Archaeomagnetism abstracts from AGU, Toronto

Here are the abstracts on archaeomagnetism from the session I co-organized at the just completed American Geophysical Union Joint Assembly:

Archaeomagnetic Study performed on Early Medieval Buildings from western France
Chauvin, A (, Géosciences-Rennes, Université Rennes 1, CNRS UMR 6118, Campus de Beaulieu, Rennes, France et al.

A multiple dating study, involving a collaboration between specialists of dating techniques (thermoluminescence (TL) and radiocarbon), historians of art and archaeologists, has been carried out on several early medieval buildings from western France. The early medieval period is not well known especially in France where there is a lack of visible evidence that identifies pre-Romanesque architecture. The majority of buildings to have survived from this period are religious ones, considered important enough to be made of strong, non-perishable material such as stone or brick, as for example the churches of Notre-Dame-sous- Terre in the Mont-Saint-Michel or St Martin in Angers. Due to their significance in architectural history, it is imperative to position them accurately in the chronology of the history of art. Bricks are often used to build up round-headed arches or to reinforce the frame of a wall with bonding courses in those churches. TL dating and archeomagnetic analysis were performed on cores drilled within bricks while radiocarbon dating were undertaken on coals found within mortars. In order to increase the number of data during the early Middle Ages, archeointensity determinations using the classical Thellier technique with anisotropy of thermal remanence and cooling rate corrections were performed. Archaeomagnetic directions were used to recognize the firing position of bricsk during manufacture. Reliable and precise ages were obtained on the church Notre-Dame-sous-Terre; they indicate two phases of building in 950±50AD and 990±50AD. Mean archeointensities obtained on 17 (21) samples from the first (second) phases appears very closed 69.1±1.2 and 68.3±1.6 microTesla. Ages and archeomagnetic results obtained on 4 other sites will be presented and compared to the available data in western Europe.
News story on Chauvin presentation

European Archaeomagnetism: Progress and Problems
Evans, M E (, Institute for Geophysical Research, University of Alberta, Edmonton, AB T6G 2G7, Canada et al.

Much progress has been made since the seminal work of Giuseppe Folgheraiter (1856-1913) in the late 19th century. So much so that recent advances now make it possible to draw up complete isogonic and isoclinic maps for Europe and adjacent areas spanning the last three millennia (Pavon-Carrasco et al., 2009). Results based on multiple independent studies, with high precision and good age control are crucial and should be recognized as "anchor points" (e.g. Pompeii). On the other hand, the nagging problem of outliers persists. Among the possible causes are magnetic refraction, physical distortion, and inadequate chronological control. Some examples, drawn from our own investigations over the last 30 years, will be discussed in detail. These include previously unpublished data from a detailed study (more than 100 samples) of a kiln in southern Italy, and an apparently good (but aberrant) archaeodirection from a kiln in southern Spain.

Regional Archeomagnetic Model for Europe for the Last 3000 Years: Application to Dating

Pavón-Carrasco, F (, Grupo de Paleomagnetismo. Dpto. de Geofísica y Meteorología, Universidad Complutense de Madrid., Avda. Complutense, s/n., Madrid, 28040, Spain et al.

Recently a new regional archeomagnetic model in Europe for the last three millennia has been proposed. This model, SCHA.DIF.3K (Pavón-Carrasco et al., 2009, Geochem. Geophys. Geosyst., doi:10.1029/2008GC002244, in press), is based on a Spherical Cap Harmonic Analysis (SCHA) for spatial representation and sliding windows method in time. The model provides information of both directional and intensity variation of the Earth's Magnetic Field for the last 3000 years in the European region. One of the immediate applications of SCHA.DIF.3K regional model is its use as tool for archeomagnetic dating. So far the PalaeoSecular Variation Curve (PSVC) determined for a region has been used for archeomagnetic dating. The limitation of this application is the distance from the dating point to the location of the reference curve (the relocation error). In addition it must be borne in mind that the PSVC are individually generated for each region, so there is no consistency enforced between curves from neighboring areas. The use of the SCHA.DIF.3K model as a tool for archeomagnetic dating represents an improvement for several reasons. First of all, the regional model has been generated considering all elements of the geomagnetic field (declination, inclination and intensity). Second, the regional model is built with an in situ archeomagnetic database. Furthermore, the database covers the whole time period from 1000 BC to 1900 AD, while the database used in the PSVC has gaps of data for any time interval. Finally, and more important, we can generate a PSVC at the location of the archeological structure, avoiding in this way the relocation error associated with traditional PSVC. To demonstrate the utility of the regional SCHA.DIF.3K model, we have used it to date several archeological structures and we have compared results with the archaeological information and/or archeomagnetic dating provide by the use of the PSVC.

