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

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Highlights

  • The revised and extended Bulgarian archaeomagnetic database is presented.

  • 28 newly studied features are included as reference points.

  • Magnetic characteristics of unpublished reference points are summarized.

  • Reference geomagnetic curves for Bulgaria are presented.

  • The specified data are important contribution to the global archaeomagnetic database.

Abstract

The 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.

Introduction

Archaeological baked clay structures and volcanic rocks are the most valuable source of data bearing information about the past behaviour of full geomagnetic vector. On the basis of this kind of data, treated with Bayesian (Lanos, 2004, Lanos et al., 2005, Lanos et al., in preparation) or bivariate (Le Goff et al., 2002) statistics, enable secular variation curves (SVCs) to be built for a given territory. These curves should be created from sufficiently precise archaeomagnetic determinations obtained from well dated materials preferably by independent methods. In such cases, SVCs can be used in archaeology for dating purposes (Kovacheva et al., 2004, Lanos, 2004, Herve et al., 2011). The global and regional geomagnetic field modeling (Korte et al., 2009, Korte and Constable, 2011, Pavón-Carrasco et al., 2009) is the most important key for elucidating the geomagnetic field origin and fluid motions in the Earth’s interior. These complicated geophysical problems can be solved on the basis of reliable data sets of archaeomagnetic and palaeomagnetic (volcanic and sedimentary environments) determinations of geomagnetic field elements. Each database compilation is a long, continuous process and periodical updates, revision or re-examinations are required. The accumulation of the Bulgarian data set started in 1967 and several updates have been published (Kovacheva, 1992, Kovacheva, 1997, Kovacheva et al., 2009a). In previous compilations archaeomagnetic determination from all studied collections in Sofia Palaeomagnetic laboratory (Bulgarian, Serbian, Greek, etc) were included. Now the data from non-Bulgarian archaeological sites are excluded and those obtained after 2009 (28 different features) are added. Furthermore, in the present compilation, the dating intervals for some sites were corrected according to the new 14C dates that are becoming available or last archeological conceptions. The aim of this paper is to communicate the enlarged and revised after 2009 local data set and the obtained from it local reference curves for the three geomagnetic field elements during the last 8000 years.

Section snippets

General description of the extended database

The basic feature of Bulgarian archaeomagnetic data set is that it summarizes the simultaneous determination of the geomagnetic field direction (D, I) and intensity (F) – both parameters being obtained from the same material. It covers almost the entire prehistoric and historic periods. The methodology applied in the Sofia Palaeomagnetic laboratory was gradually improved and optimized over the years. Detailed information about sampling technique, experimental protocols, selection and quality

General magnetic characteristics of the unpublished sites, included after 2009

The new sites included in the last version of Bulgarian database are 28. The archaeomagnetic results from 11 of them are published (corresponding references are given in the last column of the Table 1 – the first reference corresponds to that of Geomagia 50.v3 (in preparation) and the second one corresponds to the original (detailed) publication). The main properties of the other 17 unpublished reference points are summarized in this section.

For all studied materials different types of magnetic

Discussion of the raw data

The main difficulty in archaeomagnetic studies aiming to describe the palaeosecular variations of the geomagnetic field elements is the reliable distribution of the experimental results on the absolute scale of time (Fig. 7). This distribution depends entirely on the obtained information for dating interval of the studied structures. The last 2000 years are historically well known for the Western Europe and SVCs are obtained for France (Gallet et al., 2002) and Germany (Schnepp and Lanos, 2005).

Bulgarian SVCs of the three geomagnetic field elements for the last 8000 years and discussion

The main goal of each local data set is the elaboration of the local palaeosecular variations of the geomagnetic field elements in the archaeological past. The here presented extended and revised Bulgarian database is used to provide the searched secular variations over the last 8000 years. They are given in Fig. 9 (Lanos, 2004, pp. 43–78; Lanos et al. in preparation).

The evident larger error bands immediately mark the time periods where not sufficient materials were found and studied (Fig. 7),

Conclusions

As a result of this study the following conclusions can be drown:

  • 1.

    Each local database requires a constant maintenance, filling up and revisions.

  • 2.

    Obviously the newly obtained archaeomagnetic data are close to the previously published. The later in combination with the revised points describe better the general trend of the three geomagnetic field elements.

  • 3.

    The Bayesian evaluation of the local secular variation curves of the geomagnetic elements over the last 8000 years are valuable contribution to

Acknowledgements

This study was supported by the Project No DMU 03/42 granted from the Bulgarian National Fund. The authors expressed their gratitude to the entire archaeological community in Bulgaria for their collaboration during sampling and for the exchange of useful information. Anonymous reviewer and Prof. Tarling are acknowledged for the valuable comments, suggestions and English style corrections.

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