Beta Analytic Inc. (Headquarters)
4985 SW 74 Court
Miami, Florida 33155
USA
Tel: (1) 305-667-5167
Fax: (1) 305-663-0964
Email: lab@radiocarbon.com

Beta Analytic Limited
London Bioscience Innovation Centre
2 Royal College Street
London NW10NH
United Kingdom
Tel: (44) 207 617 7490
Fax: (44) 207 160 5350
Email: lab@radiocarbon.com

Introduction to Radiocarbon Determination by the
Accelerator Mass Spectrometry Method
AMS Counting
There are three principal isotopes of carbon which occur naturally - 12C, 13C (bothstable) and 14C (unstable or radioactive). These isotopes are present in thefollowing amounts 12C - 98.89%, 13C - 1.11%, and 14C - 0.00000000010%. Thus,
one carbon-14 atom exists in nature for every 1,000,000,000,000 or (1 in atrillion) carbon-12 atoms in living material. The radiocarbon method is based on
the rate of decay of the radioactive or unstable carbon isotope 14 (14C), which isformed in the upper atmosphere through the effect of cosmic ray neutrons upon
nitrogen-14.
The reaction is:14

N + n => 14C + p(Where n is a neutron and p is a proton)
The 14C formed is rapidly oxidized to 14CO2 and enters the earth's plant andanimal life ways through photosynthesis and the food chain. The rapidity of thedispersal of 14C into the atmosphere has been demonstrated by measurementsof radioactive carbon produced from thermonuclear bomb testing. 14C also entersthe earth's oceans in an atmospheric exchange and as dissolved carbonate [theentire 14C inventory is termed the carbon exchange reservoir (Aitken, 1990)].
Plants and animals which utilize carbon in biological food chains take up 14C
during their lifetimes. They exist in equilibrium with the 14C concentration of theatmosphere; that is, the number of 14C atoms and non-radioactive carbon atoms
stays approximately the same over time. As soon as a plant or animal dies, theycease the metabolic function of carbon uptake; there is no replenishment ofradioactive carbon, only decay.
Libby, Anderson and Arnold (1949) were the first to measure the rate of thisdecay. They found that after 5568 years, half the 14C in the original sample will
have decayed and after another 5568 years, half of that remaining material will
have decayed, and so on (see figure 1 below). The half-life (t 1/2) is the namegiven to this value which Libby measured at 5568±30 years. This became known
as the Libby half-life. After 10 half-lives, there is a very small amount ofBeta Analytic

radioactive carbon present in a sample. At about 50,000-60 000 years, then, thelimit of the technique is reached (beyond this time, other radiometric techniquesmust be used for dating). By measuring the 14C concentration or residual
radioactivity of a sample whose age is not known, it is possible to obtain the
count rate or number of decay events per gram of Carbon. By comparing thiswith modern levels of activity (1890 wood corrected for decay to 1950 AD) andusing the measured half-life, it becomes possible to calculate a date for the deathof the sample.
As 14C decays it emits a weak beta particle (b), or electron, which possesses anaverage energy of 160keV. The decay can be shown:
14

C => 14N + b

Thus, the 14C decays back to 14N. There is a quantitative relationship between
the decay of 14C and the production of a beta particle. The decay is constant but
spontaneous. That is, the probability of decay for an atom of 14C in a discrete
sample is constant, thereby requiring the application of statistical methods for the
analysis of counting data.
It follows from this that any material which is composed of carbon may be dated.
Herein lies the true advantage of the radiocarbon method – it is able to beuniformly applied throughout the world on any material that contains residualcarbon.
Because of this, laboratories from around the world are producing radiocarbon
assays for the scientific community. The 14C technique has been and continues
to be applied and used in many different fields including hydrology, atmosphericscience, oceanography, geology, palaeoclimatology, archaeology, biomedicine
and materials science.
The Accelerator Mass Spectrometry method of direct 14C isotope counting was
first performed in 1977 by scientific research teams at Rochester and Toronto,
the General Ionex Corporation, and soon after additional measurements werecarried out at Simon Fraser and McMaster Universities (Gove, 1994).
Radiocarbon dating using Accelerator Mass Spectrometry (AMS) differs from the14

