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BY: [2]THOMAS HIGHAM

"Everything which has come down to us from heathendom is wrapped in
a thick fog; it belongs to a space of time we cannot measure. We
know that it is older than Christendom, but whether by a couple of
years or a couple of centuries, or even by more than a millenium,
we can do no more than guess." [Rasmus Nyerup, (Danish
antiquarian), 1802 (in Trigger, 1989:71)].

Nyerup's words illustrate poignantly the critical power and
importance of dating; to order time. Radiocarbon dating has been
one of the most significant discoveries in 20th century science.
Renfrew (1973) called it 'the radiocarbon revolution' in describing
its impact upon the human sciences. Oakley (1979) suggested its
development meant an almost complete re-writing of the evolution
and cultural emergence of the human species. Desmond Clark (1979)
wrote that were it not for radiocarbon dating, "we would still be
foundering in a sea of imprecisions sometime bred of inspired
guesswork but more often of imaginative speculation" (Clark,
1979:7). Writing of the European Upper Palaeolithic, Movius (1960)
concluded that "time alone is the lens that can throw it into
focus".

The radiocarbon method was developed
by a team of scientists led by the late Professor
Willard F. Libby of the University of Chicago in immediate post-WW2
years.
Libby later received the Nobel Prize in Chemistry in 1960: "for his
method to use Carbon-14 for age determinations in archaeology,
geology, geophysics, and other branches of science."
According to one of the scientists who nominated Libby as a
candidate for this honour;

"Seldom has a single discovery in chemistry had such an impact on
the thinking of so many fields of human endeavour. Seldom has a
single discovery generated such wide public interest."

(From Taylor, 1987).

Today, there are over 130 [3]radiocarbon dating laboratories around
the world producing radiocarbon assays for the scientific
community. The C14 technique has been and continues to be applied
and used in many, many different fields including hydrology,
atmospheric science, oceanography, geology, palaeoclimatology,
archaeology and biomedicine.

The 14C Method

There are three principal isotopes of carbon which occur naturally
- C12, C13 (both stable) and C14 (unstable or radioactive). These
isotopes are present in the following amounts C12 - 98.89%, C13 -
1.11% and C14 - 0.00000000010%. Thus, one carbon 14 atom exists in
nature for every 1,000,000,000,000 C12 atoms in living material.
The radiocarbon method is based on the rate of decay of the
radioactive or unstable carbon isotope 14 (14C), which is formed in
the upper atmosphere through the effect of cosmic ray neutrons upon
nitrogen 14. The reaction is:

14N + n => 14C + p

(Where n is a neutron and p is a proton).
The 14C formed is rapidly oxidised to 14CO2 and enters the earth's
plant and animal lifeways through photosynthesis and the food
chain. The rapidity of the dispersal of C14 into the atmosphere has
been demonstrated by measurements of radioactive carbon produced
from thermonuclear bomb testing. 14C also enters the Earth's oceans
in an atmospheric exchange and as dissolved carbonate (the entire
14C inventory is termed the carbon exchange reservoir (Aitken,
1990)). Plants and animals which utilise carbon in biological
foodchains take up 14C during their lifetimes. They exist in
equilibrium with the C14 concentration of the atmosphere, that is,
the numbers of C14 atoms and non-radioactive carbon atoms stays
approximately the same over time. As soon as a plant or animal
dies, they cease the metabolic function of carbon uptake; there is
no replenishment of radioactive carbon, only decay. There is a
useful diagrammatic representation of this process given [4]here

Libby, Anderson and Arnold (1949) were the first to measure the
rate of this decay. They found that after 5568 years, half the C14
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 name given 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 radioactive carbon present in a sample. At about 50 - 60
000 years, then, the limit of the technique is reached (beyond this
time, other radiometric techniques must be used for dating). By
measuring the C14 concentration or residual radioactivity of a
sample whose age is not known, it is possible to obtain the
countrate or number of decay events per gram of Carbon. By
comparing this with modern levels of activity (1890 wood corrected
for decay to 1950 AD) and using the measured half-life it becomes
possible to calculate a date for the death of the sample.

