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Radiometric dating
Radiometric dating (often called radioactive dating ) is a technique used to date materials such as rocks or carbon, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. [ 1 ] The use of radiometric dating was first published in 1907 by Bertram Boltwood [ 2 ] and is now the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials. Together with stratigraphic principles. radiometric dating methods are used in geochronology to establish the geological time scale. [ 3 ] Among the best-known techniques are radiocarbon dating. potassium-argon dating and uranium-lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.
Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.
Radioactive decay [ edit ]
Example of a radioactive decay chain from lead-212 ( 212 Pb) to lead-208 ( 208 Pb). Each parent nuclide spontaneously decays into a daughter nuclide (the decay product ) via an ? decay or a ? ? decay. The final decay product, lead-208 ( 208 Pb), is stable and can no longer undergo spontaneous radioactive decay.
All ordinary matter is made up of combinations of chemical elements. each with its own atomic number. indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes. with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including alpha decay (emission of alpha particles ) and beta decay (electron emission, positron emission, or electron capture ). Another possibility is spontaneous fission into two or more nuclides.
For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant. It is not affected by external factors such as temperature. pressure. chemical environment, or presence of a magnetic or electric field. [ 4 ] [ 5 ] [ 6 ] The only exceptions are nuclides that decay by the process of electron capture. such as beryllium-7. strontium-85. and zirconium-89. whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.
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Radiocarbon dating
From Wikipedia, the free encyclopedia
Radiocarbon dating is a method of determining the age of an object by using the properties of radiocarbon. a radioactive isotope of carbon. The method was invented by Willard Libby in the late 1940s and soon became a standard tool for archaeologists. It depends on the fact that radiocarbon, often abbreviated as 14
C. is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide. This is then incorporated into plants by photosynthesis. and animals acquire 14
C by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point the amount of 14
C it contains begins to reduce as the 14
C undergoes radioactive decay. Measuring the amount of 14
C in a sample from a dead plant or animal such as piece of old wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The oldest dates that can be reliably measured by radiocarbon dating are around 50,000 years ago, though special preparation methods occasionally permit dating of older samples.
While the idea behind radiocarbon dating is straightforward, years of additional work were required to develop the technique to the point where accurate dates could be obtained. Research has been going on since the 1960s to determine what the proportion of 14
C in the atmosphere has been over the past fifty thousand years. The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's actual calendar age. In addition to this curve, other corrections must be made to account for different proportions of 14
C in different types of organism (fractionation) and different 14
C levels in different parts of the biosphere (reservoir effects).
Measurement of radiocarbon was originally done by beta-counting devices, so called because they counted the amount of beta radiation emitted by decaying 14
C atoms in a sample. More recently, accelerator mass spectrometry has become the method of choice; it can be used with much smaller samples (as small as individual plant seeds), and gives results much more quickly.
The development of radiocarbon dating has had a profound impact on archaeology. In addition to permitting more accurate dating within archaeological sites than did methods previously in use, it also allows comparison of dates of events across great distances. Histories of archaeology often refer to the early impact of the new method as the “radiocarbon revolution”. Occasionally, the method is used for items of popular interest such as the Shroud of Turin. which is claimed to show an image of the body of Jesus Christ. A sample of linen from the shroud was tested in 1988 and found to date from the 1200s or 1300s, casting doubt on its authenticity.
