Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .186
2. The Birth of Radiocarbon Dating . . . . . . . . . . . . . . . . . . . . .186
2.1 Standards and Validation . . . . . . . . . . . . . . . . . . . . . . .189
3. Natural Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
4. The Bomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .192
4.1 Excess
14
C as a Global Geochemical Tracer . . . . . . . .193
4.2 The Second (Geochemical) Decay Curve
of
14
C: Isotopic-Temporal Authentication . . . . . . . .193
5. Anthropogenic Variations; âTrees Polluteâ . . . . . . . . . . . . . .195
5.1 Fossil-Biomass Carbon Source Apportionment . . . . . .196
6. Accelerator Mass Spectrometry . . . . . . . . . . . . . . . . . . . . . .199
6.1 The Invention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .199
6.2 The Shroud of Turin . . . . . . . . . . . . . . . . . . . . . . . . . . .200
7. Emergence of
”
-Molar
14
C Metrology . . . . . . . . . . . . . . . . .204
7.1 Long-Range Transport of Fossil
and Biomass Aerosol . . . . . . . . . . . . . . . . . . . . . . . .205
7.2 Isotopic Speciation in Ancient Bones
and Contemporary Particles . . . . . . . . . . . . . . . . . . .210
7.2.1 Urban Dust (SRM 1649a); a Unique
Isotopic-Molecular Reference Material . . . .211
8. Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .215
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
185
J. Res. Natl. Inst. Stand. Technol.
109
, 185-217 (2004)]
The Remarkable Metrological History of
Radiocarbon Dating [II]
Volume 109
Number 2
March-April 2004
Lloyd A. Currie
National Institute of Standards
and Technology,
Gaithersburg, MD 20899-8370
U.S.A.
lloyd.currie@nist.gov
This article traces the metrological history
of radiocarbon, from the initial break-
through devised by Libby, to minor (evolu-
tionary) and major (revolutionary)
advances that have brought
14
C measure-
ment from a crude, bulk [8 g carbon] dating
tool, to a refined probe for dating tiny
amounts of precious artifacts, and for
âmolecular datingâ at the 10 ”g to 100 ”g
level. The metrological advances led to
opportunities and surprises, such as the
non-monotonic dendrochronological cali-
bration curve and the âbomb effect,â that
gave rise to new multidisciplinary areas of
application, ranging from archaeology and
anthropology to cosmic ray physics to
oceanography to apportionment of anthro-
pogenic pollutants to the reconstruction of
environmental history.
Beyond the specific topic of natural
14
C,
it is hoped that this account may serve as a
metaphor for young scientists, illustrating
that just when a scientific discipline may
appear to be approaching maturity, unanti-
cipated metrological advances in their own
chosen fields, and unanticipated anthro-
pogenic or natural chemical events in the
environment, can spawn new areas of
research having exciting theoretical and
practical implications.
Key words:
accelerator mass spectro-
metry; apportionment of fossil and biomass
carbon; âbombâ
14
C as a global tracer; dual
isotopic authentication; metrological
history; molecular dating; radiocarbon
dating; the Turin Shroud; SRM 1649a.
Accepted:
February 11, 2004
Available online:
http://www.nist.gov/jres
1. Introduction
This article is about metrology, the science of
measurement. More specifically, it examines the
metrological revolutions, or at least evolutionary mile-
stones that have marked the history of radiocarbon
dating, since its inception some 50 years ago, to the
present. The series of largely or even totally unantici-
pated developments in the metrology of natural
14
C is
detailed in the several sections of this article, together
with examples of the consequent emergence of new
and fundamental applications in a broad range of
disciplines in the physical, social, and biological
sciences.
The possibility of radiocarbon dating would not have
existed, had not
14
C had the âwrongâ half-lifeâa fact
that delayed its discovery [1]. Following the discovery
of this 5730 year (half-life) radionuclide in laboratory
experiments by Ruben and Kamen, it became clear to
W. F. Libby that
14
C should exist in nature, and that it
could serve as a quantitative means for dating artifacts
and events marking the history of civilization. The
search for natural radiocarbon was itself a metro-
logical challenge, for the level in the living biosphere
[ca. 230 Bq/kg] lay far beyond the then current state of
the measurement art. The following section of this
article reviews the underlying concepts and ingenious
experimental approaches devised by Libby and his
students that led to the establishment and validation of
the âabsoluteâ radiocarbon technique.
That was but the beginning, however. Subsequent
metrological and scientific advances have included: a
major improvement in
14
C decay counting precision
leading to the discovery of natural
14
C variations; the
global tracer experiment following the âpulseâ of
excess
14
C from atmospheric nuclear testing; the grow-
ing importance of quantifying sources of biomass and
fossil carbonaceous contaminants in the environment;
the revolutionary change from decay counting to atom
counting (AMS: accelerator mass spectrometry) plus
its famous application to artifact dating; and the
demand for and possibility of
14
C speciation (molecular
dating) of carbonaceous substances in reference
materials, historical artifacts, and in the natural
environment.
2. The Birth of Radiocarbon Dating
The year before last marked the 50th anniversary of
the first edition of Willard F. Libbyâs monograph,
Radiocarbon Dating
âpublished in 1952 [2]. Eight
years later Libby was awarded the Nobel Prize in
Chemistry. In a very special sense that small volume
(111 pages of text) captured the essence of the path to
discovery: from the initial stimulus, to both conceptual
and quantitative scientific hypotheses, to experimental
validation, and finally, to the demonstration of highly
significant applications. The significance of Libbyâs
discovery, from the perspective of the Nobel
Committee, is indicated in Fig. 1, which includes also a
portrait of Libby in the year his monograph was pub-
lished [3].
1
The statement of the Nobel Committee
represents an unusual degree of foresight, in light of
unsuspected scientific and metrological revolutions
that would take place in ensuing years.
Like many of the major advances in science,
Radiocarbon Dating
was born of Scientific Curiosity.
As noted by Libby in his Nobel Lecture, âit had its
origin in a study of the possible effects that cosmic rays
might have on the earth and on the earthâs atmosphereâ
[4]. Through intensive study of the cosmic ray and
nuclear physics literature, Libby made an important
series of deductions, leading to a quantitative predic-
tion of the natural
14
C concentration in the living bio-
sphere. As reviewed in chapter I of Libbyâs monograph,
and in the Nobel Lecture, the deductive steps included:
(1) Serge Korffâs discovery that cosmic rays generate
on average about 2 secondary neutrons per cm
2
of the
earthâs surface per second; (2) the inference that the
large majority of the neutrons undergo thermalization
and reaction with atmospheric nitrogen to form
14
C via
the nuclear reaction
14
N(n,p)
14
C; (3) the proposition that
the
14
C atoms quickly oxidize to
14
CO
2
, and that this
mixes with the total exchangeable reservoir of carbon
in a period short compared to the ca. 8000 year mean
life of
14
C. Based on the observed production rate of
neutrons from cosmic rays (ca. 2 cm
â2
s
â1
), their near
quantitative transformation to
14
C, and an estimate of
the global carbon exchangeable reservoir (8.5 g/cm
2
),
Libby estimated that the steady state radioactivity con-
centration of exchangeable
14
C would be approximate-
ly [(2
Ă
60)/8.5] or about 14 disintegrations per minute
(dpm) per gram carbon (ca. 230 mBq g
â1
). Once living
matter is cut off from this steady state, exponential
nuclear decay will dominate, and âabsolute datingâ will
follow using the observed half-life of
14
C (5568 years).
2
186
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
1
Figure 1 shows Libby as the author first met him, shortly after the
latter entered the University of Chicago as a graduate student in
chemistry.
2
Production rate and reservoir parameters are taken from the Nobel
lecture [4]; these values differ somewhat from those used by Libby
in [5] and in the first edition of his book [2]. The half-life (5568 a) is
the âLibby half-lifeâ which by convention is used to calculate âradio-
carbon ages;â the current accepted value for the physical half-life is
(5730 ± 40) a [5a].
Two critical assumptions are needed for absolute
14
C
dating: constancy of both the cosmic ray intensity and
size of the exchangeable reservoir on average for many
thousands of years. A graphical summary of the above
points is given in Fig. 2.
Libby first postulated the existence of natural
14
C in
1946, at a level of 0.2 to 2 Bq/mol carbon (1 dpm/g to
10 dpm/g) [5]. His first experimental task was to
demonstrate this presence of ânaturalâ
14
C in living
matter. The problem was that, even at 10 dpm/g, the
14
C
would be unmeasurable! The plan was to search for
natural
14
C in bio-methane, but the background of his
well-shielded 1.9 L Geiger counter (342 counts per
minute) exceeded the expected signal by a factor of
400. Libby and coworkers did succeed in demonstrat-
ing the presence of
14
C in living matter, however. For an
account of their creative approach to the problem, see
their one page article in
Science
, âRadiocarbon from
Cosmic Radiationâ [6].
3
Having detected
14
C in the living biosphere, Libby
and his colleagues had to develop a measurement
technique that was both quantitative and practical. The
thermal diffusion enrichment technique [6] was not: it
demanded very large samples and thousands of (1946)
US dollars âto measure the age of a single mummyâ
[4]. Development of an acceptable technique was
formidable, as outlined in Table 1. A substantial in-
crease in signal was achieved by converting the sample
to solid carbon, which coated the inner wall of a
specially designed âscreen wall counter;â but the back-
ground/signal ratio (16:1) still eliminated the possibili-
ty of meaningful measurements. At this point, Libby
had an inspiration, from the analysis of the nature of the
background radiation [4]. He concluded that it was
primarily due to secondary, ionizing cosmic radiation
having great penetrating powerânegative mu mesons
(
”
â
). By surrounding the sample counter with cosmic
ray guard counters operating in an anti-coincidence
mode, most of the
”
â
counts could be eliminated, result-
ing in a further background reduction by a factor of
twenty, to approximately 5 counts per minute (cpm).
