The specific conditions needed for absolute dating techniques such as dendrochronology and glacial seriation long limited the ability of archaeologists to provide a specific range of dates for many sites. That changed in the mid-twentieth century when studies of radioactivity led to tools for measuring the natural rate of radioactive decay, the loss of radioactivity, of elements in archaeological deposits. In fact, dates determined using radioactive decay are calculated from 1950, the year in which this dating method was developed. Radioactive materials, such as uranium, decay at a consistent rate known as a half-life—the number of years it takes for half of that radioactive element to decay (converting it into a non-radioactive element). Each radioactive element has a specific, known half-life, and these dating methods measure the amount of the radioactive element and of its stable decay product, called the daughter element, to determine how many half-lives (years) have passed since the decay process began. These methods are collectively called radiometric dating.
Radiocarbon Dating
One of the most widely known radiometric dating techniques is radiocarbon dating, which measures the decay of Carbon‑14 (C‑14). Many elements exist in both stable and unstable (radioactive) forms called isotopes. For example, carbon has an atomic number of 6, which is the number of protons. Carbon isotopes vary by the number of neutrons they contain. Carbon‑12 is a stable (non-radioactive) carbon isotope, named for its atomic weight, which is the total number of protons (6) and neutrons (6). Carbon-14 is a radioactive isotope that has 6 protons and 8 neutrons. Its instability leads it to decay, and it has a half-life of 5,730 years.
radiocarbon dating
An absolute dating technique based on the knowledge that living organisms build up organic carbon. When the organism dies, Carbon-14 (C-14) atoms disintegrate at a known rate, which makes it possible to calculate the date of an organic object by measuring the amount of C-14 in the sample (NPS Archeology Program: Archeology for Interpreters).
Carbon‑14 is significant for archaeology because it is common in archaeological deposits. It is produced when cosmic radiation strikes the atmosphere and is incorporated into molecules of carbon dioxide. As plants naturally absorb the carbon dioxide, they incorporate Carbon‑14 into their structures, and organisms that consume the plants incorporate Carbon‑14 into their tissues. Organic material found in archaeological deposits, including wood, plants, baskets, textiles, as well as human and animal remains, all contain this carbon. Over time, the Carbon‑14 in the deposits decays at the rate of its half-life of 5,730 years so samples can be taken from organic remains in archaeological deposits to determine how much time has passed since their deaths. The more recently the organic matter died, the greater the ratio of Carbon‑14 to its non-radioactive carbon by-product since there has been less time for decay to occur. Small amounts of Carbon‑14 relative to its non-radioactive by-product indicate that the organic matter died in the more distant past. Essentially, archaeologists can use anything found in the archaeological record that was once living and ingesting carbon to obtain a date using radiocarbon dating.
Radiocarbon dating is performed by chemists, who analyze samples sent to them by archaeologists. The samples must be kept free from contamination so recent sources of carbon (such as paper tags) must not be bagged with anything that will undergo C‑14 analysis. This technique can date objects and materials with a high degree of accuracy but requires calibration as we now know that carbon concentrations in the atmosphere have not remained constant over time. The concentration of C‑14 in the atmosphere at the time affects the amount of C‑14 that is incorporated into the cells of plants and animals. Additionally, our ability to date accurately with this technique is limited to samples that are between 400 and 50,000 years old; accuracy declines beyond that range. There are other issues with C‑14 dating as well, including the marine reservoir effect, which affects radiocarbon dating of shells. Many marine organisms ingest both atmospheric carbon from the environment and older carbon from materials they consume that come from deep within the ocean and are transported to the surface by circulating water and currents. Radiocarbon dating performed on the remains of aquatic life requires calibration to account for these complexities.
marine reservoir effect
A phenomenon whereby the radiocarbon content of terrestrial organisms is not the same as marine organisms and organisms that ingest them.
Other Radiometric Dating Methods
Dating Technique
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Material Dated
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How It Works
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Potassium-Argon (K/Ar)
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Igneous (volcanic) rock, which contains radioactive Potassium‑40
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The ratio of radioactive Potassium‑40 to its daughter product, Argon‑14, is measured in rock samples to determine the number of half-lives that have passed.
The half-life of Potassium-40 is 1.3 billion years so this method is most accurate for materials that are older than 1 million years.
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Uranium series
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Travertine (calcium carbonate), which is found in cave walls and floors
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Provides highly accurate dates for materials that are between 50,000 and 500,000 years old.
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Fission track
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Obsidian and other glassy volcanic materials
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Determines age based on the natural splitting (fission) of Uranium‑238, which leaves tracks behind in the surface of the material.
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Other Absolute Dating Techniques
Dating Technique
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Material Dated
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How It Works
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Thermoluminescence (TL)
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Ceramics and glass
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Over time, ceramics and glass trap electrons that have been released by natural radiation. Heating the material beyond a critical point allows it to release the electrons as light energy, which can be measured. This method is used to determine the last time the material was heated (such as when a ceramic was fired).
Effectively dates materials that are 100 to 500,000 years old.
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Electron spin resonance (ESR)
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Materials that decompose when heated, such as tooth enamel
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Similar to TL dating but less sensitive.
Effective for confirming dates obtained using other methods.
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Archaeomagnetic dating
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Clay
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Earth’s magnetic fields have changed over time, causing the location of magnetic north to shift. Magnetic particles in clay record the direction of magnetic north at the time the clay was heated.
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mtDNA
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Mitochondrial DNA
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Compares the DNA of individuals and populations found in their cells’ mitochondria (an organelle responsible for energy processing) to establish patterns of migration over time.
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Y chromosome
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Y chromosomes
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Compares the DNA from Y chromosomes (male sex chromosomes) of individuals and populations to establish patterns of migration over time.
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