1.2 Physical and chemical details
2 Dating considerations
2.1 Atmospheric variation 2.2 Isotopic fractionation 2.3 Reservoir effects 2.4 Contamination
3.1 Material considerations 3.2 Preparation and size
4 Measurement and results
4.1 Beta counting 4.2 Accelerator mass spectrometry 4.3 Calculations 4.4 Errors and reliability 4.5 Calibration 4.6 Reporting dates
5 Use in archaeology
5.1 Interpretation 5.2 Notable applications
6 See also 7 Notes 8 References 9 Sources 10 External links
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 14C using the
laboratory's cyclotron accelerator and soon discovered that the atom's
half-life was far longer than had been previously thought. This was
followed by a prediction by Serge A. Korff, then employed at the
n + 14 7N → 14 6C + p
where n represents a neutron and p represents a proton.
Once produced, the 14C quickly combines with the oxygen in the
atmosphere to form carbon dioxide (CO
14 6C → 14 7N + e− + ν e
By emitting a beta particle (an electron, e−) and an electron antineutrino (ν e), one of the neutrons in the 14C nucleus changes to a proton and the 14C nucleus reverts to the stable (non-radioactive) isotope 14N. Principles During its life, a plant or animal is exchanging carbon with its surroundings, so the carbon it contains will have the same proportion of 14C as the atmosphere. Once it dies, it ceases to acquire 14C, but the 14C within its biological material at that time will continue to decay, and so the ratio of 14C to 12C in its remains will gradually decrease. Because 14C 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 14C will be left. The equation governing the decay of a radioactive isotope is:
− λ t
displaystyle N=N_ 0 e^ -lambda t ,
where N0 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. λ 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. The mean-life, denoted by τ, of 14C is 8,267 years, so the equation above can be rewritten as:
t = 8267 ⋅ ln (
displaystyle t=8267cdot ln(N_ 0 /N) text years
The sample is assumed to have originally had the same 14C/12C ratio as
the ratio in the atmosphere, and since the size of the sample is
known, the total number of atoms in the sample can be calculated,
yielding N0, the number of 14C atoms in the original sample.
Measurement of N, the number of 14C atoms currently in the sample,
allows the calculation of t, the age of the sample, using the equation
The half-life of a radioactive isotope (usually denoted by t1/2) 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 14C's half-life than its mean-life.[note 1] The
currently accepted value for the half-life of 14C is 5,730 years.
This means that after 5,730 years, only half of the initial 14C will
remain; a quarter will remain 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 14C in the atmosphere has remained constant over time. In
fact, the level of 14C in the atmosphere has varied significantly and
as a result the values provided by the equation above have to be
corrected by using data from other sources. This is done by
calibration curves, which convert a measurement of 14C in a sample
into an estimated calendar age. The calculations involve several steps
and include an intermediate value called the "radiocarbon age", which
is the age in "radiocarbon years" of the sample: an age quoted in
radiocarbon years means that no calibration curve has been used −
the calculations for radiocarbon years assume that the 14C/12C ratio
has not changed over time. Calculating radiocarbon ages also requires
the value of the half-life for 14C, which for more than a decade after
Libby's initial work was thought to be 5,568 years. This was revised
in the early 1960s to 5,730 years, which meant that many calculated
dates in papers published prior to this were incorrect (the error in
the half-life is about 3%). For consistency with these early papers,
and to avoid the risk of a double correction for the incorrect
half-life, radiocarbon ages are still calculated using the incorrect
half-life value. A correction for the half-life is incorporated into
calibration curves, so even though radiocarbon ages are calculated
using a half-life value that is known to be incorrect, the final
reported calibrated date, in calendar years, is accurate. 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 14C, and because
no correction (calibration) has been applied for the historical
variation of 14C in the atmosphere over time.[note 2]
Simplified version of the carbon exchange reservoir, showing proportions of carbon and relative activity of the 14C in each reservoir[note 3]
variations in the 14C/12C ratio in the atmosphere, both geographically and over time; isotopic fractionation; variations in the 14C/12C ratio in different parts of the reservoir; contamination.