decay counting methods in that the amount of C in the sample is measured
directly, rather than by waiting for the individual radioactive decay events tooccur. This makes the technique 1,000 to 10,000 times more sensitive thandecay counting. The enhanced sensitivity is achieved by accelerating sample
atoms as ions to high energies using a particle accelerator and using nuclear
particle detection techniques. Additionally because of the increased sensitivity,
counting times are greatly reduced (minutes to hours, instead of days) and submilligram sample sizes can be routinely measured. Because of this, a greatly2Beta Analytic

increased range of sample types and sizes can now be measured that previously
gas-proportional or LSC counting techniques could not.
14

In the C AMS technique, the element of interest (sample carbon) is chemicallyseparated from the original sample, converted to graphite, pressed into acathode (sample target holder) where it forms a solid graphite plug or layer, andis then placed into a sputter ion source of an accelerator.
This methodology outlines the general graphite sample preparation (chemistry)and AMS counting procedures at Beta Analytic Inc. for organic and carbonate
samples. Through this process, the CO2 produced from carbonaceous raw
materials is cryogenically purified (separated from non-combustible gases) andreduced to solid graphite for measurement in an AMS.
Conversion of Sample Carbon to Graphite
Sample Pretreatment
Each sample must first be pretreated or the material of interest isolated to ensurethat only the primary carbon of interest will be analyzed. As many different typesof carbon-containing compounds are present, different pretreatment regimes
have been developed to concentrate and isolate the particular carbon fraction ofinterest prior to dating (See Pretreatment Glossary – Standard Pretreatment
Protocols at Beta Analytic).
CO2 Generation
The pretreated sample carbon is first oxidized to CO2 either by combustion in an
Oxygen stream or through direct reaction with reduced Cupric Oxide (metal wire /powder) for organics or through acid hydrolysis for carbonates.
Combustion OR Acid Hydrolysis
Organic + O2 + ∆H => CO2 Carbonate + H3PO4 => CO2(Sample) (900°C) (Sample)
Organic + CuO + ∆H => CO2(Sample) (900°C)
The CO2 generated is then cryogenically purified by removing water vapor andany non-combustible/condensable gases by passing through a series of dry-ice /Methanol water traps (~ -78°C) and depending on the sample type a series ofliquid Nitrogen / Pentane slush traps (~ -129°C).

3

Beta Analytic

Conversion of CO2 to Graphite (Graphitization reaction)
To produce the small amounts of elemental carbon from CO2 for measurement in
an AMS, we use the Bosch reaction (Manning and Reid, 1977) that is a chemical
reaction between carbon dioxide and hydrogen that produces elemental carbon(graphite), water and heat.
The reaction takes place as two successive reductions; first to carbon monoxide
and then to carbon, which permeates and adheres to the surface of the Cobalt
powder (catalyst). More details on graphitization techniques and catalysts can befound in Vogel et al, 1984.
The overall reaction is as follows:(550 to 650°C)
CO2 (g) + 2 H2 (g) → C(s) + 2 H2O (l)(Co Catalyst)
However, the above reaction is actually the result of two separate reactions. The
first reaction - the water-gas shift reaction is a fast one:
CO + H → CO + H O
2

2

2

The second reaction controls the overall reaction rate:
CO + H → C + H O
2

2

During the graphitization procedure, cryogenically cleaned, dry CO2 produced
from the combustion or acid hydrolysis is transferred cryogenically (with Liquid
Nitrogen @ -196°C)* to a specially designed graphitization cell under vacuum
that is composed of Vycor glass, Pyrex glass and Stainless Steel (modified
design after Lloyd et al, 1991).
* NOTE: Prior to the introduction of the CO2 over the cobalt catalyst in the graphitization cell, thecobalt is pre-conditioned to remove any carbon contaminants by being evacuated then purged
with Hydrogen multiple times and finally reduced over ¾ atmosphere of hydrogen for 1-2 hours at
550°C to 650°C.

