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Radiocarbon dating
A method of
obtaining age estimates on organic materials which has been used to date samples
as old as 75,000 years. The method was developed immediately following World War
II by Willard F. Libby and coworkers, and has provided age determinations in
archeology, geology, geophysics, and other branches of science.
Radiocarbon
(14C) determinations can be obtained on wood; charcoal; marine and fresh-water
shell; bone and antler; peat and organic-bearing sediments; carbonate deposits
such as tufa, caliche, and marl; and dissolved carbon dioxide (CO2) and
carbonates in ocean, lake, and ground-water sources. Each sample type has
specific problems associated with its use for dating purposes, including
contamination and special environmental effects. While the impact of 14C dating
has been most profound in archeological research and particularly in prehistoric
studies, extremely significant contributions have also been made in hydrology
and oceanography. In addition, beginning in the 1950s the testing of
thermonuclear weapons injected large amounts of artificial 14C (“bomb 14C”) into
the atmosphere, permitting it to be used as a geochemical tracer.
Basis of the Method
Carbon (C) has
three naturally occurring isotopes. Both 12C and 13C are stable, but 14C decays
by very weak beta decay (electron emission) to nitrogen-14 (14N) with a
half-life of approximately 5700 years. Naturally occurring 14C is produced as a
secondary effect of cosmic-ray bombardment of the upper atmosphere (Fig. 1). As
14CO2, it is distributed on a worldwide basis into various atmospheric,
biospheric, and hydrospheric reservoirs on a time scale much shorter than its
half-life. Metabolic processes in living organisms and relatively rapid turnover
of carbonates in surface ocean waters maintain 14C levels at approximately
constant levels in most of the biosphere. The natural 14C activity in the
geologically recent contemporary “prebomb” biosphere was approximately 13.5
disintegrations per minute per gram of carbon. See also: Cosmogenic nuclide; Isotope
Fig. 1 Generation, distribution, and decay of
14C.
To the degree
that 14C production has proceeded long enough without significant variation to
produce an equilibrium or steady-state condition, 14C levels observed in
contemporary materials may be used to characterize the original 14C activity in
the corresponding carbon reservoirs. Once a sample has been removed from
exchange with its reservoir, as at the death of an organism, the amount of 14C
begins to decrease as a function of its half-life. A 14C age determination is
based on a measurement of the residual 14C activity in a sample compared to the
activity of a sample of assumed zero age (a contemporary standard) from the same
reservoir. The relationship between the 14C age and the 14C activity of a sample
is given by the equation below, where t
is radiocarbon
years B.P. (before the present), λ is the decay constant of 14C (related to the
half-life t1/2 by the expression t1/2 = 0.693/λ), Ao is the activity of the
contemporary standards, and As is the activity of the unknown age samples.
Conventional radiocarbon dates are calculated by using this formula, an
internationally agreed half-life value of 5568 ± 30 years, and a specific
contemporary standard. Most laboratories define the contemporary standard value
by using one of the standards prepared by the U.S. National Bureau of Standards
[NBS; now known as the U.S. National Institute of Standards and Technology
(NIST)], or a standard with a known relationship to the NBS/NIST oxalic acid
preparations.
Measurement of
Radiocarbon
The naturally
occurring isotopes of carbon occur in the proportion of approximately 98.9% 12C,
1.1% 13C, and 10−10% 14C. The extremely small amount of radiocarbon in natural
materials was one reason why 14C was one of the isotopes which had been produced
artificially in the laboratory before being detected in natural concentrations.
The routine development of the radiocarbon method was made possible by the
development by Libby of a practical method of low-level counting. To detect the
very weak beta-decay characteristic of 14C, a means had to be devised to
introduce the sample directly into the sensitive volume of a detector. In all of
Libby's early work, the sample was converted to solid carbon (amorphous
elemental carbon) and deposited on a sleeve which fitted inside a screen-wall
type of Geiger counter. The counting rate of an unshielded screen-wall counter
was on the order of 500 counts per minute.
Since the
activity from 14C decay of a modern sample was expected to be about six or seven
counts per minute, the total background counting rate had to be radically
reduced. This was accomplished initially by placing the instrument in an iron
shield with 20-cm (8-in.) walls. This reduced the activity in the detector to
120 counts per minute, still unacceptably high. The final reduction was made
possible by enclosing the sample counter in a ring of smaller Geiger counters.