In the early years of using the technique, it was understood that it
depended on the atmospheric 14C/12C 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. Over time, however, discrepancies began
to appear between the known chronology for the oldest Egyptian
dynasties and the radiocarbon dates of Egyptian artefacts. Neither the
Atmospheric 14C, New Zealand and Austria. 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 14C in the Northern
Hemisphere. The date that the
Partial Test Ban Treaty
Coal and oil began to be burned in large quantities during the 19th
century. Both are sufficiently old that they contain little detectable
14C and, as a result, the CO
2 released substantially diluted the atmospheric 14C/12C ratio. Dating
an object from the early 20th century hence gives an apparent date
older than the true date. For the same reason, 14C 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 14C 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.
A much larger effect comes from above-ground nuclear testing, which
released large numbers of neutrons and created 14C. From about 1950
until 1963, when atmospheric nuclear testing was banned, it is
estimated that several tonnes of 14C were created. If all this extra
14C had immediately been spread across the entire carbon exchange
reservoir, it would have led to an increase in the 14C/12C ratio of
only a few per cent, but the immediate effect was to almost double the
amount of 14C in the atmosphere, with the peak level occurring in
about 1965. The level has since dropped, as this bomb pulse or "bomb
carbon" (as it is sometimes called) percolates into the rest of the
s a m p l e
s t a n d a r d
displaystyle delta ce ^ 13 C =left( frac left( frac ce ^ 13 C ce ^ 12 C right)_ sample left( frac ce ^ 13 C ce ^ 12 C right)_ standard -1right)times 1000
where the ‰ sign indicates parts per thousand. Because the PDB standard contains an unusually high proportion of 13C,[note 6] most measured δ13C values are negative.
North Ronaldsay sheep
Material Typical δ13C range
Marine plankton −22‰ to −17‰
C3 plants −30‰ to −22‰
C4 plants −15‰ to −9‰
Atmospheric CO 2 −8‰
Marine CO 2 −32‰ to −13‰
For marine organisms, the details of the photosynthesis reactions are
less well understood, and the δ13C values for marine photosynthetic
organisms are dependent on temperature. At higher temperatures, CO
2 has poor solubility in water, which means there is less CO
2 available for the photosynthetic reactions. Under these conditions,
fractionation is reduced, and at temperatures above 14 °C the
δ13C values are correspondingly higher, while at lower temperatures,
2 becomes more soluble and hence more available to marine
organisms. The δ13C value for animals depends on their diet. An
animal that eats food with high δ13C values will have a higher δ13C
than one that eats food with lower δ13C values. The animal's own
biochemical processes can also impact the results: for example, both
bone minerals and bone collagen typically have a higher concentration
of 13C than is found in the animal's diet, though for different
biochemical reasons. The enrichment of bone 13C also implies that
excreted material is depleted in 13C relative to the diet.
Since 13C makes up about 1% of the carbon in a sample, the 13C/12C
ratio can be accurately measured by mass spectrometry. Typical
values of δ13C have been found by experiment for many plants, as well
as for different parts of animals such as bone collagen, but when
dating a given sample it is better to determine the δ13C value for
that sample directly than to rely on the published values.
The carbon exchange between atmospheric CO
2 and carbonate at the ocean surface is also subject to fractionation,
with 14C in the atmosphere more likely than 12C to dissolve in the
ocean. The result is an overall increase in the 14C/12C ratio in the
ocean of 1.5%, relative to the 14C/12C ratio in the atmosphere. This
increase in 14C concentration almost exactly cancels out the decrease
caused by the upwelling of water (containing old, and hence 14C
depleted, carbon) from the deep ocean, so that direct measurements of
14C 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 440 years for
ocean surface water.
Libby's original exchange reservoir hypothesis assumed that the
14C/12C ratio in the exchange reservoir is constant all over the
world, but it has since been discovered that there are several
causes of variation in the ratio across the reservoir.
