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Interactives
Collapse: Why do civilizations fall?
Collapse: Why do civilizations fall?Chaco Canyon
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Glossary of Archaeological Terms
What is chert? Why is dendrochronology important? Browse archaeological terms in this glossary.

How Carbon-14 Dating Works
Examine the methods by which scientists use radioactivity to determine the age of objects, most notably carbon-14 dating.


Dating the Evidence

Accurate dating is important for putting events and objects in sequence. For example, the arrival of sophisticated carbon-14 dating methods in the 1940s caused scientists to revise their interpretation of events in Europe in prehistoric times. Ancient sites in England, Malta, and elsewhere turned out to be older than once thought. With this new information, scientists were able to paint a more accurate picture of European prehistory.

Dating methods can be relative or absolute. An absolute dating method tells the excavator the specific date of the material being studied (plus or minus a margin of error). Imagine an archaeologist needs to assign a date to a bone recovered during an excavation. Turning to carbon-14 dating, the archaeologist might discover that the bone dates from 3,500 years ago. Adding the margin of error for carbon-14 (in this case, +/- 150 years), the archaeologist can give a reliable date range for the bone: 1655-1405 B.C.

Relative dating is more inexact, but still useful. It involves putting things into a sequence based on their relative ages. A bone found deep in the ground will generally be older than one found close to the surface, for example. Two bones found in the same archaeological deposit are likely to be the same age. This sort of information helps archaeologists to put objects into an age sequence. Relative dating does not give archaeology absolute dates that can be tied into a calendar.

Carbon-14 dating

Probably the most well known dating method is carbon-14 (C-14) dating. This method of absolute dating is based on the measurement of the radioactive decay of the C-14 atom. All living beings, both plants and animals, have a mixture of radioactive C-14 and nonradioactive C-12 atoms in their organic material. Once organisms die, they stop assimilating C-14. The remaining C-14 atoms decay at a constant rate, allowing this process to be used reliably as an indicator of age.

Obsidian hydration dating

This dating method is used to date obsidian, a kind of volcanic glass. Obsidian makes a useful knife blade or cutting tool, and archaeologists have found these tools while excavating settlements such as those of the ancient Maya. Obsidian hydration dating is based on the way obsidian absorbs water from its surroundings (a process called hydration). Obsidian absorbs water at a known rate, which makes obsidian hydration dating a reliable method. By viewing obsidian under a microscope, scientists can measure what is called a hydration rim, a layer under the surface of the obsidian that shows exactly how much water has been absorbed. Obsidian hydration dating has provided invaluable information about population patterns in the ancient Maya center of Copán.

Seriation

Seriation is a relative dating method that is used to place types of artifacts (or sites with certain artifact types) into a chronological sequence. This is especially significant when archaeologists have no other dating information. This method assumes that artifact styles change over time, but that the changes are gradual. For example, imagine that there are a number of different sites in an excavation area. Each site contains evidence of different artifact styles (such as pottery of different styles). These stylistic changes happen gradually, so the sites can be arranged in order by time. The rise and fall of each style helps to date the sites. Any artifact that might reflect stylistic changes, such as pottery or tombstone art, can be dated using this relative method.

Dendrochronology

Dendrochronology can determine absolute and relative dates of wood, its origin, and what climates were like in the past. Year-to-year variations in climate, especially rainfall amounts, are reflected in tree rings. A lot of rain in a given year causes thick rings, and little rainfall yields thin rings. In a given time period within a given area, the pattern of thick and thin rings is distinctive. It's like a fingerprint. All trees in the same area will show the same pattern of rings. By examining both living trees and archaeological remains of trees in a given area, scientists can document ring patterns hundreds or even thousands of years into the past.

Once they know the pattern of tree rings in a given area for a given time period, archaeologists can match any new samples to this pattern. This can help date the sample with a high degree of accuracy. If a tree trunk was used as a house beam, and archaeologists can use dendrochronology to date when the tree was cut down, they have a concrete reference point for dating that house. Unless the beam was put in later, the house was built when the tree was cut down, or slightly later. In dendrochronology, it is essential to have a specimen that includes the outermost tree ring, since this shows the year the tree was cut. This method is generally accurate to one year and, in certain instances, it can even determine the season.

There are numerous dating methods, both relative and absolute, available to archaeologists. Regardless of what methods are used, it's important that care be taken in preparing and analyzing the samples. It may be necessary to use more than one dating method or to run a dating method more than once to check for accuracy. Dating provides archaeology with one of the most important ways to provide context: time. Without the context that time can provide, an archaeologist's understanding is incomplete.

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