The final background to signal ratio of 0.8 for living
carbon, made possible the measurement of natural
(biospheric)
14
C with a precision under 2 % (Poisson
relative standard deviation) with a total (sample, back-
ground) counting time of just 2 d ([2], Chap. V). Fig. 3
shows the low-level counting apparatus devised by
Libby, with which the seminal
14
C dating measurements
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
187
Fig. 1.
Portrait of W. F. Libby, about the time of publication of the first edition of his monograph,
Radiocarbon Dating
(1952), and statement of the Nobel Committee (1960) [3].
3
To fully appreciate the nature of the experimental impediments and
flashes of insight along the path to discovery, students are encour-
aged to study the original scientific literature, as given here, rather
than restricting attention to subsequent summaries in textbooks.
were made. The
14
C screen wall counter is visible
through the open, 8 inch thick cantilevered steel doors
having a wedge-like closure. The steel âtombâ reduces
the background by about a factor of five. The bundle
of anticoincidence cosmic ray guard counters, seen
surrounding the central counter in the figure, eliminates
some 95 % of the residual background from the
penetrating
”
â
radiation, through electronic cancella-
tion.
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
188
Fig. 2.
Graphical representation of the production, distribution, and decay of natural
14
C
(courtesy of D. J. Donahue).(Parameter values are approximate.)
Table 1
. Libbyâs Measurement Challenge
âą
Cosmic ray neutron intensity: 2 n cm
â2
s
â1
âą
Exchangeable carbon reservoir: 8.5 g cm
â2
âą
Estimated
14
C activity: 14 dpm g
â1
(0.23 Bq g
â1
)
âą
Sample size (detector efficiency): 8 g carbon (5.5 %)
âą
Estimated modern carbon rate 6.2 cpm (min
â1
)
âą
Background rate: 500 cpm (unshielded), 100 cpm (20 cm Fe)
Assumptions:
Constant production rate
Fixed exchangeable C reservoir (uniform distribution)
Perhaps the most valuable metrological lesson from
Libbyâs early work was the extreme importance of
formulating a realistic theoretical estimate for the
sought-after âsignal.â Without that as a guideline for
designing a measurement process with adequate detec-
tion or quantification capabilities, there is essentially
no possibility that natural radiocarbon could have
been found by chance with the then current radiation
instrumentation.
2.1 Standards and Validation
Once the measurement of natural
14
C became
feasible, the immediate task tackled by Libby and his
colleagues was to test the validity of the radiocarbon
dating model. The first step consisted of determining the
zero point of the natural radiocarbon decay curveâ i.e.,
the radioactivity concentration (dpm
14
C per gram C) in
living matter, and to test for significant geographic varia-
tion. This was a major component of the PhD thesis of
E. C. Anderson [7]; the result (
R
o
) was (15.3 ± 0.5) dpm/g
[255 Bq/kg] with no significant deviation from the
hypothesis of a uniform global distribution.
4
The next
step was to measure the
14
C concentrations in selected
historical artifacts of known age, and compare them to
the âabsoluteâ
14
C age. The latter was accomplished by
comparing the artifact
14
C concentration (dpm/g C) to
that of the living biosphere. The absolute age derives
from the inversion of first order nuclear decay relation,
using 15.3 dpm/g and 5568 a as the parameters of the
âabsoluteâ natural
14
C decay curve.
The famous result, utilizing known age tree rings and
independently-dated Egyptian artifacts, is shown in
Chapter I of Libbyâs 1952 monograph and Fig. 4 in this
article. Although the relative measurement uncertain-
ties are moderately large (ca. 1 % to 5 %), the data
provide a striking validation for the radiocarbon dating
method over a period of nearly 5000 years. Note that
the curve shown is
not fit to the data
! Rather, it repre-
sents the absolute, two-parameter nuclear decay func-
tion. (See [8] for detailed information on the validation
samples selected.)
This initial absolute dating function served to estab-
lish the method, but it indicated the need for a univer-
sal radiocarbon dating standard, since the reference
value for the intercept (here 15.3 dpm/g) would vary
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
189
Fig. 3.
Low-level anticoincidence counting apparatus devised by
Libby for the original
14
C measurements that led to the establishment
of the radiocarbon dating technique (Ref. [2], and
Radiocarbon
Dating
(jacket cover) R. Berger and H. Suess, eds., Univ. California
Press, Berkeley (1979).)
Fig. 4.
Radiocarbon dating validation curve (1952): the âcurve of
knownsâ that first demonstrated that absolute radiocarbon dating
âworked.â The validation points represent tree rings and historical
artifacts of known age. The exponential function is
not fit to the data,
but derived from the independently measured half-life and the
14
C
content of living matter ([2], Fig. 1).
4
The neutron intensity in the atmosphere, and hence the
14
C produc-
tion profile, has major variations vertically (because of cosmic ray
absorption with atmospheric depth) and latitudinally (because of
geomagnetic shielding)âSee Figs. 2 and 3 in Ref. [2]. Because
14
C
has such a long mean life (
â
8000 a), however, it was expected that
any residual gradients in the global exchange reservoir would be
undetectable, given the 3 % to 5 % uncertainties of Libbyâs original
measurements (Ref. [2], Chap. I).
among laboratories, if they each made their own
standards. The problem was tackled by the internation-
al radiocarbon community in the late 1950s, in cooper-
ation with the U.S. National Bureau of Standards. A
large quantity of contemporary oxalic acid di-hydrate
was prepared as NBS Standard Reference Material
(SRM) 4990B. Its
14
C concentration was ca. 5 % above
what was believed to be the natural level, so the
standard for radiocarbon dating was defined as 0.95
times the
14
C concentration of this material, adjusted to
a
13
C reference value of â19 per mil (PDB). This value
is defined as âmodern carbonâ referenced to AD 1950.
Radiocarbon measurements are compared to this
modern carbon value, and expressed as âfraction of
modernâ (
f
M
); and âradiocarbon agesâ are calculated
from
f
M
using the exponential decay relation and the
âLibby half-lifeâ 5568 a. The ages are expressed in
years before present (BP) where âpresentâ is defined as
AD 1950. A published estimate for the
14
C concentra-
tion of âmodern carbonâ is given as (13.53 ± 0.07)
dpm/g [9]. In July 1983, a replacement SRM 4990C
was substituted for the nearly exhausted SRM 4990B.
It was prepared from oxalic acid derived from the
fermentation of French beet molasses from harvests of
1977. A copy of the Certificate Analysis of SRM
4990C, together with pertinent references, may be
obtained from the website:
http://nist.gov/srm
[10].
5
Libbyâs successful development of the science of
radiocarbon dating led to the rapid establishment of
more than a hundred dating laboratories world-wide,
the initiation of a journal supplement that later became
the journal Radiocarbon, and the establishment of a
continuing series of triennial RADIOCARBON confer-
ences, the first of which took place in Andover,
Massachusetts in 1954.
3. Natural Variations
Already, by the time the Nobel Prize was awarded,
Radiocarbon Dating appeared to be approaching matu-
rity, with a rich future in application as opposed to new
fundamental discovery. This all changed, however,
when some of the fundamental assumptions proved to
be invalidâwhat might be considered as the âfailure of
Radiocarbon Dating.â
This âfailureâ resulted from basic advances in
14
C
metrology. New approaches to low-level counting
yielded measurement imprecision that ultimately
approached 0.2 % (rsd);
6
and construction of the
âradiocarbon dating calibration curveâ from meticu-
lously counted annual tree ring segments showed that
assumptions of constancy within different geochemical
compartments of the exchangeable carbon reservoir,
and over time, were invalid. (This is a classic example
demonstrating that one cannot prove the ânull hypo-
thesis;â the validation curve that established the
radiocarbon dating method demonstrated consistency
(validity) only within the errors (uncertainties) of the
validation measurements.)
The failure of the absolute
dating model was, in fact, a notable success.
The revo-
lutionary discovery of natural radiocarbon variations
literally arose out of the ânoiseâ of absolute radiocar-
bon dating, and it transformed the study of natural
14
C
into a multidisciplinary science, giving rise to totally
new scientific disciplines of
14
C solar and geophysics.
At his opening address at the 12th Nobel Symposium
on
Radiocarbon Variations and Absolute Chronology
[12] in Uppsala, Nobelist Kai Siegbahn emphasized
that âThis subject is [now] interesting to specialists
in many different fields, as can be seen from the list
of participants, showing archaeologists, chemists,
dendrochronologists, geophysicists, varved-clay geolo-
gists, and physicistsâ (Ref. [12], pp. 19f). An early
version of the dendrochronological
14
C calibration
curve, presented by Michael and Ralph at the
Symposium, is given in Fig. 5 (Ref. [12], p. 110).
7
The
Bristlecone pine, as shown in the figure, has made a
seminal contribution to the science of dendrochronolo-
gy, and through that, to the study of natural
14
C varia-
tions. It is considered by some to be the worldâs âoldest
living thing,â with a single tree containing annual rings
going back 4000 years or more. It is clear from Fig. 5
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
190
5
Several secondary standards for
14
C dating are available through
the International Atomic Energy Agency. These materials, designated
IAEA C1 â C8, consist of wood, cellulose, sucrose, and carbonate;
they cover a range of 0.00 pMC to 150.6 pMC, and have been sub-
ject to an international comparison [11]. Note that pMC (percent
modern carbon) refers to
f
M
expressed as a percentage.
6
The deciding factor for high precision
14
C measurement was the
successful development of CO
2
gas proportional counting, after
several failed attempts. Compared to Libbyâs solid sample (graphite)
technique, the CO
2
method resulted in smaller sample sizes and
efficiency enhancement by nearly a factor of twenty.
7
The relatively imprecise dendro-calibration curve in Fig. 5 extends
to ca. 5000 BC. Meanwhile, the radiocarbon dating calibration func-
tion has undergone considerable refinement: it now comprises an
extensive database, and it has become an essential element of
all
radiocarbon dating. The 1986 Calibration Issue of the journal
Radiocarbon
[13] has a compilation going back to ca. 8000 BC.