The pre-conditioned graphitization cell contains either a 4.5 mg or 3.5 mg aliquotof Cobalt metal powder (a mixture of 85% Alfa Aesar -400 mesh, and 15% Alfa
Aesar Puratronic -22 mesh) that is specific to the size of the sample being
graphitized.
Based on the amount of CO2 generated during the combustion, the CO2 iscryogenically transferred to a graphitization cell containing and amount of Cobalt

4

Beta Analyticthat is 3x greater in weight than the amount of carbon (3:1 ratio), then an aliquotof Ultra-Pure Research Grade Hydrogen is added such that the amount ofHydrogen is 3x greater in volume as the sample CO2 (3:1 ratio). The
graphitization cell is then placed into a 550°C to 650°C oven for a period of 10-12
hours while the water produced during the graphitization is continuously
removed, via a cold finger that is part of the graphitization cell, by a dry-ice
Methanol slush (-78°C).
After 10-12 hours have passed, the pressure in the graphitization cell isinspected to ensure that the reaction has gone to completion (that the CO2 hasbeen converted to graphite with a minimum yield falling in the range of 80-100%).
If the reaction has not reached completion, the graphitization cell is returned tothe oven, and reacted again for a period of 4-10 hrs. If the reaction does notreach an 80-100% completion following the second graphitization attempt, thesample is analyzed again starting from combustion.
If the graphitization has gone to completion, the graphitization cell is placed
under a vacuum and allowed to warm to room temperature causing all water
vapor formed during the graphitization reaction to be pumped away, leaving thegraphite dry. The graphite is then purged 1x with ultra-pure (99.999) Argon whichhas passed through an ascarite molecular sieve to remove any CO2 and asilicone drying agent to remove any water vapor. This is then followed bypumping to a vacuum and then purged again with ultra-pure (99.999) Argon toequalize with atmospheric pressure. The graphitization tube is then removed
from the graphitization cell and capped with an Al-foil cover and placed in a test
tube rack ready to be loaded into a cathode for AMS counting.
AMS Cathode (Target) Loading
The prepared graphite sample is placed into an AMS cathode (target) by pouring
the graphite directly from the graphitization tube into the cathode. The graphite
powder is then compressed to a minimum of 150 psi (gauge reading) in an Arborpress or by hammering with a press-pin.
The surface of the graphite (target face) is optically inspected to ensure that thegraphite appears to be smooth and homogenous (i.e. the graphite does notappear uneven or mottled in coloration). The cathode (target) is then placed intoa cathode wheel and placed into the AMS along with the necessary Modern
Standards (OXI and/or OXII, organic and carbonate blanks, TIRI known age
standards (organic and or carbonate) and sufficient blind QA samples (samples
previously measured by LSC and/or AMS) for C14/13 and C14/12 detection.
AMS Counting
As is common with other kinds of mass spectrometry, AMS counting is performedby converting the atoms in the sample (graphite) into a beam of fast moving ions5

Beta Analytic(charged atoms). The mass of these ions is then separated by the application ofmagnetic and electric fields and measured by nuclear particle detection
techniques. The measurement of radiocarbon by mass spectrometry is very
difficult because its concentration is less than one atom in 1,000,000,000,000(one-trillion) atoms. Thus the accelerator is used to help remove ions that might
be confused with radiocarbon before the final detection.
The sample is put into the ion source (as graphite) and is ionized by bombarding
it with Cesium ions and then focused into a fast-moving beam. The ions14

14

produced are negative which prevents the confusion of C with N sincenitrogen does not form a negative ion. The first bending magnet is used in thesame way as the magnet in an ordinary mass spectrometer to select ions ofmass 14 (this will include large number of
14

12

-

2-

CH and13-

CH ions and a very few

C ions). The ions then enter the accelerator. As they travel to the terminal
detector they are accelerated sufficiently such that when they collide with the gas
molecules in the stripper, all of the molecular ions (such as

12

CH and213CH) arebroken up allowing most of the Carbon ions to pass through to a second bending
magnet which further separates the ions with the mass and momentum expected
of

14

C,

13

C, and12

Finally the filteredC.
14

C ions enter the detector where their velocity and energy are
14

checked so that the number of C ions in the sample can be counted. Not all of
the radiocarbon atoms put into the ion source end up reaching the detector andso the stable isotopes,

