Potential of archeomagnetism as a dating tool for archeology: Examples from France
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Potential of archeomagnetism as a dating tool for archeology:
Examples from France
Maxime Le Goff 1, Yves Gallet 1, Nicolas Warmé 1,2, Agnès Genevey 3
IPGP1, INRAP2, C2RMF3
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Archeomagnetic sampling following Thellier’s technique
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« Big » sample inductometer at Saint Maur IPGP Laboratory
Magnetic measurements
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Methodology:
The directional results are determined and selected on the basis of viscosity experimentsviscosity experiments (Thellier, 1981)
By vectorial substraction VRM and TRM
1) Direct position during 15 days measurements
2) Inverse position during 15 days measurements
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Example of archeomagnetic results (Rungis, France)
TRM
VRM
Mean TRM direction: D=-0.7°, I=70.6°, 95=0.4° (N=14)
2000
fichier RUN06.AMZ
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A reference directional curve is needed
Archeomagnetic dating
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Bucur, PEPI, 1994
Changes in direction of the Earth’s magnetic field in France as deduced from archeomagnetic data
Sliding window of 80 years shifted every 25 years
Thellier, PEPI, 1981
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Age distribution of the French archeomagnetic results selected by Bucur (1994)
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Constructing a reference curve taking into account the non-homogeneous
age distribution of the data
The mean directions (i) are estimated using the bivariate extension of Fisher’s statistics ( wj Tj )
For each time window (i), intercepted data (j) are weighted following the proportion of time contained in the window (0< wj <1)
Each individual datum is defined by a direction (D, I, k, N)and an age bracket
A total weight is obtained for each time window ( wi )
The width of the window is step-by-step increased until the minimum weight required is attained
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(Le Goff, Gallet, Genevey, Warmé, PEPI, 2002)
Comparison between the French curves constructed
using moving windowsof varying (double line) and fixed (dashed line) duration
Weight and duration of each time window consideringa threshold value of 2.5
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Determining an archeomagnetic age…
The angular distances between the respective means allow to determine an archeomagnetic age bracket at 95%
We can also estimate the probability P (in %) of making an error if an undated archeomagnetic direction is assumeddifferent from any sliding window direction
Use the rejection test developed by McFadden and McElhinny (1990)
Modified in order to compare a Fisherian mean (to be dated) and non Fisherian means (ellipses defining the reference curve)
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but low quality
high qualityPerfect agreement
Archeomagneticdating
still possible
Circumvent a contradiction…
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Dating the end of use of a Roman water conduit (Rungis, France) supplying the south of Paris
Water conduit
kilnkiln
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Age of the oven found in Rungis
Archeomagnetic age: AD 625-725(more probable: AD 685-725)
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The case of a kiln found with a filling containing several tens of
Roman and some undetermined potsherds
Has this kiln a Roman age ?
(Argenteuil,France)
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No, the kiln is Merovingian (AD 625-725)
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The story of a domestic kiln…
1) Excavating a ditch
2) Digging the kiln in silt
3) Heating the kiln
4) Cooking meals
5) Abandonment of the kiln And digging a new one
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Is there a significant time interval between several domestic kilns sharing the same working area ?
Site of Marines (Val d’Oise, France)
1 m
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Dating of 4 kilns
same archeomagnetic age: AD 705-895 (more probable: AD 705-845)
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Constraining the historical change in the use of kilns:
Site of Marines
Archeomagnetic ages at 95%
Most probable archeomagnetic ages
Nb.
Str
uctu
res
Nb.
Str
uctu
res
500 600 700 800 900 1000
Age (AD)
Individual to groupIndividual to group
500 600 700 800 900 1000
Isolated kiln Non isolated Kiln
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Conclusions:
The archeomagnetic dating technique is already operational in France,
But
We still need additional well-dated archeomagnetic directions to define better
the reference archeomagnetic secular variation curve.