As 14C decays it emits a weak beta particle (b ), or electron,
which possesses an average energy of 160keV. The decay can be
shown:

14C => 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 be uniformly applied throughout the world.
Included below is an impressive list of some of the types of
carbonaceous samples that have been commonly radiocarbon dated in
the years since the inception of the method:

* [5]Charcoal, wood, twigs and seeds.
* [6]Bone.
* [7]Marine, estuarine and riverine shell.
* Leather.
* [8]Peat
* Coprolites.
* [9]Lake muds (gyttja) and sediments.
* [10]Soil.
* Ice cores.
* Pollen.
* Hair.
* Pottery.
* Metal casting ores.
* Wall paintings and rock art works.
* [11]Iron and meteorites.
* [12]Avian eggshell.
* Corals and foraminifera.
* Speleothems.
* Tufa.
* Blood residues.
* Textiles and fabrics.
* Paper and parchment.
* Fish remains.
* Insect remains.
* Resins and glues.
* Antler and horn.
* Water.

The historical perspective on the development of radiocarbon dating
is well outlined in Taylor's (1987) book "Radiocarbon Dating: An
archaeological perspective". Libby and his team intially tested the
radiocarbon method on samples from prehistoric Egypt. They chose
samples whose age could be independently determined. A sample of
acacia wood from the tomb of the pharoah [13]Zoser (or Djoser; 3rd
Dynasty, ca. 2700-2600 BC) was obtained and dated. Libby reasoned
that since the half-life of C14 was 5568 years, they should obtain
a C14 concentration of about 50% that which was found in living
wood (see Libby, 1949 for further details). The results they
obtained indicated this was the case. Other analyses were conducted
on samples of known age wood (dendrochronologically aged). Again,
the fit was within the value predicted at ±10%. The tests suggested
that the half-life they had measured was accurate, and, quite
reasonably, suggested further that atmospheric radiocarbon
concentration had remained constant throughout the recent past. In
1949, Arnold and Libby (1949) published their paper "Age
determinations by radiocarbon content: Checks with samples of known
age" in the journal Science. In this paper they presented the first
results of the C14 method, including the "Curve of Knowns" in which
radiocarbon dates were compared with the known age historical dates
(see figure 1). All of the points fitted within statistical range.
Within a few years, other laboratories had been built. By the early
1950's there were 8, and by the end of the decade there were more
than 20.
______________________________________________________________

Figure 1: The "Curve of Knowns" after Libby and Arnold (1949). The
first acid test of the new method was based upon radiocarbon dating
of known age samples primarily from Egypt (the dates are shown in
the diagram by the red lines, each with a ±1 standard deviation
included). The Egyptian King's name is given next to the date
obtained. The theoretical curve was constructed using the half-life
of 5568 years. The activity ratio relates to the carbon 14 activity
ratio between the ancient samples and the modern activity. Each
result was within the statistical range of the true historic date
of each sample.

In the 1950s, further measurements on Mediterranean samples, in
particular those from Egypt whose age was known through other
means, pointed to radiocarbon dates which were younger than
expected. The debate regarding this is outlined extensively in
Renfrew (1972). Briefly, opinion was divided between those who
thought the radiocarbon dates were correct (ie, that radiocarbon
years equated more or less to solar or calendar years) and those
who felt they were flawed and the historical data was more
accurate. In the late 1950's and early 1960's, researchers
measuring the radioactivity of known age tree rings found
fluctuations in C14 concentration up to a maximum of ±5% over the
last 1500 years. In addition to long term fluctuations, smaller
'wiggles' were identified by the Dutch scholar Hessel de Vries
(1958). This suggested there were temporal fluctuations in C14
concentration which would neccessitate the calibration of
radiocarbon dates to other historically aged material. Radiocarbon
dates of sequential dendrochronologically aged trees primarily of
US bristlecone pine and German and Irish oak have been measured
over the past 10 years to produce a calendrical / radiocarbon
calibration curve which now extends back over 10 000 years (more on
[14]Calibration). This enables radiocarbon dates to be calibrated
to solar or calendar dates.

Later measurements of the Libby half-life indicated the figure was
ca. 3% too low and a more accurate half-life was 5730±40 years.
This is known as the Cambridge half-life. (To convert a "Libby" age
to an age using the Cambridge half-life, one must multiply by
1.03).

