synthetic samples, although the value of the palaeofield could be, sometimes, overestimated

Transdimensional inference of archeomagnetic intensity change

One of the main goals of archeomagnetism is to document the secular changes of Earth's magnetic field by laboratory analysis of the magnetization carried by archeological artefacts. Typical techniques for creating a time-dependent model assume a prescribed temporal discretisation which, when coupled with sparse data coverage, require strong regularisation generally applied over the entire time series in order to ensure smoothness. Such techniques make it difficult to characterise uncertainty and frequency content, and robustly detect rapid changes. Key to proper modelling (and physical understanding) is a method that places a minimum level of regularisation on any fit to the data. Here we apply a transdimensional Bayesian technique based on piecewise linear interpolation to sparse archeointensity datasets, in which the temporal complexity of the model is not set a priori, but is self-selected by the data. The method produces two key outputs: (i) a posterior distribution of intensity as a function of time, a useful tool for archeomagnetic dating, whose statistics are smooth but formally unregularised; (ii) by including the data ages in the model of unknown parameters, the method also produces posterior age statistics of each individual contributing datum. We test the technique using synthetic datasets and confirm agreement of our method with an integrated likelihood approach. We then apply the method to three archeomagnetic datasets all reduced to a single location: one temporally well-sampled within 700km from Paris (here referred to as Paris700), one that is temporally sparse centred on Hawaii, and a third (from Lübeck, Germany and Paris700) that has additional ordering constraints on age from stratification. Compared with other methods, our average posterior distributions largely agree, however our credible intervals appear to much better reflect the uncertainty during periods of sparse data coverage. Because each ensemble member of the posterior distribution is piecewise linear, we only fit oscillations when required by the data. As an example, we show that an oscillatory signal, associated with temporally-localised intensity maxima reported for a sparse Hawaiian dataset, is not required by the data. However, we do recover the previously reported oscillation of period 260 yrs for the Paris700 dataset and compute the probability distribution of the period of oscillation. We further demonstrate that such an oscillation is unresolved when accounting for age uncertainty by using a fixed age and with an artificially inflated error budget on intensity

Изследване магнетизма на археологически структури. Практически указания при работа на терен: Study of the magnetism of archaeological structures. Fieldwork practical guidance

The application of archaeomagnetic method as a dating tool in archaeology has intensified considerably in the last few years. The successful use of archaeomagnetic methodology depends on different factors and many efforts have been made for their clarification. Sampling is the first stage of each archaeomagnetic study. The careful selection and accurate orientation of the collected materials in the field are crucial for the obtained archaeomagnetic results. That is why detailed description of the fieldwork techniques used in Palaeomagnetic laboratory in Sofia is given and the advantages and disadvantages of the different baked clay types of materials are summarized

Early Neolithic settlement Yabalkovo (Maritsa valley, Bulgaria) in the context of archaeomagnetic studies: Раннонеолитното селище Ябълково (долината на Марица) в контекста на археомагнитно проучване

Five fired clay structures from the Northeast sector of the Early Neolithic settlement of Yabalkovo in the Maritsa valley, Bulgaria were a subject of an archaeomagnetic investigation. The results of the detailed rock-magnetic experiments carried out show that the collected materials are suitable for archaeomagnetic study and reliable determinations for past geomagnetic field elements can be obtained. According to the mean values calculated for the ancient geomagnetic field direction and intensity, two different periods of time can be undoubtedly distinguished for the investigated structures. This coincides exactly with the two chronological horizons suggested by the excavators. The archaeological observations point that the oven in sq. K36, the hearth in sq. F16 and the hearth in sq. H31 are related to Horizon II, while the hearths in sq. I31 and sq. F18 are connected with Horizon I.Based on the Bulgarian reference curves (latest version), the following archaeomagnetic dating intervals are received: Yabalkovo – Horizon II – (5685–5489 BC) and Yabalkovo – Horizon I – (5472–5427 BC). These results agree well with the archaeological data that structures from Horizon II are older than structures from Horizon I. The existing 14C dates (for materials taken mainly from the Southwest sector of the settlement) show considerably older dating intervals but they are not able to differentiate chronologically the different structures. The obtained archaeomagnetic determinations for Yabalkovo are compared with these available for other archaeomagnetically studied Early Neolithic sites from Bulgaria and Northern Greece. This comparison indicates that Horizon II of Yabalkovo can be synchronised with the younger layers of Kovatchevo (Karanovo I), Stara Zagora (Karanovo II) and Dobriniste. Horizon I of Yabalkovo is chronologically comparable with Horizons VII+VIII of Samovodene and Avgi (Nortern Greece)

