Abstract
The objective of this study is to provide a well-dated point for a future palaeosecular variation (PSV) reference curve for western Russia. For this purpose archaeomagnetic and magnetic property analyses were carried out on a pottery kiln unearthed at the UNESCO World Heritage site of ancient Bolgar, having a rather precise age dating. The archaeological context provided an age between 1340 and 1360 C.E. The characteristic remanence vector was determined through alternating field demagnetisation and Thellier-Thellier palaeointensity experiments. Some innovations were introduced regarding palaeointensity. The check testing the equality of blocking and unblocking temperature was redefined. This allowed waiving the commonly used additional zero-field cooling steps during the Thellier-Thellier experiment. Another innovation concerns the calculation of archaeointensity at structure level. A Bayesian approach was introduced for averaging individual specimen archaeointensities using a prior probability distribution of unknown uncertainties. Next, an additional prior probability distribution was used to correct for cooling rate effects. This resulted in a lower uncertainty compared to common practice and in eluding time consuming cooling rate experiments. The complex magnetic mineralogy consists of maghaemite, multi-domain haematite and Al-substituted haematite. Some samples contained also some non-stoichiometric magnetite. The magnetic mineralogy was determined through hysteresis loops, backfield and remanence decay curves, measurements of the frequency dependence of magnetic susceptibility and through low temperature magnetisation curves. Accompanying high-temperature thermomagnetic analyses revealed an excellent thermo-chemical stability of the studied specimens. Directions obtained from alternating field demagnetisation and those extracted from archaeointensity experiments are congruent and have low uncertainties. The obtained archaeomagnetic results are fairly in agreement with global geomagnetic field models and contemporary PSV data of the wider area. The geomagnetic field vector obtained for ancient Bolgar is of high quality, deserving thus its inclusion in a future PSV reference curve for European Russia.
Highlights
Isothermal remanent magnetization (IRM) acquisition and backfield curves show a significant change in slope between 60 and 100 mT; field strengths of 1500 mT are insufficient to saturate the magnetisation
There is evidence for at least two magnetic mineral populations with significantly different coercive forces, i.e. a dominating low coercivity phase with coercivities mainly between 5 and 60 mT, and a minor high coercivity phase with coercive forces mainly between 200 and 600 mT. This is confirmed by hysteresis loops carried out in fields up to ±5 T (Fig. 3c,d) evidencing a wasp-waisted shape, which indicate the presence of at least two magnetic mineral populations with strongly contrasting coercivities
The remarkable slope change around 80 K seen in Fig. 3e is probably caused by viscous grains which become unblocked during warming and which lose partly their magnetisation
Summary
G. Folgheraiter is considered the founder of archaeomagnetic dating, since he was the first to conduct investigations of antique ceramics suggesting that burned natural or synthetic materials can be used to study variations of past geomagnetic field intensity and direction (Folgheraiter, 1899). Folgheraiter studied a Roman pottery dating back to the 1st millennium B.C.E. J. Thellier (Koenigsberger, 1938a,b; Thellier, 1938; Thellier and Thellier, 1959) further continued these studies of geomagnetic field intensity and direction using burned archaeological material which had acquired a thermo-remanent magnetisation
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