12

C and

13

C are measured as well in order to monitor the14

13

detection efficiency. For each sample, a ratio of C/ C is calculated andcompared to measurements made on standards with known ratios.
Uncertainties in AMS
Soon after the first AMS spectrometers were developed, the quality of AMS
measurements was demonstrated through comparisons of the error in the meanof a series of n AMS measurements for a sample (external error) to the counting
statistics of the measured total counts, N, in that series of measurements(internal error). If μ is the mean of a group of individual measurements, each withvariance σ2 (here assumed equivalent for all measurements), the fractional
precisions were shown to be equivalent:
2

2

2

2

σ ext = σ / n (n −1)μ = σ int = 1/ N

total

Indeed, equivalence of the standard error in the mean of AMS measurements tothe precision expected from counting statistics demonstrated the degree to which

6Beta Analytic

the spectrometer and its operation are free of systematic error. (Wölfli, et al.,
1983; Donahue, Jull, & Zabel, 1984; Farwell, et al., 1984; Suter, et al., 1984).
The development of a uniform sample material for 14C AMS, filamentous or
fullerene graphite (Vogel et al, 1984), provided intense ion beams for all samples
and standards, bringing the internal and external uncertainties into routine
equivalence for precise (σ ≤1%) AMS quantification. (Bonani, et al., 1987; Vogel,at al., 1987).
Sample Isotopic Fractionation (Stable Isotope Ratios 13/12C)
In order to provide radiocarbon determinations that are both accurate andprecise, it is necessary to measure the stable isotopes of 13C and 12C and theirratio. This is performed by extracting a small amount of the CO2 generated during
the combustion or acid hydrolysis and measuring the 13/12C ratio relative to the
PDB mass-spectrometry standard. This ratio is later used in the calculation of theradiocarbon age and error to correct for isotopic fractionation in nature.
Fractionation during the geochemical transfer of carbon in nature produces
variation in the equilibrium distribution of the isotopes of carbon (12C, 13C and
14
C). Craig (1953) first identified that certain biochemical processes alter theequilibrium between the carbon isotopes. Some processes, such as
photosynthesis for instance, favors one isotope over another, so after
photosynthesis, the isotope 13C is depleted by 1.8% in comparison to its natural
ratios in the atmosphere (Harkness, 1979). Conversely the inorganic carbon
dissolved in the oceans is generally 0.7% enriched in 13C relative to atmosphericcarbon dioxide.
The extent of isotopic fractionation on the 14C/12C ratio, which must be measured
accurately, is approximately double that for the measured 13C/12C ratio. If isotopic
fractionation occurs in natural processes, a correction can be made bymeasuring the ratio of the isotope 13C to the isotope 12C in the sample being
dated. The ratio is measured using an ordinary mass spectrometer. The isotopic
composition of the sample being measured is expressed as delta13C which
represents the parts per thousand difference (per mille) between the sample
carbon-13 content and the content of the international PDB standard carbonate(Keith et al., 1964; Aitken, 1990). A d13C value, then, represents the per mille(part per thousand) deviation from the PDB standard. PDB refers to theCretaceous belemnite formation at Peedee in South Carolina, USA. Thisnomenclature has recently been changed to VPDB (Coplen, 1994).
In summary, isotopic fractionation refers to the fluctuation in the carbon isotope
ratios as a result of natural biochemical processes as a function of their atomic
mass (Taylor, 1987). Variations as such are unrelated to time and naturalradioactive decay. It is common practice in radiocarbon laboratories to correct
radiocarbon activities for sample fractionation. The resultant ages are termed