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. This exchange process brings14C from the atmosphere into the
surface waters of the ocean, but the 14C thus introduced takes a long
time to percolate through the entire volume of the ocean. 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). This effect is not uniform –
the average effect is about 440 years, but there are local deviations
of several hundred years for areas that are geographically close to
each other. The effect also applies to marine organisms such
as shells, and marine mammals such as whales and seals, which have
radiocarbon ages that appear to be hundreds of years old.
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. The atmospheric
14C/12C ratio is lower in the southern hemisphere, with an apparent
additional age of 30 years for radiocarbon results from the south as
compared to the north. This is probably because the greater surface
area of ocean in the southern hemisphere means that there is more
carbon exchanged between the ocean and the atmosphere than in the
north. Since the surface ocean is depleted in 14C because of the
marine effect, 14C is removed from the southern atmosphere more
quickly than in the north.
If the carbon in freshwater is partly acquired from aged carbon, such
as rocks, then the result will be a reduction in the 14C/12C ratio in
the water. 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. These rocks are usually so old that they
no longer contain any measurable 14C, so this carbon lowers the
14C/12C ratio of the water it enters, which can lead to apparent ages
of thousands of years for both the affected water and the plants and
freshwater organisms that live in it. This is known as the hard
water effect because it is often associated with calcium ions, which
are characteristic of hard water; other sources of carbon such as
humus can produce similar results. The effect varies greatly and
there is no general offset that can be applied; additional research is
usually needed to determine the size of the offset, for example by
comparing the radiocarbon age of deposited freshwater shells with
associated organic material.
It is common to reduce a wood sample to just the cellulose component
before testing, but since this can reduce the volume of the sample to
20% of its original size, testing of the whole wood is often performed
as well. Charcoal is often tested but is likely to need treatment to
Unburnt bone can be tested; it is usual to date it using collagen, the
protein fraction that remains after washing away the bone's structural
material. Hydroxyproline, one of the constituent amino acids in bone,
was once thought to be a reliable indicator as it was not known to
occur except in bone, but it has since been detected in
For burnt bone, testability depends on the conditions under which the
bone was burnt. If the bone was heated under reducing conditions, it
(and associated organic matter) may have been carbonized. In this case
the sample is often usable.
Shells from both marine and land organisms consist almost entirely of
calcium carbonate, either as aragonite or as calcite, or some mixture
of the two.
Preparation and size
Particularly for older samples, it may be useful to enrich the amount
of 14C in the sample before testing. This can be done with a thermal
diffusion column. The process takes about a month and requires a
sample about ten times as large as would be needed otherwise, but it
allows more precise measurement of the 14C/12C ratio in old material
and extends the maximum age that can be reliably reported.
Once contamination has been removed, samples must be converted to a
form suitable for the measuring technology to be used. Where gas
is required, CO
2 is widely used. For samples to be used in liquid
scintillation counters, the carbon must be in liquid form; the sample
is typically converted to benzene. For accelerator mass spectrometry,
solid graphite targets are the most common, although iron carbide and
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 (0.35 ounces) is typically
Accelerator mass spectrometry
Measuring 14C is now most commonly done with an accelerator mass spectrometer
For decades after Libby performed the first radiocarbon dating
experiments, the only way to measure the 14C in a sample was to detect
the radioactive decay of individual carbon atoms. In this
approach, what is measured is the activity, in number of decay events
per unit mass per time period, of the sample. This method is also
known as "beta counting", because it is the beta particles emitted by
the decaying 14C atoms that are detected. In the late 1970s an
alternative approach became available: directly counting the number of
14C and 12C atoms in a given sample, via accelerator mass
spectrometry, usually referred to as AMS. AMS counts the 14C/12C
ratio directly, instead of the activity of the sample, but
measurements of activity and 14C/12C ratio can be converted into each
other exactly. For some time, beta counting methods were more
accurate than AMS, but as of 2014 AMS is more accurate and has become
the method of choice for radiocarbon measurements. In addition
to improved accuracy, AMS has two further significant advantages over
beta counting: it can perform accurate testing on samples much too
small for beta counting; and it is much faster – an accuracy of
1% can be achieved in minutes with AMS, which is far quicker than
would be achievable with the older technology.