More recent attempts at extending the record much further back in
time have utilized
14
C comparisons with other dating methods,
notably U/Th disequilibrium dating. By this means, calibration data
have been given for periods beyond 20 000 BC [14].
that the dendrochronological age shows a significant
departure from the absolute
14
C (nuclear) age, begin-
ning about three thousand years ago, and continuing
through the end of this series of measurements (ca.
5000 BC). These newly discovered deviations from the
absolute dating model, of course, posed new scientific
questions: what are the causes of the deviations, and
can we use them to better understand Nature? In fact,
the dendro-calibration curve serves dual purposes. For
more classic âdatingâ disciplines, such as archaeology,
anthropology, and geology (event dating), it gives an
empirical correction function for the simple radio-
carbon ages (BP) derived from the first order decay
relation. For solar and geophysics and related disci-
plines, it gives the potential for the quantitative investi-
gation of the causes of the variations.
The Nobel Symposium serves as a rich resource for
information about the natural
14
C variations. An excel-
lent exposition of the three prime causative factors is
given by Hans Suess (Ref. [12], pp. 595-605). These
are: â(1) changes in the
14
C production rate due to
changes in the intensity of the [earthâs] geomagnetic
field; (2) ... modulation of the cosmic-ray flux by solar
activity; (3) changes in the geochemical radiocarbon
reservoirs and rates of carbon transfer between them.â
The major departure (ca. 10 %) seen in Fig. 5 is consid-
ered to be due to the geomagnetic field, corresponding
to a factor of two change in its intensity over the past
8000 years [15]. This has given major impetus to the
science of archaeomagnetism. The other two factors
are considered responsible for the partly periodic
fine structure exhibited in the curve, with varying
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
191
Fig. 5.
Radiocarbon Variations, discovered by comparison of high precision radiocarbon âdatesâ
with high (annual) accuracy tree ring dates. The plot, which covers the period from about
5000 BC to the present, represents an early version of the radiocarbon dating calibration curve
([12], p.110). The photo shows the Bristlecone pine, the major source of dendrodates extending
back many millennia (Photo is courtesy of D. J. Donahue).
amplitudes of about 1 % to 2 %. (See Figs. 1, 2 in the
Suess article, respectively, for plots of the first order
(geomagnetic) and second order (fine structure) devia-
tions from the ideal exponential decay function (âradio-
carbon ageâ).)
A fascinating link exists between dendrochronology
and radiocarbon age, related to climate. That is, tree
rings by their width time series, like ice cores by their
18
O time series, give insight into ancient climate [16].
This, in turn, may be linked to the aforementioned
14
C
variations from changing solar activity and/or varia-
tions in geochemical reservoirs. Fig. 6 represents a
famous example of the inter-relationships among solar
activity (sunspots), natural radiocarbon variations, and
climate (Ref. [15], Fig. 5a; Ref. [16], p. 615). The upper
part of the figure shows the correlation between the
sunspot record (circles, and ca. 11 year cycles) and the
14
C variations. The period of low solar activity, and
correspondingly increased
14
C activity, peaking at about
1500 AD and 1700 AD is striking. The lower part of
the figure suggests a strong link to global climate,
represented here by the âlittle ice age.â
4. The Bomb
Atmospheric nuclear testing had an unintended but
profound impact on
14
C geoscience. It approximately
doubled the
14
C concentration in atmospheric CO
2
, and
consequently in living matter, by the mid-1960s. This
came about because neutrons released from nuclear
fission (or fusion) react with atmospheric nitrogen by
exactly the same reaction,
14
N(n,p)
14
C, as the secondary
neutrons from cosmic rays. The âbomb pulseâ of excess
14
C was recorded in all parts of the living biosphere,
from vintage wine [17] to contemporary tree rings [18].
It was characterized by a sharp injection of
14
C in the
early 1960s, followed by relatively slow geochemical
decay after the limited (atmospheric) nuclear test ban
treaty. Totally new and unanticipated opportunities to
perform global tracer experiments resulted from this
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
192
Fig. 6.
Radiocarbon Variations and Climate: the influence of solar activity (sunspot record) (top) on
14
C concentrations (cosmic ray production
rates) and climate (Maunder Minimum temperature record) (bottom) [15, 16].
sudden, widespread injection of anthropogenic
14
C into
the biogeochemical system.
4.1 Excess
14
C as a Global Geochemical Tracer
An extensive world-wide program of monitoring the
excess atmospheric
14
CO
2
began with the onset of
nuclear testing and continues today. Results of precise
measurements of the input function for excess
14
CO
2
are shown in Fig. 7 (Ref.[19]; Ref. [20], Chap. 31,
(I. Levin, et al.)). Use of this known pulse of excess
14
C
as a tracer has allowed scientists to study exchange and
transport processes in the atmosphere, the biosphere,
and the oceans on a scale that would otherwise have
been nearly impossible. Simple visual examination of
Fig. 7 shows, for example, that the excess atmospheric
14
C injected in the northern hemisphere gave an attenu-
ated signal in the southern hemisphere, and that there
was a lag time of approximately 2 years.
Nowhere has the bomb pulse been more important
than in furthering our understanding of the dynamics
of the ocean. A comprehensive program (GEOSECS:
Geochemical Ocean Section Study) to follow the plume
of excess
14
C as it diffused in the Atlantic and Pacific
oceans was initiated in the 1970s. A small example of
the findings is given in Fig. 8, where we find a nearly
uniform distribution below the mixed layer, indicating
rapid vertical transport in the North Atlantic, in contrast
to model predictions [19, 21]. The scientific impact of
this massive tracer study of ocean circulation is strik-
ing, considering, for example, the new knowledge it
brings regarding the effects of the oceans on pollutant
and heat transport and climate [22].
8
4.2 The Second (Geochemical) Decay Curve of
14
C: Isotopic-Temporal Authentication
Geochemical relaxation of the excess atmospheric
14
C after about 1970 has resulted in a second (short-
lived) âdecay curveâ for
14
C (tail of the input function,
Fig. 7). This has made possible a new kind of radio-
carbon dating, where modern artifacts and forgeries,
food products, forensic biology samples, and industrial
bio-feedstocks can be dated with near annual resolution
[24]. As a result of the new submilligram measurement
capability (Sec. 6), short-term radiocarbon dating is
beginning to achieve commercial importance, as
exemplified by its application to the dual isotopic
(
13
C,
14
C) fingerprinting and time stamping of industrial
materials.
A case in point is the Cooperative Research and
Development project between the NIST Chemical
Science and Technology Laboratory and the DuPont
Central Research and Development Laboratory [25].
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Journal of Research of the National Institute of Standards and Technology
193
Fig. 7.
Input function of excess (âbombâ)
14
C: a global tracer for carbon cycle dynamics
in the atmosphere, biosphere, and oceans [19].
8
The advent of accelerator mass spectrometry, as discussed in
Sec. 6 of this article, has given a major boost to our knowledge of
ocean circulation. Information gained through the GEOSECS
program has been greatly amplified in the World Ocean Circulation
Experiment (WOCE), where requisite sample sizes were reduced
from 200 L of sea water each, to less than 1 L; and the
14
C ocean
circulation database grew by more than 10 000 dates during the
1990s [23].
The goal of the project was to demonstrate the capabil-
ity to authenticate and date renewable (biosourced)
feedstocks, chemical intermediates, and finished
industrial products using high accuracy dual isotopic
(
13
C-
14
C) âfingerprinting,â traceable to NIST. The
specific project, as outlined in Fig. 9, was directed
toward the unambiguous identification of the copoly-
mer polypropylene terephthalate (3GT)) produced from
the biosourced monomer 1,3-propanediol (3G), which
was derived from corn as feedstock. (Terephthalic acid
(TPA) served as the complementary monomer.)
Isotopic discrimination was essential because it is not
possible chemically to distinguish the biosourced 3G
and 3GT from existing industrial materials that are
fossil feedstock (petroleum) based. The ability to estab-
lish a unique isotopic fingerprint for the DuPont
biotechnology materials was critical for the identifica-
tion of the product as a unique composition of matter,
and to track it in commerce. The work represents a
frontier of high accuracy, dual isotope metrology, with
13
C data
u
r
< 0.01 %) serving to discriminate among
different photosynthetic cycles, and
14
C data (
u
r
< 0.5 %)
serving both for quantitative fossil-biomass apportion-
ment and for dating the year of growth of the biomass
feedstock.
A graphical summary of the results of the project is
presented in Fig. 10, which shows the dual isotopic
signatures of the copolymer (3GT) and bio-sourced
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
194
Fig. 8.
Excess
14
C and ocean circulation (GEOSECS). Model (left) and experimental
(right) vertical transects of bomb
14
C in the North Atlantic [19].
monomer (3G); as well as values for isotopic reference
materials (S1: SRM 4990B [oxalic acid]; S2: IAEA C6
[ANU sucrose]; S3: SRM 1649a [urban dust])., and
pre-existing materials (3G
âČ
, 3G
âł
). The dashed line
joining the copolymer end members (3G, TPA) demon-
strates
isotopic-stoichiometric mass balance.
Rec-
tangular regions in red define the âscope of claimsâ
(authentication regions) for the new isotopic composi-
tions. The blue âxâ in the figure represents data for an
independent batch of the monomerâsent to NIST
âblindâ to test the validity of the authentication region
for bio-sourced 3G. The results show both that the
test was successful and that the separate production
batches of the 3G monomer had unique isotopic signa-
tures. The approximately ten-fold expansion of the
isotopic data for two independent batches (A, B) of
corn-glucose (bottom right) demonstrates the dual
isotopic discrimination capability of the technique. In
fact, using the short term âdecayâ curve of
14
C (Fig. 11),
it was possible to date the two batches to the nearest
year of growth, 1994 (A) and 1996 (B), respectively.
(Standard uncertainty bars shown.)
