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The older a sample is, the less 14 C there is to be detected, and because the half-life of 14 C the period of time after which half of a given sample will have decayed is about 5, years, the oldest dates that can be reliably measured by this process date to approximately 50, years ago, although special preparation methods occasionally make accurate analysis of older samples possible.
Research has been ongoing since the s 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 calendar age. Other corrections must be made to account for the proportion of 14 C in different types of organisms fractionation , and the varying levels of 14 C throughout the biosphere reservoir effects.
Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the s and s. Because the time it takes to convert biological materials to fossil fuels is substantially longer than the time it takes for its 14 C to decay below detectable levels, fossil fuels contain almost no 14 C. As a result, beginning in the late 19th century, there was a noticeable drop in the proportion of 14 C as the carbon dioxide generated from burning fossil fuels began to accumulate in the atmosphere.
Conversely, nuclear testing increased the amount of 14 C in the atmosphere, which reached a maximum in about of almost double the amount present in the atmosphere prior to nuclear testing. Measurement of radiocarbon was originally done by beta-counting devices, which 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 counts all the 14 C atoms in the sample and not just the few that happen to decay during the measurements; it can therefore 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 previous methods, it allows comparison of dates of events across great distances.
Histories of archaeology often refer to its impact as the "radiocarbon revolution". Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice age , and the beginning of the Neolithic and Bronze Age in different regions. In , Martin Kamen and Samuel Ruben of 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.
They synthesized 14 C using the laboratory's cyclotron accelerator and soon discovered that the atom's half-life was far longer than had been previously thought. Korff , then employed at the Franklin Institute in Philadelphia , that the interaction of thermal neutrons with 14 N in the upper atmosphere would create 14 C.
In , Libby moved to the University of Chicago , where he began his work on radiocarbon dating. He published a paper in in which he proposed that the carbon in living matter might include 14 C as well as non-radioactive carbon.
By contrast, methane created from petroleum showed no radiocarbon activity because of its age. The results were summarized in a paper in Science in , in which the authors commented that their results implied it would be possible to date materials containing carbon of organic origin.
Libby and James Arnold proceeded to test the radiocarbon dating theory by analyzing samples with known ages. For example, two samples taken from the tombs of two Egyptian kings, Zoser and Sneferu , independently dated to BC plus or minus 75 years, were dated by radiocarbon measurement to an average of BC plus or minus years. These results were published in Science in December In nature, carbon exists as two stable, nonradioactive isotopes : carbon 12 C , and carbon 13 C , and a radioactive isotope, carbon 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, years, so its concentration in the atmosphere might be expected to decrease over thousands of years, but 14 C is constantly being produced in the lower stratosphere and upper troposphere , primarily by galactic cosmic rays , and to a lesser degree by solar cosmic rays. Once produced, the 14 C quickly combines with the oxygen in the atmosphere to form first carbon monoxide CO ,  and ultimately 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 C to 12 C is approximately 1. The equation for the radioactive decay of 14 C is: . During its life, a plant or animal is in equilibrium with its surroundings by exchanging carbon either with the atmosphere or through its diet. It will, therefore, have the same proportion of 14 C as the atmosphere, or in the case of marine animals or plants, with the ocean. 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 decrease.
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. The equation governing the decay of a radioactive isotope is: . 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.
The above calculations make several assumptions, such as that the level of 14 C in the atmosphere has remained constant over time. Calculating radiocarbon ages also requires the value of the half-life for 14 C. Radiocarbon ages are still calculated using this half-life, and are known as "Conventional Radiocarbon Age". Since the calibration curve IntCal also reports past atmospheric 14 C concentration using this conventional age, any conventional ages calibrated against the IntCal curve will produce a correct calibrated age.
When a date is quoted, the reader should be aware that if it is an uncalibrated date a term used for dates given in radiocarbon years it may differ substantially from the best estimate of the actual calendar date, both because it uses the wrong value for the half-life of 14 C , and because no correction calibration has been applied for the historical variation of 14 C in the atmosphere over time.
Carbon is distributed throughout the atmosphere, the biosphere, and the oceans; these are referred to collectively as the carbon exchange reservoir,  and each component is also referred to individually as a carbon exchange reservoir. 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.
This affects the ratio of 14 C to 12 C in the different reservoirs, and hence the radiocarbon ages of samples that originated in each reservoir. There are several other possible sources of error that need to be considered. The errors are of four general types:. 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.
Over time, however, discrepancies began to appear between the known chronology for the oldest Egyptian dynasties and the radiocarbon dates of Egyptian artefacts. The question was resolved by the study of tree rings :    comparison of overlapping series of tree rings allowed the construction of a continuous sequence of tree-ring data that spanned 8, years.