Libby's first detector was a
Simplified schematic layout of an accelerator mass spectrometer used for counting carbon isotopes for carbon dating
AMS counts the atoms of 14C and 12C in a given sample, determining the 14C/12C ratio directly. The sample, often in the form of graphite, is made to emit C− ions (carbon atoms with a single negative charge), which are injected into an accelerator. The ions are accelerated and passed through a stripper, which removes several electrons so that the ions emerge with a positive charge. The C3+ ions are then passed through a magnet that curves their path; the heavier ions are curved less than the lighter ones, so the different isotopes emerge as separate streams of ions. A particle detector then records the number of ions detected in the 14C stream, but since the volume of 12C (and 13C, needed for calibration) is too great for individual ion detection, counts are determined by measuring the electric current created in a Faraday cup. Some AMS facilities are also able to evaluate a sample's fractionation, another piece of data necessary for calculating the sample's radiocarbon age. The use of AMS, as opposed to simpler forms of mass spectrometry, is necessary because of the need to distinguish the carbon isotopes from other atoms or molecules that are very close in mass, such as 14N and 13CH. As with beta counting, both blank samples and standard samples are used. Two different kinds of blank may be measured: a sample of dead carbon that has undergone no chemical processing, to detect any machine background, and a sample known as a process blank made from dead carbon that is processed into target material in exactly the same way as the sample which is being dated. Any 14C 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 12CH 2 or 13CH. A 14C 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. Calculations Main article: Calculation of radiocarbon dates 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 most common standard sample material is oxalic acid, such as the HOxII standard, 1,000 lb of which was prepared by NIST in 1977 from French beet harvests. The results from AMS testing are in the form of ratios of 12C, 13C, and 14C, which are used to calculate Fm, the "fraction modern". This is defined as the ratio between the 14C/12C ratio in the sample and the 14C/12C ratio in modern carbon, which is in turn defined as the 14C/12C ratio that would have been measured in 1950 had there been no fossil fuel effect. Both beta counting and AMS results have to be corrected for fractionation. This is necessary because different materials of the same age, which because of fractionation have naturally different 14C/12C ratios, will appear to be of different ages because the 14C/12C ratio is taken as the indicator of age. To avoid this, all radiocarbon measurements are converted to the measurement that would have been seen had the sample been made of wood, which has a known δ13C value of −25‰. Once the corrected 14C/12C ratio is known, a "radiocarbon age" is calculated using:
A g e = − 8033 ⋅ ln ( F m )
displaystyle Age=-8033cdot ln(Fm)
The calculation uses Libby's half-life of 5,568 years, not the more
accurate modern value of 5,730 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.
Errors and reliability
The reliability of the results can be improved by lengthening the
testing time. For example, if counting beta decays for 250 minutes is
enough to give an error of ± 80 years, with 68% confidence, then
doubling the counting time to 500 minutes will allow a sample with
only half as much 14C to be measured with the same error term of 80
The stump of a very old bristlecone pine. Tree rings from these trees (among others) are used in building calibration curves.
The calculations given above produce dates in radiocarbon years: i.e. dates that represent the age the sample would be if the 14C/12C ratio had been constant historically. Although Libby had pointed out as early as 1955 the possibility that this assumption was incorrect, it was not until discrepancies began to accumulate between measured ages and known historical dates for artefacts that it became clear that a correction would need to be applied to radiocarbon ages to obtain calendar dates. 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. Hans Suess used this data to publish the first calibration curve for radiocarbon dating in 1967. The curve showed two types of variation from the straight line: a long term fluctuation with a period of about 9,000 years, and a shorter term variation, often referred to as "wiggles", with a period of decades. 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. These short term fluctuations in the calibration curve are now known as de Vries effects, after Hessel de Vries. 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.