5. Anthropogenic Variations; âTrees
Polluteâ
The achievement of high precision, low background
counting, discussed in Sec. 3, led also to the first
isotopic evidence of global pollution with fossil CO
2
â
named the âSuess Effect,â after its discoverer. A
dramatic monotonic drop in the
14
C/
12
C ratio in tree
rings beginning in the late 19th century, reflecting the
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
195
Fig. 9.
Polypropylene Terephthalate: biomass and fossil feedstocks. The 1,3, propane-
diol monomer is derived from a renewable (biomass) feedstock via laboratory biotech-
nology: conversion of glucose or cornstarch using a single microorganism. The copoly-
mer has potential large volume demand, and is useful as a fiber, film, particle, and a
molded article [25].
use of coal during the Industrial Revolution, showed a
2.5 % fossil carbon dilution effect by the 1950s (Ref.
[12], p. 289), after which it was eclipsed by the vast
injection of âbombâ carbon. Thus began still another
field of
14
C science: the investigation of
anthropogenic
variations,
particularly as related to environmental
pollution.
5.1 Fossil-Biomass Carbon Source Apportionment
Research on more specific local or even regional
carbonaceous pollution began slowly, because of the
massive samples required. Heroic sampling efforts in
the late 1950s demonstrated the principle by measure-
ments of particulate carbon pollution in U.S. urban
atmospheres [26, 27]. After a lapse of two decades,
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
196
Fig. 10.
Unique Isotopic Signatures: the
14
C-
13
C plane [25]. The main panel shows dual isotopic signatures f:or (1) NIST (S1, S3) and IAEA (S2)
traceability standards, and (2) glucose from biomass (a), the new bio-sourced monomer 3G (b) (from cornstarch), the resulting copolymer 3GT
(d), and pre-existing products 3G
âČ
, 3G
âł
(c, e). Expanded views of the authentication regions (red rectangles) for the copolymer (left) and monomer
(center) are given in the bottom panels, plus
â
10-fold expansion (right) of the isotopic data for independent batches (A, B) of a biomass feedstock
(glucose from corn). The blue âxâ represents a blind (3G) validation sample.
research in this area was renewed by the author, stimu-
lated by a 1975 article in
Science
reporting that the
culprit for a severe case of urban pollution in tidewater
Virginia might be hydrocarbon emissions from trees
[28]. The evidence was chemical and controvertible:
plausible, but circumstantial evidence suggested that
the air pollution was due to hydrocarbon emissions
from trees rather from automobile exhaust or evapora-
tion from nearby industrial and military storage tanks.
The article concluded that âthe relatively unsophisti-
cated monitoring of [organic] pollutant concentra-
tions ... will rarely be of value in identifying [pollutant]
sources ...â Recognizing immediately that
14
C could
function as an undisputed discriminator, we decided to
design miniature low level counters, capable of meas-
uring just 10 mg carbon samples, more than two orders
of magnitude smaller than those used in the two earlier
studies. Apart from forest fires, we found that the trees
were
not
the prime culprits, except for the case where
humans were using the trees for fuel! A review of
research in this area in the ensuing 20 years is given in
Ref. [29].
One illustration of
14
C aerosol science is given in
Fig. 12. It is drawn from perhaps the most extensive
study to date of urban particulate pollution using
14
C.
The multi-year, multidisciplinary study of the origins of
mutagenic aerosols in the atmospheres of several
U.S. cities, focussed on Albuquerque, New Mexico
during the winter of 1984-1985. The photos show the
tremendous impact on visibility from particulate pollu-
tion from rush hour traffic. Results of the two month
study of particulate carbon proved that daytime pollu-
tion (up to
â
65 %) was dominated by motor vehicle
emissions (fossil carbon), and nighttime pollution
(up to
â
95 %), by residential woodburning (biomass
carbon), with the mutagenicity (potency) of the motor
vehicle particles more severe by a factor of three [30].
Particulate carbon aerosols are now widely recognized
as an extreme health hazard in a number of U.S. cities;
and except for periods dominated by wildfires, major
studies including
14
C measurements have produced
incontrovertible evidence that the urban episodes are
dominated by fossil carbon, largely from motor vehicle
exhaust [31].
Quantitative apportionment of natural and anthro-
pogenic sources of particulate carbon, methane, carbon
monoxide, and volatile organic ozone precursors in
the atmosphere, meanwhile, has seen a significant
expansion thanks to the sensitivity enhancement
of accelerator mass spectrometry (AMS) [32, 33].
Most recently, with the emergence of micromolar
14
C
AMS, and GC/AMS, the ability to âdateâ individual
chemical fractions in small samples is having important
impacts on both artifact age accuracy, and our under-
standing of perturbations of the human and natural
environments by fossil and biomass carbonaceous
species. (See Section 7).
197
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Fig. 11.
Short term
14
C âdecayâ curve, representing geochemical relaxation of excess
atmospheric
14
C from nuclear testing [Levin et al., in (Ref. [19]; Ref. [20], Chap. 31).
Information critical for the discussion in Sec. 7.2.1 is indicated by the arrowânamely, the
sampling date and corresponding biomass
14
C enrichment for SRM 1649a (urban dust).
198
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Journal of Research of the National Institute of Standards and Technology
Fig. 12.
Anthropogenic
14
C variations: fossil-biomass carbon apportionment of particu-
late air pollution in Albuquerque, New Mexico. (Photos showing visibility reduction in
early morning (top) and mid-afternoon (bottom) are courtesy of R .K. Stevens [30].).
14
C
measurements quantified atmospheric soot from motor vehicles and residential wood-
burning, and helped apportion concomitant data on particulate mutagenicity.
6. Accelerator Mass Spectrometry
6.1 The Invention
The second revolution in
14
C measurement science
was the discovery of a means to count
14
C
atoms
, as
opposed to
14
C
decays
(beta particles). The potential
impact on sensitivity was early recognized: inverting
the first order nuclear decay relation, one finds that the
ratio of the number of
14
C atoms to the number of
14
C
decays for any given sample is simply (
Ï
/t
), where
Ï
is
the mean life (8270 a for
14
C), and
t
is the counting time
used for measurement of the disintegrations. Allowing
for the difference in relative detection efficiency
between AMS and low-level counting, and setting
t
to 2 d,
gives a sensitivity enhancement of roughly 10
4
, in favor
of AMS. This implies a dating capability of sub-
milligram amounts of modern carbon.
The prize of radiocarbon dating at the milligram
level was so great that major efforts were made
to refine mass spectrometric techniques to render the
1.2
Ă
10
â12 14
C/
12
C ratio of modern carbon measurable;
but, like Libbyâs initial attempt to count natural radio-
carbon (without enrichment), natural
14
C proved
unmeasurable by conventional mass spectrometry.
Impediments from molecular ions and the extremely
close isobar (
14
N:
â
m/m
= 1.2
Ă
10
â5
) were over-
whelming. Success came in 1977, however, when high
energy (megavolt) nuclear accelerators were used as
atomic ion mass spectrometers [34-36]. Two measure-
ment ideas held the key: (1) Negative carbon ions are
produced by a sputter ion source, using graphite as the
target. (2) Following low energy mass selection, atom-
ic and molecular negative ions are injected into an
accelerator tube with a megavolt potential. The major
isobar is eliminated because nitrogen does not form a
stable negative ion. Passage of the high energy ions
through a stripper gas or foil destroys all molecular ions
through the âcoulomb explosion,â leaving only atomic
carbon ions in the +3 or +4 charge state.
14
C/
12
C ratio
measurements down to ca. 10
â15
are thus made possible.
Typical sample sizes are 0.5 mg to 1 mg; modern
carbon yields 10 000 counts in just a few minutes;
and instrument backgrounds are negligible (
â€
0.2 %
modern, equivalent to a
14
C age of
â„
50 000 years BP).
A diagram of the accelerator at one of the leading
facilities is given in Fig. 13 [37]. The dramatic impact
of high energy (atomic ion) mass spectrometry is
shown in Fig. 14, where it is clear that natural
14
C is
quite unmeasurable by low energy (conventional) mass
spectrometry due to molecular ions exceeding the
14
C signal by more than eight orders of magnitude
(Ref. [20], Chap. 16]! Excellent reviews of the history,
principles, and applications of AMS are given in
Ref. [20] by H. Gove (Chap. 15) and R. Beukens
(Chap. 16).
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Fig. 13.
AMS: tandem accelerator at ETH, ZĂŒrich. Negative carbon ions, produced with a Cs
+
sputter ion source, undergo low energy mass
resolution and then are injected into the 4.5 MV accelerator tube. Molecular ions are destroyed by the stripper gas, and emerging 18 MeV C
+3
beams of
12
C,
13
C, and
14
C are mass analyzed and measured in current (stable C ions) and event (
14
C ions) detectors [37].
As noted in the reviews by Gove and Beukens, the
AMS revolution has extended well beyond
14
C, spawn-
ing a totally new research area in long-lived isotopic
and ultra trace stable cosmo- and geo-chemistry and
physics through its capability to measure
3
H,
14
C,
26
Al,
36
Cl,
41
Ca, and
129
I, and most recently, selected actinides.
Within one year of the publications announcing suc-
cessful
14
C AMS, another continuing series of interna-
tional conferences was born. The first international
AMS conference took place in 1978 in Rochester, New
York. These conferences have continued on a triennial
basis, with each proceedings occupying a special AMS
conference issue of the journal,
Nuclear Instruments
and Methods in Physics Research
.
6.2 The Shroud of Turin
The radiocarbon dating of the Turin Shroud is
arguably the best known dating application of acceler-
ator mass spectrometry, at least to the lay public. It
could not, or at least it would not have taken place with-
out AMS, because most decay (beta) counting tech-
niques would have consumed a significant fraction of
this artifact. Although still a destructive analytical tech-
nique, AMS required only âa postage stampâ amount of
the linen cloth (Ref. [20], Chap. 15). This particular
exercise is having a metrological impact well beyond
the radiocarbon date,
per se.