The major developments in the radiocarbon method up to the present
day involve improvements in measurement techniques and research
into the dating of different materials. Briefly, the initial solid
carbon method developed by Libby and his collaborators was replaced
with the [15]Gas counting method in the 1950's. [16]Liquid
scintillation counting, utilising benzene, acetylene, ethanol,
methanol etc, was developed at about the same time. Today the vast
majority of radiocarbon laboratories utilise these two methods of
radiocarbon dating. Of major recent interest is the development of
the [17]Accelerator Mass Spectrometry method of direct C14 isotope
counting. In 1977, the first AMS measurements were conducted by
teams at Rochester/Toronto and the General Ionex Corporation and
soon after at the Universities of Simon Fraser and McMaster (Gove,
1994). The crucial advantage of the AMS method is that milligram
sized samples are required for dating. Of great public interest has
been the AMS dating of carbonacous material from prehistoric rock
art sites, the Shroud of Turin and the Dead Sea Scrolls in the last
few years. The development of high-precision dating (up to ±2.0 per
mille or ±16 yr) in a number of gas and liquid scintillation
facilities has been of similar importance (laboratories at Belfast
(N.Ireland), Seattle (US), Heidelberg (Ger), Pretoria (S.Africa),
Groningen (Netherlands), La Jolla (US), Waikato (NZ) and Arizona
(US) are generally accepted to have demonstrated radiocarbon
measurements at high levels of precision). The calibration research
undertaken primarily at the Belfast and Seattle labs required that
high levels of precision be obtained which has now resulted in the
extensive calibration data now available. The development of small
sample capabilities for LSC and Gas labs has likewise been an
important development - samples as small as 100 mg are able to be
dated to moderate precision on minigas counters (Kromer, 1994) with
similar sample sizes needed using [18]minivial technology in Liquid
Scintillation Counting. The radiocarbon dating method remains
arguably the most dependable and widely applied dating technique
for the late Pleistocene and Holocene periods.

[19][LINK] 

INDEX | [20]Introduction | [21]Measurement | [22]Applications |
[23]WWW Links | [24]k12 | [25]Publication | [26]Corrections | 
[27]Age calculation | [28]Calibration | [29]Pretreatment |
[30]References | [31]Awards | [32]Credits | [33]What's new? |
[34]Email | 

HTML DOCUMENT BY T.HIGHAM.

References

1. LYNXIMGMAP:file://localhost/www/jnocook.net/saturn/new/int.html#map
2. mailto:thomas.higham at archaeology-research.oxford.ac.uk
3. http://www.radiocarbon.org/Info/#labs
4. http://webmuseen.de/14C.html
5. http://www.c14dating.com/charc.html
6. http://www.c14dating.com/bone.html
7. http://www.c14dating.com/shell.html
8. http://www.c14dating.com/peat.html
9. http://www.c14dating.com/mud.html
10. http://www.c14dating.com/soil.html
11. http://rsphysse.anu.edu.au/%7Ergc103/people/rgc_res.html#iron
12. http://www.c14dating.com/egg.html
13. http://ccat.sas.upenn.edu/arth/zoser/zoser.html
14. http://units.ox.ac.uk/departments/rlaha/calib.html
15. http://www.c14dating.com/meths.html
16. http://www.c14dating.com/meths.html
17. http://www.c14dating.com/meths.html
18. http://www2.waikato.ac.nz/c14/vials.html
19. http://www.c14dating.com/
20. http://www.c14dating.com/int.html
21. http://www.c14dating.com/meths.html
22. http://www.c14dating.com/applic.html
23. http://www.c14dating.com/www.html
24. http://www.c14dating.com/k12.html
25. http://www.c14dating.com/publication.html
26. http://www.c14dating.com/corr.html
27. http://www.c14dating.com/agecalc.html
28. http://www.rlaha.ox.ac.uk/orau/calibration.html
29. http://www.c14dating.com/pret.html
30. http://www.c14dating.com/bib.html
31. http://www.c14dating.com/awards.html
32. http://www.c14dating.com/credits.html
33. http://www.c14dating.com/whtsnew.html
34. http://www.c14dating.com/email.html