Accepted Manuscript
Title: Radiocarbon: a chronological tool for the recent past
Authors: Quan Hua
PII:
S1871-1014(09)00056-9
DOI:
10.1016/j.quageo.2009.03.006
Reference:
QUAGEO 196
To appear in:
Quaternary Geochronology
Received Date: 2 April 2007
Revised Date:
3 March 2009
Accepted Date: 6 March 2009
Please cite this article as: Hua, Q. Radiocarbon: a chronological tool for the recent past, Quaternary
Geochronology (2009), doi: 10.1016/j.quageo.2009.03.006
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Radiocarbon: a chronological tool for the recent past
Quan Hua
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Australian Nuclear Science and Technology Organisation (ANSTO), PMB 1, Menai, New South
Wales 2234, Australia
E-mail: [email protected]
Tel: +61 2 9717 3671
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Fax: +61 2 9717 9265
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Keywords: Radiocarbon dating, Chronology, Recent past, Carbon-14 wiggle-matching, Bombpulse carbon-14 dating.
Abstract
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The past few hundred years have seen large fluctuations in atmospheric 14C concentration. In part,
these have been the result of natural factors, including the climatic changes of the Little Ice Age,
and the Spörer and Maunder solar activity minima. In addition, however, changes in human
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activity since the middle of the 19th century have released
14
C-free CO2 to the atmosphere.
Moreover, between c. 1955 and c. 1963, atmospheric nuclear weapons testing resulted in a
14
C in the atmosphere. This was followed by a
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dramatic increase in the concentration of
significant decrease in atmospheric 14C as restrictions on nuclear weapons testing began to take
effect and as rapid exchange occurred between the atmosphere and other carbon reservoirs. The
large fluctuations in atmospheric 14C that occurred prior to 1955 mean that a single radiocarbon
date may yield an imprecise calibrated age consisting of several possible age ranges. This
difficulty may be overcome by obtaining a series of 14C dates from a sequence and either wigglematching these dates to a radiocarbon calibration curve or using additional information on dated
materials and their surrounding environment to narrow the calibrated age ranges associated with
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each
14
C date. For the period since 1955 (the bomb-pulse period), significant differences in
atmospheric
14
C levels between consecutive years offer the possibility of dating recent samples
with a resolution of from one to a few years. These approaches to dating the recent past are
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illustrated using examples from peats, lake and salt marsh sediments, tree rings, marine organisms
and speleothems.
1. Introduction
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The past few centuries have been characterised by dramatic and significant environmental
changes. Sub-millennial scale climatic variations have resulted in a shift from the Little Ice Age,
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which began around the 14th century and may have continued to the mid-19th century, to the
warm episode of the last half century. Human activity has also contributed to changes in the
Earth’s environment via land clearing, urbanisation and industrialisation, especially since 1850.
Studies of these changes require a precise and accurate chronological framework, to which
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radiocarbon dating can contribute. Sixty years after the discovery of radiocarbon dating by W.F.
Libby (1946), the method provides one of the most reliable and well-established means of dating
the Holocene and Late Pleistocene. This is indicated by the world-wide existence of over 150
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radiocarbon laboratories (Anon., 2008) that deliver tens of thousands of radiocarbon dates every
year (Geyh, 2005). After a short description of basic radiocarbon dating, this paper discusses the
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features, potential and limitations of the method for dating the past few hundred years. Examples
of the use of the technique in constructing chronologies of recent environmental archives and
dating recent materials are presented.
2. Radiocarbon dating method
2.1. Principles
Carbon has three natural isotopes:
~1.1%, and
14
12
C and
13
C, with relative abundances of ~98.9% and
C or radiocarbon, which occurs only in minute amounts (~1.2 x 10–10% in the
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troposphere, for example) (Olsson, 1968). Carbon-12 and
13
14
C are stable isotopes, while
C is
radioactive. Radiocarbon is produced continuously in the atmosphere by the interaction of the
secondary neutron flux from cosmic rays with atmospheric
14
N, following the reaction
14
N+n
the upper troposphere (Gäggeler, 1995). Following its production,
14
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(neutron) → 14C + p (proton). About 55% of 14C is formed in the lower stratosphere and 45% in
14
C is oxidised to produce
CO2, which is quickly dispersed throughout the atmosphere. The 14C is then transferred to other
carbon reservoirs, such as the biosphere and oceans, via photosynthesis and air-sea exchange of
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CO2 respectively. Living organisms take up radiocarbon through the food chain and via metabolic
processes. This provides a supply of 14C that compensates for the decay of the existing 14C in the
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organism, establishing an equilibrium between the 14C concentration in living organisms and that
of the atmosphere. When an organism dies, this supply is cut off and the 14C concentration of the
organism starts to decrease by radioactive decay at a rate given by the radiocarbon half-life. This
rate is independent of other physical and environmental factors. The time t elapsed since the
t=−
T1/2 ⎛ N(t) ⎞
⎟
ln⎜
ln 2 ⎜⎝ N o ⎟⎠
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organism was originally formed can be determined from:
where T1/2 is the radiocarbon half-life, No is the original
(1)
14
C concentration in the organism and
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N(t) is its residual 14C concentration at time t.