This is shown, in part, by
widely accepted statements (1) concerning scientific
investigations of the Shroud, and (2) following publi-
cation of the
Nature
article announcing radiocarbon
dating results (Fig. 15; Ref. [38]).
1: âThe Shroud of Turin is the single, most studied
artifact in human history.â
2: âThe
Nature
(
14
C) article has had more impact on
Shroud research than any other paper ever written on
the subject.â
The article, which was prepared by three of the most
prestigious AMS laboratories, is available to the gener-
al public on the web (
www.shroud.com/nature.htm
).
Together with public television [39], it is helping to
create a broad awareness and understanding of the
nature and importance of the AMS measurement capa-
bility. Secondly, because of controversy surrounding
the meaning of the radiocarbon result, measurement
aspects of artifact dating have been given intense
scrutiny. Such scrutiny is quite positive, for it gives the
possibility of added insight into unsuspected phenome-
na and sources of measurement uncertainty.
The Turin Shroud is believed by many to be the
burial cloth of Christ. The documented record, how-
ever, goes back only to the Middle Ages, to Lirey,
France (ca. 1353 AD) with the first firm date being
1357 AD when it was displayed in a Lirey church.
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Journal of Research of the National Institute of Standards and Technology
Fig. 14.
Conventional (top) vs accelerator (high energy) (bottom) mass
spectrometry:
14
C/
12
C sensitivity is enhanced by more than eight orders
of magnitude through destruction of molecular ions (and unstable N
â
)
(Ref. [20], Chap. 16).
201
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Journal of Research of the National Institute of Standards and Technology
Fig. 15.
The Turin Shroud. Shown in the montage are: (15a, upper left), the cover of the issue of
Nature
(16 February 1989) reporting the results
of the
14
C measurements by AMS laboratories in Tucson, ZĂŒrich, and Oxford; and three singular features of the artifact: (15b, lower left),
the
â
50 mg dating sample received by the Tucson laboratory, showing the distinctive weave (3:1 herringbone twill), with dimensions about
1 cm
Ă
0.5 cm; (15c, upper right), the characteristic negative image, considered by some as a remarkable piece of mediaeval art; and (15d, lower
right), a microphotograph by Max Frei showing individual fibers supporting pollen grains of presumed unique origin [38, 39].
202
Radiocarbon dating was seen immediately as a defini-
tive method to decide whether the âLirey Shroudâ
could have come from flax grown in the 1st century
AD. The Shroud image, considered by some to be the
skilled work of a mediaeval artist, shows a full length
image of a crucified man; but as a
negative image
[Fig.
15c].
9
Prior to the AMS measurements, the Shroud was
subject to intensive examination by photography, spec-
troscopy, art and textile analysis, and palynology [38-
40]. The unique herringbone twill [Fig. 15b] is consid-
ered consistent with a 1st Century date; and pollen
grains found on the cloth [Fig. 15d] are stated by Max
Frei to have originated from a plant found only in the
region of Jerusalem. Radiocarbon dating of the cloth,
however, yielded a result of 1262 to 1384 AD (95 %
confidence interval) [38].
Apart from sampling,
10
the AMS measurements were
performed taking the strictest quality control measures.
Three highly competent laboratories were selected: the
University of Arizona, Oxford University, and the
Swiss Federal Institute of Technology [ETH] in
ZĂŒrich. Samples of the Shroud, plus three control
samples of known age, were distributed blind to the
three laboratories. Control of this operation (distribu-
tion of samples, collection of results) was the responsi-
bility of Michael Tite of the British Museum. The
accuracy and precision of the interlaboratory data for
the control samples were outstanding, leaving no doubt
as to the quality of the AMS measurement technique
(Fig. 16). Sample-1 (Shroud) results, however, were
just marginally consistent among the three laboratories,
prompting the authors of Ref. [38] to state that âit is
unlikely that the errors quoted by the laboratories for
sample-1 fully reflect the overall scatter.â Consistent
with the discussion in Sec. 2, the
14
C age measurements
are reported in â
14
C years BP.â Transformation of these
ages to calendar ages must take into account the natu-
ral
14
C variations, using the dendrochronological
calibration curve [13]. The transformation is shown in
Fig. 17, which demonstrates also an interesting aspect
of the non-monotonic calibration function: namely,
exclusion of the period between 1312 AD and 1353 AD
from the 95 % confidence interval. In addition, an
interesting link exists between this figure and
Fig. 6 (Maunder Minimum), in that the same solar-
activity-induced
14
C variations are represented. A com-
parison of the two figures shows that the radio-
carbon date (691 BP), near the end of a significant
calibration curve protrusion (Fig. 17), corresponds to
the end of the 13th century warm period having high
solar activity (Fig. 6).
Consistency of the AMS results with the existing
(Lirey) documentation seems compelling, but a wave
of questioning has followedânot of the AMS method,
but of possible artifacts that could have affected the
linen and invalidated the
14
C result (Ref. [40], Chap. 1,
Refs. [41], [42]). A sampling of the creative hypotheses
put forward is given in Table 2. The first, for example,
is based on the premise that nuclear reactions involving
the substantial amount of deuterium contained in a
human body could produce neutrons, which might then
produce excess
14
C through the (n,p) reaction, making
the age too young. The proposed deuteron reactions,
however, are either qualitatively or quantitatively
inaccurateâbarring an unnatural burst of high energy
photons (photofission). The third proposal raises the
question of non-contemporaneous organic matterâ
whether from incompletely removed carbon contami-
nation from âoil, wax, tears, and smokeâ that the cloth
had been exposed to, or from bacterial attack and
Volume 109, Number 2, March-April 2004
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9
Figure 15c and 15d images are from the documentary prepared by
the British Broadcasting Corporation which is hereby acknowledged
[39]; Fig. 15b is courtesy of D. J. Donahue.
10
The critical, non-AMS issue relates to sample validity. The origi-
nally agreed upon sampling protocol was to have involved seven
laboratories, two measurement techniques (decay and atom [AMS]
counting), and multiple samples representing different regions of the
cloth. Shortly before the event, however, the scheme was changed to
restrict the number of laboratories (all AMS) and the number of
samples to three,
all taken from the same location.
The sampling
location, near a corner of the Shroud, and near an area damaged by
the fire of 1532 AD, is considered an unfortunate choice, because
of the possibility of exogenous carbon from the fire, repairs, and
organic contamination from handling through the ages [40, 41].
Organic contamination cannot be dismissed. Recent observations
indicate the presence of a bacterially-induced âbioplasticâ coating on
Shroud fibers, as has sometimes been found on mummy wrapping
fabric (leading to erroneous dates). According to [42] (Gove, et al.),
such bioplastic contamination would not have been removed by the
conventional pre-treatment methods applied to the Shroud samples.
Qualitatively, such contamination would lead to a more recent date;
quantitatively, if the contamination were all from the 16th Century, it
would need to represent roughly 70 % of the carbon present, to shift
a first century date to the observed result. (For recent, late 20th
Century contamination, roughly 40 % contamination carbon would
be required.) In a 2002 review article posted to the shroud website,
www.shroud.com/pdfs/rogers2.pdf
, [38], Rogers and Arnoldi
question the bioplastic hypothesis, on the basis of detailed chemical
analysis of fibers from the âRaes sampleâ which was taken from a
region adjacent to that of the
14
C samples. Quantitatively, these
authors suggest that the coating would contribute only a few percent
to the sample carbon; qualitatively, they believe that it is a poly-
saccharide gum (probably Gum Arabic) that would be removed by
the
14
C pretreatment chemistry. Nevertheless, Rogers and Arnoldi
question the validity of the
14
C sample, partly because of the
presence of cotton and other chemical differences between the
adjacent (Raes) sample and the main shroud material.
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Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 16.
AMS
14
C dating results (âblindâ) for the Turin Shroud (sample-1) and three control samples of known
age (samples-2,3,4), from the three AMS laboratories: Z (ZĂŒrich), O (Oxford), and A (Arizona). Dates are
expressed as âRadiocarbon Yearsâ before present (BP); uncertainties represent 95 % confidence intervals [38].
Fig. 17.
Transformation of the Radiocarbon Age (BP) to the Calendar
Age (AD) of the Shroud. The
14
C age (95 % CI) of (691 ± 31)
BP corresponds to a two-valued calendar age as a result of the non-
monotonic radiocarbon dating calibration curve. As indicated in the
figure, the projected calendar age ranges are: (1262â1312) AD and
(1353â1384) AD [38].
deposit over the ages. Apart from the effects of such
factors on the Shroud, the issue of organic reactions and
non-contemporaneous contamination of ancient materi-
als can be a very serious and complex matter, deserving
quantitative investigation of the possible impacts on
measurement accuracy.
10
Research questions of this
sort, including the classic problem of dating ancient
bone, form one of the key stimuli for the development
of âmolecular datingââthe topic of the following sec-
tion.
7. Emergence of ”-Molar
14
C Metrology
Radiocarbon metrology is at the very moment in the
midst of still another revolution, involving the dating
(or isotopic speciation) of pure chemical fractions:
"molecular dating." For trace species, such as poly-
cyclic aromatic hydrocarbons (PAHs), or remote, low
concentration samples, such as the soot or pollen in the
free troposphere or in ice cores, the sensitivity of AMS
is challenged to its ultimate. In order to understand the
nature of the challenge it is interesting to consider the
limiting factors. In a recent study it was shown that
10 % Poisson "error" (standard uncertainty) can be
achieved with 0.9 ”g modern carbon, whereas machine
background is equivalent to 0.2 ”g or less [43]. Sample
processing blanks, however, may range from 1 ”g to
15 ”g or more, and they may consist of both biomass
carbon and fossil carbon [44]. Thus, the ultimate limit-
ing factor for very small sample AMS is the overall
isotopic-chemical blank. Environmental studies of
14
C
in individual chemical compounds can be successful at
the 1 ”g to 10 ”g level, but only with stringent control
of the variability of the blank. This is in sharp contrast
with small sample, low-level counting where the
Poisson modern carbon limit (ca. 3 mg) and back-
ground limit (ca. 5 mg equivalent) far exceed the typ-
ical sample preparation blank (ca. 40 ”g) [29].