Contents
Background [ edit ]
History [ edit ]
In the early 1930s Willard Libby was a chemistry student at the University of Berkeley. receiving his Ph. D. in 1933. He remained there as an instructor until the end of the decade. In 1939 the Radiation Laboratory at Berkeley began experiments to determine if any of the elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research. It was soon discovered that 14
C 's half-life was far longer than had been previously thought, and in 1940 this was followed by proof that the interaction of slow neutrons with 14
N was the main pathway by which 14
C was created. It had previously been thought 14
C would be more likely to be created by deuterons interacting with 13
C. At about this time Libby read a paper by W. E. Danforth and S. A. Korff, published in 1939, which predicted the creation of 14
C in the atmosphere by neutrons from cosmic rays which had been slowed down by collisions with molecules of atmospheric gas. It was this paper that first gave Libby the idea that radiocarbon dating might be possible. [ 1 ]
In 1945, Libby moved to the University of Chicago. He published a paper in 1946 in which he proposed that the carbon in living matter might include 14
C as well as non-radioactive carbon. [ 2 ] [ 3 ] Libby and several collaborators proceeded to experiment with methane collected from sewage works in Baltimore, and after isotopically enriching their samples they were able to demonstrate that they contained radioactive 14
C. By contrast, methane created from petroleum had no radiocarbon activity. The results were summarized in a paper in Science in 1947, and the authors commented that their results implied it would be possible to date materials containing carbon of organic origin. [ 2 ] [ 4 ]
Libby and James Arnold proceeded to experiment with samples of wood of known age. For example, two wood samples taken from the tombs of two Egyptian kings, Zoser and Sneferu. independently dated to 2625 BC plus or minus 75 years, were dated by radiocarbon measurement to an average of 2800 BC plus or minus 250 years. These results were published in Science in 1949. [ 5 ] [ 6 ] In 1960, Libby was awarded the Nobel Prize in Chemistry for this work. [ 2 ]
Physical and chemical details [ edit ]
In nature, carbon exists as two stable, nonradioactive isotopes. carbon-12 ( 12
C ), and a radioactive isotope, carbon-14 ( 14
C ), also known as "radiocarbon". The half-life of 14
C (the time it takes for half of a given amount of 14
C to decay ) is about 5,730 years, so its concentration in the atmosphere might be expected to reduce over thousands of years. However, 14
C is constantly being produced in the lower stratosphere and upper troposphere by cosmic rays. which generate neutrons that in turn create 14
C when they strike nitrogen-14 ( 14
N ) atoms. [ 2 ] The 14
C creation process is described by the following nuclear reaction :
Once produced, the 14
C quickly combines with the oxygen in the atmosphere to form carbon dioxide ( CO
2 ). Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is taken up by plants via photosynthesis. Animals eat the plants, and ultimately the radiocarbon is distributed throughout the biosphere. The ratio of 14
Principles [ edit ]
During its life, a plant or animal is exchanging carbon with its surroundings, so the carbon it contains will have the same proportion of 14
C as the biosphere and the carbon exchange reservoir. Once it dies, it ceases to acquire 14
C. but the 14
C within its biological material at that time will continue to decay, and so the ratio of 14
C to 12
C in its remains will gradually reduce. Because 14
C decays at a known rate, the proportion of radiocarbon can be used to determine how long it has been since a given sample stopped exchanging carbon—the older the sample, the less 14
C will be left. [ 8 ]
The equation governing the decay of a radioactive isotope is: [ 2 ]
where N 0 is the number of atoms of the isotope in the original sample (at time t = 0, when the organism from which the sample was taken died), and N is the number of atoms left after time t . [ 2 ] ? is a constant that depends on the particular isotope; for a given isotope it is equal to the reciprocal of the mean-life — i. e. the average or expected time a given atom will survive before undergoing radioactive decay. [ 2 ] The mean-life, denoted by ? . of 14
C is 8,267 years, so the equation above can be rewritten as: [ 11 ]
The sample is assumed to have originally had the same 14
C / 12
C ratio as the ratio in the biosphere, and since the size of the sample is known, the total number of atoms in the sample can be calculated, yielding N 0 . the number of 14
C atoms in the original sample. Measurement of N . the number of 14
C atoms currently in the sample, allows the calculation of t . the age of the sample, using the equation above. [ 8 ]
The half-life of a radioactive isotope (the time it takes for half of the sample to decay, usually denoted by t 1/2 ) is a more familiar concept than the mean-life, so although the equations above are expressed in terms of the mean-life, it is more usual to quote the value of 14
C 's half-life than its mean-life. [ note 1 ] The currently accepted value for the half-life of 14
C is 5,730 years. [ 2 ] This means that after 5,730 years, only half of the initial 14
C will have remained; a quarter will have remained after 11,460 years; an eighth after 17,190 years; and so on.