The sample counter and the outer guard ring were connected together
electronically so that any pulse from any of the outer Geiger tubes would
inactivate the sample counter for about 10−3 s. This anticoincidence system
reduced the background in the center detector to about five counts per minute.
With this
system, the maximum age that could be measured was about 23,000 years and
required the use of 10–12 g (0.35–0.42 oz) of carbon from sample materials.
Because of self-absorption of the weak betas in the sample, the efficiency of
the detector was only about 5%. Because of this and the susceptibility of the
carbon black to contamination from airborne radioactive fallout, the solid
carbon technique was replaced by either gas counters or liquid scintillation
systems. See also: Low-level
counting; Radioactivity
Gas counters
In the early
1950s, both proportional (Fig. 2) and Geiger gas counters were employed in 14C
work, using carbon dioxide, carbon disulfide, acetylene, methane, or ethane as
counting gases. As in the case of the solid carbon system, the center counter
containing the sample was surrounded by individual Geiger tubes or an annular or
continuous ring guard, all housed within an iron or lead shield assembly.
Efforts to reduce the background values in gas detectors have resulted in
various types of experimental arrangements, including the location of counters
in underground vaults. In such underground facilities, the contribution of the
meson flux, the major contributor to the background rate, can be significantly
reduced. Because of the 90–95% efficiency in most gas detector systems, the
typical maximum age limits were extended to 40,000–60,000 years, depending on
the experimental configuration, including the volume of the detectors and the
level of the background count rates in specific detectors. Isotopic enrichment
of sample gases permits the maximum age attainable to be extended several
additional half-lives. In general, sample-size requirements with gas detectors
were reduced from that required with the solid carbon method—special systems
being designed to permit the measurement of a sample with as little as 0.1 g
(3.5 × 10−3 oz) of carbon. See
also: Geiger-Müller counter; Ionization chamber; Meson
Fig. 2 Large-volume proportional counter, which
is used for carbon-14 measurements. The outer shield is closed by rolling doors.
The sample is introduced into the radiation counter in the form of carbon
dioxide gas. (Geochemical Laboratory, Lamont-Doherty Geological Observatory,
Liquid scintillation
systems
Current liquid
scintillation systems involve the conversion of samples to benzene. The addition
of a scintillator chemical allows beta-decay events to be monitored by
photomultiplier tubes. Earlier liquid scintillation systems used for 14C
measurements generally required larger samples. However, currently the amounts
required are comparable to gas counting systems. Continuing developments in
liquid scintillation technology for low-level measurement have provided the
ability to monitor counter performance in much greater detail than typically is
possible in gas counting, and have also resulted in reduced background values.
In these systems, the maximum ages that can be measured can be extended beyond
that possible with typical gas systems.
Direct detection
Both of the
conventional decay counting methods share a common problem in that they employ
an inherently inefficient means of monitoring 14C concentrations in samples. In
1 g of modern carbon, for every decay per minute there are about 4 × 109 atoms
of 14C. Counting methods have been developed which employ particle accelerators
as very sensitive mass spectrometers, thus counting the 14C atoms directly.
It has long been
recognized that if the 14C atoms could be detected directly, rather than by
waiting for their decay, smaller samples could be used for dating and older
dates could be measured. A simple hypothetical example to illustrate this point
is a sample containing only one atom of 14C. To measure the age (that is, the
abundance of 14C), the sample can be placed into a mass spectrometer and that
atom counted, or the sample can be placed into a Geiger counter and counted,
requiring a wait on the average of 8000 years (the mean life of 14C) for the
decay. In practice, neither the atoms nor the decays can be counted with 100%
efficiency, but the huge advantage for atom counting remains.
Until 1977,
attempts at direct atom counting for 14C and other natural radioisotopes failed,
because of the extremely low concentration of 14C. Ordinary mass spectrometers
could not see the tiny 14C signal in the background of other atoms and molecules
in the sample. Even trace amounts of nitrogen would swamp the 14C signal, since
14N forms ions with nearly identical mass and identical charge to that of the
14C atoms.