11
Some illustrations of pure compound "dating" by
NIST and collaborators are given in Table 3. The first
item refers to the aforementioned 1 ”g capability, using
"dilution AMS." For thermally stable species such as
soot and pollen, we have the possibility of controlling
the sample preparation blank to less than 0.2 ”g by
applying a "thermal discriminator" at a critical stage of
the process. Microgram level
14
C soot studies have
already been successful in Greenland snow; and pollen
studies hold great promise for ice core dating, and per-
haps even for dating the pollen found by Max Frei on
the Turin Shroud.
12
An important measurement issue
for ice core pollen relates to the amount needed for a
given dating precision. To give a rough estimate:
assuming 50 ng carbon per pollen grain, a pollen
age of 2000 years, and 5 % Poisson imprecision
(
Ïâ
400 years); one would need to collect about 100
pollen grains. This might be accomplished in a few
hours, using the "hand picking" microscope technique
of Long et al. [48].
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Table 2.
Creative Hypotheses
âą
Excess
14
C from deuterium
spontaneous fission; cold fusion
âą
14
C isotopic fractionation/ exchange (fire of 1532 AD)
biased sampling; âageâ depends on location
âą
Bioplastic coating; non-contemporaneous with linen
pretreatment chemistry
11
There is a profound difference between background-limited decay
counting and blank-limited AMS, that may not be widely appreciat-
ed. Although the ultimate limitation in each case is âBâ variability,
when B represents the instrumental background it tends to be reason-
ably well controlled, and under the best of circumstances, Poissonian
[45]. An extra degree of caution is needed, however, when the limit-
ing âBâ is an isotopic-chemical blank. At best, it might be assumed
normal; then replicate-based detection tests and confidence intervals
can be constructed using Studentâs-t. If the blank does not represent
a homogeneous or stationary state (as a reagent blank, well-mixed
environmental or biological compartment, etc.), such tests and inter-
vals can be totally misleading. Non-stationary blanks may exhibit
(geochemically meaningful) structure, or they may be erratic, reflect-
ing a transient source of contamination [46].
12
Molecular dating" of the pure cellulose fraction of the Shroud, or
of the associated pollen, could furnish an interesting consistency test
for the published radiocarbon date. It would be especially interest-
ing to put a "time stamp" on pollen whose point of origin has already
been ascribed to a location 10 km to 20 km east and west of
Jerusalem [47]. Such measurements are made feasible by the reduc-
tion of requisite sample sizes by a factor of ten or more, from what
AMS
14
C dating required sixteen years ago. The question of non-
contemporaneous fiber from 16th Century repairs, for example,
could be addressed by new
14
C measurements on just 100 ”g of
fibers (
â
50, 1 cm linen fibers) from the main part of the Shroud. The
expected standard uncertainty would be equivalent to approximately
120 radiocarbon years ([43], Eq. 1).
Table 3.
Molecular Dating (
14
C AMS at the microgram level)
âą
Dilution AMS
quantifies 0.9 ”g modern carbon (1999)
â soot/ pollen blank controllable to ~0.2 ”g (
Ï â
60 ng)
â challenge: dating pure pollen grains from the Shroud
âą
Fossil and biomass aerosol sources
characterized in remote
atmosphere/ cryosphere (2.9 ”g biomass soot quantified)
âą
Individual amino acids
dated in mammoth bones (LC/AMS)
âą
Individual polycyclic aromatic hydrocarbons
dated in atmo-
spheric particles and marine sediment (GC/AMS)
7.1 Long-Range Transport of Fossil and Biomass
Aerosol
Ongoing multidisciplinary, multi-institutional research
on soot particles in remote and paleo-atmospheres,
which is absolutely dependent on the small sample
dating capability, is indicated in Fig. 18. The upper
portion of the figure relates to climate oriented research
on the sources and transport of fossil and biomass
aerosol to the remote Arctic [49]; the lower portion
relates to atmospheric and paleoatmospheric research at
Alpine high altitude stations and ice cores [50,51]. In the
remainder of this section we present some of the high-
lights and measurement challenges of the first project,
on the long-range transport of carbonaceous particles to
Summit, Greenland.
Cooperative research on this project, between NIST
and the Climate Change Research Center at the
University of New Hampshire (UNH), began in 1994.
It was catalyzed by the discovery of an unusually heavy
loading of soot on one of the air filters used for
7
Be
sampling at Summit, Greenland by Jack Dibb of UNH
[52]. The Summit soot had been ascribed to the combi-
nation of intense boreal wildfire activity in the lower
Hudsonâs Bay region of Canada and exceptional atmos-
pheric transport to central Greenland. Measurement of
14
C in the filter sample yielded definitive evidence for
biomass burning as the source of the soot. On one day
only (5 August 1994), the biomass carbon increased by
nearly an order of magnitude, with scarcely any change
in the fossil carbon concentration on the filter.
Supporting data for the origin of the biomass burning
carbon came from backtrajectory analysis, AVHRR
(infrared) satellite imagery of the source region, and
TOMS (ultraviolet) satellite imagery that was able to
chart the course of the soot particles from the source
wildfires to Summit. The several parts of this remark-
able event are assembled in Figs. 19, 20 [29,49,52].
13
Since snow and ice can serve as natural archives for
atmospheric events, one may expect to find chemical
evidence of prior yearsâ fire seasons in snowpits, firn,
and ultimately ice cores. This is illustrated in the upper
right portion of Fig. 18, which shows depth profile
sampling in a snowpit at Summit, overlaying an energy
dispersive spectrum and SEM image of a char particle
found near the 1994 fire horizon in a 1996 snowpit
[29]. An organic tracer of conifer combustion, methyl
dehydroabietate, was found also at the same depth [53].
Atmospheric science entered a new phase at Summit
during the âWinter-Overâ project (1997-1998) [54].
For the first time, direct sampling of air and surface
snow took place over the polar winter, extending from
June 1997 to April 1998. A special achievement of
micro-molar
14
C âdatingâ was the first seasonal data for
carbonaceous particles, deposited with the surface
snow.
14
The seasonal record for biomass carbon
particles, shown in Fig. 21, was striking [55]. The large
spring peaks, in particular, consisted primarily of
biomass carbon: 0.76 (
u
= 0.03) modern carbon mass
fraction (
f
M
) for sample-1 (WO1), and 0.94 (0.01) mass
fraction for sample-8 (WO8). Beyond the fossil-
biomass apportionment, however, lay questions about
the nature and origin of the carbonaceous aerosol.
Especially intriguing are contrasts between the samples
showing summer [sample-4 (WO4)] and spring
[sample-8 (WO8)] biomass-C maxima in Fig. 21. To
explore these, a âmulti-spectroscopicâ approach was
taken, through which insights and supporting evidence
were derived from a variety of analytical techniques.
Results for one of the microanalytical techniques
employed, laser microprobe mass spectrometry
(LAMMS), are shown in Fig. 22.
15
The figure uses a
principal component projection to summarize multi-
variate (multi-mass) contrasts between the summer and
spring biomass peaks. It shows that the three summer
(WO4) sub-samples tend to favor C
n
â
cluster ions
(n-even), typical of condensed carbon structure (and
graphite), whereas the three spring (WO8) sub-samples
exhibit a more complex, oxygenated structure such as
occurs with biopolymers.
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Journal of Research of the National Institute of Standards and Technology
13
The observed (Fig. 20) vs inferred (Fig. 19) paths of the smoke
plume present an interesting contrast. The TOMS satellite image
shows the smoke approaching the southern tip of Greenland on 3, 4
August 1994 and departing toward Iceland on 6 August. The back-
trajectory model employed in Ref. [52] places the approach at a
somewhat higher latitude, and of course provides no departure
information.
14
The micro-molar
14
C capability was essential for this work
because of the extremely small concentrations of particulate carbon
in the surface snow, especially during the winter
(<10
”g C/ kg snow).
15
Microanalytical methods, such as LAMMS, are crucial for gaining
chemical insight on individual particles, or when only very small
snow (or ice) samples of remote aerosols are available, or needed for
high resolution studies. In contrast to the ng capability of the most
sensitive bulk analysis techniques, LAMMS, can provide useful
chemical data on as little as 20 pg of carbon species [57].
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Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 18.
Submicromolar
14
C apportionment of anthropogenic and natural carbonaceous aerosols at remote sites in Europe and Greenland provides
knowledge of their impacts on present and paleoclimate [49-51].
207
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 19.
Massive (6
d, 3000
km) transport of soot from boreal wildfires in Canada to Summit, Greenland. Left inset [1]:
A
VHRR satellite im
age of wildfire region [29]; Center [2]:
6
d backtrajectories [52]; Right inset [3]: seven-fold,1
d increase in biomass-C (upper
, red curve) at Summit [S] on 5
Aug 1994;
fossil-C (lower
, black curve), was little changed [49, 29]).
208
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 20.
Direct, time lapse observation of the track of the
August 1994 smoke parcel by
T
OMS (dif
ferential UV) satellite imagery [49]. C
onsistent with the
14
C (biomass
carbon) data, the cloud of smoke, indicated by the light turquoise circles, is present over central Greenland for 1 day only
, 5
August
1994.
Findings from other techniques:
âą
Thermal-optical analysis.
Distinctive seasonal
volatilization/decomposition patterns were seen as
samples were heated in a stream on helium. The
summer sample (WO4) had a predominant high
temperature peak at
â
560 °C and little evidence of
charring (4 %), whereas the spring sample (WO8)
had a predominant peak at
â
410 °C and major
charring (19 %). Thermal analysis of a powdered
wood (oak) reference material showed a thermal
peak at the approximately same temperature as
WO8, with 21 % charring, implying the presence
of a major cellulosic component in this sample.
