Paleomagnetic and Archaeomagnetic Dating

Chronological Methods 11 - Paleomagnetic and Archaeomagnetic Dating

After World War II, geologists developed the paleomagnetic dating technique to measure the movements of the magnetic north pole over geologic time. In the early to mid 1960s, Dr. Robert Dubois introduced this new absolute dating technique to archaeology as archaeomagnetic dating.

How does Magnetism work?

Magnetism occurs whenever electrically charged particles are in motion. The Earth's molten core has electric currents flowing through it. As the earth rotates, these electric currents produce a magnetic field that extends outward into space. This process, in which the rotation of a planet with an iron core produces a magnetic field, is called a dynamo effect.

The Earth's magnetic core is generally inclined at an 11 degree angle from the Earth's axis of rotation. Therefore, the magnetic north pole is at approximately an 11 degree angle from the geographic north pole. On the earth's surface, when you hold a compass and the needle points to north, it is actually pointing to magnetic north, not geographic (true) north.

The Earth's magnetic north pole can change in orientation (from north to south and south to north), and has many times over the millions of years that this planet has existed. The term that refers to changes in the Earth's magnetic field in the past is paleomagnetism. Any changes that occur in the magnetic field will occur all over the world; they can be used to correlate stratigraphic columns in different locations. This correlation process is called magnetostratigraphy.

Lava, clay, lake and ocean sediments all contain microscopic iron particles. When lava and clay are heated, or lake and ocean sediments settle through the water, they acquire a magnetization parallel to the Earth's magnetic field. After they cool or settle, they maintain this magnetization, unless they are reheated or disturbed. This process is called thermoremanent magnetization in the case of lava and clay, and depositional remanent magnetization in the case of lake and ocean sediments.

In addition to changing in orientation, the magnetic north pole also wanders around the geographic north pole. Archaeomagnetic dating measures the magnetic polar wander.

For example, in the process of making a fire pit, a person can use clay to create the desired shape of the firepit. In order to harden the clay permanently, one must heat it above a certain temperature (the Curie point) for a specified amount of time. This heating, or firing, process resets the iron particles in the clay. They now point to the location of magnetic north at the time the firepit is being heated. When the firepit cools the iron particles in the hardened clay keep this thermoremanent magnetization. However, each time the firepit is reheated above the Curie point while being used to cook something, or provide heat, the magnetization is reset. Therefore, you would use archaeomagnetic dating to date the last time the firepit was heated above the Curie point temperature.

Paleomagnetic and Archaeomagnetic Profile

Paleomagnetism and Archaeomagnetism rely on remnant magnetism,as was explained above. In general, when clay is heated, the microscopic iron particles within it acquire a remnant magnetism parallel to the earth's magnetic field. They also point toward the location around the geographic north pole where the magnetic north pole was at that moment in its wandering. Once the clay cools, the iron particles maintain that magnetism until the clay is reheated. By using another dating method (dendrochonology, radiocarbon dating) to obtain the absolute date of an archaeological feature (such as a hearth), and measuring the direction of magnetism and wander in the clay today, it is possible to determine the location of the magnetic north pole at the time this clay was last fired. This is called the virtual geomagnetic pole or VGP. Archaeologists assemble a large number of these ancient VGPs and construct a composite curve of polar wandering (a VGP curve). The VGP curve can then be used as a master record, against which the VGPs of samples of unknown age can be compared to and assigned a date.

How are Paleomagnetic and Archaeomagnetic Samples Processed?

Geologists collect paleomagnetic samples by drilling and removing a core from bedrock, a lava flow, or lake and ocean bottom sediments. They make a marking on the top of the core which indicates the location of the magnetic north pole at the time the core was collected. This core is taken back to a laboratory, and a magnetometer is used to measure the orientation of the iron particles in the core. This tells the geologist the orientation of the magnetic pole when the rock was hot.

Archaeologists collect archaeomagnetic samples by carefully removing samples of baked clay from a firepit using a saw. A nonmagnetic, cube-shaped mold (aluminum) is placed over the sample, and it is filled with plaster. The archaeologist then records the location of magnetic north on the cube, after the plaster hardens. The vertical and horizontal placement of the sample is also recorded. Eight to twelve samples are collected and sent to a laboratory for processing. A magnetometer is used to measure the orientation of the iron particles in the samples. The location of the magnetic pole and age are determined for that firepit by looking at the average direction of all samples collected.

The Limitations of Paleomagnetic and Archaeomagnetic Dating

Using this technique, a core or sample can be directly dated. There are a number of limitations, however.

First, it is necessary to know the approximate age of the sample to avoid miscorrelations. The K-Ar method has been used to place the sample in an approximate age range. However, sometimes the error associated with K-Ar date is greater than the time span being studied using Paleomagnetic or Archaeomagmetic Dating techniques.
Second, when studying depositional remanent magnetization, in the case of lake and ocean sediments, disturbance of the sediments by currents, slumping of sediments, or burrowing animals is a problem. Any of these disturbances can churn up sediments and change the orientation of the iron particles in the sediments, or remove parts of the sedimentary record altogether. Therefore, paleomagnetism studies of sediments should be used as an average record of long term changes in the Earth's magnetic field to reduce error in the interpretation of the record.
Third, the microscopic iron particles in some sediments undergo chemical changes after they have settled through the water into strata. These chemical changes cause the iron particles to realign themselves with the Earth's magnetic field at the time of the chemical change. This is called chemical remanent magnetization. The identification of the particular iron minerals that are susceptible to this change can be an early warning that errors can be expected.
Fourth, paleomagnetic dating can only date deposits that are hundreds of thousands to millions of years old. This is useful when studying early fossil hominids, but is not useful when studying modern human beings.
Finally, the skill of the archaeologist collecting the sample, and the number of the samples used to calibrate the archaeomagnetic master curve affect the precision with which archaeologists can determine a date for a feature.

Links

Archaeometry Journal Home Page

Paleomagnetic Data at NOAA National Data Center

Centre for Environmental Magnetism and Palaeomagnetism (CEMP)

Fort Hoofddijk Paleomagnetic Laboratory, Utrecht University, Netherlands

Institute for Rock Magnetism, University of Minnesota

Rock-Magnetism & Paleomagnetism Lab, Geological Survey of Japan

Los Hornos: A Case Study in Chronology

Laboratory of Earth's Magnetism, Saint-Petersburg State University, Russia

CSU Archaeometric Laboratory

References

Eighmy, J.L. 1980. Archaeomagnetic Dating: A Handbook for Archaeologists.

Eighmy, J.L., and R.S. Sternberg, eds. 1990. Archaeomagnetic Dating.

Butler, R.F. 1992. Paleomagnetism: Magnetic Domains to Geologic Terrains.


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