The technique
for detecting 14C atoms [and other natural radioisotopes such as tritium,
beryllium-10 (10Be), and chlorine-36 (36Cl)] is a combination of mass
spectrometry and accelerator technology, called accelerator mass spectrometry
(AMS). The approach was first demonstrated using a cyclotron. This accelerator,
which sent particles along a spiral trajectory, was used as an ultrasensitive
mass spectrometer to distinguish ionized carbon isotopes by their charge-to-mass
ratio. Detecting 14C by this means was possible, but consistent results proved
difficult to achieve despite years of effort. See also: Mass spectrometry
Another type of
AMS technology uses a tandem electrostatic accelerator (Fig. 3). The device
employs two stages. First, a negative ion beam is accelerated and passed through
a stripper, which removes the electrons, converting the beam to positive ions.
Then the particles are further accelerated. The stripping process breaks up
molecules of mass 14, which would otherwise interfere with the detection of 14C,
and the negative ion beam eliminates 14N since there are no known stable
negative nitrogen ions. Almost all AMS 14C applications currently use tandem
accelerators to accomplish routine measurements. See also: Particle accelerator
Fig. 3 Accelerator mass spectrometry system for
the direct detection of 14C atoms. (Center for Mass Spectrometry,
The advent of
AMS technology in the late 1970s brought about an enormous boost in detection
efficiency that promised three important advantages for 14C dating. First the
amount of carbon required was reduced from grams to milligrams. Second, counting
times were reduced from days, weeks, or even months to minutes. Finally, it was
initially thought that the detection sensitivity would increase so that the
maximum age datable with 14C might be extended to 100,000 years. However,
sensitivity is limited by very small amounts of contamination introduced during
sample preparation. Much of this contamination stems from the requirement in
most laboratories that samples be converted to graphitic carbon for measurement.
In routine operation, current AMS technology can measure between 40,000 and
50,000 years, and rarely, 60,000 years.
The ability to
use milligram (rather than gram) samples is very important for dating. Certain
irreplaceable objects (for example, parchments, cloth, and chips of wood) would
have had to be destroyed in order to extract the gram of carbon required for a
date; for the accelerator method, only a small piece of the artifact is
required. In addition, the ability to date by using only milligrams of carbon
allows careful selection of the sample used. Any part of the object which may
have been contaminated by modern carbon can be ignored; small seeds trapped in
the object, or even specific amino acid compounds which are less likely to come
from modern carbon contamination, can be selected.
Despite the
sensitivity of the accelerator technique, decay dating will probably continue to
be used for 14C dating when gram amounts of carbon are available. However, the
technology of atomic mass spectrometry continues to be developed and will
increasingly be used for routine 14C analysis. The accuracy of 14C values based
on atomic mass spectrometry has become essentially comparable to that obtained
with decay counting.
Accuracy of Radiocarbon
Determinations
A measurement of
the 14C content of an organic sample will provide an accurate determination of
the sample's age if it is assumed that (1) the production of 14C by cosmic rays
has remained essentially constant long enough to establish a steady state in the
14C/12C ratio in the atmosphere, (2) there has been a complete and rapid mixing
of 14C throughout the various carbon reservoirs, (3) the carbon isotope ratio in
the sample has not been altered except by 14C decay, and (4) the total amount of
carbon in any reservoir has not been altered. In addition, the half-life of 14C
must be known with sufficient accuracy, and it must be possible to measure
natural levels of 14C to appropriate levels of accuracy and precision. Studies
have shown that the primary assumptions on which the method rests have been
violated both systematically and to varying degrees for particular sample types.
Several approaches have been developed to provide calibration and corrections of
conventional 14C values. The basis of the calibration and correction procedures
will be discussed in the context of a brief review of the assumptions of the
method.
Constancy in radiocarbon production
rates
Carbon-14
determinations on known-age samples have revealed systematic discrepancies in
the 14C time scale. The first hint of such anomalies came from early 14C
measurements on Egyptian archeological materials. Samples which, on historical
grounds, should have dated to the early part of the third millennium B.C.
yielded 14C values some 700–800 years too young. Carbon-14 determinations
carried out on dendrochronologically dated wood, periglacial varves, and lake
sediments confirmed the fact that there have been systematic variations in 14C
values over time. The data which first contributed most directly to the study of
these anomalies was the dendrochronological time scale provided by the
bristlecone pine (Pinus longaeva), from the White Mountains of east-central
California, developed by C. W. Fergusson. His data provide an unbroken tree-ring
series back to almost 6700 B.C. An independently developed bristlecone tree-ring
chronology from a different locality in the southern portion of the White
Mountains, developed by V. C. La Marche and T. P. Harlan, supports the accuracy
of the Fergusson chronology at least as far back as about 3500 B.C. Carbon-14
determinations on bristlecone pine as well as tree-ring-dated samples from the
sequoia (Sequoia gigantea) and European oaks (Quercus spp.) have been undertaken
by a number of laboratories, and 14C determinations on these samples provide
data over the last 10,000 years.