âą
Ion chromatography.
Fire tracers (NH
4
+
, K
+
)
accompanied WO4; soil tracers (Ca
++
, Mg
++
)
accompanied WO8
âą
Backtrajectories.
For WO4, strong transport was
indicated from regions of annual wildfires in the
Canadian Northwest; for WO8, strong transport
was indicated from the agricultural regions of the
upper Midwestâboth representing transport
distances of some 8 Mm.
âą
Electron probe microanalysis.
For WO4, up to
90 % C (mass fraction) was observed in individ-
ual, ”m size particles, with C > O for the most
abundant (core) particles; for WO8, maximum C
particles had a C:O ratio consistent with cellulosic
biopolymer, and C < O for the core particles.
The weight of multi-spectroscopic evidence thus
indicates that the summer (WO4) and spring (WO8)
biomass particles
do not represent the same type of
biomass.
Rather, the WO4 particles appear to include a
soot component from high temperature combustion
(motor vehicles, wildfires). The WO8 particles, whose
carbon derives
almost
entirely from biomass, appear to
have a major biopolymer component, such as cellulose
and other bio-materials associated with soil and vegeta-
tive carbon. These findings are consistent with work by
Puxbaum and colleagues, who have found by direct
chemical analysis, significant amounts of cellulose,
bacteria, and fungal spores in atmospheric particles [58,
59].
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Journal of Research of the National Institute of Standards and Technology
Fig. 21.
First evidence of a seasonal pattern in biomass carbon aerosol in surface snow in central Greenland
[55, 56]. Fundamental differences were found between the biomass carbon peaks in summer (sample-4 [WO4]),
and spring (sample-8 [WO8]) via âmulti-spectroscopicâ macro- and micro-chemical analysis.
7.2 Isotopic Speciation in Ancient Bones and
Contemporary Particles
The dating of ancient bones has been notably
unreliable because of diagenesis and isotopic contami-
nation that occur with millennia of environmental
exposure. Molecular dating of individual amino acids
in such bones has proven to be one of the most effec-
tive means to overcome this problem. Figure 23 shows
dramatically how the apparent radiocarbon age of
the Dent Mammoth changed from ca. 8000 BP to
ca. 11 000 BP, as the dated chemical fraction was
refined from the crude collagen fraction to the individ-
ual amino acids. The known radiocarbon age is given as
â
11,000 BP, based on association with Clovis culture
artifacts, and biostratigraphy [60]. (Note that the
âcalibratedâ or corrected calendar age, derived from
the radiocarbon calibration curve [13], is roughly
1500 years older than the radiocarbon age for this time
period.) The commonly dated organic fractions from
bones (weak acid insoluble collagen [COLL] and
gelatin [GEL]) gave ages that were at odds with the
archaeological evidenceâsuggesting recent humate
contamination. When the diagenesis-resistant molecu-
lar components were isolated (individual amino acids
and the collagen hydrolysates [XAD-HYD]), age
concordance among the individual amino acids
and with the archaeological evidence indicated relia-
bility. Had contamination from bio-intrusive material
having a different chemical (amino acid) pattern
occurred, amino acid age heterogeneity would have
been expected [60]. This work could not have been
accomplished without the ability to date 80 ”g carbon
fractions.
An historical footnote related to this work involves
the question of the ancestors of the North American
Clovis culture. Since the Clovis sites give the earliest
unequivocal data on the âpeoplingâ of the Americas,
it has been of enormous interest to find a geochrono-
logical link to an earlier culture. The most popular
belief that the Clovis progenitors had arrived over
the âBering Land Bridgeâ from Siberia has recently
been put into doubt, however, with new
14
C evidence
that one of the most likely pre-Clovis sites in northeast-
ern Siberia is 4000 years younger than previously
believed. Dating at
â
13 000 calendar years ago, it is
doubtful that migration could have transpired quickly
enough to give rise to the Clovis culture (13 600 to
12 600 calendar years BP) in the North American
Southwest [61].
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Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 22.
PCA biplot of laser microprobe mass spectral data; compositional contrast between particles
from the summer biomass peak (WO4, red: C
n
â
cluster ions favored) and the spring biomass peak
(WO8, green: oxygenates favored) [55].
7.2.1 Urban Dust (SRM 1649a): a Unique
Isotopic-Molecular Reference Material
SRM 1649a is NISTâs most highly characterized
natural matrix Standard Reference Material, and it is
the
only one
for which there are certificate values for
14
C in individual chemical fractions and pure molecular
species. The âcarbonâ portion of the Certificate of
Analysis was developed through an extensive inter-
national interlaboratory comparison, involving eighteen
teams of analytical experts from eleven institutions
[62]. The particle-based SRM, which has been charac-
terized for nearly 200 chemical species and properties,
serves as an essential quality assurance material for a
remarkably broad range of disciplines, from the moni-
toring of pesticides, PCBs, and particulate mutagenic
activity to basic organic geochemistry to isotopic
apportionment of carbonaceous particles. A dramatic
illustration of the
14
C isotopic heterogeneity in this
reference material is given in Fig. 24. The biomass
carbon mass fractions are seen to range from about 2 %
(aliphatic extract) to 38 % (total carbon). Thus, the
aliphatic fraction derives essentially (
â
98 %) from
fossil fuel emissions, and, on average, fossil sources
account for some 60 % of the carbon in these particles.
Note that the Certificate of Analysis [63] provides
14
C data expressed in the proper reference units as
fraction of modern carbon (
f
M
). To emphasize the more
meaningful fossil-biomass carbon source dichotomy,
however, we have chosen to present the information
here in terms of the fraction of biomass carbon.
Conversion is based on the âpost-bombâ enrichment of
14
C in the living biosphere, as shown in Fig. 11.
Sampling for SRM 1649a took place in 1976-1977;
the enrichment factor for biomass carbon at that time,
indicated by the red arrow in the figure, was 1.35.
One of the most important outcomes of the SRM
1649a intercomparison exercise was the set of data
obtained for âelemental carbonâ (EC). EC (sometimes
known as âblack carbonâ) is routinely monitored in
urban and rural aerosols, and it is of major concern
because of its presumed impacts on health, visibility,
and climate (radiation absorption). SRM 1649a
potentially can serve as a key laboratory quality
assurance reference material for EC measurement.
Results of the largest intercomparison to date of EC in
a uniform reference material, however, indicate a
severe measurement problem: relative values for the
reported data span a range of 7.5, showing very signif-
icant method dependence. Three clusters of results
for the mass fraction of EC (relative to total-C), reported
as
information values
on the Certificate of Analysis,
are 0.075, 0.28, and 0.46. (For the
14
C data in Fig. 24,
cluster-1 EC has been labeled âsootâ and cluster-3 EC,
âchar.â
14
C was not determined in cluster-2 EC.)
The fundamental problem is that EC is
not a pure
substance,
so a unique âtrue valueâ for EC may not
exist, in principle.
16
Some interesting insights into the
meaning of certain of the EC results follow, however,
from the
14
C EC speciation data.
211
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 23.
âMolecular Datingâ of individual amino acids in ancient
bone. Radiocarbon ages of commonly dated (collagen, gelatin)
fractions were 2000 to 3000 years too young as a result of environ-
mental degradation; pure molecular fractions (amino acids) were
self-consistent and in agreement with the Clovis culture age [60].
16
Although a âtrueâ (Certified) EC value may be beyond reach, com-
patibility of results from laboratories using the same method suggests
the possibility of method-specific (âoperationalâ) EC
Reference
Values
for this SRM.
Isotopic consistency.
Measurement of
14
C in multiple
chemical fractions offers the possibility of two very
interesting and important consistency tests: (1) assess-
ment of
isotopic-chemical consistency
among chemi-
cally-related fractions, and (2) assessment of overall
isotopic-mass balance.
The first test is illustrated by
comparison of the
14
C content of the EC fraction with
that of the PAH fraction (on average). To the extent that
both components originate from the same source,
acetylenic free radicals that generate polyaromatic
structures in the flaming stage of combustion, one
would expect similar
14
C composition. Such is the case
for
14
C in cluster-1 EC (labeled âsootâ in Fig. 24), but
not for cluster-3 EC (labeled âcharâ). The lack of
isotopic consistency for cluster-3 EC is the stimulus for
the different label, since this manifestation of EC
necessarily reflects a different mix of fossil-biomass
sources than the flaming stage EC, which derives
primarily from fossil fuel carbon.
Regarding the second test, the
14
C data in Fig. 24
demonstrate that isotopic-mass balance cannot be
achieved with the current isotopic-chemical data. Since
the biomass carbon fraction on average (38 % mass
fraction) exceeds that of
all
other measured fractions,
there
must be a significant missing biomass carbon
component
. This matter is addressed in [62], where it is
suggested that unmeasured biopolymers may account
for more than 45 % of the residual (non-extractable,
non-EC) carbon mass. Cellulose is one excellent candi-
date [58].
GC/AMS.
Finally, the âmolecular datingâ of individual
PAH in SRM 1649a epitomizes one of the latest
advances in micromolar
14
C measurement science: the
capability to link chromatographic isolation of pure
chemical compounds to AMS determination of
14
C
/12
C.
Results of applying off-line GC/AMS to six PAHs
recovered from the aromatic fraction of SRM 1649a are
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Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 24.
NIST Standard Reference Material 1649a (âurban dustâ). Photograph of the bulk reference material and derived âfilter samplesâ for QA
of atmospheric elemental carbon (EC).
14
C data listed indicate the mass fraction (%) of biomass-C in the several chemical fractions [29, 62].
shown in Fig. 25. The critical first step was the sequen-
tial isolation of tens of micrograms of the six PAHs in
separate traps by automated preparative scale capillary
gas chromatography [66]. The individually trapped
PAHs were then oxidized and converted to AMS tar-
gets. These results represent the first such data ever
available for an atmospheric particulate SRM, and
although such compounds are only trace constituents of
atmospheric particles (
â
10 ”g/g), they are of great con-
sequence due to their mutagenic and carcinogenic prop-
erties. In this case, as shown in Fig. 25, radiocarbon
dating of the individual PAHs revealed these congeners
to be isotopically heterogeneous, and demonstrated a
basic flaw in the conventional wisdom that the heavier
PAHs, in particular, are more likely to be produced
strictly from fossil fuel combustion sources.