The above calculations make several assumptions, such as that the level of 14
C in the biosphere has remained constant over time. [ 2 ] In fact, the level of 14
C in the biosphere has varied significantly and as a result the values provided by the equation above have to be corrected by using data from other sources in the form of a calibration curve, which is described in more detail below. [ 12 ] For over a decade after Libby's initial work, the accepted value of the half-life for 14
C was 5,568 years; this was improved in the early 1960s to 5,730 years, which meant that many calculated dates in published papers were now incorrect (the error is about 3%). However, it is possible to incorporate a correction for the half-life value into the calibration curve, and so it has become standard practice to quote measured radiocarbon dates in "radiocarbon years", meaning that the dates are calculated using Libby's half-life value and have not been calibrated. [ 13 ] [ note 2 ] This approach has the advantage of maintaining consistency with the early papers, and also avoids the risk of a double correction for the Libby half-life value. [ 15 ]
Carbon exchange reservoir [ edit ]
Simplified version of the carbon exchange reservoir, showing proportions of carbon and relative activity of the 14
The different elements of the carbon exchange reservoir vary in how much carbon they store, and in how long it takes for the 14
C generated by cosmic rays to fully mix with them. [ 2 ] The atmosphere, which is where 14
C is generated, contains about 1.9% of the total carbon in the reservoirs, and the 14
C it contains mixes in less than seven years. [ 16 ] [ 17 ] The ratio of 14
C to 12
C in the atmosphere is taken as the baseline for the other reservoirs: if another reservoir has a lower ratio of 14
C to 12
C. it indicates that the carbon is older and hence that some of the 14
C has decayed. [ 12 ] The ocean surface is an example: it contains 2.4% of the carbon in the exchange reservoir, [ 16 ] but there is only about 95% as much 14
C as would be expected if the ratio were the same as in the atmosphere. [ 2 ] The time it takes for carbon from the atmosphere to mix with the surface ocean is only a few years, [ 18 ] but the surface waters also receive water from the deep ocean, which has over 90% of the carbon in the reservoir. [ 12 ] Water in the deep ocean takes about 1,000 years to circulate back through surface waters, and so the surface waters contain a combination of older water, with depleted 14
C. and water recently at the surface, with 14
C in equilibrium with the atmosphere. [ 12 ]
Creatures living at the ocean surface have the same 14
C ratios as the water they live in, and as a result of the reduced 14
C / 12
C ratio, the radiocarbon age of marine life is typically about 400 years. [ 19 ] [ note 4 ] Organisms on land, however, are in closer equilibrium with the atmosphere and have the same 14
C / 12
C ratio as the atmosphere. [ 2 ] These organisms contain about 1.3% of the carbon in the reservoir; sea organisms have a mass of less than 1% of those on land and are not shown on the diagram. [ 16 ] Accumulated dead organic matter, of both plants and animals, exceeds the mass of the biosphere by a factor of nearly 3, and since this matter is no longer exchanging carbon with its environment, it has a 14
C / 12
Dating considerations [ edit ]
The variation in the 14
C / 12
C ratio in different parts of the carbon exchange reservoir means that a straightforward calculation of the age of a sample based on the amount of 14
C it contains will often give an incorrect result. There are several other possible sources of error that need to be considered. The errors are of four general types:
variations in the 14
C / 12
C ratio in the atmosphere, both geographically and over time;
isotopic fractionation;
variations in the 14
C / 12
C ratio in different parts of the reservoir;
contamination.
Atmospheric variation [ edit ]
In the early years of using the technique, it was understood that it depended on the atmospheric 14
C / 12
C ratio having remained the same over the preceding few thousand years. To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects. However, in 1958, Hessel de Vries was able to demonstrate that the 14
C / 12
C ratio had changed over time by testing wood samples of known ages and showing there was a significant deviation from the expected ratio. This discrepancy, often called the de Vries effect, was resolved by the study of tree rings. [ 20 ] [ 21 ] Comparison of overlapping series of tree rings allowed the construction of a continuous sequence of tree-ring data that spanned 8,000 years. [ 20 ] (Since that time the tree-ring data series has been extended to 13,900 years.) [ 22 ] Carbon-dating the wood from the tree rings themselves provided the check needed on the atmospheric 14
C / 12
C ratio: with a sample of known date, and a measurement of the value of N (the number of atoms of 14
C remaining in the sample), the carbon-dating equation allows the calculation of N 0 – the number of atoms of 14
C in the sample at the time the tree ring was formed – and hence the 14
C / 12
C ratio in the atmosphere at that time. [ 20 ] Armed with the results of carbon-dating the tree rings, it became possible to construct calibration curves designed to correct the errors caused by the variation over time in the 14
C / 12
C ratio. [ 23 ] These curves are described in more detail below .