7

Beta Analytic"normalized", meaning the measured activity is modified with respect to -25 permille wrt VPBD. The correction factor must be added or subtracted from theconventional radiocarbon age.
The deltaC13 value for a sample can yield important information regarding theenvironment from which the sample comes or the mixtures of materials used to
produce it because the isotope value of the sample reflects the isotopic
composition of the immediate environment. In the case of shellfish, for example,
marine shells typically possess a dC13 value of between -1 and +4 per mille,
whereas river shells possess a value of between -8 and -12 per mille. Therefore,
in a case where the precise environment of the shell is not known, it is possible
to determine the most likely by analysis of the dC13 result.
Fractionation also describes variations in the isotopic ratios of carbon brought
about by non-natural causes. For example, samples may be fractionated in thelaboratory through a variety of means. Usually, this is due to lack of attention todetail and incomplete conversion of the sample from one stage to another orfrom one part of the laboratory to another. In Liquid Scintillation Counting, forexample, incomplete synthesis of acetylene during lithium carbide preparation
may result in a low yield and concurrent fractionation. Similarly, the transfer ofgases in a vacuum system may involve fractionation error if the sample gas is notallowed to equilibrate throughout the total volume. Atoms of larger or smaller
mass may be favored in such a situation. If, however, the entire sample isconverted completely from one form to another (e.g. solid to gas, acetylene tobenzene) then no fractionation will occur.
Radiocarbon Age and Error Calculation
Much of the information presented in this section is based upon the paper
Stuiver, M. and Polach, H.A. 1977. Discussion: Reporting of 14C data.
Radiocarbon 19; 355-63.
The radiocarbon age of a sample is obtained by measurement of the residual
radioactivity. This is calculated through careful measurement of the residual
activity (per gram C) remaining in a sample whose age is unknown, comparedwith the activity present in Modern and Background samples.
Modern Standard
The principal modern radiocarbon standard is N.I.S.T (National Institute ofStandards and Technology; Gaithersburg, Maryland, USA) Oxalic Acid I(C2H2O4). Oxalic acid I is N.I.S.T designation SRM 4990 B and is termed HOx1.
This is the International Radiocarbon Dating Standard. Ninety-five percent of theactivity of Oxalic Acid from the year 1950 is equal to the measured activity of theabsolute radiocarbon standard which is 1890 wood. The 1890 wood was chosen8Beta Analytic

as the radiocarbon standard because it was growing prior to the fossil fuel effectsof the industrial revolution.
The activity of 1890 wood is corrected for radioactive decay to 1950. Thus 1950,
is year 0 BP by convention in radiocarbon dating and is deemed to be the'present'. The year 1950 was chosen for no particular reason other than to honor
the publication of the first radiocarbon dates calculated in December 1949
(Taylor, 1987;97).
The Oxalic acid standard was made from a crop of 1955 sugar beet. There were1000 lbs made. The isotopic ratio of HOx I is -19.3 per mille with respect to (wrt)
the PBD standard belemnite (Mann, 1983). The Oxalic acid standard which wasdeveloped is no longer commercially available. Another standard, Oxalic Acid II
was prepared when stocks of HOx 1 began to dwindle. The Oxalic acid II
standard (HOx 2; N.I.S.T designation SRM 4990 C) was made from a crop of1977 French beet molasses. In the early 1980s, a group of 12 laboratories
measured the ratios of the two standards. The ratio of the activity of Oxalic acid II
to 1 is 1.2933±0.001 (the weighted mean) (Mann, 1983). The isotopic ratio of
HOx II is -17.8 per mille.
According to Stuiver and Polach (1977), all laboratories should report theirresults either directly related to NBS Oxalic acid or indirectly using a substandard which is related to it.
Background Detection
It is vital for a radiocarbon laboratory to know the contribution to routine sample
activity of non-sample radioactivity. Obviously, this activity is additional and must
be removed from calculations. In order to make allowances for backgroundcounts and to evaluate the limits of detection, materials which radiocarbon
specialists can be fairly sure contain no activity are measured under identicalcounting conditions as normal samples.
Background samples usually consist of geological samples of infinite age such ascoal, lignite, limestone, ancient carbonate, anthracite, marble or swamp wood. Bymeasuring the activity of a background sample, the normal radioactivity present
while a sample of unknown age is being measured can be accounted for anddeducted.
Conventional radiocarbon ages (BP)A radiocarbon measurement termed a conventional radiocarbon age (or CRA) is
obtained using a set of parameters outlined by Stuiver and Polach (1977), in thejournal Radiocarbon. A time-independent level of 14C activity for the past is
assumed in the measurement of a CRA. The activity of this hypothetical level of
14
C activity is equal to the activity of the absolute international radiocarbon

