Archeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits

Archeomagnetic dating developed at St. Maur laboratory has been applied to the Mediterranean volcanoes Etna, Vesuvius and Ischia. The method involves samples from lava flows or high temperature emplaced pyroclasts (welded scoriae, pumice, etc.) weighing 0.5-1 kg each, that allows reaching a precision of a few tenths of a degree on the direction of their thermoremanent magnetization, and hence a semi-angle of the Fisher 95 percent confidence cone between 0.6 and 1.8 for every volcanic unit. Among the factors reducing precision on the mean magnetic direction, the most important appears to be a distortion of the ambient field induced by magnetization of the cooling lava, which means that a number of samples should be collected over a large area. Age determination is based upon similarity between variation curves of the Direction of Earth's Magnetic Field (DEMF) reconstructed in France from 120 well-dated archeological sites, and on Italian volcanoes from historically dated eruptions. A total of 63 lava flows and pyroclastic units, such as cinder cones or nuee ardente deposits, are shown to be dated with an overall precision of +/-40 years for the last 1500 years, and +/-50 to +/-100 years from AD 500 to 150 BC, this lesser precision resulting from both an increasing uncertainty on the shape of the DEMF curve and a smaller variation of the DEMF itself. This irregularity of the DEMF path plus an increasing number of ambiguities, related to similarity of the DEMF at different times further into the past, are the most serious limitations of the method. Though well-dated eruptions are known for the last two millennia, retrieval of their products is often misleading and about 50 percent of volcanics presumed of known date prior to the 17th century are in fact of older age, discrepancies usually reaching several hundreds of years. Owing to good agreement between the DEMF curves of France and southern Italy, the method may confidently be extended to volcanic materials from the whole of Mediterranean Europe, provided there are firm constraints that they were erupted within the last 2100 years.