Journal of Archaeological Science 35 (2008) 1389e1398 http://www.elsevier.com/locate/jas Recent achievements in archaeomagnetic dating in the Iberian Peninsula: application to Roman and Mediaeval Spanish structures M. Go´mez-Paccard a,*, E. Beamud b a Research Group of Geodynamics and Basin Analysis, Department of Stratigraphy, Paleontology and Marine Geosciences, Universitat de Barcelona, Campus de Pedralbes, E-08028 Barcelona, Spain b Research Group of Geodynamics and Basin Analysis, Paleomagnetic Laboratory (UB-CSIC), Institute of Earth Sciences ‘‘Jaume Almera’’, Sole´ i Sabarı´s, E-08028 Barcelona, Spain Received 18 May 2007; received in revised form 25 September 2007; accepted 8 October 2007 Abstract Archaeomagnetic studies in Spain have undergone a significant progress during the last few years and a reference curve of the directional variation of the geomagnetic field over the past two millennia is now available for the Iberian Peninsula. These recent developments have made archaeomagnetism a straightforward dating tool for Spain and Portugal. The aim of this work is to illustrate how this secular variation curve can be used to date the last use of several burnt structures from Spain. Four combustion structures from three archaeological sites with ages ranging from Roman to Mediaeval times have been studied and archaeomagnetically dated. The directions of the characteristic remanent magnetization of each structure have been obtained from classical thermal and alternating field (AF) demagnetization procedures, and a mean direction for each combustion structure has been obtained. These directional results have been compared with the new reference curve for Iberia, providing archae- omagnetic dates for the last use of the kilns. The inferred ages obtained from this study are consistent with the archaeological evidence available. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Dating; Archaeomagnetism; Iberian Peninsula; Secular variation; Earth’s magnetic field 1. Introduction the angle formed between the geographic north and the hori- zontal component of the geomagnetic field, ranging from Archaeomagnetism is mainly known by archaeologists as 0 to 360. I is the angle between the horizontal plane and a dating tool. This technique is based on two physical phenom- the geomagnetic field vector, it ranges from À90 to 90 and ena: first, the Earth’s magnetic field varies in space and time; is defined as positive downwards. The Earth’s magnetic field and second, certain archaeological structures heated to high intensity (F ) is usually measured in microTeslas (mT). temperatures can acquire a stable thermoremanent magnetisa- Measurements of TRM allow the determination of the di- tion (TRM) which is parallel and proportional to the ambient rection and intensity of the Earth’s magnetic field at the mo- magnetic field at the time of cooling. The mechanism of TRM ment of cooling of the burnt structure. Therefore, the past acquisition relies on the capacity of some naturally occurring secular variation (SV) of the Earth’s magnetic field can be de- minerals (principally iron oxides) to retain a permanent or termined for any given region by studying well dated and remanent magnetisation. heated archaeological structures of different ages. The archae- The direction of the surface geomagnetic field is usually de- ological structures suitable for this kind of studies include fined by the magnetic declination (D) and inclination (I ). D is kilns, furnaces, domestic ovens, thermal baths, burnt walls or soils, bricks, pottery, or any heated material that has ac- quired a TRM, recording the Earth’s magnetic field present * Corresponding author. Tel.: þ34 93 403 4888; fax: þ34 93 402 1340. at the time of cooling. It is worth pointing out that the direc- E-mail address: [email protected] (M. Go´mez-Paccard). tion of the ancient geomagnetic field can be obtained only 0305-4403/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2007.10.005 1390 M. Go´mez-Paccard, E. Beamud / Journal of Archaeological Science 35 (2008) 1389e1398 from structures found in situ since the last firing. The other 2006b). The Bayesian statistical approach allows the window finds, like ceramics or other displaced baked clays objects, width to be automatically adapted to the density of points can inform about the archaeointensity but not about the direc- along the time axis, making the points movable within their re- tion of the ancient geomagnetic field. Once determined, these spective dating error ranges. This method adds some prior SV reference curves can be used as a dating tool by comparing knowledge on the global nature of the curve to be estimated, the archaeomagnetic field information obtained from the ar- as it is assumed that the studied physical phenomenon varies chaeological material to be dated (direction and/or intensity in a smooth way. The Bayesian approach allows the fitting of the TRM) with the available SV curve of the Earth’s mag- of a spherical spline function based on roughness penalty to netic field for the region concerned. This analysis