9Beta Analytic

standard. The Conventional Radiocarbon Age BP is calculated using theradiocarbon decay equation:t = -8033 ln (Asn/Aon)
Where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977). Aon
is the activity in counts per minute of the modern standard. Asn is the equivalent
cpm for the sample. 'ln' represents the natural logarithm.
A CRA embraces the following recommended conventions:• a half-life of 5568 years;• the use of Oxalic acid I or II as the modern radiocarbon standard;• correction for sample isotopic fractionation (deltaC13) to a normalized or basevalue of -25.0 per mille relative to the ratio of 12C/13C in the carbonate standard
VPDB (more on fractionation and deltaC13);• the use of 1950 AD as 0 BP, ie all 14C ages head back in time from 1950;• the assumption that all 14C reservoirs have remained constant through time.
Additional terms are sometimes requested to be or reported with, or in lieu of thestandard Conventional Radiocarbon Age BP result from which all others aremathematically derived. These are the “Measured Radiocarbon Age BP”, Percent
Modern Carbon (pMC), Mean Biobased Result (expressed in %), Percent Mean
Biogenic Carbon Content, Percent Biomass CO2, Fraction Modern Carbon (fmdn
or fMC) as well as d14C, D14C, delta 14C, ∆14C and delta 13/12C (all of which
are expressed in per mille notation (%) rather than per cent notation.)
d14C represents the per mille depletion in sample carbon 14 prior to isotopic
fractionation correction and is measured by;
d14C=((Asn/Aon) - 1)1000 per mille
D14C represents the 'normalized' value of d14C. 'Normalized' means that theactivity is scaled in relation to fractionation of the sample, or its deltaC13 value.
All D14C values are normalized to the base value of - 25.0 per mille with respect
to the standard carbonate (VPDB). D14C is calculated using:
D14C=d14C - 2(dC13 + 25)(1 + d14C/1000) per milleThis value can then be used to calculate the CRA using the equation given
above.

10

Beta Analytic

Figure 1 - Decay curve for 12C showing the activity at one half-life (t/2). The terms "%Modern" or"pMC" and D14C are shown related in this diagram along with the Radiocarbon age in years BP(Before 1950 AD).

Age reporting
If the reservoir corrected conventional radiocarbon age calculated is within thepast 200 years, it should by convention be termed 'Modern' (Stuiver and Polach,
1977; 362). If a sample age falls after 1950, it is termed greater than Modern, or>Modern. Absolute percent modern (%M or pMC - 'percent modern carbon') is
calculated using:%M=100 x Asn/Aabs
or
Asn/Aon(1/8267(y-1950)) x 100 percent
Where Aabs is the absolute international standard activity, 1/8267 is the lifetime
based on the new half life (5730 yr), Y = the year of measurement of theappropriate standard. This is an expression of the ratio of the net modern activity
against the residual normalized activity of the sample, expressed as apercentage and it represents the proportion of radiocarbon atoms in the sample
compared to that present in the year 1950 AD. Thus, %Modern becomes a usefulterm in describing radiocarbon measurements for the past 45 years when, due to11

Beta Analytic

the influx of artificial radiocarbon into the atmosphere as a result of nuclear bomb
testing, the 'age' calculation becomes a 'future' calculation.
If the sample approaches D14C = -1000 per mille within 2 standard deviations, it
is considered to be indistinguishable from the laboratory background, i.e., notable to be separated with confidence from the laboratory count-rates which result
from a sample which contains no radionuclide. In this instance, a Greater-Than
Age is calculated. An example of a Greater-Than Age is >55, 000 yr or >50, 000
yr (Gupta and Polach, 1985).
Samples whose age falls between modern and background and are given finite
ages. Standard errors released with each radiocarbon assay are usually roundedby convention (Stuiver and Polach, 1977). Again, not all laboratories subscribe tothese conventions, some do not round up ages.
Standard Error and Sources of Error
Statistical analysis is necessary in radiocarbon dating because the decay of 14C
although constant, is spontaneous. It is not possible to measure the entireradioactivity in a given sample, hence the need for some kind of statisticalanalysis of counted data. The distribution of counted 14C decay events will, overtime, yield a pattern. The pattern is termed a "normal distribution curve".
A normal or "Gaussian" distribution describes the symmetrical bell shaped clusterof events around the average or mean of the data. In a normal distribution, 2 outof 3, or 68% of the values or counts observed will fall within one standard
deviation of the average of the data. At two standard deviations, 95% of theobserved counts will fall within the range and at three standard deviations, 99%of the counts which comprise the normal distribution will fall within this region.
Each radiocarbon date is released as a conventional radiocarbon age with'standard error'. This is the '±' value and by convention is ± 1 sigma. The
standard error is based principally upon counting statistics, however othersources of error are possible and their effects are listed below:
Summary; The accuracy ofradiocarbon dates (modified
from Polach, H.A. 1976). Sourcesof Error
1. Precision of age
determination
2. Inherent a. 14C half-life
b. 12C/13C fractionation