The Northern hemisphere curve from INTCAL13. As of 2014 this is the most recent version of the standard calibration curve. The diagonal line shows where the curve would lie if radiocarbon ages and calendar ages were the same.
Over the next thirty years many calibration curves were published using a variety of methods and statistical approaches. These were superseded by the INTCAL series of curves, beginning with INTCAL98, published in 1998, and updated in 2004, 2009, and 2013. The improvements to these curves are based on new data gathered from tree rings, varves, coral, plant macrofossils, speleothems, and foraminifera. The INTCAL13 data includes separate curves for the northern and southern hemispheres, as they differ systematically because of the hemisphere effect; there is also a separate marine calibration curve. For a set of samples with a known sequence and separation in time such as a sequence of tree rings, the samples' radiocarbon ages form a small subset of the calibration curve. The resulting curve can then be matched to the actual calibration curve by identifying where, in the range suggested by the radiocarbon dates, the wiggles in the calibration curve best match the wiggles in the curve of sample dates. This "wiggle-matching" technique can lead to more precise dating than is possible with individual radiocarbon dates. Wiggle-matching can be used in places where there is a plateau on the calibration curve, and hence can provide a much more accurate date than the intercept or probability methods are able to produce. The technique is not restricted to tree rings; for example, a stratified tephra sequence in New Zealand, known to predate human colonization of the islands, has been dated to 1314 AD ± 12 years by wiggle-matching. The wiggles also mean that reading a date from a calibration curve can give more than one answer: this occurs when the curve wiggles up and down enough that the radiocarbon age intercepts the curve in more than one place, which may lead to a radiocarbon result being reported as two separate age ranges, corresponding to the two parts of the curve that the radiocarbon age intercepted. Bayesian statistical techniques can be applied when there are several radiocarbon dates to be calibrated. For example, if a series of radiocarbon dates is taken from different levels in a given stratigraphic sequence, Bayesian analysis can help determine if some of the dates should be discarded as anomalies, and can use the information to improve the output probability distributions. When Bayesian analysis was introduced, its use was limited by the need to use mainframe computers to perform the calculations, but the technique has since been implemented on programs available for personal computers, such as OxCal. Reporting dates Several formats for citing radiocarbon results have been used since the first samples were dated. As of 2014, the standard format required by the journal Radiocarbon is as follows. Uncalibrated dates should be reported as "<laboratory>: <14C year> ± <range> BP", where:
<laboratory> identifies the laboratory that tested the sample, and the sample ID <14C year> is the laboratory's determination of the age of the sample, in radiocarbon years <range> is the laboratory's estimate of the error in the age, at 1σ confidence. BP stands for "before present", referring to a reference date of 1950, so that 500 BP means the year 1450 AD.
For example, the uncalibrated date "UtC-2020: 3510 ± 60 BP" indicates that the sample was tested by the Utrecht van der Graaf Laboratorium, where it has a sample number of 2020, and that the uncalibrated age is 3510 years before present, ± 60 years. Related forms are sometimes used: for example, "10 ka BP" means 10,000 radiocarbon years before present (i.e. 8,050 BC), and 14C yr BP might be used to distinguish the uncalibrated date from a date derived from another dating method such as thermoluminescence. Calibrated 14C dates are frequently reported as cal BP, cal BC, or cal AD, again with BP referring to the year 1950 as the zero date. Radiocarbon gives two options for reporting calibrated dates. A common format is "cal <date-range> <confidence>", where:
<date-range> is the range of dates corresponding to the given confidence level <confidence> indicates the confidence level for the given date range.
