Achaeometrical Research in Hungary II., 1988

PROSPECTING and DATING - Péter MÁRTON: Magnetic directional data for Hungary and their application for archaeomagnetic dating

Péter MÁRTON MAGNETIC DIRECTIONAL DATA FOR HUNGARY AND THEIR APPLICATION FOR ARCHAEOMAGNETIC DATING The archaeomagnetic record for Hungary consists of magnetic directional data (declination and inclination couplets) obtained by sampling and measuring the magnetisation of remains of baked clay structures which have remained in place since their last use in antiquity. Suitable material for such studies have come from excavations at various places in Hungary and the age estimates, which are based in the majority of cases on archaeological evidence, have been provided by the sites' archaeologists. The main body of the magnetic directional data originate from the last 2000 years giving an almost continuous coverage for this time period (Fig. 1). Similar records of various length are known for other regions in Europe, Asia and North America. Though all European records show the same broad features the details may be different owning to the regional variation of the geomagnetic field. From the geomagnetic point of view each archaeomagnetic data set is equivalent to an extension of the geomagnetic record back into antiquity for the so-called secular variation (SV) of the local geomagnetic field, i.e. the long-term changes of the geomagnetic elements with time within a region. The observation (direct measurement) of the direction of the geomagnetic field is a relatively recent achievement. The longest continuous records (from the London and Paris observatories) extend back only to the end of the 16

th century. For Hungary, there is no similar record available, though there are a number of well documented measurements and ancient map readings from which the SV of the direction of the local geomagnetic field can be traced back to AD 1700 with confidence (Fig. 1). For times preceding 1700, it is the archaeomagnetic directional record which contains all information that is available at present concerning the secular variation of geomagnetic directions (Fig 1). The broad features are already clear from the data despite of their scattering due to uncertainties both in their age estimates and measured values. The inclination varies with time between 55 and 75 degrees in an almost monochromatic manner with a nearly 900 year period. The variation of declination is less regular than that of the inclination but even so some diagnostic features are clearly distinguishable as e. g. the significant negative values during the Avar Period and towards the end of the Middle Ages and the large positive values that decrease with time during the Middle Ages. In order to quantify the directional variations, the data were subjected to a numerical smoothing procedure in which both of the aforementioned uncertainties were accounted for by using a suitable weighting scheme. The resulting curves (Fig. 1) are now considered as the current model of the directional S V of the geomagnetic field for our region. Clearly, this model is not the final one. It is subject to improvements, especially in those periods in which the data are scarce as e.g. the Roman Period. It is also hoped that the 200 year wide gap representing the „Dark Ages" can be filled with data in the near future. Geophysics Department Loránd Eötvös University (H1083 Budapest Ludovika tér 2)