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Archeomagnetism: Paleointensity Data from Archeological Artifacts

English title Archeomagnetism: Paleointensity Data from Archeological Artifacts
Applicant Hirt Ann M.
Number 128695
Funding scheme R'EQUIP
Research institution Institut für Geophysik ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geophysics
Start/End 01.12.2009 - 30.11.2010
Approved amount 92'000.00
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All Disciplines (2)

Discipline
Geophysics
Archaeology

Keywords (5)

Archeomagnetism; Paleointensity; Rock magnetism; earth's magnetic field; archeological artifacts

Lay Summary (English)

Lead
Lay summary
The Earth's magnetic field has both a magnitude and a direction. Paleomagnetism exploits the fact that rocks and sediments record the dipole field if averaged over about 10,000 years. Materials that acquire their magnetization in less time (e.g. weeks to few centuries) record also the non-dipolar part of the Earth's magnetic field; this is known as the secular variation. Secular variation is found both in the intensity of the field and its direction. Archeomagnetism studies the evolution of the field during the recent past millennia, and uses the change in the absolute intensity and direction of the geomagnetic field as a tool for dating archeological artifacts or young lavas. Absolute field intensity requires materials whose magnetization has cooled through its Curie temperature in the ambient field. Techniques have been developed to extract the paleofield intensity from these materials through a method of incremental heating of the artifact in zero-field and a known applied field. The oven purchased under this project will be used to obtain absolute paleointensity data for central Europe from archeological artifacts in this time period over the past several thousand years.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Associated projects

Number Title Start Funding scheme
150629 Long core SQUID Rock Magnetometer: High-resolution lake studies, assessing efficacy of drug targeting, and the search for magnetic monopoles 01.12.2013 R'EQUIP

Abstract

Background The Earth’s magnetic field has both a magnitude and a direction. Gauss described the potential of the field in his 1838 memoir Allgemeine Theorie des Erdmagnetismus as an infinite series of spherical harmonic functions, which mathematically describes how the field varies on a spherical surface of constant radius. He recognized that there are both internal and external sources to the field, but that the external sources are insignificant, and that the internal field is predominantly a dipole. Paleomagnetism exploits the fact that rocks and sediments record the dipole field if averaged over about 10,000 years. Materials that acquire their magnetization in less time (e.g. weeks to a few centuries) record also the non-dipolar part of the Earth’s magnetic field; this is known as the secular variation. Archeomagnetism studies the evolution of the field during the recent past millennia, and uses the secular variation of the geomagnetic field as a tool for dating archeological artifacts or young lavas. The groups Earth and Planetary Magnetism and Climate Geology at the ETH-Zürich and the Radioactive Tracers Group at the Department of Surface Waters, EAWAG are presently collaborating under an ETH CHIRP (Collaborative, Highly Interdisciplinary Research Project) to investigate the link among solar activity, magnetic field behavior and climate. Part of this project is aimed at modeling past behavior of the Earth’s field in high resolution over the past two millennia, and requires information for both the intensity and direction of the field. The CHIRP study is focusing on lacustrine sediment as archives for field change. Lake sediments are good recorders of field direction, but only provide information on relative change in the intensity of the Earth’s magnetic field in the past, i.e. relative paleointensity. Absolute field intensity requires materials whose magnetization has a thermal remanent origin (thermal remanent magnetization, TRM), i.e., the material has cooled through its Curie temperature in the ambient field. Techniques have been developed to extract the paleofield intensity from these materials. In nature TRM is found in igneous rocks that cool from a molten state or materials that have been heated to a high temperature and then cooled through their Curie temperature in the ambient field, e.g., fired pottery or bricks. Archeological artifacts are an invaluable source for information of paleofield intensity for the past few millennia. An additional project, which has been submitted by Dr. Fabio Donadini under the Ambizione program to the Swiss National Science Foundation, will be dedicated to obtaining absolute paleointensity data for central Europe from archeological artifacts in this time period. Dr. Donadini will be working in the Institute of Geophysics, ETH-Zürich, but will be also collaborating with Finnish, Bulgarian, Swiss, and American partners. Justification Extraction of the absolute paleointensity of the Earth’s field is based on reproducing its acquisition under laboratory conditions. For archeological materials it is often extracted by using the Thellier-Thellier technique (Thellier and Thellier, 1959), which involves the acquisition of a TRM, a partial TRM (pTRM) in successively higher temperature compared to the natural remanent magnetization (NRM) of the material in the same temperature range. The manner in which a material acquires its magnetization in a weak magnetic field is very dependent on the properties of the minerals carrying the remanent magnetization. It is also important that the original ferromagnetic mineralogy does not alter during the successive heating. For this reason we are requesting funds for an oven dedicated for TRM acquisition and a susceptibility bridge with oven unit for characterizing the magnetic mineralogy and its possible alteration during heating. The Laboratory for Natural Magnetism (LNM) of the ETH Zürich is a fully functional laboratory, however, the available thermal specimen demagnetizers do not allow palaeointensity measurements because they lack the possibility of inducing a direct field simulating the geomagnetic field.
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