Laboratory for Ion Beam Physics
Prof. Dr. Hans-Arno Synal
+41 44 633 20 27
synal@phys.ethz.ch
Vorlesung Analytische Strategie
STRATEGISCHE ÜBUNG
HS 2015
Radiocarbon dating: Physical age determinations using natural radionuclides
The dating method
The radiocarbon method (also called the 14C dating) is based on the radioactive decay of the
heaviest naturally occurring carbon isotope 14C, which has a half-life of 5700 years. 14C is
generated by the interaction of cosmic rays with atmospheric nitrogen and passes through the
carbon dioxide circuit in organic materials. The constant production of 14C nuclides and their
radioactive decay lead to a secular equilibrium in the ratio of carbon isotopes (12C and 14C). In the
reference year of radiocarbon dating, 1950, this equilibrium ratio was about 1*10-12 (14C / 12C).
Organic material incorporated in the CO2 cycle, having a corresponding isotope ratio. Separates
this material from the CO2 cycle (for example, by dead), then the decaying 14C is no longer be
replaced. That means, the originally 14C/12C ratio decreases corresponding to the radioactive decay
law.
Measurement parameters
Radiocarbon dating is made by measuring the 14C/12C isotope ratio of a sample to be examined.
The measurements are made relative to standard reference materials (samples with a known
14 12
C/ C ratio). Also blank samples (samples that originally contained no 14C, but going through the
same chemical processing, which is applied to the test sample) are analyzed to detect a possibly
occurring 14C contamination during sample preparation. For each sample a measurement
uncertainty is always specified. This consists of the following components: a) Statistical
fluctuations arising from the finite number of detected 14C atoms. b ) Uncertainty of the
normalization to standard reference samples. c ) Uncertainty of the blank correction. d)
Uncertainty introduced due to variation in sample preparation procedures.
For the measurements, a complex procedure is used, which allows a control the random variations
of the measurement device and ensures that the final uncertainty is consistent with the statistical
uncertainty of the counting procedure. In addition to the 14C/12C ratio, the ratio of stable carbon
isotopes 13C/12C is measured. This measurement is performed with respect to standard reference
samples, too. The result is expressed as δ13C with the resulting measurement uncertainty. From
these measured values and their respective uncertainties, the so-called radiocarbon age (14C age)
and a corresponding measurement uncertainty according to an internationally established
procedures determined [ 1]. This age does not correspond to the historical age. However, it is
useful for comparing different samples.
The determination of the historical age
It has been shown that the 14C production was changeable by cosmic radiation over time. This and
other effects are responsible for variations of the equilibrium 14C/12C ratios and thus, it cannot be
assumed to be constant in biological material over time. Consequently, a direct translation of a
measured 14C/12C ratio in an historic age is not allowed. However, one can reconstruct with the
help of tree-ring archives the equilibrium 14C/12C ratios as a function of the historical age (the
historical age is obtained by counting the annual rings. Dendroage it is also called). In the past, a
calibration curve has been prepared by measuring the 14C concentration in timber samples
(Dendroarchives [2]). The calculation of the age of a sample whose radiocarbon age is determined
by measuring its ratio of 14C/12C, now takes place by comparison with the calibration curve. This
is done with the help of calibration programs (e.g. OxCal [3 ] ) that perform the convolution of the
radiocarbon age and the calibration curve mathematically correct . As a result, we obtain
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probability statements over the time period (historical age) from which the sample may originates.
It should be noted that the calibration data are continuously improved. Depending on which data
and calibration program which is used, there may be slight differences in the analysis. It is
therefore recommended that source of the calibration curve are indicated. The graphical
representation of the resulting probability distribution as a function of age is the base of the
historical age determination. In general, this distribution cannot be converted directly into historic
ages.
Examples
The climatic changes of the past years have caused dramatic shrinking to the glaciers of the Alps.
With the retreating glaciers areas are accessible, which were covered by ice thousands of years. In
the recent past, passes over the Alps are accessible again, which were heavily used in earlier
millennia. Examples of these prehistoric Alpine crossings are the Hauslabjoch in the Oetztal Alps
and the Schnidejoch in the Bernese Oberland.
The retreating ice sheets open access to valuable archaeological objects dating from prehistoric
times. So in 1991 at Hauslabjoch, a male corpse was found well preserved (The Iceman "Ozi" ).