Atmospheric 14
C. New Zealand [ 24 ] and Austria. [ 25 ] The New Zealand curve is representative of the Southern Hemisphere; the Austrian curve is representative of the Northern Hemisphere. Atmospheric nuclear weapon tests almost doubled the concentration of 14
C in the Northern Hemisphere. [ 9 ] The date that the Partial Test Ban Treaty (PTBT) went into effect is marked on the graph.
Coal and oil began to be burned in large quantities during the 1800s. Both coal and oil are sufficiently old that they contain little detectable 14
C and, as a result, the CO
2 released substantially diluted the atmospheric 14
C / 12
C ratio. Dating an object from the early 20th century hence gives an apparent date older than the true date. For the same reason, 14
C concentrations in the neighbourhood of large cities are lower than the atmospheric average. This fossil fuel effect (also known as the Suess effect, after Hans Suess. who first reported it in 1955) would only amount to a reduction of 0.2% in 14
C activity if the additional carbon from fossil fuels were distributed throughout the carbon exchange reservoir, but because of the long delay in mixing with the deep ocean, the actual effect is a 3% reduction. [ 20 ] [ 26 ]
A much larger effect comes from above-ground nuclear testing, which released large numbers of neutrons and created 14
C. From about 1950 until 1963, when atmospheric nuclear testing was banned, it is estimated that several tonnes of 14
C were created. If all this extra 14
C had immediately been spread across the entire carbon exchange reservoir, it would have led to an increase in the 14
C / 12
C ratio of only a few per cent, but the immediate effect was to almost double the amount of 14
C in the atmosphere, with the peak level occurring in about 1965. The level has since dropped, as the "bomb carbon" (as it is sometimes called) percolates into the rest of the reservoir. [ 20 ] [ 26 ] [ 27 ]
Isotopic fractionation [ edit ]
Photosynthesis is the primary process by which carbon moves from the atmosphere into living things. In both photosynthetic pathways (C3 and C4 ) 12
C is absorbed slightly more easily than 13
C. which in turn is more easily absorbed than 14
C ratios in plants that differ from the ratios in the atmosphere. This effect is known as isotopic fractionation. [ 28 ] [ 29 ]
To determine the degree of fractionation that takes place in a given plant, the amounts of both 12
C and 13
C isotopes are measured, and the resulting 13
C / 12
C ratio is then compared to a standard ratio known as PDB. [ note 5 ] The 13
C / 12
C ratio is used instead of 14
C / 12
C because the former is much easier to measure, and the latter can be easily derived: the depletion of 13
C relative to 12
C is proportional to the difference in the atomic masses of the two isotopes, so the depletion for 14
C is twice the depletion of 13
C. [ 12 ] The fractionation of 13
C. known as ? 13 C. is calculated as follows: [ 28 ]
where the ‰ sign indicates parts per thousand. [ 28 ] Because the PDB standard contains an unusually high proportion of 13
C. [ note 6 ] most measured ? 13 C values are negative.
Radiometric dating
Radiometric dating (often called radioactive dating ) is a technique used to date materials such as rocks or carbon, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. [ 1 ] The use of radiometric dating was first published in 1907 by Bertram Boltwood [ 2 ] and is now the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials. Together with stratigraphic principles. radiometric dating methods are used in geochronology to establish the geological time scale. [ 3 ] Among the best-known techniques are radiocarbon dating. potassium-argon dating and uranium-lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.
Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.
Radioactive decay [ edit ]
Example of a radioactive decay chain from lead-212 ( 212 Pb) to lead-208 ( 208 Pb). Each parent nuclide spontaneously decays into a daughter nuclide (the decay product ) via an ? decay or a ? ? decay. The final decay product, lead-208 ( 208 Pb), is stable and can no longer undergo spontaneous radioactive decay.
All ordinary matter is made up of combinations of chemical elements. each with its own atomic number. indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes. with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including alpha decay (emission of alpha particles ) and beta decay (electron emission, positron emission, or electron capture ). Another possibility is spontaneous fission into two or more nuclides.
For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant. It is not affected by external factors such as temperature. pressure. chemical environment, or presence of a magnetic or electric field. [ 4 ] [ 5 ] [ 6 ] The only exceptions are nuclides that decay by the process of electron capture. such as beryllium-7. strontium-85. and zirconium-89. whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.
