Archaeomagnetic dating

Archaeomagnetism describes a dating method that can date an object using the terminus ante quem by comparing the magnetic information stored in it with the state of a past geomagnetic field . Archaeomagnetic dating is less widely used than radiocarbon dating , but it can also be used to date non-organic materials. This makes them a valuable, complementary dating technique.

One of the greatest disadvantages of archaeomagnetic dating is that there are still no complete magnetic calibration curves for certain regions in Europe , particularly those relating to the past intensity of the earth's magnetic field. Archaeomagnetism has its basic methodology from the geological discipline of geophysics .

Theoretical foundations

The earth's magnetic field

A magnet is a body made up of a magnetic north and south that attract each other. Magnetism arises from the movement of charged particles. Between the magnetic north and south lies a magnetic field that can be described by the so-called field lines.

The earth's magnetic field has the same properties as an ordinary bar magnet. It originates largely in the outer core of the earth and consists of slowly moving molten iron that is trapped between the solid inner core and the solid mantle . Because the outer core is made of molten iron, electrons can freely circulate in it. The earth's magnetic field now arises because iron, as a conductive material, is moved both by the thermodynamic phenomenon of convection currents and by the rotation of the earth . This creates a helical movement of the liquid iron, which creates a magnetic field like an electric coil. This whole process is called geodynamo because of its positive feedback that leads to self-preservation .

The strength and direction of the earth's magnetic field can be described with a vector at any point in the field. One differentiates within this vector the declination, the inclination and the field strength. Declination is the angle between true north and the horizontal component. The inclination is the slope or slope of the vector compared to the horizontal plane. The field strength is represented by the length of the vector.

The earth's magnetic field is subject to constant change. For one thing, it seems to precess around the earth's axis of rotation like a top . Due to the slightly faster rotation of the earth's mantle compared to the earth's core, it also supposedly moves to the west. In addition, phenomena sometimes called regional, secular variation disturb the magnetic field . They can be triggered, for example, by eddy currents in the outer core of the earth, charged particles that often reach the earth through solar winds , or by iron deposits in the earth's crust. In addition, about every 500,000 years there is a decrease in the earth's magnetic field with subsequent polarity reversal. It is precisely this variability of the earth's magnetic field that archaeomagnetic dating makes use of.

Recording mechanisms

There are materials that, after being magnetized, retain magnetization. This is called magnetic remanence . The most common of these materials are iron oxides. They are found in floors, in clay, including ceramics, and in many different stones as trace elements. In iron oxide -Mineralien, magnetic domains, so-called form white districts . Because an atom imposes its own magnetic moment on all electrons of neighboring atoms in a quantum mechanical exchange, a typical magnetic direction results in each domain. In the initial state, each domain is now magnetized in a random direction, so that the resulting magnetic moment is very small. But when a mineral is heated, the magnetization is gradually reduced until it disappears completely at a certain temperature. This temperature is called the Curie temperature , it depends on the purity, size and shape of the mineral. When the mineral cools down, the domains remagnetize and take over the direction and strength of the magnetic environment, which is often equal to the earth's magnetic field. Although every domain will never align itself with the magnetic environment, the result of this process known as thermoremanence is a magnetic field that is representative of the earth's magnetic field.

In order to extinguish the remanent magnetization again, the mineral must be reheated to at least approximately the temperature of the first remanence . If the Curie temperature of a mineral is below 200 ° Celsius, there is a good chance that it will give up its magnetization again over time. One then speaks of viscous remanence. Such minerals are not suitable for dating.

Dating method

Methodological basics

One can date based on the direction or strength of the magnetic field. The following requirements must be met in order to be able to date an artifact archaeomagnetically:

• The object to be examined must contain remanently magnetizable minerals.
• The object to be examined must have been exposed to high heat so that it could be magnetized thermoremanently.
• The object to be examined must, in the case of the investigation of the field strength, have been used regionally or, in the case of the investigation of the magnetic direction, must not have been moved.
• A calibration curve of the earth's magnetic field, i.e. a representation of the change in the earth's magnetic field, must be available for the region of origin of the object to be examined. The data for the calibration curve are obtained through the magnetic analysis of already dated archaeological samples or sediments.

Determining the direction of the magnetic field

Because the sample must not be rotated when determining the direction, the direction of true north must be noted on the sample.

In order to neutralize the interfering viscous-magnetic domains contained in the sample, these areas are demagnetized either thermally or with an alternating magnetic field. In thermal demagnetization, the sample is heated above the Curie temperature of the viscous magnetic but below the Curie temperature of the stable domains. During demagnetization with an alternating magnetic field, the sample is placed in a weak magnetic field, the viscous-magnetic domains taking over this and then being neutralized by gradually lowering the magnetic field.

