Archaeometry is the analysis of archeological materials using analytical techniques borrowed from the physical sciences and engineering. Examples include trace element analysis to determine the source of obsidian used to manufacture arrowheads, and chemical analysis of the growth rings of fossilized sea shells to determine seasonal variations in local temperature over time.

Modern archaeometry began with the discovery of radiocarbon dating in the 1950s. Today, artifact analyses use excavation techniques, remote sensing, and dating methods that all draw on archaeometry.

Archaeometricians are currently using sophisticated computer techniques to handle the masses of data this field continues to generate.

Archaeomagnetic and paleomagnetic dating

Because shifts in the molten core of the planet cause Earth’s magnetic field to vary, and because this causes our planet’s magnetic North Pole to change position over time, magnetic alignments in archeological specimens can be used to date specimens.

In paleomagnetism, rocks are dated based on the occurrence of reversal’s in Earth’s magnetic poles. These types of pole reversals have occurred with irregular frequency every hundred thousand years or so in Earth’s history. Geologists collect samples to be analyzed by drilling into bedrock, removing a core, and noting the relative alignment to Earth’s present magnetic field. The sample is then analyzed in the laboratory to determine its remnant magnetism—the pole’s alignment when the sample crystallized. Using a compiled master chronology of pole reversals, scientists can then date the specimen. Because the time between pole reversals is so large, this technique can only be used to date objects to an accuracy of a few thousand to tens of thousands of years. The technique has been used to date human remains in the Siwalki Hills of India, in the Olduvai Gorge in Kenya, and in the Hadar region of Ethiopia.

Archaeomagnetism makes use of the fact that the magnetic North Pole has shifted position over time. When clay in an object is heated to a sufficiently high temperature, the iron particles in the clay will align to the magnetic pole. If the clay has remained undisturbed since it was fired, it will indicate the position of the pole when it was made. Archaeomagnetism can therefore be used to date fixed objects such as lined fire pits, plaster walls, and house floors. Other techniques, such as radiocarbon dating and dendrochronology, can be used to date wood from the fire. By comparing data, a master curve showing the position of the magnetic North Pole over time can be generated. This master curve then provides a basis for assigning dates to undated clay samples based on where their remnant magnetism indicates the pole was when they were fired. Because the pole position can be determined rather exactly for the last 100,000 years or so, dates for materials of this age and younger can be quite accurate. However, disturbances occur at times in Earth’s magnetic field at various geographical locations, so it has been necessary to develop separate master curves for different regions. In the southwestern United States, where dendrochronology was used to help calibrate the master curve, archaeomagnetism can yield dates with accuracy as great as +/– 50 years, a precision unmatched by radiocarbon dating.

Dendrochronology

Dendrochronology is the extraction of chronological and environmental information from the annual growth rings of trees. This technique uses well established tree ring sequences to date events. Reconstruction of environmental occurrences, droughts for example, which took place when the trees were growing, is also possible based on traits such as changes in tree ring thickness. Tree-ring dating allows dates to be assigned to archeological artifacts; reconstructed environmental events shed light on the ways that human societies have changed in response to environmental conditions.

Dendrochronology was developed in the early 1900s by the American astronomer Andrew Ellicott Douglas as part of his research on the effects of sunspots on Earth’s climate. Douglas developed a continuous 450-year record of tree ring variability, which he succeeded in correlating with the winter rainfalls preceding the growth years.

The technique very quickly proved useful for dating wood and charcoal remains found in the American southwest. By 1929, dendrochronology had become the first independent dating technique to be used in archeology. Since then, approximately 50, 000 tree ring dates from about 5, 000 sites have yielded the finest prehistoric dating controls anywhere in the world. Tree ring dating later proved successful in other parts of North America, including Alaska and the Great Plains. Today, the technique is practiced in one form or another throughout the world.

The key to successful dendrochronolgical dating is cross-dating—comparing one tree’s rings with other trees in the area. This may be done by looking for covariations in tree ring width, or comparing other tree ring attributes to identify overlapping sequences.

By incorporating overlapping sequences from multiple trees, it has been possible to produce chronologies that go back further than any of the individual tree ring specimens. In this way, it has been possible to extend the chronology for the southwest as far back as 322 BC. The longest individual tree ring chronologies developed to date have been for an 8, 700-year California bristlecone pine sequence, and a 10,000-year sequence in Europe.

Besides chronological information, archeological tree ring dating yields information about the way wood was used in an ancient culture, and about past climates.

Fission-track dating

Radioactive decay (fission) of uranium U-238 causes microscopic tracks of subatomic particles to develop in minerals and glass. By measuring the number of these present in an artifact, which is a function of the sample’s age and the amount of uranium present, scientists can determine the absolute age of an artifact.

Fission-track dating has been used to determine the age of glaze coverings on 400-500 year old Japanese bowls. A glass shard dating to Gallo-Roman times was determined to date from AD 150, but the precision of that date was only +/– 20% (a possible date range from AD 120–180) Nineteenth-century glass produced in central Europe, on the other hand, was dated very precisely. The technique has occasionally proven useful for pottery analysis when the objects contained inclusions of materials such as obsidian in which the fission tracks had not been erased over time by the high temperatures of glazing.