Effect upon Age
Determination
Statistical;
Typically±1%Modern or
less
Libby half life 3%
too low
Variable, up to

12

Measures to minimize
the error incurred
Big samples, longer
count times, repeat
sample assays
Multiply CRA's by 1.03 if
necessary
Stable isotope analyses

Beta Analytic

450 yr for shell.
c. 14C Modern standard

Variable > 80 yr

d. Variation in past 14C
production rates

0-800 yr, beyond
ca12 ka notdetermined

e. Distribution of 14C in nature

f. Changes of 14C concentrationin the atmosphere.

3. Contamination

Surface ocean
latitudinal
dependence 400 to -750 yr.
Deep ocean 1800 yr.
Industrial effectca -2.5% andatom bomb
effect +160% in
atmosphere
Nil to 300 yr up
to 15 ka; >20 ka
possible beyond
25 ka.

4. Biological age of material<10 yr to>1000yr

5. Association of sample andevent

Intermediate

6. HumanIntermediate

7. Interpretation of results

Intermediateusing Mass Spec.
International crosscheckof secondary standards.
Tree ring calibration;otherwise interpret
results in radiometrictimescale.

Interpretation of results.

Interpretation of results
Interpretation of results,
analysis and dating ofextracted pretreatedfractions.
Identification of species
of material in the case ofwood and charcoal toshort lived samples only.
Interpretation of results
Care in field andlaboratory
Care in interpretation,interdisciplinary
approach andcollaboration

Accuracy and Precision in Radiocarbon Dating
It is important to note the meaning of "accuracy" and "precision" in radiocarbon
dating. Accuracy refers to the date being a 'true' estimate of the age of a sample
within the range of the statistical limits or ± value of the date. Thus, for the sakeof argument, if we were radiocarbon dating a sample of human bones from anindividual who we knew died in 1066 AD and obtained a date of 1040±40 AD, wewould have dated the event of his death accurately. If however the date obtained
were 1000±15 AD, we would be inaccurate.

13

Beta Analytic

In terms of precision, however, the former is imprecise in comparison to the latterbecause of the larger stated error value. As such it can be seen that the date of
1000±15 AD while being highly precise is, in this instance, inaccurate.
Reservoir Effects
A Conventional Radiocarbon Age or CRA does not take into account specificdifferences between the activity of different carbon reservoirs. A CRA is derived
using an age calculation based upon the decay corrected activity of the absolute
radiocarbon standard (1890 AD wood) which is in equilibrium with atmosphericradiocarbon levels (as mentioned previously, 1890 wood is no longer used as theprimary radiocarbon standard, instead Oxalic Acid standards I and II werecorrelated with the activity of the original standard). In order to ascertain the agesof samples which were formed in equilibrium with different reservoirs to thesematerials, it is necessary to provide an age correction. Implicit in theConventional Radiocarbon Age BP is the fact that it is not adjusted for thiscorrection.
Radiocarbon samples which obtain their carbon from a different source (orreservoir) than atmospheric carbon may yield what is termed apparent ages. A
shellfish alive today in a lake within a limestone catchment, for instance, will yielda radiocarbon date which is excessively old. The reason for this anomaly is thatthe limestone, which is weathered and dissolved into bicarbonate, has noradioactive carbon. Thus, it dilutes the activity of the lake meaning that theradioactivity is depleted in comparison to 14C activity elsewhere. The lake, in thiscase, has a different radiocarbon reservoir than that of the majority of theradiocarbon in the biosphere and therefore an accurate radiocarbon age requiresthat a correction be made to account for it.
One of the most commonly referenced reservoir effects concerns the ocean. The
average difference between a radiocarbon date of a terrestrial sample such as atree, and a shell from the marine environment is about 400 radiocarbon years(see Stuiver and Braziunas, 1993). This apparent age of oceanic water is caused
both by the delay in exchange rates between atmospheric CO2 and ocean
bicarbonate, and the dilution effect caused by the mixing of surface waters withupwelled deep waters which are very old (Mangerud 1972). A reservoir
correction must therefore be made to any conventional shell dates to account forthis difference. Human bone may be a problematic medium for dating in someinstances due to human consumption of fish, whose 14C label will reflect theocean reservoir. In such a case, it is very difficult to ascertain the precise
reservoir difference and hence apply a correction to the measured radiocarbon
age.
Spurious radiocarbon dates caused by volcanic emanations of radiocarbondepleted CO2 probably also come under the category of reservoir corrections.
Plants which grow in the vicinity of active volcanic fumaroles will yield a14