The Mesoamerica Secular Variation Curve. A continuous research since 1999
Soler-Arechalde, A M (, Universidad Nacional AUtonoma de Mexico, Instituto de Geofisica, Ciudad Universitaria, Del. Coyoacan, Mexico, DF 04510, Mexico et al.

Since 1999 we have been working on improving the Mesoamerica Secular Variation Curve. We re-sampled some sites that Wolfman initially used in its first proposal of this Curve, such as Teotihuacan, Teopancazco and Tula. Wolfman 's curve only had 4 radiocarbon dates directly associated with the sampled sites; the other dating were actually stratigraphic and ceramic correlations. More than 28 radiocarbon dates have been incorporated from AD 60 to 560 from samples associated to Teotihuacan civilization, 13 more from Xochicalco and 10 from Tula. More than 1000 specimens, from 12 twelve sites were fully processed with alternated field demagnetization We have been working during the excavation campaigns and training the archeologist to get their samples. A 100-year moving window was employed to get the average poles. A Bayesian statistic has been employed in order to improve our curve. We still denote a lack of data from two time intervals: from 0 to 200 and from 1000 to 1600. We are now working on it, collecting samples from these periods, such as those from La Joya, Ver, which are now been processing and from which some preliminary results will be reported.

Dating Post-Medieval Archaeology: Which Global Geomagnetic Field Model to use?
Lodge, A (, University of Liverpool, Department of Earth and Ocean Sciences, Jane Herdman Laboratories, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom et al.

The scientific dating of Post-Medieval archaeology (16th Century onwards) is problematic as most methods cannot provide any better resolution than may be apparent from contextual or stylistic considerations. As high resolution global geomagnetic field models exist for this period, archaeomagnetism offers the possibility of bi-decadal dating of burnt in situ structures, with implications for the management of cultural heritage. The question arises as to which global geomagnetic field model is most appropriate for this dating? Should the high resolution historical field model, gufm (Jackson et al., 2000, Four centuries of geomagnetic secular variation from historical records, Phil. Trans. Roy. Soc. Lond. A, 358, 957- 90.) which covers the period 1590-1990 AD and is based on data from ship's logs be used, or should an archaeomagnetic model such as GMADE2K.2 (Lodge & Holme, 2008, Developing a global geomagnetic field model for archaeomagnetic dating in Europe for the last 2000 years (updating GMADE2K.1 to GMADE2K.2), Geophys. Res. Abstr., 10, Abstract EGU2008-A-03470) be used? In general a higher accuracy can be expected from the historical model, but the modeling strategy for gufm is aimed at investigating the magnetic field evolution at the core-mantle boundary, whilst GMADE2K.2 is developed to serve as an archaeomagnetic dating tool. If we compare secular variation curves in Europe for declination at this time, then the two models agree very well. For inclination however, there is a discrepancy pre-1800 AD between the two models, with the historical model tending to higher inclinations. Here we study the possible causes of this discrepancy: How reliable are the early historical inclination data? How reliable is the historical model at this time - is the inclination being affected by the domination of declination data? Finally, are the archaeomagnetic data systematically low, possibly caused by undetected magnetic refraction? The advantage of constructing global geomagnetic field models is that the inter-dependence of the components is taken into account. However, if we cannot reconcile the archaeomagnetic models and data with the historical model and data, then dating remains problematic.