provides ar- the data in three dimensions (declination, inclination and chaeomagnetic dates for the last use of the archaeological time). Moreover, when available, this method can take into ac- structures, when the TRM was acquired. Obviously, this can count previous archaeologic/stratigraphic constraints during only be reliably applied in those regions for which reference the calculation process. The results are expressed as a mean SV curves are available. curve and an envelope (error) at a 95% confidence level. Several SV curves for the last millennia are available for The ‘‘real’’ curve will lie somewhere inside the error band. Europe, including France (Thellier, 1981; Bucur, 1994; Gallet The Bayesian SV curve obtained for Iberia (Go´mez-Paccard et al., 2002), Bulgaria (Kovacheva et al., 1998), Hungary et al., 2006b) spans approximately 2000 years, from À100 to (Ma´rton, 2003; Ma´rton and Ferencz, 2006), Germany 1959 AD. Several directions are available for each century (Schnepp and Lanos, 2005), Great Britain (Batt, 1997; Zana- throughout most of the record. However, there is still a need niri et al., 2007), Austria (Schnepp and Lanos, 2006) and Italy to increase the amount of data, especially between the 6th (Tema et al., 2006). These curves are currently used as a useful and 10th centuries AD and prior to 0 AD, where most data dating tool for archaeological studies in some of these coun- come from neighbouring countries. Despite these limitations, tries (see for example Kovacheva et al., 2004). these studies (Go´mez-Paccard et al., 2006a,b) finally made it Several archaeomagnetic studies carried out in Spain in possible to use archaeomagnetism as a dating tool for the Ibe- recent years allowed the establishment of the first SV curve rian Peninsula. for the Iberian Peninsula (Go´mez-Paccard et al., 2006b). The Obviously the reliability of the archaeomagnetic method aim of this work is to illustrate how this new curve can now used as a dating tool depends on the fidelity of the SV curve be used to date the last use of different burnt structures from used for comparison. The precision of archaeomagnetic dating Spain. To this purpose, four combustion structures from three depends on several factors and varies from case to case. It is archaeological sites with ages ranging from Roman to Medi- constrained by the rate of variation of the geomagnetic field aeval times have been studied and archaeomagnetically (declination and inclination), the envelope errors of the SV dated. curve, and the error of the archaeomagnetic direction of the structure to be dated. It is worth pointing out that the error 2. Archaeomagnetism in Spain of SV curve depends on the precision of the independent dat- ing of the archaeological structures on which the SV curve is Spain has a rich historical past that has been the subject of based, and the number of structures available for each period. intensive archaeological research, making this country a fa- This indicates not only the need to obtain more data but also vourable place to obtain detailed records of the variation of the need to study well dated materials in order to construct the Earth’s magnetic field during historical time. Despite this SV curves. Then, the success of the archaeomagnetic method remarkable potential, until recently, the number of archaeo- in Iberia requires a close collaboration between archaeologists magnetic studies for this region was limited. New studies and geophysicists from this region. This collaboration remains have increased the amount of data and it is now possible to re- fundamental to improve the method and the precision of trace the evolution of the geomagnetic field direction in this archaeomagnetic dating. region for at least the past two millennia. In order to document variation of the full geomagnetic vec- Go´mez-Paccard et al. (2006a) presented a compilation of tor, detailed and reliable measurements of the Earth’s mag- previously available archaeomagnetic directions for Spain netic field intensity are also needed. The establishment of an (Thellier, 1981; Pare´s et al., 1992; Oyamburu et al., 1996) intensity reference curve can also help to dating purposes, es- along with the results from 58 structures not studied previ- pecially in the case of displaced objects, where only the ar- ously, providing a catalogue of 63 archaeomagnetic directions chaeointensity, and occasionally the inclination can be spanning the last two millennia. In order to construct the SV obtained. Some archaeointensity studies have been performed curve for Iberia, Go´mez-Paccard et al. (2006b) compiled in Spain (Kovacheva et al., 1995; Nachasova et al., 2002a,b, a data set of archaeomagnetic directions from sites that fell 2007; Burakov et al., 2005, 2006; Burakov et al., in press; within a 900 km circle centred on Madrid. Madrid was used Go´mez-Paccard et al., 2006c). Despite these efforts, the exist- as the reference point because it is close to the geographic cen- ing archaeointensity data set for Iberia remains poor and only tre of the Iberian Peninsula (40.4 N, 3.7 W).