2.2. Contamination
In general, any organism containing carbon and that once lived in equilibrium with
atmospheric 14C can be dated by the radiocarbon method. Typical material for radiocarbon dating
includes wood, charcoal and bones. Before samples are processed for dating, any contaminants
(carbon-containing materials that do not belong to the original sample) must be removed,
otherwise incorrect ages may be determined. According to Hedges (1992), the removal of
contamination, known as the pre-treatment step, can be carried out using two strategies. The first
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involves the physical and chemical removal of contaminants such as soils and sediments from the
surrounding environment, roots and rootlets that may have penetrated from higher up the
sequence, dissolved carbonates carried by groundwater, and humic acids derived from
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decomposed organic materials in the upper part of the sequence (Olsson, 1979; Mook and
Streurman, 1983). This approach is usually employed in the case of samples of charcoal and
wood. The second strategy involves the extraction of a specific contamination-free component
from the sample, such as collagen from bones (Longin, 1971; Hedges and van Klinken, 1992) and
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alpha-cellulose from woods (Head, 1979; Hoper et al., 1998; Hua et al., 2004a; Anchukaitis et al.,
2.3. Measurement methods
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2008).
Two different methods have been used for the measurement of
14
C concentration in a
sample: decay counting and accelerator mass spectrometry (AMS). Since
14
C is radioactive,
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emitting β¯ particles with a maximum energy of about 156 keV, 14C can be measured by detecting
these particles. This decay counting or radiometric method involves measuring 14C by either gas
proportional or liquid scintillation counters (Taylor, 1987). In the case of gas proportional
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counters, pretreated samples are converted to CO2, while liquid scintillation counters employ
benzene synthesised from the samples. By contrast, rather than counting the β¯ particles resulting
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from 14C decay, whose rate is controlled by the long 5730 year half-life of 14C (Godwin, 1962),
the AMS method counts 14C atoms directly (relative to those of the stable carbon isotopes 13C and
12
C in the samples). Compared to the radiometric method, AMS has advantages in terms of
measurement time (from tens of minutes to a few hours for AMS compared to a few days for the
radiometric method) and the quantity of material required for dating (0.1–2 mg of carbon for
AMS compared to 0.5–2 g or more of carbon for the radiometric method) (Tuniz et al., 1998; Jull
and Burr, 2006). For AMS, pretreated samples are converted to CO2 and then graphite. After
three decades of development, samples containing as little as 10–20 µg of carbon can now be
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reliably analysed by AMS (Hua et al., 2004b; de Jong et al., 2004; Santos et al., 2007; Smith et
al., 2007; Petrenko et al., 2008). The ability to analyse small samples using AMS techniques has
opened up opportunities for radiocarbon dating of new materials such as specific amino acids
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extracted from bones (Hedges and van Klinken, 1992), pollen extracted from peats and lake
sediments (Brown et al., 1989, 1992), macrofossils from lake sediments (Goslar et al., 2000;
Kitagawa and van der Plicht, 2000), foraminifera from marine sediments (Broecker et al., 1990;
Hughen et al., 2000) and specific skeletal components of carbonate sediments (Woodroffe et al.,
2.4. Radiocarbon conventional ages
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1999, 2007).
Radiocarbon ages are reported in years before present (BP), where ‘present’ is
conventionally defined as AD 1950. In radiocarbon age calculations, the atmospheric
14
C
concentration in 1950, a hypothetical value, is conventionally set at 100 percent Modern Carbon
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(pMC) (Stuiver and Polach, 1977) or 1 fraction modern carbon (F) (Donahue et al., 1990;
McNichol et al., 2001; Reimer et al., 2004a). As isotopic fractionation (differentiation against
heavier isotopes) occurs in nature, for example during photosynthesis and air-sea exchange of
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CO2, different carbon materials have different δ13C values (Hoefs, 1987). In addition, the
depletion in 14C relative to 12C as a result of fractionation is approximately twice the depletion in
C relative to
12
C (Craig, 1954). Measured
14
C concentrations must therefore be corrected for
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13
isotopic fractionation using δ13C. Conventionally, this is achieved by the normalisation of
measured
14
C values from measured δ13C to δ13C = –25‰ PDB (an average δ13C value for C3
plants) before age calculations are performed.
To simplify the calculation of radiocarbon ages, atmospheric 14C concentration is assumed to
be constant through time, with the implication that all living terrestrial materials have an initial
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14
C concentration of F = 1. From equation (1), the conventional radiocarbon age of a sample S is
defined as:
T1/2
ln(FS )
ln 2
(2)
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t=−
t = − 8033 ln (FS )
or
(3)
where Fs is the 14C concentration in sample S in fraction modern carbon after correction to δ13C =
–25‰ PDB. This value is determined by measuring sample S against
14
C standard reference
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materials such as oxalic acid I (Olsson, 1970), oxalic acid II (Stuiver, 1983) or ANU-sucrose
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(Polach, 1979). T1/2 is the Libby half-life of radiocarbon of 5568 years. This half-life is
approximately 3% shorter than the correct half-life of 5730±40 years (Godwin, 1962). The
discrepancy between the two half-lives is corrected during the radiocarbon calibration process.
Ages up to about 50 000 years (~9–10 half-lives of 14C) can be determined by radiocarbon
2.4. Calibration
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dating.
It is well known that the 14C concentration of the atmosphere has not been constant in the
14
C concentrations are mainly due to
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past (Reimer et al., 2004b). Variations in atmospheric
variations in the rate of radiocarbon production in the atmosphere, caused by changes in the
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Earth’s magnetic field, variability in solar activity and changes in the carbon cycle (Taylor, 1987;
Damon and Sonett, 1991). Long-term (103–104 years) fluctuations in atmospheric
14
C are the
result of changes in the Earth’s magnetic field. Short- and medium-term (101–102 years)
variations in atmospheric 14C are mainly due to variability in solar activity, although centennialscale variability may also be due to changes in geomagnetic field intensity (St-Onge et al., 2003).
