Archaeomagnetic dating accuracy and precision

Chronological Methods 11 - Paleomagnetic and Archaeomagnetic Dating this new absolute dating technique to archaeology as archaeomagnetic dating. . used to calibrate the archaeomagnetic master curve affect the precision with which. Archaeomagnetic dating is a method for dating fired materials and sediments precise in periods where other dating methods, e.g., radiocarbon dating, are. archaeomagnetic dating, although it must be emphasized that the point has not yet successfully and very precise geomagnetic time-scales can be expected.

The more information there is, the better we will understand how the Earth's magnetic field has changed over time, which may allow more precise archaeomagnetic dates to be produced. A number of secular variation curves have been produced for Britain over the last 50 years, reflecting the inclusion of additional information as well as improved methods used to construct the curves.

The measurement process can be divided into three stages: The initial measurement of the samples determines the natural remanent magnetisation NRM. This relates to the archaeological signal plus the signal held by less stable magnetic particles, referred to as the viscous component. The less stable component needs to be removed to produce an accurate date for the archaeological event of interest.

The pilot demagnetisation of a subset of the samples determines information about the stability of the magnetic signal recorded within the material, and identifies the point at which the viscous point is removed from the samples.

This is carried out using one of two methods: The partial demagnetisation of the remaining samples uses the information produced during the pilot study to remove the viscous component from the samples to leave the archaeological signal of interest.

The samples can then be re-measured to determine the direction of the archaeological signal recorded by the samples. This provides the minimum information needed to produce an archaeomagnetic date. Calibration The process of calibration translates the measured magnetic vector into calendar years. A record of how the Earth's magnetic field has changed over time is required to do this, and is referred to as a secular variation or a calibration curve.

A date is obtained by comparing the mean magnetic vector, defined by the declination and inclination values, with the secular variation curve; the potential age of the sampled feature corresponds to the areas where the magnetic vector overlaps with the calibration curve. Unfortunately, the Earth's magnetic poles have reoccupied the same position on more than one occasion, and can result in multiple age ranges being produced.

Alternative chronological information is required in these situations to identify the archaeological significant age range. The current British secular variation curve was produced by Zananiri et al.

Paleomagnetic and Archaeomagnetic Dating

By using another dating method dendrochonology, radiocarbon dating to obtain the absolute date of an archaeological feature such as a hearthand 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.

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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.

Archaeomagnetic Dating

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.

Archaeomagnetic Dating

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.

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  • Archaeomagnetic dating accuracy

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.