For example, "cal 1220–1281 AD (1σ)" means a calibrated date for
which the true date lies between 1220 AD and 1281 AD, with the
confidence level given as 1σ, or one standard deviation. Calibrated
dates can also be expressed as BP instead of using BC and AD. The
curve used to calibrate the results should be the latest available
INTCAL curve. Calibrated dates should also identify any programs, such
as OxCal, used to perform the calibration. In addition, an article
in Radiocarbon in 2014 about radiocarbon date reporting conventions
recommends that information should be provided about sample treatment,
including the sample material, pretreatment methods, and quality
control measurements; that the citation to the software used for
calibration should specify the version number and any options or
models used; and that the calibrated date should be given with the
associated probabilities for each range.
Use in archaeology
A key concept in interpreting radiocarbon dates is archaeological
association: what is the true relationship between two or more objects
at an archaeological site? It frequently happens that a sample for
radiocarbon dating can be taken directly from the object of interest,
but there are also many cases where this is not possible. Metal grave
goods, for example, cannot be radiocarbon dated, but they may be found
in a grave with a coffin, charcoal, or other material which can be
assumed to have been deposited at the same time. In these cases a date
for the coffin or charcoal is indicative of the date of deposition of
the grave goods, because of the direct functional relationship between
the two. There are also cases where there is no functional
relationship, but the association is reasonably strong: for example, a
layer of charcoal in a rubbish pit provides a date which has a
relationship to the rubbish pit.
Contamination is of particular concern when dating very old material
obtained from archaeological excavations and great care is needed in
the specimen selection and preparation. In 2014,
Thomas Higham and
co-workers suggested that many of the dates published for Neanderthal
artefacts are too recent because of contamination by "young
As a tree grows, only the outermost tree ring exchanges carbon with
its environment, so the age measured for a wood sample depends on
where the sample is taken from. This means that radiocarbon dates on
wood samples can be older than the date at which the tree was felled.
In addition, if a piece of wood is used for multiple purposes, there
may be a significant delay between the felling of the tree and the
final use in the context in which it is found. This is often
referred to as the "old wood" problem. One example is the Bronze
Age trackway at Withy Bed Copse, in England; the trackway was built
from wood that had clearly been worked for other purposes before being
re-used in the trackway. Another example is driftwood, which may be
used as construction material. It is not always possible to recognize
re-use. Other materials can present the same problem: for example,
bitumen is known to have been used by some
Part of the Great Isaiah Scroll, one of the
In 1947, scrolls were discovered in caves near the
Dating methodologies in archaeology
^ The mean-life and half-life are related by the following equation:
= 0.693 ⋅ τ
displaystyle T_ frac 1 2 =0.693cdot tau
^ The term "conventional radiocarbon age" is also used. The definition of radiocarbon years is as follows: the age is calculated by using the following standards: a) using the Libby half-life of 5568 years, rather than the currently accepted actual half-life of 5730 years; (b) the use of an NIST standard known as HOxII to define the activity of radiocarbon in 1950; (c) the use of 1950 as the date from which years "before present" are counted; (d) a correction for fractionation, based on a standard isotope ratio, and (e) the assumption that the 14C/12C ratio has not changed over time. ^ The data on carbon percentages in each part of the reservoir is drawn from an estimate of reservoir carbon for the mid-1990s; estimates of carbon distribution during pre-industrial times are significantly different. ^ The age only appears to be 440 years once a correction for fractionation is made. This effect is accounted for during calibration by using a different marine calibration curve; without this curve, modern marine life would appear to be 440 years old when radiocarbon dated. ^ "PDB" stands for "Pee Dee Belemnite", a fossil from the Pee Dee formation in South Carolina. ^ The PDB value is 11.2372‰.
^ a b Taylor & Bar-Yosef (2014), p. 268.
^ a b Taylor & Bar-Yosef (2014), p. 269.
^ "Radiocarbon Dating – American Chemical Society". American
Chemical Society. Retrieved 2016-10-09.
^ "Radiocarbon Dating – American Chemical Society". American
Chemical Society. Retrieved 2016-10-09.
^ a b c d e f g h i j k l m n o p q Bowman (1995), pp. 9–15.
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The Wikibook Historical
RADON – database for European 14C dates
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