See also: Dendrochronology; Varve
Main trend
Upon examination
of the data in Fig. 4 and other similar plots, it becomes apparent that
radiocarbon years and calendar years are not necessarily equivalent. If such had
been the case, all of the data points plotted on Fig. 4 would lie along the
horizontal 0 line. In fact, some of the points lie above the line, indicating
that 14C values in these periods are too old. Conversely, those below the 0 line
are too young when compared to the tree-ring data. This plot indicates that
there are two major components to the deviations. The first is a general
main-trend secular variation phenomenon (the curved line) exhibiting during the
Holocene, a sine-wave function with an apparent period of about 8500–9000 years,
with a maximum deviation of about 800 years, approximately 8000 years ago.
Fig. 4 Secular variation/major trend;
relationship between radiocarbon and dendrochronological age of wood samples.
(After J. Klein et al., Calibration of radiocarbon dates, Radiocarbon,
24(2):103–150, 1982)
The
characteristic of the secular variation anomalies in the period before about
10,000 years ago cannot, at present, be documented by tree-ring/14C data. It has
been argued, however, that the major part of the effect may be estimated by
examining the record of the Earth's dipole geomagnetic field over time.
Variations in the intensity of the dipole field modulate the cosmic-ray flux in
the vicinity of the Earth. An increase in the field strength, for example,
diverts more of the cosmic-ray particles away from the Earth, resulting in a
decrease in the production of 14C.
Geophysicists
have collected data which document changes in the intensity of the Earth's
dipole field for the last few hundred thousand years. Because of the apparent
inverse relationship between the intensity of the field and the 14C production
rate, it would, in theory, be possible to extrapolate the maximum and minimum
secular variation deviations back to the limit of the 14C method. Unfortunately,
such data are not yet as precise as might be wished. However, comparisons of 14C
ages with uranium-thorium (U-Th) ages obtained on cores from coral deposits
support conclusions based on the 14C/tree-ring data up to the limit of the
current dendrochronological data. Uranium-thorium values can be used to continue
to examine the 14C deviations over the last 30,000 years. Such data indicate
that radiocarbon ages earlier than 10,000 years B.P. continue to be
systematically younger than U-Th ages, with a maximum difference of about 3500
years approximately 20,000 years ago.
See also: Paleomagnetism; Rock magnetism
De Vries effect
In addition to
the long-term secular variation phenomenon, the bristlecone pine data have
revealed the presence of high-frequency components to the variation in 14C
activity. These short-term oscillations or wiggles have sometimes been called
the De Vries effect after the pioneering Dutch researcher, Hessel de Vries, who
was one of the first to call attention to the existence of systematic anomalies
in 14C values. Although almost all investigators concerned with the issue have
agreed that the tree-ring/14C data definitely reveal the presence of a number of
short-term perturbations, the frequency and magnitude of earlier episodes during
the Pleistocene, (that is, > 10,000 years B.P.) have not been resolved.
Likewise, there are uncertainties as to the causes of the De Vries effect,
although variation in solar activity (heliomagnetic effects) has been seen as an
important factor.
Calibration of radiocarbon
dates
The existence of
main trend and De Vries deviations has important implications in the
interpretation of 14C determinations. The long-term variations result in the
necessity to calibrate conventional 14C dates in terms of the known variation
between radiocarbon time and real or calendar time as documented by the
dendrochronological/14C values. The magnitude of the calibration varies
depending on from what time period a sample is derived. For the period back to
about 1000 B.C., corrections required by virtue of the secular-variation
deviations do not exceed about 150 years. Prior to 1000 B.C., the magnitude of
the correction steadily increases. By using data such as those presented in Fig.
4, various approaches have been developed to “calibrate” radiocarbon values. In
this context, calibration involves taking a 14C age value expressed as a
conventional radiocarbon date and adding or subtracting the number of years
required to bring the conventional age into conformity with the 14C
determinations on known-age tree-ring-dated samples.