Coal and oil began to be burned in large quantities during the 19th century. 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 would only amount to a reduction of 0. A much larger effect comes from above-ground nuclear testing, which released large numbers of neutrons into the atmosphere, resulting in the creation of 14 C.
From about until , when atmospheric nuclear testing was banned, it is estimated that several tonnes of 14 C were created. The level has since dropped, as this bomb pulse or "bomb carbon" as it is sometimes called percolates into the rest of the reservoir.
Photosynthesis is the primary process by which carbon moves from the atmosphere into living things. In photosynthetic pathways 12 C is absorbed slightly more easily than 13 C , which in turn is more easily absorbed than 14 C.
This effect is known as isotopic fractionation. At higher temperatures, CO 2 has poor solubility in water, which means there is less CO 2 available for the photosynthetic reactions. The enrichment of bone 13 C also implies that excreted material is depleted in 13 C relative to the diet. The carbon exchange between atmospheric CO 2 and carbonate at the ocean surface is also subject to fractionation, with 14 C in the atmosphere more likely than 12 C to dissolve in the ocean.
This increase in 14 C concentration almost exactly cancels out the decrease caused by the upwelling of water containing old, and hence 14 C depleted, carbon from the deep ocean, so that direct measurements of 14 C radiation are similar to measurements for the rest of the biosphere.
Correcting for isotopic fractionation, as is done for all radiocarbon dates to allow comparison between results from different parts of the biosphere, gives an apparent age of about years for ocean surface water. The marine effect : The CO 2 in the atmosphere transfers to the ocean by dissolving in the surface water as carbonate and bicarbonate ions; at the same time the carbonate ions in the water are returning to the air as CO 2.
The deepest parts of the ocean mix very slowly with the surface waters, and the mixing is uneven. The main mechanism that brings deep water to the surface is upwelling, which is more common in regions closer to the equator. Upwelling is also influenced by factors such as the topography of the local ocean bottom and coastlines, the climate, and wind patterns. Overall, the mixing of deep and surface waters takes far longer than the mixing of atmospheric CO 2 with the surface waters, and as a result water from some deep ocean areas has an apparent radiocarbon age of several thousand years.
Upwelling mixes this "old" water with the surface water, giving the surface water an apparent age of about several hundred years after correcting for fractionation. The northern and southern hemispheres have atmospheric circulation systems that are sufficiently independent of each other that there is a noticeable time lag in mixing between the two. Since the surface ocean is depleted in 14 C because of the marine effect, 14 C is removed from the southern atmosphere more quickly than in the north.
For example, rivers that pass over limestone , which is mostly composed of calcium carbonate , will acquire carbonate ions. Similarly, groundwater can contain carbon derived from the rocks through which it has passed. Volcanic eruptions eject large amounts of carbon into the air. Dormant volcanoes can also emit aged carbon. Any addition of carbon to a sample of a different age will cause the measured date to be inaccurate.
Contamination with modern carbon causes a sample to appear to be younger than it really is: the effect is greater for older samples. Samples for dating need to be converted into a form suitable for measuring the 14 C content; this can mean conversion to gaseous, liquid, or solid form, depending on the measurement technique to be used. Before this can be done, the sample must be treated to remove any contamination and any unwanted constituents.
Particularly for older samples, it may be useful to enrich the amount of 14 C in the sample before testing. This can be done with a thermal diffusion column. Once contamination has been removed, samples must be converted to a form suitable for the measuring technology to be used.
For accelerator mass spectrometry , solid graphite targets are the most common, although gaseous CO 2 can also be used. The quantity of material needed for testing depends on the sample type and the technology being used. There are two types of testing technology: detectors that record radioactivity, known as beta counters, and accelerator mass spectrometers.
For beta counters, a sample weighing at least 10 grams 0. For decades after Libby performed the first radiocarbon dating experiments, the only way to measure the 14 C in a sample was to detect the radioactive decay of individual carbon atoms. Libby's first detector was a Geiger counter of his own design. He converted the carbon in his sample to lamp black soot and coated the inner surface of a cylinder with it. This cylinder was inserted into the counter in such a way that the counting wire was inside the sample cylinder, in order that there should be no material between the sample and the wire.
Libby's method was soon superseded by gas proportional counters , which were less affected by bomb carbon the additional 14 C created by nuclear weapons testing. These counters record bursts of ionization caused by the beta particles emitted by the decaying 14 C atoms; the bursts are proportional to the energy of the particle, so other sources of ionization, such as background radiation, can be identified and ignored.
The counters are surrounded by lead or steel shielding, to eliminate background radiation and to reduce the incidence of cosmic rays. In addition, anticoincidence detectors are used; these record events outside the counter and any event recorded simultaneously both inside and outside the counter is regarded as an extraneous event and ignored.