The result is that radiocarbon and calendar ages are not identical, and the former ages have to be
converted to the latter using a calibration curve, which describes the atmospheric
14
C
concentration in the past measured in precisely and independently dated materials. The current
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internationally-ratified calibration curve IntCal04 covers the past 26 000 calendar years (cal) BP
(Reimer et al., 2004b). This curve is based on dendrochronologically-dated tree rings for the
period 0–12 400 cal BP. For the remaining period 12 400–26 000 cal BP, the curve is derived
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from independently dated marine samples such as foraminifera and corals, with an assumption of
a constant marine reservoir effect for each sampling site. Beyond the IntCal04 timescale, there are
several published age calibration data sets. These are derived from independently dated materials
such as corals (Fairbanks et al., 2005), foraminifera in marine sediments (Hughen et al., 2006),
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speleothems (Beck et al., 2001) and terrestrial macrofossils in varved sediments (Kitagawa and
van der Plicht, 2000). However, these data sets are not in good agreement beyond 26 000 cal BP
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(van der Plicht et al., 2004) either because the archives from which the data sets are derived have
their own problems and/or because the data sets are based on a simple assumption of a constant
radiocarbon reservoir effect through time (van der Plicht, 2002).
Calibration of 14C ages is usually undertaken using a computer program. Several calibration
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programs are available on-line. These include CALIB (http://radiocarbon.pa.qub.ac.uk/), Oxcal
(http://c14.arch.ox.ac.uk/embed.php?File=oxcal.html)
and
CalPal
(http://www.calpal.de/).
Additional calibration programs can be found on the Radiocarbon journal website at
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http://www.radiocarbon.org/Info/index.html.
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2.5. Reservoir effects
The deep ocean has a much lower 14C content than that of the atmosphere. This is because
deep ocean waters experience long periods when they are not in contact with the atmosphere (the
residence time of carbon in the deep ocean is ~800 years (Broecker, 2000)). During this time the
14
C content of deep ocean waters is depleted by radioactive decay. As a result, materials drawing
carbon from deep ocean reservoirs may have lower initial
14
C concentrations than
contemporaneous materials of terrestrial origin. This may result in their appearing older than
contemporaneous terrestrial materials. In the case of the surface ocean, by contrast, interaction
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with both the atmosphere and the deep ocean means that surface waters have 14C concentrations
that are intermediate between these two reservoirs. Organisms that live in the surface ocean, such
as shells, corals and planktonic foraminifera, therefore appear younger than contemporaneous
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deep ocean materials, but older than contemporaneous terrestrial samples. The offset between
surface ocean and terrestrial samples is known as the marine reservoir age (R). To calibrate a
radiocarbon date for a surface ocean sample, the IntCal04 curve can be used with a known value
of R. Alternatively, the current internationally-ratified marine calibration curve, Marine04
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(Hughen et al., 2004), can be used, with a known value of regional offset from the global marine
model age for that sample, defined as ΔR. The latter method is generally preferred and an on-line
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data base of ΔR for different regions is available (Reimer and Reimer, 2001). For age calibration,
the ΔR and R of a location are usually assumed to be constant through time (Stuiver et al., 1986).
However, recent studies have reported variations of several hundred to a couple of thousand years
in these values during the Late-glacial (for the southwest Pacific (Sikes et al., 2000), the
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Mediterranean (Siani et al., 2001), northern and equatorial Atlantic (Kromer et al., 2004;
Bondevik et al., 2006; Cao et al., 2007; Sarnthein et al., 2007), and northern and tropical Pacific
(Sarnthein et al., 2007)) and the Holocene (for the tropical Pacific (Yu et al., 2007; McGregor et
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al., 2008)). These variations are due to changes in ocean circulation and the carbon cycles
associated with climatic changes. Temporal variations in ΔR and R values should therefore be
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considered when calibrating 14C ages of marine samples from these regions.
Aquatic plant fragments, freshwater shells and lake sediments are also influenced by
reservoir effects. These materials can appear older than contemporaneous terrestrial samples
because a portion of the carbon in lakes comes from depleted
14
C-carbon sources, such as
dissolved inorganic carbon from groundwater and carbonates from limestone (Deevey et al.,
1954). This reservoir age lies between several hundred to more than a thousand years (Colman et
al., 2000; Zoppi et al., 2001) and can vary significantly with time (Geyh et al., 1998).
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Radiocarbon dates from these materials must therefore be corrected for any reservoir effect
before being calibrated using the IntCal04 curve.
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3. Radiocarbon dating of the recent past: features, potential and limitations
During the past few hundred years the carbon cycle has experienced both human disturbance
and natural variation. The natural variations are largely a product of climatic change (for
example, the Little Ice Age from approximately the 14th to the mid-19th centuries) and changes
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in solar activity (for example, the Spörer, Maunder and Dalton minima). Disturbances due to
human activities include anthropogenic CO2 perturbation resulting from the combustion of fossil
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fuel, changes in land use since the middle 19th century and 14C disturbances due to atmospheric
nuclear explosions beginning in 1945. These variations and disturbances have led to changes in
atmospheric 14C concentration through time.
