New constraints on geomagnetic field intensity variations in the Balkans during the Early Byzantine period from ceramics unearthed at Thasos and Delphi, Greece

International audience; We report on five new archeomagnetic field intensity data obtained in Greece from groups of pottery fragments precisely dated to between the middle of the fourth century and the beginning of the seventh century CE. These potsherds were unearthed on the islands of Thasos (Northern Greece) and Delphi (Central Greece). Their dating is primarily ensured by typo-morphological arguments, combined with archeological and historical constraints. Archeointensity measurements were performed using the Triaxe protocol, which involves continuous magnetization measurements at high temperatures and which allows us to overcome the thermoremanent magnetization anisotropy and cooling rate effects. Magnetic mineralogy measurements such as low-field magnetic susceptibility versus temperature and thermal demagnetization of three orthogonal IRM components have identified magnetite with possible impurities as the main carrier of the magnetization. The new data range from 52.0μT to 61.5μT after reduction to Thessaloniki and show an increase in geomagnetic field intensity in Greece during the Early Byzantine period. They appear in good agreement with previous intensity results satisfying a set of quality criteria and obtained in a region of 700 km around Thessaloniki, therefore incorporating data from Bulgaria, Greece and South Italy. This study is part of an ongoing effort to better constrain the evolution in geomagnetic field intensity in the Balkans over the past few millennia, with potential use for dating in archeology. The rapid intensity variations documented here during the Early Byzantine period are clearly of interest in this respect.