There are a variety of magnetometers that can be used to determine the direction of the magnetic field. The most common is the spinner magnetometer. The sample is rotated on a platform in a room protected from the external magnetic field. Because of the magnetic field, current is then induced in a coil that runs around this platform. This current can be measured and is proportional to the strength of the magnetic field in one direction of the sample. In order to cover all three dimensions of the direction of the magnetic field, the measurements are repeated twice, each time rotated by 90 °. The three specific vectors can now be added to a vector resulting for the sample.

The measured resulting vectors of all examined samples of an object can then finally also be added again. Different measurement results of the individual samples can occur due to a slight displacement of the examined object, different magnetic minerals with different Curie temperatures within the object or the proximity of the object to other strongly magnetic objects. Similar to the standard deviation in Gaussian statistics, a range (α = 95%) can also be determined for the resulting vector, in which the chance that the actual vector of the earth's magnetic field lies within is 95%. This range is usually ± 2–3 ° deviation from the calculated vector.

The calculated area can now be placed on the calibration curve of the declination and inclination of the location and thus determine the age of the object.

Determination of the strength of the magnetic field

By examining the intensity of the magnetic field, portable objects that are only used regionally can be examined. It is based on the assumption that there is a linear relationship between the strength of the induced magnetic field and the earth's magnetic field, which can be expressed as follows:

${\displaystyle {M_{\text{alt}}}}$= ∙ ${\displaystyle k}$ ${\displaystyle {B_{\text{alt}}}}$

M stands for the magnetic field, k for a constant, B for the induced magnetic field. In a laboratory, the old magnetic field of a sample can now be neutralized and magnetized again. Assuming that the new induced magnetic field is also in the same linear relationship to the external magnetic field, one can determine the constant k:

${\displaystyle k={\frac {M_{\text{Labor}}}{B_{\text{Labor}}}}}$

Since k is now determined and can also be determined experimentally, one can also calculate the old earth's magnetic field. The linear relationship can be disturbed by differences between the original atmosphere and the atmosphere prevailing in the laboratory and by inhomogeneity of the sample. To find out at which temperatures the relationship between the magnetic fields is linear, the sample is gradually heated and the remaining magnetization is measured. After the induction of the new field, this is repeated in the same way. The strength of the old earth's magnetic field can now be compared with a calibration curve and the object can be dated. ${\displaystyle B_{\text{alt}}}$

Web links

• Archaeomagnetic Dating. Historic England. Retrieved January 3, 2018.

literature

• Eighmy, Jeffery; Sternberg, Robert: Archaeomagnetic Dating. Tucson: The University of Arizona Press, ISBN 978-0816511327 .
• Heinrich Christoph Soffel: Paleomagnetism and Archaeomagnetism. Springer, Berlin 1991, ISBN 3-540-53890-9 .
• Kathrin Lisa Kapper: Earth Paleofield in the Alpine Region during the past 8000 years. Zurich 2014.
• Martin Jim Aitken : Science-based dating in archeology. Routledge, London 1990, ISBN 978-0582493094 .

Individual evidence

1. ↑ Eighmy, Jeffery; Sternberg, Robert: Archaeomagnetic Dating. Tucson: The University of Arizona Press. ISBN 978-0816511327 , p. 5.
2. ^ Martin Jim Aitken : Science-based dating in archeology. Routledge, London 1990, ISBN 978-0582493094 , p. 3.
3. ^ Kathrin Lisa Kapper: Earth Paleofield in the Alpine Region during the past 8000 years. Zurich 2014, pp. 1–3.
4. ↑ Eighmy, Jeffery; Sternberg, Robert: Archaeomagnetic Dating. Tucson: The University of Arizona Press. ISBN 978-0816511327 , pp. 6-7.
5. ↑ Eighmy, Jeffery; Sternberg, Robert: Archaeomagnetic Dating. Tucson: The University of Arizona Press. ISBN 978-0816511327 , pp. 10-13.
6. ^ Archaeomagnetic Dating. ( Memento of the original from November 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Historic England. Retrieved January 3, 2018, p. 4. 
7. ^ Martin Jim Aitken : Science-based dating in archeology. Routledge, London 1990, ISBN 978-0582493094 , p. 240.
8. ^ Archaeomagnetic Dating. ( Memento of the original from November 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Historic England. Retrieved January 3, 2018, pp. 8–9. 
9. ^ Archaeomagnetic Dating. ( Memento of the original from November 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Historic England. Retrieved January 3, 2018, p. 8. 
10. ^ Archaeomagnetic Dating. ( Memento of the original from November 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Historic England. Retrieved January 3, 2018, p. 10. 
11. ^ Kathrin Lisa Kapper: Earth Paleofield in the Alpine Region during the past 8000 years. Zurich 2014, pp. 35–42.