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3.1. Radiocarbon dating during the period before the onset of bomb 14C
Natural short- and medium-term variations in atmospheric
14
C are largely attributable to
solar variability, especially to changes in the magnetic field strength of the solar wind. These alter
the magnitude of deflection (the shielding effect) of galactic cosmic rays travelling towards the
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Earth, resulting in variations in secondary neutron flux and in the production rate of 14C in the
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atmosphere (Stuiver and Quay, 1980). As a result, during intervals of high solar activity,
shielding of the Earth’s atmosphere from cosmic rays increases, causes a decrease in
14
C
production. By contrast, during periods of low solar activity, the shielding effect decreases,
leading to an increase in 14C production. Climatic change also contributes to short- and mediumterm variations in atmospheric 14C via the redistribution of 14C between carbon reservoirs (mainly
the atmosphere and oceans) due to changes in air-sea exchange of CO2, variations in deep water
formation and changes in the ocean’s thermohaline circulation (THC) (Stocker and Wright, 1996;
Broecker, 1997; Broecker et al., 1999, Clark et al., 2002). A weaker THC results in higher
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atmospheric 14C as less 14C from the surface ocean/atmosphere is carried away to the deep ocean,
while a stronger THC results in lower atmospheric 14C.
The centuries before the first appearance of bomb
14
C in the mid-20th century are
Increases in atmospheric
14
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characterised by large fluctuations in atmospheric 14C concentration. These are depicted in Fig. 1.
C, which are centred on AD 1500, 1700 and 1815, are largely the
product of the Spörer, Maunder and Dalton solar activity minima respectively, although a small
portion of these increases may be attributed to changes in the carbon cycle associated with
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climatic change during the LIA (Stuiver and Quay, 1980; Damon and Sonnet, 1991; Bard et al.,
1997). The large decrease in atmospheric 14C after c. AD 1900 is mainly due to the continuous
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release of 14C-free CO2 to the atmosphere as a consequence of the combustion of fossil fuels since
AD 1850. This is known as the Suess effect (Suess, 1955). It is worth noting that the combustion
of fossil fuels in the period c. AD 1850–1900 was too small to cause an obvious decrease in
atmospheric 14C (Marland et al., 2008). These 14C variations result in large fluctuations (wiggles)
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in the IntCal04 curve for recent periods, particularly from AD 1650 to 1950 (Fig. 1a). Any
attempt to determine the calibrated age of a sample with a 14C age of few hundred years may thus
yield several possible age ranges. As an illustration, a single sample with a conventional
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radiocarbon age of 150±40 years BP is shown in Fig. 2. The calibrated age of this sample
contains five possible age ranges from AD 1660 to 1950, indicated by grey boxes. Even if the
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precision of the age were improved, the range of calibrated ages would not change. This provides
a limit to the capacity of radiocarbon methods for dating the recent past when only a single
sample is dated. In other words, the existence of large wiggles in the calibration curve during the
last few centuries markedly decreases the precision of single radiocarbon dates.
In order to avoid this problem and to obtain more precise calibrated ages, two approaches
have been applied, both using a series of 14C dates instead of a single date. The first approach is
known as 14C wiggle-matching. This is illustrated in Fig. 3. A series of samples, each separated
by known time-spans (ti), is dated by radiocarbon. These radiocarbon dates form a block of
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wiggles that can be compared with those in the radiocarbon calibration curve. This block of 14C
dates is shifted along the x-axis. When the total difference between these radiocarbon dates and
the calibration curve reaches a minimum, the best fit of the two data sets is approached and a
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more precise calibrated age associated with each single date in the series is obtained (Pearson,
1986; Bronk Ramsey et al., 2001; Geyh, 2005). Radiocarbon wiggle-matching between a series of
14
C dates and a radiocarbon calibration curve can be performed using the OxCal calibration
program (Bronk Ramsey, 2001). The
14
C wiggle-matching method has been used to date peat
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profiles on the assumption that the peat accumulated in a piece-wise linear fashion (Blaauw and
Christen, 2005; Yeloff et al., 2006). The second approach also uses multiple
14
C dates in
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sequence, but additional information on dated materials and their surrounding environment (for
example, changes in peat composition) is required to narrow the calibrated age ranges associated
with each
14
C date (Turetsky et al. 2004; Goslar et al., 2005; Yeloff et al., 2006). In this way,
more precise calibrated ages can be achieved. Examples of these approaches are illustrated in
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Section 4.
There are small differences in the natural atmospheric
14
C concentration between the
Northern and Southern Hemispheres. These are known as inter-hemispheric
14
C offsets. The
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Southern Hemisphere has a larger surface ocean area than the Northern Hemisphere (~60%
compared to ~40%) with greater wind velocities. As a result, more
14
C in the southern
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troposphere is transported to the oceans through air-sea exchange of CO2 and more 14C-depleted
CO2 from the oceans is transported to the southern troposphere. Natural 14C levels in the southern
troposphere are therefore usually lower than those in the northern troposphere, and the
radiocarbon ages of terrestrial materials in the Southern Hemisphere for a particular period of
time are usually older than those in the Northern Hemisphere. The calibration curves for the
northern (IntCal04) (Reimer et al., 2004b) and southern (SHCal04) (McCormac et al., 2004)
temperate regions for the past few hundred years are shown in Fig. 4. For the period AD 1500–
1920, the SHCal04 radiocarbon ages are older than their IntCal04 counterparts with a difference
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varying from 3 to 63
14
C years. However, for the period from AD 1920 to 1955, the SHCal04
radiocarbon ages are almost equal to or younger than their Northern Hemisphere counterparts,
with a maximum 14C offset of 35 years in AD 1940. This is due to a weaker dilution effect of 14C-
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free CO2 from the combustion of fossil fuels in the southern troposphere compared to that in the
northern troposphere (the primary source of this anthropogenic CO2 is in the Northern
Hemisphere) (McCormac et al., 1998; Stuiver and Braziunas, 1998). There is currently no
radiocarbon calibration curve that may be applied to tropical regions. A decadal
14
C data set
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derived from dendrochronologically-dated tree rings from northern Thailand for the period AD
1620–1780 (Hua et al., 2004a) is shown in the inset diagram of Fig. 4 in comparison with
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IntCal04 and SHCal04. The Thai data differ little from IntCal04 for the period of steep decrease
in 14C age (AD 1630–1710), but are similar to SHCal04 for the period AD 1720–1780. A simple
calibration data set for the tropics based on average values of IntCal04 and SHCal04 may
therefore not be ideal, especially for the use of radiocarbon to reconstruct high-precision tropical
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chronologies.