Archaeomagnetic study of two medieval ovens discovered in the Pliska Palace, North-eastern Bulgaria: Археомагнитно проучване на две средновековни пещи от Двореца в Плиска, Североизточна България

Two dome ovens from the archaeological complex of Pliska Palace, North-Eastern Bulgaria, were sampled and studied using archaeomagnetic method. These two ovens are well dated archaeologically, and their archaeomagnetic results will elucidate better the sought geomagnetic field variations for the given time period. On the other hand, we have the possibility to demonstrate the applicability of the archaeomagnetic method for dating purposes. Detailed rock-magnetic analyses were performed in order to establish the magnetic properties of the collected materials (stability of carried remanence, type of dominant magnetic minerals, domain state of magnetic particles and degrees of mineralogical transformations during heating). In general, the investigated materials are suitable for archaeomagnetic determination. The applied experiments show that they have not been heated to temperatures over 460oC as the temperature of heating in oven No2 was probably slightly higher in comparison to that in oven No1. Taking into account the determined mean values for the geomagnetic field elements, it is obvious that both ovens have been used in the past within two different time periods. The mean declination of oven No1 is more than 20 degrees lower than the declination of oven No2. Differences between the two other geomagnetic field parameters, inclination and intensity, are less pronounced: 5 degrees for inclination and 3 μT for intensity. An archaeomagnetic dating was done on the basis of Bulgarian reference curves from 2013. There is a very good agreement between the determined archaeomagnetic dating intervals (894–993) AD (for oven No1) and (1001–1075) AD (for oven No2), and the archaeological assumptions. These results confirm the importance of the archaeomagnetic method in archaeology as a reliable dating tool

Early Neolithic settlement Yabalkovo (Maritsa valley, Bulgaria) in the context of archaeomagnetic studies

Five fired clay structures from the Northeast sector of the Early Neolithic settlement of Yabalkovo in the Maritsa valley, Bulgaria were a subject of an archaeomagnetic investigation. The results of the detailed rock-magnetic experiments carried out show that the collected materials are suitable for archaeomagnetic study and reliable determinations for past geomagnetic field elements can be obtained. According to the mean values calculated for the ancient geomagnetic field direction and intensity, two different periods of time can be undoubtedly distinguished for the investigated structures. This coincides exactly with the two chronological horizons suggested by the excavators. The archaeological observations point that the oven in sq. K36, the hearth in sq. F16 and the hearth in sq. H31 are related to Horizon II, while the hearths in sq. I31 and sq. F18 are connected with Horizon I. Based on the Bulgarian reference curves (latest version), the following archaeomagnetic dating intervals are received: Yabalkovo – Horizon II – (5685–5489 BC) and Yabalkovo – Horizon I – (5472–5427 BC). These results agree well with the archaeological data that structures from Horizon II are older than structures from Horizon I. The existing 14C dates (for materials taken mainly from the Southwest sector of the settlement) show considerably older dating intervals but they are not able to differentiate chronologically the different structures. The obtained archaeomagnetic determinations for Yabalkovo are compared with these available for other archaeomagnetically studied Early Neolithic sites from Bulgaria and Northern Greece. This comparison indicates that Horizon II of Yabalkovo can be synchronised with the younger layers of Kovatchevo (Karanovo I), Stara Zagora (Karanovo II) and Dobriniste. Horizon I of Yabalkovo is chronologically comparable with Horizons VII+VIII of Samovodene and Avgi (Nortern Greece)