Lithics

Lithics are stone tools. Stone tools are capable of revealing information about sources of raw materials and ancient trade routes, usage (through wear patterns), function (from residues such as blood and plant material), and the evolution of craft specializations.

To determine how a tool was made, the archeologist may attempt to reproduce the tool in the laboratory (the traditional method), or take an analytic approach using models based on physics, fracture mechanics, and the physical properties of various materials.

Luminescence dating

When certain materials such as quartz, feldspar, and flint are buried, they store trapped electrons that are deposited by background sources of nuclear and cosmic radiation. As long as the material is buried, the population of trapped electrons accumulates at a constant rate. Once the material is exposed to daylight or heat, however, the trapped electrons are released from their traps. By monitoring the luminescence produced by the released electrons, it is possible to determine the length of time that an object has been underground.

Metals analysis

Archeologists analyze metal artifacts to determine the sources of ores used to produce the artifacts, and to learn more about trade patterns and fabrication technologies. Metal analyses are also used to authenticate artifacts. Analytical techniques frequently used to determine elemental compositions include x-ray fluorescence spectrometry, atomic absorption spectrometry, and neutron activation analysis. Leadisotope analysis is the technique of choice for determining sources of ores containing copper, silver, and tin (all of which contain trace amounts of lead).

Obsidian hydration dating

In many cultures, obsidian was the preferred material for working into stone tools. When obsidian, which is a volcanic glass, is fractured, the fresh surfaces absorb water. The thickness of the water-absorbing edge, or rind, increases with time. Measurement of the rind with powerful microscopes thus yields a dimension that can be correlated with the age of the tool.

Although this technique has been widely used in California and the Great Basin, it remains a relatively inaccurate technique when used alone to date artifacts.

Paleobotany and paleoethnobotany

In paleobotany, the remains of plants recovered from prehistoric soil deposits are analyzed to determine the species of plants that were present, the parts of the plant used, the time of year they were collected, and genetic changes in the plant species over time. In order to use this technique, the paleobotanist must have access to a complete reference collection indicating the changes in a plant species over time. Paleobotanists have, using this technique, been able to reconstruct information about prehistoric climates, patterns of plant use, seasonal patterns of site occupation, vegetables included in diets, and transitions from plant-gathering to plant-cultivation practices.

The paleoethnobotanist, like the paleobotanist, studies plant remains in the context of archeology, but in addition looks at the interactions between the plant materials and the people who used them. The first techniques used for plant recovery involved methods of flotation to separate organic from inorganic matter.

KEY TERMS

Artifact— A man-made object that has been shaped and fashioned for human use.

Atomic absorption spectrometry— Method of analysis in which the specimen is placed in a flame and the light emitted is analyzed.

Cosmic radiation— Electrons and atomic nuclei that impinge upon Earth from outer space.

Flotation— A method of separating organic remains by causing them to float to the surface.

Fracture mechanics— Analysis of the way an objects breaks.

Luminescence— Light emission from a body that is not due only to that body’s temperature. Luminescence is frequently produced by chemical reactions, irradiation with electrons or electromagnetic radiation, or by electric fields.

Magnetic field— The electromagnetic phenomenon produced by a magnetic force around a magnet.

Neutron activation analysis— Method of analysis in which a specimen is bombarded with neutrons, and the resultant radio isotopes are measured.

Nuclear radiation— Particles emitted from the atomic nucleus during radioactive decay or nuclear reactions.

Radioactivity— Spontaneous release of subatomic particles or gamma rays by unstable atoms as their nuclei decay.

X-ray fluorescence spectrometry— A nondestructive method of analysis in which a specimen is irradiated with x rays and the resultant spectrum is analyzed.

Modified flotation techniques are still used to extract carbonized plant fragments from sediment. Modern analytical techniques for examining recovered plant materials, based on genetic and DNA research, permit the identification of plant proteins, isotopes, starches, and lipids. With these methods, it has been possible to determine the sequences of domestication of such plants as maize, wheat, barley, and rice.

Potassium-argon dating

The potassium-argon method of dating allows scientists to date rocks that were formed between 50,000 and two billion years ago. The rate at which argon 40 forms from the decay of potassium 40, one of the most common minerals in Earth’s crust, is known. Therefore, it is possible to determine the age of an object based on the accumulation of argon 40 in the specimen. The first archeological site to be dated by this method (using lava samples) was at Olduvai Gorge in Tanzania.

Radiocarbon dating

Radiocarbon dating allows archeologists to date materials, formed between 300 and 40-50,000 years ago, that contain organic carbon. Carbon 14 is a naturally occurring radioisotope of ordinary carbon (carbon-12) that is created in the upper atmosphere when carbon-12 is bombarded by cosmic rays. On earth, living organisms metabolize carbon 14 in the same percentage that it exists in the atmosphere. Once the plant or animal dies, however, the carbon-14 atoms began to decay at a known rate. Consequently, the age of a carbon-containing specimen such as charcoal, wood, shells, bone, antlers, peat, and sediments with organic matter, can be determined. One of the first applications of this technique was to assign a date to the beginning of the postglacial period of about 10,000 years ago.