3.2. Bomb-pulse radiocarbon dating
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As a result of hundreds of atmospheric nuclear detonations almost entirely in the Northern
Hemisphere in the late 1950s and early 1960s, there was a dramatic increase in the concentration
14
C in the atmosphere. Atmospheric
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of
14
C reached a maximum in the Northern Hemisphere in
AD 1963–1964, almost double its pre-bomb level (Fig. 5). Since then, atmospheric
14
C
concentrations have decreased due to rapid exchange between the atmosphere and other carbon
reservoirs (mainly the biosphere and oceans). Although several atmospheric nuclear bomb tests
were carried out in the period AD 1945–1951, these were too small to increase atmospheric 14C
(Hua et al., 1999) and the take-off in atmospheric 14C began only in AD 1955 with the injection
of moderate amounts of 14C into the atmosphere in association with atmospheric nuclear bomb
tests during AD 1952–1954. For the period since AD 1955, significant differences in atmospheric
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14
C levels between consecutive years offer the possibility of dating recent samples with a
resolution of from one to a few years. Bomb-pulse
14
14
C dating, which is based on the
C
concentration in materials at the time of their formation, therefore differs from conventional
14
C concentration in dated samples due to
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radiocarbon dating, which is based on the residual
radioactive decay. For the bomb period, measured 14C concentration is usually reported in pMC
or F rather than as radiocarbon ages, and a bomb 14C curve is employed to convert the measured
pMC or F values to calendar years. A simple illustration of bomb-pulse 14C dating is shown in
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Fig. 5, in which a sample S with a 14C concentration of Fs possesses two possible calendar age
ranges T1 and T2.
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Hua and Barbetti (2004) recently reviewed all available atmospheric 14C data for the bomb
period from AD 1955 onwards and provided a comprehensive compilation of tropospheric bomb
14
C concentration for use in bomb-pulse
14
C dating. They compiled four zonal data sets of
tropospheric bomb 14C data at (mostly) monthly resolution (three in the Northern Hemisphere and
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C
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one in the Southern Hemisphere), which are depicted in Fig. 6. Significant differences in
between the four zones are evident for the early bomb period from the late 1950s to the late
1960s. As almost all the sources of bomb
14
C were located in the Northern Hemisphere, the
C during this period reflects the major zones of atmospheric circulation
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distribution of bomb
14
and their boundaries as excess
C was transferred southwards from the northern high-latitudes
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(Hua and Barbetti, 2004, 2007). Since AD 1980, the atmospheric
14
C level of the Southern
Hemisphere has been slightly higher than that of the Northern Hemisphere. This is a result of
greater contamination by
14
C-free anthropogenic CO2 in the Northern Hemisphere. Several
calibration programs have been devised for bomb-pulse
(http://radiocarbon.pa.qub.ac.uk/)
(Stuiver
and
14
C dating. These include CALIBomb
Reimer,
1993)
and
OxCal
v4.0
(http://c14.arch.ox.ac.uk/embed.php?File=oxcal.html) (Bronk Ramsey, 2001). The bomb pulse
14
C method has been used to date recent skeletons for forensic studies (Wild et al., 1998), to
establish the age of drugs and wines (Zoppi et al., 2004), and to date human cells and teeth for
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biological studies (Spalding et al., 2005; Bhardwaj et al., 2006). Further applications of this
dating method are described in the next section.
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4. Examples of radiocarbon dating of the recent past
4.1. Dating of peat profiles
The radiocarbon dating of recent peat profiles has been undertaken by a number of
investigators (Gallagher et al., 2001; Goodsite et al., 2001; Mauquoy et al., 2002; Blaauw et al.,
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2003; Donders et al., 2004; Charman and Garnett, 2005; van der Linden and van Geel, 2006). A
comprehensive review of dating recent peats was carried out by Turetsky et al. (2004), who
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discussed several dating methods, including 14C and the use of chronostratigraphic age markers.
They concluded that radiocarbon dating of recent peat profiles may offer accurate and precise
results at decadal-scale resolution for the few hundred years before the onset of bomb 14C and at
1–2 year resolution during the bomb period. Here, two examples of radiocarbon dating of recent
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peats using different age-depth models are presented.
Yeloff et al. (2006) radiocarbon-dated a number of peat profiles in northwest Europe. For
each core a series of radiocarbon dates on terrestrial plant remains, composed of seeds and
Sphagnum stems and branches, was obtained at 1 cm intervals. The authors used two different
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age models based on Bayesian statistics to estimate peat accumulation rates: 14C wiggle-matching
AC
C
using the Bpeat program (Blaauw and Christen, 2005) and multiple dates from a stratigraphic
sequence using BCal (Buck et al., 1999). Bpeat assumes piece-wise linear accumulation of peat
deposits, while BCal uses constraints in the chronological ordering of dates (deeper samples are
older than shallower samples) to reduce the calibrated age ranges of individual
14
C dates. The
authors dated five northern European ombrotrophic peat bogs. The accumulation rates at each site
derived using the two methods were generally similar. A comparison of the rates estimated using
the two methods for the top part (30–50 cm depth) of a peat core from Lille Vildmose (Denmark)
is shown in Fig. 7.
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Goslar et al. (2005) dated a number of peat profiles in Europe spanning the past 400 years.