The
documentation of the presence of the De Vries or short-term anomalies has
introduced a second problem in the calibration of 14C values. Periods of rapid
change in the 14C content of the atmosphere result in situations where a single
14C value may reflect two or more points in real time. The characteristics of
the short-term anomalies are illustrated in Fig. 5. During periods of
particularly rapid change in 14C activity, it is usually not possible to use 14C
data to document temporal intervals in units of less than a few hundred years.
Thus the dendrochronologically based calibration data can be used to identify
the general degree of deviation of 14C values from real time and also the degree
of maximum precision which is possible for specific temporal intervals.
Fig. 5 Secular variation/De Vries effect.
Example of short-term variations in 14C activity; detail of De Vries effects
using high-precision measurements for period of approximately 4500–5100 14C
years B.P. (After A. F. M. de Jong and W. G. Mook, Medium-term atmospheric 14C
variations, Radiocarbon, 22(2):267–272, 1980)
The impact of
the De Vries effects on the precision of 14C values, as applied to archeological
and historical problems, must be considered. The need to take into account these
shorter-term variations have, for example, been demonstrated in the evaluation
of 14C values on twelfth- and fourteenth-century European medieval archeological
materials. See also: Archeological
chronology
Lack of geographical and altitude
variations
Concern has been
expressed as to whether variation in the 14C content of wood samples taken from
a small number of localities in the Northern Hemisphere can be used to document
worldwide secular variation effects. This question has been specifically
answered as a result of studies of 14C concentrations in tree rings from
Studies have
also shown the lack of any significant altitude effect on 14C concentrations in
wood. A concern had been expressed that solar protons of appropriate energies
would interact with nitrogen to form 14C directly in wood samples growing at
high elevations. The projected effect would be to inflate the 14C content of a
sample so that the 14C age would appear to be significantly younger than its
true age. However, calculations indicate that the maximum effect that could be
obtained, assuming the most advantageous parameters, would not exceed 40 years.
Other evidence suggests the actual effect is much less. The bristlecone pine
wood of the
Variability in radiocarbon
distribution
An important
feature of the 14C method is its potential to provide directly comparable age
determinations on a worldwide basis for a wide variety of organic samples. For
this potential to be realized, 14C, following its production, has to be mixed
rapidly and completely throughout all of the carbon-containing reservoirs on a
time scale not exceeding a few tens of years. To the degree that such conditions
prevail, the contemporary 14C content of all organic samples will be essentially
identical. It was quickly determined that such is not the case. The initial 14C
content of samples could be significantly affected as a result of environmental
conditions. A classic illustration of the problem was the discovery of living
organisms from a fresh-water lake exhibiting 14C ages of approximately 2000
years. In this case a large percentage of the carbon used by the organisms was
derived from dissolved CO2 from the limestone bed of the lake. The fictional age
of the modern samples had been produced as a result of the dilution of
contemporary 14C activity by “dead” carbon (that is, containing no 14C) from the
limestone. Another example is provided from trees growing in active volcanic
areas. The CO2 emitted during volcanic discharges is characteristically depleted
of its 14C. Living trees exhibiting apparent ages as much as 1000 years from
such environments have been reported.
One effect of
the recognition that the geochemical environment of a sample can affect its
initial 14C concentration has been to cast doubt on the reliability of
particular types of samples. The use of shells in 14C studies has been affected,
since a tradition arose that their use should be discouraged. Terrestrial shells
(gastropods) from most fresh-water environments generally merit this negative
evaluation, since they typically take up carbonate which is not in equilibrium
with atmospheric 14C. The reputation of marine shells was adversely affected
primarily as a result of early experiences with shells taken from several
archeological sites along the Peruvian coast. Marine shell samples were found to
have 14C values that exhibited an apparent age as much as 900 years greater than
that of charcoal samples assumed to have been deposited contemporaneously. It
was therefore assumed that marine shells would consistently yield anomalous
values.
