On the Reliability of Archaeomagnetic Dating in IberiaTwo Case Studies from Portugal and Zamora

Resumen

Archaeomagnetic investigations have been carried out on 22 combustion structures from Portugal and Spain, along the Duero River Valley at the archaeological sites of Crestelos and Oliva! Poço da Barca (NE Portugal) and at El Castillon (NW Spain). The age of most of the investigated structures at Portuguese sites ranges from the 3rd century BC up to Roman times according to archaeological information. At El Castillon the investigated kilns are considered to cover the 5th - 7th centuries AD. Stepwise thermal and alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions. Mean site directions were well grouped in most sites. Only the two most poorly preserved structures showed a relatively high directional dispersion. Classical Thellier paleointensity experiments were conducted on pilot specimens. Successful results were obtained in specimens from el Castillon and from Oliva! Poço da Barca. In contrast, alterations occurred during thermal treatment at Crestelos kilns and hearths. Archaeomagnetic dating was conducted in all sites by using six reference Palaeosecular Variation (PSV) curves provided by local studies (from Iberia and France) and by regional (SCHA.DIF.3k) and global models (ARCH3k.1 and SHA.DIF.14k). The best PSV master curves for dating purposes on the investigated structures are the French curve for the old structures (older than 200 AD) and the regional SCHA.DIF.3k model for the younger structures. The new archaeomagnetic dating results suggest that the Crestelos site was active since 265±87 BC up to 252±138 AD, with a mean occupation period between 198 BC-43 AD. There could have been a previous occupation of the site but it could not be precisely dated. Distributions of the directions from Crestelos describe a PSV trend consistent with the French curve, suggesting that the archaeomagnetic dating tool could be improved in the near future by dating selected structures by independent methods. Archaeomagnetic dating at El Castillon site provide two phases of occupation of the site, at 441±12 AD and 488±53 AD, consistent with recent radiocarbon ages from the site. The effect of TRM anisotropy on directions has been evaluated, with the results highlighting the necessity for TRM anisotropy corrections if accurate dating is to be obtained.