At each site, high sampling resolution was achieved by collecting materials at 3–5 mm intervals.
In most cases Sphagnum was used for
14
C analysis. The maximum values of bomb
14
C in peat
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profiles varied between sites, and in all cases were lower than the atmospheric bomb peak value.
This implies that the peat sections contain a mixture of 14C assimilated over a period longer than
the resolution of atmospheric records. This period was estimated for each site and taken into
account when an age-depth model was built. Goslar et al. used multiple dates from a stratigraphic
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sequence and lithological information (accumulation rates may change when lithological
conditions change) to constrain their age-depth model. Fig. 8 illustrates an age-depth model for
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peat core Mauntschas-03 from southeast Switzerland and its evolution after three stages of
dating. The results show that the denser the sequence of
depth model.
14
14
C
C dates, the more accurate the age-
Although many studies have reported good results from the 14C dating of peats, the method
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is not free of problems. Charman and Garnett (2005) dated two peat profiles from Butterburn
Flow in England. They found that many of the ages were older than expected and that the oldest
ages occurred at the top of the profiles. This may have been because the most recent peat samples
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had been affected by inputs of older carbon from industrial emissions such as coal burning and
the fallout of particulates. These contaminants may have included spheroidal carbonaceous
AC
C
particles, which would not have been removed completely during sample pretreatment as a result
of their bonding to Sphagnum leaves (they may be retained within the pores on the cells: see
Charman and Garnett (2005) and references therein). This implies that peats from areas close to
industrial activity should not be used for radiocarbon dating.
4.2. Dating of lake and salt marsh sediments
Accurate radiocarbon dating of lake and salt marsh sediments is not simple due to the
presence in such deposits of organic and inorganic carbon materials from various sources
15
ARTICLE IN PRESS
(McGeehin et al., 2004). Dating is complicated by reservoir effects (in the case of aquatic plant
macrofossils and inorganic materials) and by the reworking of older material into recent
sediments. In order to determine which materials from modern lake sediments are most suitable
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for 14C dating, Davidson et al. (2004) dated a range of organic materials from a core from Sky
Lake, Mississippi, USA. They found twigs (representing 1–2 years of growth) of local plant
species to be more reliable for
14
C dating than either fine (>250 μm) organic debris or wood
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fragments (from large branches or tree trunks). The 14C content of the twigs showed a clear bomb
signal that was always higher than that of either the fine organic debris or the wood fragments at
the same depths (Fig. 9a). They therefore used twigs to date two cores from the same lake (Fig.
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9b). The results showed a good match between the 14C content of the twigs and that in nearby tree
rings for the bomb period. Prior to this, there was a generally decreasing trend in
14
C
concentration with depth. Although a few data points from one of the cores did not match the
bomb curve very well and variations are evident in the pre-bomb data, perhaps the result of minor
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bioturbation or the occasional influx of reworked materials from the surrounding forest (Davidson
et al., 2004), this method shows considerable promise for dating recent lake sediments.
Marshall et al. (2007) constructed a chronology of deposition for a 76 cm sequence of salt
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marsh sediments from Poole Harbour in southern England. They radiocarbon-dated fragments of
grass stem, which lay horizontally in the clayey sediment matrix. This sampling strategy was used
AC
C
to minimise the chance of younger roots being selected for dating. As grass stems are fragile, the
use of these samples for dating also minimised the possibility that the materials had been
reworked. The 14C values of the grass fragments from the top 26.5 cm of the sequence followed
both the rising and the falling limbs of the pulse in bomb
14
C. Chronological ordering (deeper
samples are older than shallower samples) and independent time markers (obtained from the
analysis of pollen and spheroidal carbonaceous particles) were used as constraints to eliminate
unlikely calibrated age ranges of individual
14
C ages, allowing the construction of a reliable
chronology of deposition.
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ARTICLE IN PRESS
Another approach to dating lake sediments involves the direct analysis of the deposit.
McGeehin et al. (2004) extracted humin (<63 µm) from two lake sediment cores from Grenada
Lake, north-central Mississippi, USA. Humin samples were oxidised to CO2 using a stepped-
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combustion method, with samples combusted at 400°C and then 900°C. In all cases, the bomb 14C
values of the low temperature (400°C) fractions were much higher than those of the high
temperature (900°C) fractions, indicating an improvement in dating of sediment by combusting at
low temperature. The authors argued that the combustion of sediments at low temperature could
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reduce the contribution of reworked carbon bound to clay minerals. However, the bomb
14
C
values of the low temperature fractions were still significantly lower than those of the
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atmosphere, indicating a reservoir effect problem in direct dating of sediments.
4.3. Dating of tree rings
Bomb radiocarbon has been used to validate the annual nature of distinct growth zones or
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rings in some species of tropical and temperate trees and mangroves (Worbes and Junk, 1989;
Fichtler et al., 2003; Menezes et al., 2003; Biondi et al., 2007). The 14C concentration in wood
reflects the atmospheric
14
C at the time it was formed. By measuring
14
C concentrations in the
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individual growth rings of a tree and comparing them with a suitable atmospheric bomb curve,
the dates of their growth may be determined. Comparing these growth years with those estimated
AC
C
from ring counts makes it possible to establish whether the tree produces annual rings. As shown
in Fig. 5, each value of
14
C for the period after AD 1955 gives two possible (calendar) time
windows: one on the rising limb and the other on the falling limb of the bomb curve. To
overcome this problem, at least two single growth rings in each tree need to be analysed. The
most suitable period for this kind of application is from AD 1958 to 1970, when the differences in
atmospheric
14
C between consecutive years are highest and the bomb
14
C method delivers the
greatest temporal resolution (Fig. 6a). Alpha-cellulose extracted from wood should be used for
14
C analysis in order to obtain a reliable determination of 14C concentration in single growth rings
17
ARTICLE IN PRESS
(Hua et al., 1999). An example of this kind of application is shown in Fig. 10. This compares the
14
C concentration in the rings of a dendrochronologically-dated specimen of Triplochiton
scleroxylon from Cameroon and an atmospheric 14C record from the Southern Hemisphere. The
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good agreement between the two data sets indicates that T. scleroxylon has produced annual rings
for the past 50 years (Worbes et al., 2003). If the annual nature of growth rings is validated for the
bomb period, it may be reasonably assumed that this is also true for earlier times (Menezes et al.,
2003).
