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Archaeomagnetic sampling
Archaeomagnetic directional dating - an example

Archaeomagnetic sampling Top

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Figure A: Taking oriented archaeomagnetic samples.
Figure B: Typical size of an archaeomagnetic sample.

photographs by A.I.R. Herries

Archaeomagnetic directional dating - an example

After some sample preparation, the direction (declination, inclination) and intensity of the samples magnetic remanence is measured in the laboratory with a magnetometer. Several samples are measured from one archaeological artefact, in order to obtain a well defined mean and its error.

The following example is taken from Kovacheva et al. (2004) and concerns a more or less circular pottery kiln from Reinach (Switzerland) consisting of large stones lined with clay. As stones are not suitable for archaeomagnetic dating, the baked clay lining the kiln walls was sampled. The archaeological age proposed by context dating is the second half of the 8th century AD.

The measured average direction of the remanence at the archaeological site is:

Figures C and D: Inclination and declination plots versus time. The solid horizontal lines in both diagrams represent measured inclination and declination of the archaeological site of unknown age. The dashed horizontal lines are the error of the measurement. The inclination and the declination of the reference curve (= known time variation of D and I of the Earth's magnetic field) is plotted as well, together with its error. The green shaded areas correspond to the probability densities at a 95% confidence level of possible dates.

Figure. C: Inclination plot. The measured inclination meets the reference inclination curve 2 times and would suggest two possible age intervals 642-896 and 1647-1807 AD (green shaded intervals).

(modified from Kovacheva et al., 2004)

Figure D: Declination plot. The measured  declination, meets the reference declination curve also two times, suggesting two different possible age intervals 774-932 and 1042-1618 (green shaded intervals). In order to obtain the most probable solution, the probability densities of inclination and declination are combined, see Fig. E.

(modified from Kovacheva et al., 2004)

Figure E: Age probability density of inclination (top) and declination (middle) and their combination (bottom).  When both probability densities are combined, the only possible age interval is: 753 - 901 AD, which is comparable with the archaeological age (2nd half of th the 8th century AD).

(modified from Kovacheva et al., 2004)

In case that the archaeointensity of the site has also been determined, it can be used as the third geomagnetic element in the same way as it has been demonstrated here for declination and inclination. Such dating examples, using declination, inclination and intensity, can be found in Kovacheva et al. (2004).

Kovacheva, M., Hedley, I., Jordanova N., Kostadinova, M. and V. Gigov, Archaeomagnetic dating of archaeological sites from Switzerland and Bulgaria, Journal of Archaeological Science, 31, 1463-1479, 2004.

Archaeomagnetic applications