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

Archeomagnetic sampling following Thellier’s technique

« Big » sample inductometer at Saint Maur IPGP Laboratory

Magnetic measurements

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

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

A reference directional curve is needed

Archeomagnetic dating

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

Age distribution of the French archeomagnetic results selected by Bucur (1994)

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

(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

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)

but low quality

high qualityPerfect agreement

Archeomagneticdating

still possible

Circumvent a contradiction…

Dating the end of use of a Roman water conduit (Rungis, France) supplying the south of Paris

Water conduit

kilnkiln

Age of the oven found in Rungis

Archeomagnetic age: AD 625-725(more probable: AD 685-725)

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)

No, the kiln is Merovingian (AD 625-725)

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

Is there a significant time interval between several domestic kilns sharing the same working area ?

Site of Marines (Val d’Oise, France)

1 m

Dating of 4 kilns

same archeomagnetic age: AD 705-895 (more probable: AD 705-845)

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

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.