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M. Le Goff (1) , N. Warmé (2) , Y. Gallet (1) , and A. Genevey (3)

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).

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M. Le Goff (1) , N. Warmé (2) , Y. Gallet (1) , and A. Genevey (3)

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  1. 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

  2. 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 .

  3. 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

  4. N W E I = 60° I = 30° I = 0 ABOUT ACCURACY AND RELIABITY 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 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

  5. The methodology used in our laboratory, not « standard », follows the Thellier’s 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.

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

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

  8. Two Examples of directional results with v=7% VRM VRM TRM Inclination (°) Inclination (°) TRM Declination (°) Declination (°) 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

  9. Viscosity histogram Alpha 95 histogram

  10. All Results with « true » or indicative datation (533) 384 with mean date < 1000 AD 149 with mean date > 1000 AD

  11. 363 new results from France Path 1300-1750 from Lübeck All directions reduced in Paris Some examples of other European databases from archeolabs using standard methodology

  12. 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.

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

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

  15. WHICH MODEL FITS ?

  16. 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)

  17. CONCLUSION • The Thellier’s 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 !

  18. The Saint-Maur Laboratory Thank you

  19. Examples of archeological sites with a long and continuous occupation

  20. Excavating a ditch Digging the kiln in silt Heating the kiln Cooking meals and, now, before sampling Abandonment of the kiln And digging a new one

  21. Haut Empire millieu 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, Xème 450 -> 1000 dont Mérovingien Vème, VIIIème 450 -> 750 Carolingien VIIIème, Xème 750 -> 1000 Moyen Age Classique (ex Médiéval) XIème, XIIIème 1000 -> 1300 Bas Moyen Age (BMA) XIVème, XVème 1300 -> 1500 (1492) Moderne fin XVème, fin XVIIIème 1500 -> 1800 (1789) Contemporain fin XVIIIème -->> 1800 (1789) ->>>>>

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