Based on the objects found near to the body it was quickly realized that it had to be the remains of
a prehistoric man.
Somewhat less spectacular but of comparable relevance to archaeological research are more than
300 objects that archaeologists have found at the Bernese Schnidejoch. Parts of a Neolithic
hunting equipment, a box of tree bark, a bronze fibula as well as parts of a Roman belt from sheep
wool have been recovered.
As sensational as these findings may be also, its scientific significance is palpable only if they can
be placed in the correct temporal context. The classical methods of archaeological age
determination (comparison with similar objects belonging to certain well-known eras) are often
not applicable or only provide an unreliable timing information. Reliable answers to these
intriguing questions can be obtained using the radiocarbon method.
Goal of this exercise is to define the block foundations of the method, the discussion of the
measurement procedures, analysis of data and discussions of the preparative possibilities, which
are used for sample preparation.
References
[1] M. Stuiver and H. Polach, Reporting of 14C data, Radiocarbon 19(1977) 355
[2] Reimer PJ, MGL Baillie, E Bard, A Bayliss, JW Beck, C Bertrand, PG Blackwell, CE Buck,
G Burr, KB Cutler, PE Damon, RL Edwards, RG Fairbanks, M Friedrich, TP Guilderson,
KA Hughen, B Kromer, FG McCormac, S Manning, C Bronk Ramsey, RW Reimer, S
Remmele, JR Southon, M Stuiver, S Talamo, FW Taylor, J van der Plicht, and CE
Weyhenmeyer.
Intcal04, 2004 Radiocarbon 46:1029-1058.
[3] Oxcal, http://c14.arch.ox.ac.uk/
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Historic References
[4] Arnold, J.R., and W.F. Libby, 1951. Radiocarbon dates. Science, 113:111-120.
[5] C.L. Bennett, R.P. Beukens, M.R. Clover, H.E. Gove, R.B. Liebert, A.E. Litherland, K.H.
Purser, W.E. Sondheim, Radiocarbon Dating Using Electrostatic Accelerators: Negative Ions
Provide the Key, Science, 198 (1977) 508-510.
[6] Broecker, W. S. og Kulp, J.L.1956."The radiocarbon method of age determination"
American Antiquity 22, pp.1-11
[7] Libby, W.F. 1955. Radiocarbon Dating. University of Chicago Press, Chicago.
[8] Taylor, R.E, Long, A and Kra, R. (eds) 1994. Radiocarbon After Four Decades. An
Interdisciplinary Perspective. Springer - Verlag, NY. 596 pp.
Technical References
[9] R.C.Finkel and Martin Suter, AMS in the Earth Sciences: Techniques and Application,
Advances in Analytical Geochemistry, Volume 1, pages 1-114 (*eine Kopie kann bei HansArno Synal, HPK H33, abgeholt werden, synal@phys.ethz.ch)
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ISSUES / POINTS FOR DISCUSSIONS
1.
What physical property can be used to perform a dating of archaeological objects and which
element is due to its chemical and geochemical properties particularly suitable for age
determination?
2.
Explain the basic principle of the radiocarbon method. In particular, discuss the production
of 14C by cosmic radiation and the geochemical cycle of carbon. What processes determine
the 14C content in the atmosphere?
3.
What conditions must be met so that an age determination can be carried out? You can
explain what factors affect the age determination. What are the limitations of the method?.
4.
With the help of reliable calibrations, age can be determined despite the changing 14C
production. Discuss based on the calibration curve what age ranges are not particularly well
suited for dating. In how far man had massively intervened into the natural carbon cycle
with strong consequences for radiocarbon dating?
5.
What analysis can be used to study the archaeological objects? Discuss the differences in the
methods and justify why you would use one or the other method.
6.
Chemical processes lead to a shift of isotope ratios, the so-called fractionation. How does a
shift in the carbon isotope ratios of the possibility of age determination?
7.
What materials can be used for age determination by radiocarbon method?
8.
What can falsify the age determination and how you can prepare by appropriate chemical
methods an unbiased sample as good as possible?
9.
For analysis of a sample with an accelerator mass spectrometer (AMS system) a graphite
sample is required. How would you create such a sample?
10. Estimate on how much sample material they need for analysis.
Information can be found on the Internet, as always, Wikipedia is a good introduction to the
subject:
http://www.c14methode.de.vu/
http://en.wikipedia.org/wiki/Radiocarbon_dating
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