Subsequent
studies showed that marine shells can yield generally acceptable values if the
conventional 14C values can be corrected for upwelling effects. By examining 14C
concentrations in shells collected alive in the period before nuclear testing
contributed bomb 14C, it was determined that many marine shells exhibited
apparent ages ranging as high as 1200 years. Part of the reason has to do with
the fact that ocean water depleted of 14C by long residence times in the deeper
parts of the ocean is periodically upwelled or brought to the surface and mixed
with surface ocean water. The effect is to dilute the contemporary 14C activity
of the surface ocean near the westward-facing continental margins, resulting in
a spurious apparent age for the organisms utilizing surface-ocean-water
carbonates. Shells growing in locations adjacent to the outlets of major river
systems whose water is depleted of 14C as a result of exchange with limestone or
other carbonate-bearing rocks can also give spurious apparent ages. This is
probably the explanation for the false ages exhibited by shells growing in the
Gulf of California (Colorado River discharge) and the northern part of the Gulf
of Mexico (
Upwelling and
other reservoir effects are highly variable depending on location and specific
environmental conditions. For the western coasts of North and
Problems similar
to those associated with marine shells arise for a number of sample types and
geochemical environments where contemporary samples may not be in equilibrium
with the atmosphere. In each case, empirically derived values for the
contemporary standard must be obtained for each sample type or locality, or
both. In practice, this is accomplished by determining the degree of deviation
from whatever contemporary standard is used to define modern or “zero 14C age”
samples. For example, specific values are required for marine shells from
different oceanic regions, for fresh-water shells in specific terrestrial
environments, and for
Variability in carbon isotope
ratios
For the 14C
method, the basic physical measurement used to index time is the 14C/12C ratio.
However, carbon has three naturally occurring isotopes. Variation in this ratio
can be effected by influences other than the decay of 14C. The most common
problem occurs when carbon-containing compounds not indigenous to the original
samples are physically or chemically introduced into the sample matrix resulting
in the contamination of the sample. Usually less difficult to deal with are
fractionation effects in which a variation in the stable carbon ratio translates
into a change in the 14C/12C ratio.
Contamination
The sources and
effects of the introduction of foreign organics into samples are complex; they
depend on the nature and condition of the sample materials, the characteristics
of the environment to which the samples were exposed, and the period of time
over which the exposure occurred. Precautions exercised to avoid contamination
effects are unique to each sample type and source locality. A series of
procedures to remove potential contaminants in samples has been established by
research laboratories. Most sample preparation techniques are concerned with
completely removing what is assumed to have not been present when the original
sample died or was removed from exchange with its carbon reservoir. Samples such
as wood and charcoal, which can be subjected to treatment with strong acids and
bases to facilitate the removal of absorbed carbonates and soil humic and fulvic
acids and other soluble soil organic matter, are preferred. Less desirable are
cases where it is difficult to distinguish between contamination and the
original sample as with various types of carbonate samples, such as tufa and
caliche.
It is usually
possible to infer the effect of known contamination effects on a given sample in
terms of the direction that the age change will take for a given type of
contamination, but the magnitude of the errors can be calculated only if the
true age of the original sample, the age of the contaminant, and the percentage
contribution of the contaminant are all known. Usually this is difficult to
determine. However, with few exceptions, problems of contamination for samples
with ages of less than about 10,000 years can be solved, usually by applying
standard pretreatment approaches developed by 14C laboratories. For materials
with expected ages in excess of 10,000 years, sample contamination problems
typically become more serious, and laboratories must exercise even more rigorous
care in the pretreatment processes.
Fractionation effects
While all the
isotopes of carbon follow the same chemical or physical pathway, the rate at
which this occurs varies as a function of their difference in mass. The
pioneering studies of Harmon Craig pointed to the need to consider variations in
the stable isotope ratio (13C/12C) of samples to obtain precise 14C values.
Variations equivalent to up to several hundred years can result if 14C values
are not standardized in light of 13C/12C ratios. Fortunately, no significant
fractionation effects are usually observed in standard sample materials such as
charcoal or wood. Problems arise, however, when it is necessary to compare 14C
values from a variety of sample types such as grasses, grains, seeds,
succulents, and marine carbonates, as well as standard terrestrial organics. In
such cases, it is necessary to use the stable isotope ratios to correct the 14C
values onto a common scale.
Variability in amount of
carbon
In addition to
the variation in production and distribution of 14C over time and within
portions of various carbon reservoirs, variations may result from situations
where carbon not in equilibrium with the contemporary standard values is added
or removed from any reservoir. Two instances are well documented since they
occurred within the last century as a result of human intervention in the carbon
cycle. The first, beginning in the middle of the nineteenth century, is known as
the industrial or Suess effect. The combustion of fossil fuels added enough
“dead” 14C to the atmosphere to result in the reduction by about 3% in
biospheric 14C activity. In the more recent atomic bomb or Libby effect,
relating to the detonation of thermonuclear devices in the atmosphere in the
early 1950s, large amounts of artificial 14C were produced, almost doubling the
amount of 14C in the terrestrial biosphere. When combined with the
late-eighteenth-century De Vries excurses, the Suess effect makes it difficult
to distinguish 14C concentrations within the last two centuries. This is one of
the reasons why laboratories generally use 100 or 150 years as the minimum age
which can be cited. It also explains why laboratories cannot use modern wood as
a contemporary reference.
