Chronological Methods 11 - Paleomagnetic and Archaeomagnetic Dating & Carbon 14
Dating Calculator

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.

Carbon 14 Dating Calculator
To find the percent of Carbon 14 remaining after a given number of years, type
in the number of years and click on Calculate.






Years
C 14 halflife = 5730





Carbon 14 left = percent

To find the years that have elapsed from how much Carbon 14 remains, type in the
C 14 percent and click on Calculate.






Percent C 14
C 14 halflife = 5730





Years = +/-

More about Carbon Dating
In the 1940's Dr. Willard F. Libby invented carbon dating for which he received
the Nobel Prize in chemistry in 1960.
Carbon dating has given archeologists a more accurate method by which they can
determine the age of ancient artifacts. The halflife of carbon 14 is 5730 ± 30
years, and the method of dating lies in trying to determine how much carbon 14
(the radioactive isotope of carbon) is present in the artifact and comparing it
to levels currently present in the atmosphere.
Above is a graph that illustrates the relationship between how much Carbon 14 is
left in a sample and how old it is.

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