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The French archeomagnetic directional database for the past two Millennia and its application in geomagnetism and archeology M. Le Goff (1), N. Warmé (2), Y. Gallet (1), and A. Genevey (3) (1) Institut de Physique du Globe de Paris, CNRS UMR 7154, Laboratoire de Paléomagnétisme, Paris, France, (2) Institut National de Recherches Archéologiques Préventives, France, (3) Centre de Recherche et de Restauration des Musées de France, CNRS UMR 171, Paris, France

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The last article of Pr Thellier (1904-1987), in PEPI-1981, shows the directional variation curve of the Earth Magnetic Field, in France, between 0 and 1800AD, obtained with only 63 dated structures selected in a set of more than 200 ovens and 50 sets of bricks.

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13 years later, Ileana Bucur, in PEPI 1994, upgraded the French data base with 110 new data ; 67 being archeologically dated. Including the Thellier results, she computed this French reference curve with more than 120 dated directions. Since her retirement, in 1997, a young team has continued the directional data acquisition in the Saint-Maur Laboratory, with until now, nearly 400 new structures measured

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Over the past two millennia, the directional variations stand in a small aperture cone of less than 20°. Recovering these variations thus requires a high precision in measurement EW I = 0 I = 30° I = 60° N The expected parasitic magnetizations are : - partial or total TRM due to refiring - IRM due to lightning effect - and most often, significant VRM acquisition TRM directions can be biased by magnetic anisotropy Mechanical effects, such as structure dislocation or tilting, contribute to disperse the directional results ABOUT ACCURACY AND RELIABITY

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The methodology used in our laboratory, not « standard », follows the Thelliers one Extensive sampling of the fired structures, avoiding as much as possible walls, or clay mixed with pieces of bricks or tiles. Big samples (12cm in size) precisely oriented, without further cutting or induration. Systematic measurement of magnetic viscosity. AF demagnetization in selected cases.

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« Big » sample inductometer Sampling and Magnetic measurements « Big » sample AF demagnetizer Sampling Domestic Ovens in JOSSIGNY Horizontal surface and sun orientation 12 cm

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Inverse Measurement Z X Y TRM ± VRM Samples with a Viscosity ratio v (VRM/TRM) > 10 % are discarded North Lab direction Viscosity Experiments Viscosity Experiments (Thellier, 1981) By Vectorial operation VRM and TRM 2 - Samples in Inverse position during at least 2 weeks Direct Measurement 1 - Samples in Direct position during at least 2 weeks

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Two Examples of directional results with v=7% Structure : Rungis 1277 Mean TRM direction: D=-18.5°, I=64.7° a95=0.34°, N=16, k=10860 Structure : Villeparisis 4069 Mean TRM direction: D=5.4°, I=67.7° a95=1.82°, N=8, k=743 VRM TRM Declination (°) Inclination (°)

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Alpha 95 histogram Viscosity histogram

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384 with mean date < 1000 AD149 with mean date > 1000 AD All Results with « true » or indicative datation (533)

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Some examples of other European databases from archeolabs using standard methodology All directions reduced in Paris Path 1300-1750 from Lübeck 363 new results from France

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Léry, 28 results Marines 21 results Villiers le Sec 42 results Saint-Pathus 59 results The Geographical Distribution is very uneven through-out the country. The Ile de France region contains more than half of the sampling locations These 393 new sampled structures represent more than 4400 kg-sized specimens.

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130 dated (in Bucur 94) 163 new dated structures Resulting in a total of 293 dated structures CONSIDERING NOW ONLY THE WELL DATED RESULTS...

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The updated French reference curve is calculated using the self adaptable, variable width sliding windows (LeGoff et al, 2002) Inclination Declination 293 dated structures 20 < windows < 100 years

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WHICH MODEL FITS ?

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The most precise directions (small alpha95) are also the less scattered around the mean curve 82 data with Alpha95 < 0.75° 82 data with Alpha95 > 0.75° (selection of age incertitude <= ±100 years)

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CONCLUSION The Thelliers methodology combined with suitable instrumentation provides highly precise directional data. The French reference database, which now contains 293 dated directions, is currently used as an accurate dating tool for archeological purpose. Our future article (in preparation) will bring more information !

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Thank you The Saint-Maur Laboratory

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Examples of archeological sites with a long and continuous occupation

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Excavating a ditchDigging the kiln in silt Heating the kilnCooking meals Abandonment of the kiln And digging a new one and, now, before sampling

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Haut Empiremillieu Ier BC ; fin IIIème-50 -> 300 antiquités Tardive (ex Bas-Empire)déb IV, fin Vème (476, fin Emp Rom )300 -> 500 Haut Moyen Age (HMA)Vème, Xème450 -> 1000 dont MérovingienVème, VIIIème450 -> 750 CarolingienVIIIème, Xème750 -> 1000 Moyen Age Classique (ex Médiéval)XIème, XIIIème1000 -> 1300 Bas Moyen Age (BMA)XIVème, XVème1300 -> 1500 (1492) Modernefin XVème, fin XVIIIème1500 -> 1800 (1789) Contemporainfin XVIIIème -->> 1800 (1789) ->>>>>

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