On-line GC/AMS is nearly upon us. The linkage of
gas (or liquid) chromatographic separation, and
direct
injection
of microgram amounts of pure compounds
into the ion source of an accelerator mass spectrometer,
is under active investigation in several AMS laborato-
ries; and it promises a new dimension in the practice of
radiocarbon dating at the molecular level that may have
an impact on archaeology and isotopic biogeo-
chemistry comparable to that of GC/MS on analytical,
physical, organic, and biochemistry [67].
213
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
Fig. 25.
Gas chromatography/accelerator mass spectrometry (GC/AMS): AMS following automated
prep-scale capillary GC yields âdatesâ (equivalent biomass carbon mass fractions) for micromolar
amounts of individual polycyclic aromatic hydrocarbons [63-65]. (Results shown for NIST SRM 1649a;
âI.S.â denotes an internal standard; abscissa indicates retention time (min).)
8. Epilogue
Libbyâs discovery, and the remarkable developments
that followed, arose from a scientific question (freely
translated): âWhat will become of the cosmic ray
neutrons?â It is noteworthy that an âacademic sonâ of
this Nobel Laureate also posed a scientific question to
himself. F. Sherwood Rowlandâs question also led to an
unexpected discovery having major practical import for
mankind: the possible destruction of the stratospheric
ozone layer. Rowlandâs query, also culminating in a
Nobel Prize (1995), was âI began to wonder what
was going to happen to this man-made compound
[trichlorofluoromethane] newly introduced into the
atmosphereâ [68].
May this historical journey into scientific discovery,
as an outgrowth of seemingly simple scientific curiosi-
ty, and the consequent unanticipated scientific-metro-
logical revolutions, encourage students to examine the
original historical literature documenting such discov-
eries, and to realize that profound unforeseen devel-
opments may be in store for a presumably âmatureâ
scientific discipline.
Acknowledgment
This article represents an adaptation and extension of
a recent publication in the
Czechoslovak Journal of
Physics
: âThe Remarkable Metrological History of
14
C
Dating: from ancient Egyptian artifacts to particles of
soot and grains of pollenâ [Czech. J. Phys.
53,
(Suppl.
A) A137-A160 (2003)]. Permission of the Institute of
Physics, Academy of Sciences of the Czech Republic is
gratefully acknowledged. Thanks go also to Cynthia
Zeissler and Ed Mai for assistance in final preparation
of the figures for publication.
Figures are adapted, with permission, from the
following sources. Fig. 1: photo by Fabian Bachrach
(AEC-54-5123-DOE) from page 1 of: de MessiĂšres, N.:
âLibby and the interdisciplinary aspect of radiocarbon
dating.â Radiocarbon 43 (2001) 1-5; copyright 2001
Arizona Board of Regents on behalf of the University
of Arizona. Fig. 3, from Radiocarbon Dating [jacket
cover] (Eds. R. Berger and H. Suess) Univ. California
Press, Berkeley, 1979]. Fig. 4, from Fig. 1 of: Libby,
Willard F., Radiocarbon Dating, Univ. Chicago Press,
Chicago, copyright 1952 (1st edition). Cover and Fig. 5
(plot), from Fig. 1 (p. 110) in: Olsson, I.U., Ed.
Radiocarbon Variations and Absolute Chronology (12th
Nobel Symposium), Almqvist & Wiksell, Stockholm,
1970; copyright, the Nobel Foundation. Fig. 5 (photo),
courtesy of Douglas J. Donahue, University of Arizona.
Fig. 6a (top), reprinted with permission from Fig. 5a in:
Eddy, J.: âThe Maunder Minimum,â Science 192
(1976) 1189-1202; copyright 1976 American
Association for the Advancement of Science. Fig. 6b
(bottom), from the âclimateâ figure (p. 615, last seg-
ment only, labeled âPast 1000 yearsâ) in: Mathews, S.:
âWhatâs happening to our climate,â National
Geographic 150 (1976) 176-615; copyright 1976 the
National Geographic Society. Fig. 7 and Fig. 8, from:
Toggweiler, J.R., Dixon, K. and Bryan, K.:
âSimulations of Radiocarbon in a Coarse-Resolution
World Ocean Model, 2. Distributions of bomb-pro-
duced
14
C,â J Geophys Res 94 [C6] (1989) 8243-8264
(figures 1 and 17, respectively); copyright 1989
American Geophysical Union. Figures 9 and 10 are
adapted from Currie, L.A., et al., âAuthentication and
Dating of Biomass Components of Industrial Materials:
Links to Sustainable Technology,â Nuclear Instruments
and Methods in Physics Research B172 pp 281-287,
copyright (2000), with permission from Elsevier
Science. Fig. 12: photos are courtesy of Robert K.
Stevens. Fig. 13, from Fig. 1 in: Wölfli, W.: âAdvances
in accelerator mass spectrometry,â Nuclear Instruments
and Methods in Physics Research B29 [numbers 1, 2]
pp 1-13, copyright (1987), with permission from
Elsevier Science. Fig. 14, from Fig. 16.2 in: Taylor,
R.E., Long, A., and Kra, R., Eds.: Radiocarbon after
Four Decades: an Interdisciplinary Perspective; copy-
right Springer-Verlag, New York, 1992. Fig. 15c inset
(negative image) and 15d (microphotograph), from:
British Broadcasting Corporation Documentary,
âShreds of Evidenceâ (Timewatch Series), copyright
1988. Fig. 15b, courtesy of Douglas J. Donahue,
University of Arizona. Figures 15a (reprint cover), 16,
and 17, from Damon, P., et al.: âRadiocarbon dating of
the Shroud of Turin,â Nature 337 (1989) 611-615;
copyright Macmillan Magazines Ltd, 1989. Fig. 18,
from: (a) Currie, L.A., et al.: âThe pursuit of isotopic
and molecular fire tracers in the polar atmosphere and
cryosphere,â Radiocarbon 40 (1998) 381-390, 416f;
copyright 1998 Arizona Board of Regents on behalf of
the University of Arizona; and (b) Mark Twickler,
Univ. New Hampshire. Fig. 19 (center, backtrajecto-
ries), from Dibb, J.E., et al., âBiomass burning signa-
tures in the atmosphere and snow at Summit,
Greenland: an event on 5 August 1994,â Atmospheric
Environment 30 pp 553-561 copyright (1996),
with permission from Elsevier Science. Figures 19,
20 adapted from Currie, L.A., et al., âThe pursuit of
isotopic and molecular fire tracers in the polar atmos-
214
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology
phere and cryosphere,â Radiocarbon 40 (1998) 381-
390, 416f; copyright 1998 Arizona Board of Regents on
behalf of the University of Arizona. Fig. 23, from
Fig. 1 in: Stafford, T.W., Jr., Hare, P.E., Currie, L.A.,
Jull, A.J.T., and Donahue, D.: âAccuracy of North
American Human Skeleton Agesâ, Quarternary
Research, 34, pp 111-120, copyright (1990), with
permission from Elsevier Science. Fig. 24, adapted
from Fig. 4 in Currie, L.A., âEvolution and
Multidisciplinary Frontiers of
14
C Aerosol Science,â
Radiocarbon 42 (2000) 115-126, copyright 2000
Arizona Board of Regents on behalf of the University
of Arizona. Fig. 25, from Fig. 2 in: Currie, L.A.,
Klouda, G.A., Benner, Jr., B.A., Garrity, K., and
Eglinton, T.I., âIsotopic and Molecular Fractionation in
Combustion; Three Routes to Molecular Marker
Validation, including Direct Molecular âDatingâ
(GC/AMS),â Atm. Environ. 33 (1999) 2789-2806; pub-
lished by Elsevier Science Ltd.
9. References
[1] W. F. Libby, Nuclear dating: an historical perspective, Nuclear
and Chemical Dating Techniques: Interpreting the
Environmental Record, L. A. Currie, ed., American Chemical
Society Symposium Series No. 176 (1982) Chap. 1.
[2] Willard F. Libby, Radiocarbon Dating, Univ. Chicago Press,
Chicago (1952).
[3] N. de MessiĂšres, Libby and the interdisciplinary aspect of
radiocarbon dating. Radiocarbon
43,
1-5 (2001).
[4] W. F. Libby, Radiocarbon dating (Nobel Lecture), Science
133
,
621-629 (1961).
[5] W. F. Libby, Atmospheric Helium-3 and Radiocarbon from
cosmic radiation, Phys. Rev.
69,
671-672 (1946).
[5a] H. Godwin, Half-life of radiocarbon, Nature
195,
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About the author:
Dr. L.A. Currie is an NIST Fellow
Emeritus in the Chemical Science and Technology
Laboratory. The ideas behind this article were first
conceived about 15 years ago in connection with
lectures at NIH and the University of Maryland, and
as an outgrowth of the authorâs research on environ-
mental radiocarbon while leader of the Atmospheric
Chemistry Group at NIST. The concept and scope of the
article were crystallized in connection with luncheon
talks at the Measurement Science Conference (1995)
and the Radiochemical Measurement Conference
(2001), an invited NIST Sigma Xi lecture (1998), and a
plenary lecture at the Fourteenth Radiochemical
Conference (2002). At the Conference in 2002,
Dr. Currie was presented the I.M. Marci medal, the
highest award of the Czech Spectroscopic Society of the
Czech Academy of Sciences. The National Institute
of Standards and Technology is an agency of the
Technology Administration, U.S. Department of
Commerce.
217
Volume 109, Number 2, March-April 2004
Journal of Research of the National Institute of Standards and Technology