Evdokia Tema

An updated catalogue of Greek archaeomagnetic data for the last 4500 years and a directional secular variation curve

We present an updated compilation of Greek directional archaeomagnetic data for the last 4.5 millennia. The data set comprises 89 directions from archaeological artefacts and volcanic rocks. Most of the data come from the Late Bronze Age (1700-1400 BC) that is the flourishing period of the Minoan civilization in Crete, while parts of the classical (480-323 BC), Hellenistic (323-31 BC) and Roman (146 BC-330 AD) periods are also well covered. The dataset has been analysed using the Bayesian approach for curve building and a directional secular variation (SV) curve for Greece is proposed. Comparisons with regional and global model predictions show a general agreement even though some discrepancies are observed for some time intervals. The new curves together with the previously published intensity SV curve for Greece, also using the Bayesian approach, form a homogeneous set and enrich our knowledge of the full geomagnetic field vector variation in Greece during the last millennia.

Reconstructing the geomagnetic field in west africa: first absolute intensity results from Burkina Faso

We present absolute geomagnetic intensities from iron smelting furnaces discovered at the metallurgical site of Korsimoro, Burkina Faso. Up to now, archaeologists recognized four different types of furnaces based on different construction methods, which were related to four subsequent time periods. Additionally, radiocarbon ages obtained from charcoal confine the studied furnaces to ages ranging from 700–1700 AD, in good agreement with the archaeologically determined time periods for each type of furnace. Archaeointensity results reveal three main groups of Arai diagrams. The first two groups contain specimens with either linear Arai diagrams, or slightly curved diagrams or two phases of magnetization. The third group encompasses specimens with strong zigzag or curvature in their Arai diagrams. Specimens of the first two groups were accepted after applying selection criteria to guarantee the high quality of the results. Our data compared to palaeosecular variation curves show a similar decreasing trend between 900–1500 AD. However, they reveal larger amplitudes at around 800 AD and 1650 AD than the reference curves and geomagnetic field models. Furthermore, they agree well with archaeomagnetic data from Mali and Senegal around 800 AD and with volcanic data around 1700 AD.

Directional results and absolute archaeointensity determination by the classical Thellier and the multi-specimen DSC protocols for two kilns excavated at Osterietta, Italy

We present a detailed rock-magnetic and archaeomagnetic study of two brick kilns, named OSA and OSB, discovered at the location of Osterietta, in northern Italy. The magnetic properties of representative samples have been investigated to identify the nature of the magnetic carriers, their domain state and thermal stability, and investigate their suitability for archaeomagnetic determinations. Thermally stable, mainly pseudosingle domain (PSD) magnetite is identified as the main magnetic carrier. The full geomagnetic field vector has been determined for the two kilns, including directional and intensity analysis. Archaeointensities have been recovered with both the classical Thellier and the multi-specimen protocols. The multi-specimen procedure was performed with a very fast-heating oven developed at Montpellier, France. A Matlab® code for anisotropy correction during the Thellier experiment is provided. The archaeointensity results obtained from both techniques for the OSA kiln are of high quality and in good mutual agreement. For the OSB kiln, Thellier results are characterized by large standard deviation and the multi-specimen (MSP) technique was not successful. The obtained full geomagnetic field vector (declination, inclination and intensity) has been used for the archaeomagnetic dating of the two structures suggesting that the OSA kiln was for the last time used between 1761?1841 A.D. and the OSB kiln between 1752?1831 A.D., at 95% probability. This study shows that intensity determinations do not restrict the dating results when referring to the last few centuries, as this period is characterized by very small intensity variations.

Palaeotemperature estimation of the pyroclastic deposit covering the pre-Minoan palaeosol at Megalochori Quarry, Santorini (Greece): Evidence from magnetic measurements

Thermal remanent magnetization analyses were carried out on ceramic fragments and lithic clasts embedded in the first pumice fall deposits of the Minoan eruption. The aim of this study is to estimate the equilibrium temperature after deposition of these pyroclastic fall deposits and their thermal effect on the pre-Minoan surface. A total of 30 samples from 22 independent ceramic fragments and 20 samples from 14 lithic clasts have been studied. Samples were collected from the Megalochori Quarry, located at the southern part of Santorini island. Stepwise thermal demagnetization reveals that the ceramics were mostly re-heated at temperatures around 140–180°C; in few ceramics a higher temperature component is also present, probably related to the original heating or the use of the ceramics before the eruption. Thermal demagnetization of the lithic clasts shows similar results with slightly higher re-heating temperatures, around 180–240°C. The estimated temperatures represent the equilibrium temperatures obtained after the deposition of the pumice fall and show that the pyroclastic fall deposits at a distance of around 6 km from the eruption vent maintained a temperature high enough to re-heat the buried ceramics at temperatures around 140–180°C.