ISSN : 0956-540XInternational audienceA Bayesian hierarchical modelling is proposed for the different sources of scatter occurring in archaeomagnetism, which follows the natural hierarchical sampling process implemented by laboratories in field. A comparison is made with the stratified statistics commonly used up to now. The Bayesian statistics corrects the disturbance resulting from the variability in the number of specimens taken from each sample or site. There is no need to publish results at sample level if a descending hierarchy is verified. In this case, often verified by archaeomagnetic data, only results at site level are useful for geomagnetic reference curve building. Typically, a study with at least 20 samples will give an α95i 5 per cent close to the optimal α95i for a fixed site number mi and if errors are random with zero mean (no systematic errors). The precision on the curve itself is essentially controlled, through hierarchical elliptic statistics, by the number of reference points per window and by dating errors, rather than by the confidence angles α95ij at site level (if a descending hierarchy). The Bayesian elliptic distribution proposed reveals the influence of the window width. The moving average technique is well adapted to numerous and very well dated data evenly distributed along time. It is not a global functional approach, but a (linear) local one

. Extended and revised archaeomagnetic database and secular variation curves from Bulgaria for the last eight millennia. Physics of the Earth and Planetary Interiors, Elsevier, 2014, 236, pp.79-94

a b s t r a c tAvailable archaeomagnetic data indicate that during the past 2500 yr there have been periods of rapid geomagnetic field intensity fluctuations interspersed with periods of almost constant field strength. Despite Europe being the most widely covered region in terms of archaeomagnetic data the occurrence and the behaviour of these rapid geomagnetic field intensity changes is under discussion and the challenge now is to precisely describe them. The aim of this study is to obtain an improved description of the sharp intensity change that took place in western Europe around 800 AD as well as to investigate if this peak is observed at the continental scale. For this purpose 13 precisely dated early medieval Spanish pottery fragments, four archaeological French kilns and three collections of bricks used for the construction of different French historical buildings with ages ranging between 335 and 1260 AD have been studied. Classical Thellier experiments performed on 164 specimens, and including anisotropy of thermoremanent magnetisation and cooling rate corrections, gave 119 reliable results. The 10 new high-quality mean archaeointensities obtained confirm the existence of an intensity maximum of $ 85 mT (at the latitude of Paris) centred at $ 800 AD and suggest that a previous abrupt intensity change occurred around 600 AD. Together with previously published data from western Europe that we deem to be the most reliable, the new data also suggest the existence of two other abrupt geomagnetic field intensity variations during the 12th century and around the second half of the 13th century AD. High-quality archaeointensities available from eastern Europe indicate that very similar geomagnetic field intensity changes occurred in this region. European data indicate that very rapid intensity changes (of at least 20 mT/century) took place in the recent history of the Earth's magnetic field. The results call for additional high-quality archaeointensities obtained from precisely dated samples and for a selection of the previously published data if a refined description of geomagnetic field intensity changes at regional scales is to be obtained.

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This data brief reports the latest updates of archeomagnetic data obtained at the Sofia palaeomagnetic laboratory of the Geophysical Institute, Bulgarian Academy of Sciences. The current data set consists of measurements from 284 Bulgarian archeological sites covering the past 8000 years. There are also 54 sites from other European regions, namely, Serbia, Kossovo, Greece, Spain, Switzerland, Finland, and Russian Karelia, as well as five sites from Morocco in North Africa. The update of the archeomagnetic results consisted of a thorough revision of all geomagnetic field measurements as well as dating these measurements that were published in the original papers or in previous compilations. The updated results can be found in GEOMAGIA (http://geomagia.ucsd.edu) or as an Excel spreadsheet at the EarthRef.org Digital Archive (http://earthref.org/cgi-bin/erda.cgi?n = 946).

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The widespread occurrence of a novel, high coercivity magnetic phase in well‐heated archeological material is reported. Its properties are defined when it represents the dominant magnetic phase, although it is nearly always found as part of a mixture of magnetic phases. They are as follows: very high coercivity (remanence coercivity >600 mT), low unblocking temperatures (≤200°C) and high degree of thermal stability–this last property distinguishing it from goethite. The phase shows striking similarities to magnetic phases produced by thermal decomposition of nontronite (an Fe‐rich clay), where decomposition occurs after prolonged heating in air to high temperatures – conditions suffered by well‐heated archeomagnetic material. Preliminary results of Mössbauer and X‐Ray diffraction spectroscopy suggest that the phase is more likely to be a substituted hematite, rather than Fe‐cristobalite or a variant of ɛ‐Fe2O3.