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Bomb 14C is also a useful tool to complement the standard techniques of dendrochronology
in species in which annual rings are not always clearly defined. Hua et al. (2003) determined the
C content of 27 consecutive annual rings of a section of Pinus radiata (DRF 021) from
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14
Armidale, northern New South Wales, Australia. Some 14C values of single rings from DRF 021
(based on ring counts) were not in agreement with atmospheric 14C records at similar latitudes,
suggesting the possibility of two false rings and thus two mis-identified rings in the preliminary
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count for this section. This possibility was supported by a better ring-width correlation between
the revised DFR 021 count and other P. radiata chronologies in the study region.
For the pre-bomb period, precise dating of single tree rings is problematic because of the
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large fluctuations in atmospheric 14C over the past few hundred years (see section 3.1). The 14C
wiggle-matching method has successfully been used to date precisely Holocene floating tree-ring
AC
C
sequences that cannot be cross-dated by standard dendrochronological techniques (van der Plicht
et al., 1995; Kromer et al., 2001; Barbetti et al., 2004; Galimberti et al., 2004, Kuzmin et al.,
2004; Nakamura et al., 2007). Dating precision using the 14C wiggle-matching method depends
upon the shape of the radiocarbon calibration curve at the period concerned, the number of 14C
dates available and the precisions associated with the
14
C dates. Galimberti et al. (2004)
investigated cases at three different periods: AD 100–250 (a plateau in the calibration curve), AD
900–1100 (a part of the calibration curve characterised by a steep decrease in 14C age) and 1750–
1650 BC (a portion of the calibration curve dominated by large wiggles). They reported that 5–10
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ARTICLE IN PRESS
radiocarbon dates on 10-ring samples at precisions of 25–30 14C years were sufficient to achieve a
precision of less than 25 years (95% confidence level) for the wiggle-matching method. This
4.4. Estimation of the ages of marine samples
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method may therefore be applied to the precise dating of tree rings for the pre-bomb period.
When atmospheric 14C concentration increased in the mid-1950s as a result of atmospheric
nuclear weapons tests,
14
C levels in the surface ocean also increased. This is because excess
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radiocarbon from the atmosphere was incorporated in the upper ocean by the air-sea exchange of
CO2. However, the distribution of bomb 14C in the surface ocean differs from the simple zonal
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distribution of atmospheric 14C illustrated in Fig. 6. This is because levels of 14C in the surface
ocean are mainly controlled by local and regional patterns of ocean circulation. These include
horizontal advection and vertical movements such as ocean upwelling (Druffel, 1997; Gagan et
al., 2000; Hua et al., 2005; Grottoli and Eakin, 2007). The latter process may bring 14C-depleted
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waters from the deeper ocean to the surface, producing areas of lower surface ocean 14C levels.
As a result, 14C levels in the surface ocean have local and regional characteristics such as those
depicted in Fig. 11 for the Pacific Ocean. In addition, due to the dampening effect of the oceans,
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the magnitude of increases in 14C in the surface ocean after the mid-1950s was much smaller than
that in the atmosphere (the surface waters of the Pacific Ocean experienced an increase of ~0.12–
AC
C
0.24 F from pre-bomb to maximum bomb values (Fig 11) compared with an increase of ~0.6–2 F
in the troposphere (Fig. 6)). Surface ocean 14C also increased more slowly than atmospheric 14C,
with the oceanic
14
C reaching its maximum ~10 years later than atmospheric
Suess, 1983; Nydal and Gislefoss, 1996). This occurred because excess
14
14
C (Druffel and
C in the atmosphere
after the atmospheric 14C bomb peak (AD 1963–1965) continued to be transferred to the oceans.
These issues lead to difficulties in accurately dating marine samples using bomb
14
C.
Nevertheless, bomb 14C may be used to estimate the ages of marine materials formed after AD
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ARTICLE IN PRESS
1960. Such estimates are most reliable for materials formed in the period from c. AD 1960 to the
mid-1970s, when there was a significant increase in surface ocean 14C.
Kalish (1995) and Kalish et al. (1996) measured the 14C content of fish otoliths and used a
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‘regional’ surface ocean 14C bomb curve to estimate fish birth dates and consequently fish ages.
An example of this approach is shown in Fig. 12. The birth dates of specimens of Centroberyx
affinis caught off the coast of New South Wales (Australia) were first established by counting
annual increments visible in otolith thin sections. These dates were then checked by comparing
14
C content of the first annual otolith increment with a surface ocean
curve was constructed from
14
14
C bomb curve. The
SC
the
C values measured in otoliths of known age from the species
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Pagrus auratus taken off the coast of New Zealand, on the assumption that the
14
C levels of
surface waters in the two regions were similar. Kalish (1995) reported that 13 out of 16 presumed
birth dates for Centroberyx affinis fell within the 95% confidence limits of the ‘bomb-14C
calibration curves’ based on the New Zealand Pagrus auratus data. This method has also been
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successfully applied to validate shark ages by analysing 14C in growth bands of shark vertebrae
(Campana et al., 2002; Ardizzone et al., 2006).
