First archaeomagnetic data from northern Iberia

Abstract

An archaeomagnetic study has been conducted on eight archaeological structures (two kilns, four fireplaces and two saunas) from two different areas in Asturias (northern Spain). The results provide the first archaeomagnetic directions from the northern Iberian Peninsula. The main goal of this paper is to improve on the non-uniform site distribution used to construct the first archaeomagnetic Secular Variation Curve for Iberia (“SVC-I”) by studying new sites from northern Spain, an area currently not represented. The lithologies of some of the studied archaeological structures from this zone (slates, quartzites) differ from those of the rest of the peninsula. Laboratory analysis includes both thermal and alternating field stepwise demagnetization and rock-magnetic studies. A low coercivity, moderate unblocking temperature ( T ub) phase, such as magnetite/maghemite, seems to be the carrier of the archaeomagnetic signal. This sometimes overlaps partially with a (geological) high coercivity/high T ub phase. Site-mean characteristic directions have been calculated following a hierarchical approach and applying Fisherian distribution tests. Archaeomagnetic results have been compared, applying Fisherian distribution tests, among themselves and with Roman age entries of the Spanish archaeomagnetic database. They have also been compared with the reference “SVC-I” using Bayesian methods. These analyses have served to validate the archaeological dating of the structures. There is a good agreement between the results of archaeomagnetic dating and radiocarbon dating, both of which are consistent with archaeological constraints. The uncertainties in the archaeomagnetic ages are relatively large mainly because the reference curve hardly varies in declination during Roman times. This highlights the need of more high quality archaeomagnetic data (from very well-dated structures) in order to reduce the errors associated with the reference “SVC-I” and the archaeomagnetic dating technique, particularly between 0 and 500 AD.