Absolute dating
Encyclopedia
Absolute dating is the process of determining an approximate computed age in archaeology
and geology
. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty and precision. Absolute dating provides a computed numerical age in contrast with relative dating
which provides only an order of events.
In archeology, absolute dating is usually based on the physical or chemical properties of the materials of artifacts, buildings, or other items that have been modified by humans. Absolute dates do not necessarily tell us precisely when a particular cultural event happened, but when taken as part of the overall archaeological record
they are invaluable in constructing a more specific sequence of events.
In geology, absolute dating is usually based on physical or chemical properties of igneous rock formations which are closely associated with paleontological finds. Fossils are rarely dated directly.
. Given an initial and a present quantity of such an isotope and its half-life
, the time elapsed may be calculated. Various methods apply to different materials and timescales. If a very short period of time has passed, as measured in number of half-lives, a particular technique will be less accurate and more susceptible to statistical fluctuations in the inherently random decay events. If many half-lives of the isotope of interest have passed, too much of the sample may have decayed to provide an accurate reading.
) dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Carbon-14 is an unstable isotope of normal carbon, carbon-12. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon-14 stops. Then this unstable isotope starts to decay into nitrogen-14. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the proportion of carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.
. This technique usually cannot pinpoint the date of a site better than historic records.
A further issue is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry
(AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
(K-Ar dating). Potassium-40
is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological
or archeological interest to be dated. Argon
, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature
at which the trapped argon can escape the lattice. K-Ar dating was used to calibrate the geomagnetic polarity time scale.
s, producing light. This light can be measured to determine the last time the item was heated.
Because of these and other factors, Thermoluminesence is at the most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item.
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
and geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty and precision. Absolute dating provides a computed numerical age in contrast with relative dating
Relative dating
Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another...
which provides only an order of events.
In archeology, absolute dating is usually based on the physical or chemical properties of the materials of artifacts, buildings, or other items that have been modified by humans. Absolute dates do not necessarily tell us precisely when a particular cultural event happened, but when taken as part of the overall archaeological record
Archaeological record
The archaeological record is the body of physical evidence about the past. It is one of the most basic concepts in archaeology, the academic discipline concerned with documenting and interpreting the archaeological record....
they are invaluable in constructing a more specific sequence of events.
In geology, absolute dating is usually based on physical or chemical properties of igneous rock formations which are closely associated with paleontological finds. Fossils are rarely dated directly.
Radiometric techniques
Radiometric dating is based on the constant rate of decay of radioactive isotopesRadionuclide
A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...
. Given an initial and a present quantity of such an isotope and its half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
, the time elapsed may be calculated. Various methods apply to different materials and timescales. If a very short period of time has passed, as measured in number of half-lives, a particular technique will be less accurate and more susceptible to statistical fluctuations in the inherently random decay events. If many half-lives of the isotope of interest have passed, too much of the sample may have decayed to provide an accurate reading.
Radiocarbon dating
One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbonRadiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...
) dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Carbon-14 is an unstable isotope of normal carbon, carbon-12. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon-14 stops. Then this unstable isotope starts to decay into nitrogen-14. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the proportion of carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.
Limitations
Because the half-life of carbon-14 is 5730 years carbon dating is only reliable about up to 40,000 years, radiocarbon is less useful to date some recent sites. See radiocarbon datingRadiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...
. This technique usually cannot pinpoint the date of a site better than historic records.
A further issue is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry
Accelerator mass spectrometry
Accelerator mass spectrometry differs from other forms of mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the mass spectrometric methods is its power to separate a rare isotope from an abundant...
(AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
Potassium-argon dating
Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium-argon datingPotassium-argon dating
Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...
(K-Ar dating). Potassium-40
Isotopes of potassium
Potassium has 25 known isotopes from 32K to 56K. Three isotopes occur naturally: stable 39K and 41K , and the long-lived radioisotope 40K . The standard atomic mass is 39.0983 u. Naturally occurring 40K decays to stable 40Ar by electron capture or positron emission...
is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
or archeological interest to be dated. Argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature
Closure temperature
In radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature at which a system has cooled so that there is no longer any exchange of parent or...
at which the trapped argon can escape the lattice. K-Ar dating was used to calibrate the geomagnetic polarity time scale.
Thermoluminescence
Thermoluminesence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item. Heating an item to 500 degrees Celsius or higher releases the trapped electronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, producing light. This light can be measured to determine the last time the item was heated.
Limitations
Radiation levels do not remain constant over time. Fluctuating levels can skew results - for example, if an item went through several high radiation eras, thermoluminesence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.Because of these and other factors, Thermoluminesence is at the most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item.