The other common technology used for measuring 14 C activity is liquid scintillation counting, which was invented in , but which had to wait until the early s, when efficient methods of benzene synthesis were developed, to become competitive with gas counting; after liquid counters became the more common technology choice for newly constructed dating laboratories.
The counters work by detecting flashes of light caused by the beta particles emitted by 14 C as they interact with a fluorescing agent added to the benzene. Like gas counters, liquid scintillation counters require shielding and anticoincidence counters.
For both the gas proportional counter and liquid scintillation counter, what is measured is the number of beta particles detected in a given time period. Each measuring device is also used to measure the activity of a blank sample — a sample prepared from carbon old enough to have no activity. This provides a value for the background radiation, which must be subtracted from the measured activity of the sample being dated to get the activity attributable solely to that sample's 14 C.
In addition, a sample with a standard activity is measured, to provide a baseline for comparison. The ions are accelerated and passed through a stripper, which removes several electrons so that the ions emerge with a positive charge. A particle detector then records the number of ions detected in the 14 C stream, but since the volume of 12 C and 13 C , needed for calibration is too great for individual ion detection, counts are determined by measuring the electric current created in a Faraday cup.
Any 14 C signal from the machine background blank is likely to be caused either by beams of ions that have not followed the expected path inside the detector or by carbon hydrides such as 12 CH 2 or 13 CH. A 14 C signal from the process blank measures the amount of contamination introduced during the preparation of the sample. These measurements are used in the subsequent calculation of the age of the sample. The calculations to be performed on the measurements taken depend on the technology used, since beta counters measure the sample's radioactivity whereas AMS determines the ratio of the three different carbon isotopes in the sample.
To determine the age of a sample whose activity has been measured by beta counting, the ratio of its activity to the activity of the standard must be found. To determine this, a blank sample of old, or dead, carbon is measured, and a sample of known activity is measured. The additional samples allow errors such as background radiation and systematic errors in the laboratory setup to be detected and corrected for. The results from AMS testing are in the form of ratios of 12 C , 13 C , and 14 C , which are used to calculate Fm, the "fraction modern".
Both beta counting and AMS results have to be corrected for fractionation. The calculation uses 8,, the mean-life derived from Libby's half-life of 5, years, not 8,, the mean-life derived from the more accurate modern value of 5, years. Libby's value for the half-life is used to maintain consistency with early radiocarbon testing results; calibration curves include a correction for this, so the accuracy of final reported calendar ages is assured.
The reliability of the results can be improved by lengthening the testing time. Radiocarbon dating is generally limited to dating samples no more than 50, years old, as samples older than that have insufficient 14 C to be measurable. Older dates have been obtained by using special sample preparation techniques, large samples, and very long measurement times. These techniques can allow measurement of dates up to 60, and in some cases up to 75, years before the present.
This was demonstrated in by an experiment run by the British Museum radiocarbon laboratory, in which weekly measurements were taken on the same sample for six months. The measurements included one with a range from about to about years ago, and another with a range from about to about Errors in procedure can also lead to errors in the results.
The calculations given above produce dates in radiocarbon years: i. To produce a curve that can be used to relate calendar years to radiocarbon years, a sequence of securely dated samples is needed which can be tested to determine their radiocarbon age. The study of tree rings led to the first such sequence: individual pieces of wood show characteristic sequences of rings that vary in thickness because of environmental factors such as the amount of rainfall in a given year.
These factors affect all trees in an area, so examining tree-ring sequences from old wood allows the identification of overlapping sequences. In this way, an uninterrupted sequence of tree rings can be extended far into the past. The first such published sequence, based on bristlecone pine tree rings, was created by Wesley Ferguson.
Suess said he drew the line showing the wiggles by "cosmic schwung ", by which he meant that the variations were caused by extraterrestrial forces. It was unclear for some time whether the wiggles were real or not, but they are now well-established. A calibration curve is used by taking the radiocarbon date reported by a laboratory and reading across from that date on the vertical axis of the graph. The point where this horizontal line intersects the curve will give the calendar age of the sample on the horizontal axis.
This is the reverse of the way the curve is constructed: a point on the graph is derived from a sample of known age, such as a tree ring; when it is tested, the resulting radiocarbon age gives a data point for the graph. Over the next thirty years many calibration curves were published using a variety of methods and statistical approaches.
The IntCal20 data includes separate curves for the northern and southern hemispheres, as they differ systematically because of the hemisphere effect. The southern curve SHCAL20 is based on independent data where possible and derived from the northern curve by adding the average offset for the southern hemisphere where no direct data was available.
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