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radiocarbon age which is too old. Bruns et al. (1980) measured the radioactivityof modern plants growing near hot springs heated by volcanic rocks in western
Germany and demonstrated a deficiency in radiocarbon of up to 1500 years
through comparison with modern atmospheric radiocarbon levels. Similarly, thiseffect has been noted for plants in the bay of Palaea Kameni near the prehistoricsite of Akrotiri, which was buried by the eruption of the Thera volcano over 3500
years ago (see Weninger, 1989). The effect has been suggested as providing
dates in error for the eruption of Thera which has been linked to the demise of
the Minoan civilization in the Aegean. One modern plant growing near the
emanations had an apparent age of 1390 yr. The volcanic effect has a limited
distance however. Bruns et al. (1980) found that at 200 m away from the source,
plants yielded an age in agreement with that expected. They suggested that the
influence of depleted CO2 declined rapidly with increasing distance from thesource. Radiocarbon discrepancies due to volcanic CO2 emissions are a popularsource of ammunition for fundamentalist viewpoints keen to present evidence toshow that the radiocarbon method is somehow fundamentally flawed.
Suess or Industrial EffectSince about 1890, the use of industrial and fossil fuels has resulted in large
amounts of CO2 being emitted into the atmosphere. Because the source of theindustrial fuels has been predominantly material of infinite geological age ( e.g.
coal, petroleum), whose radiocarbon content is nil, the radiocarbon activity of theatmosphere has been lowered in the early part of the 20th century up until the
1950s. The atmospheric radiocarbon signal has, in effect, been diluted by about
2%. Hans Suess (1955) discovered the industrial effect (also called after him) in
the 1950s. A number of researchers found that the activity they expected frommaterial growing since 1890 AD was lower. The logical conclusion from this wasthat in order to obtain a modern radiocarbon reference standard, representing theradiocarbon activity of the 'present day', one could not very well use wood whichgrew in the 1900s since it was affected by this industrial effect. Thus it was that
1890 wood was used as the modern radiocarbon standard, extrapolated for
decay to 1950 AD.
Atom Bomb EffectSince about 1955, thermonuclear tests have added considerably to the 14C
atmospheric reservoir. This 14C is 'artificial' or 'bomb' 14C, produced because
nuclear bombs produce a huge thermal neutron flux. The effect of this has been
to almost double the amount of 14C activity in terrestrial carbon bearing materials(Taylor, 1987).
De Vries (1958) was the first person to identify this 'Atom Bomb' effect. In thenorthern hemisphere the amount of artificial carbon in the atmosphere reached apeak in 1963 (in the southern hemisphere around 1965) at about 100% above
normal levels. Since that time the amount has declined owing to exchange and15

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dispersal of 14C into the earth's carbon cycle system. The presence of bomb
carbon in the earth's biosphere has enabled it to be used as a tracer toinvestigate the mechanics of carbon mixing and exchange processes.

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