Half-life of
radiocarbon
The fundamental
constant which permits the conversion of a 14C/12C ratio into an age value is
the half-life or decay constant of 14C. Initially in the development of the
method, Libby and collaborators used the value 5720 ± 47 as the half-life
figure, but soon adopted the weighted average of three independently obtained
measurements. The average value was 5568 ± 30 and this became identified as the
Libby half-life. In 1962, at the 5th Radiocarbon Dating Conference at
The issue of the
correct half-life for 14C has lost a considerable amount of its significance
because of the discovery and documentation of similiar variation and De Vries
effects. The existence of dendrochronologically documented relationships between
14C age and calendar age, for samples up to about 10,000 years old, enables
researchers to circumvent the problem of the actual 14C half-life and proceed to
calibrate these 14C age values directly.
Statistical and contextual
uncertainties
Most 14C
determinations are expressed in the form: age value (in 14C years B.P.) ±
statistical uncertainty. The age value is calculated by using the equation
previously presented. The measurement uncertainty results from statistical
considerations inherent in the random decay process characteristic of all
radioactive isotopes. A date, for example, of 5600 ± 80 14C years B.P. reflects
the fact that the count rate of the sample is about 50% of the modern reference
standard (that is, it has decayed for a period of about one Libby half-life) and
the age value is known to about 1% or 80 years. Statistical uncertainties in 14C
work are usually cited in terms of one standard deviation errors. The expression
5600 ± 80 is a shorthand manner of stating that there are two chances out of
three that the age equivalent of the counting rate for this sample will be
contained within the range 5520 to 5680. An accurate statement of the results of
a 14C determination must include a listing of the measurement uncertainty. In
addition, some laboratories increase this value to take into consideration
changes that affect counting conditions, such as drift of electronic equipment
and changes in barometric pressure. Statistical errors are not cited only when
the counting rate of a sample is statistically indistinguishable from the
background counting rate for the counter being used. The result is expressed as
a minimum or infinite value by stating that the age is “greater than” a limit
imposed by the characteristics of the counting system being employed (for
example, >40,000).
The processing and counting of a sample to determine its 14C age is a challenging analytical procedure. However, a technically correct value which has been carelessly collected may be scientifically worthless. Often the significance and importance of a 14C determination are only as good as the attention to detail which went into documenting the geological, historical, or archeological context of the sample. It is important to be aware of what a 14C date does and does not indicate. A 14C value provides a temporal index of when the sample was removed from its reservoir. For example, a 14C determination on a piece of charcoal or wood provides an age value for the tree rings which make up the sample. A 14C date taken on a piece of wood taken from a beam excavated from a ruined structure may indicate the time when the building was constructed if the sample was taken from the outside rings of the tree used as the source of the timber and if the timber itself did not happen to be reused from an earlier structure. Problems such as these often confront geologists and archeologists as they attempt to critically interpret the dating evidence provided by the 14C method.
- E. Bard et al., Radiocarbon calibration by means of mass spectrometric 230Th/234U and 14C ages of corals: An updated base including samples from Barbados, Mururoa and Tahiti, Radiocarbon, 40:1085–1092, 1998
- D. Polach, Radiocarbon Dating Literature: The First 21 Years, 1947–1968, 1988
- M. Stuiver, J. von der Plicht, and A. Long (eds.), Calibration Issue INTCAL 98, Radiocarbon, 40:1–1160, 1998
- R. E. Taylor, Fifty years of radiocarbon dating, Amer. Scientist, 88:60–67, 2000
- R. E. Taylor, Radiocarbon dating, in R. E. Taylor and M. J. Aitken (eds.), Chronometric Dating in Archaeology, 1997
- R. E. Taylor, Radiocarbon Dating: An Archaeological Perspective, 1987
- R. E. Taylor, R. Kra, and A. Long (eds.), Radiocarbon After Four Decades: An Interdisciplinary Perspective, 1992
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