Another application of this dating method was reported by Frantz et al. (2000). They
EP
estimated growth rates for the rhodolith Lithothamnium crassiusculum, a free-living calcareous
red alga from the southern Gulf of California, by measuring a dense series of samples from the
14
C and matching them with a surface ocean
14
C bomb curve for the Galapagos,
AC
C
rhodolith for
which is located not far from the southern Gulf of California. Using this method the authors
reported an average growth rate of 0.6 mm a–1 for the rhodolith, suggesting that large L.
crassiusculum with radii in excess of 6 cm may live over 100 years. As rhodoliths occupy
extensive areas of the world’s oceans, ranging from polar deeps to tropical shallows, they may
have the potential to provide proxies for past ocean conditions (Frantz et al., 2000).
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4.5. Time markers for speleothems
Time series of
14
C in modern speleothems have been reported by Genty et al. (1998) and
Genty and Massault (1999). The authors used annually laminated stalagmites from two caves in
14
C measurements. Their results are illustrated in Fig. 13. The two
records show a clear increase in
14
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PT
Belgium and France for
C resulting from the input of excess
14
C from atmospheric
nuclear bomb tests. However, the magnitude of the increase is smaller than that found in the
atmosphere and differs between the two cave systems. Possible sources of the carbon in
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speleothems include (1) limestone carbon containing no measurable 14C and (2) soil CO2 derived
from plant root respiration, whose 14C concentration is similar to that of the atmosphere, and from
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organic matter decomposition with a turnover time varying from decades to thousands of years
(Genty et al., 1998). The maximum value of bomb
14
C in a particular speleothem is therefore
dependent on the contribution of dead carbon from limestone, and on the proportion and
14
C
content of soil organic matter incorporated into the speleothem. To quantify the total dead carbon
DCF = (1 −
Fspel
Fatm
)
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D
in a speleothem from the two sources, the Dead Carbon Fraction (DCF) is usually calculated:
(4)
where Fspel and Fatm are the measured 14C concentration in a speleothem and the contemporaneous
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atmospheric 14C concentration respectively, both expressed as fraction modern carbon. The higher
the DCF, the lower the maximum value of bomb 14C found in the speleothem.
AC
C
Although the bomb 14C profiles in the speleothems show different maxima in the two cave
systems (Fig. 13), the timing of their onset is no more than 1–2 years after the start of the rise in
atmospheric 14C in 1955 (Genty and Massault, 1999). This feature can be used as a time marker
for modern speleothems, especially when they cannot be precisely dated by the Th/U method due
to low uranium concentration (<1 μg g–1) and insufficient
230
Th. This method has successfully
been applied to help to build chronologies for young speleothems from Gibraltar (Mattey et al.,
2008) and from New South Wales in Australia (Hodge et al., 2007).
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ARTICLE IN PRESS
5. Summary
Over the last few hundred years, atmospheric
14
C has been characterised by large
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fluctuations caused by variations in solar activity (the Spörer, Maunder and Dalton minima, for
example) and climatic changes (such as those of the Little Ice Age). The injection of
14
C-free
anthropogenic CO2 into the atmosphere since the Industrial Revolution and the dramatic increase
in atmospheric
14
C due to atmospheric nuclear detonations starting in AD 1955 have also had
the onset of bomb
14
C mean that a single
14
SC
massive impacts on 14C levels. The large fluctuations in atmospheric 14C that took place before
C date may possess several possible calibrated age
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ranges, making the 14C dating method imprecise for that period. This problem may be overcome
by measuring a series of 14C dates from a sequence and locating their most likely positions on a
calibration curve using either the 14C wiggle-matching method or additional information on dated
materials and their surrounding environment. For the period from 1955 onwards, atmospheric 14C
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D
levels differ significantly from year to year, offering the possibility of dating modern samples
with a resolution of from one to a few years.
The method of analysing a series of 14C dates from a sequence has been successfully applied
to the precise dating of recent peat profiles. The most reliable components of the peats for dating
EP
are the leaves and twigs of Sphagnum. However, recent peats from regions close to industrial
AC
C
activity may not be suitable for radiocarbon dating as spheroidal carbonaceous particles from
industrial sources attached to Sphagnum leaves may not be completely removed during sample
pretreatment (Charman and Garnett, 2005). The method of analysing multiple
14
C dates from a
sequence has also been used for dating recent lake and salt marsh sediments from the pre- and
post-bomb periods. Accurate radiocarbon dating of such sediments is not simple due to the
presence of organic and inorganic carbon from various sources (McGeehin et al., 2004). The most
reliable materials for dating young sediments are short-lived macrofossils of local species, such as
small twigs and grass stems (Davidson et al., 2004; Marshall et al., 2007). Direct dating of lake
22
ARTICLE IN PRESS
and salt marsh sediments is still problematic because of reservoir effects (McGeehin et al., 2004).
Given the potentially complex array of carbon found in modern organic and inorganic sediments,
it is helpful if alternative dating methods such as
14
Cs,
210
Pb and
241
Am are used to corroborate
C chronologies (Gallagher et al., 2001; Davidson et al., 2004; McGeehin et al, 2004;
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the
137
Turetsky et al., 2004; Marshall et al., 2007).
Bomb 14C has successfully been used to validate the annual nature of distinct growth zones
or rings of some species of tropical and temperate trees and mangroves (Worbes and Junk, 1989;
SC
Fichtler et al., 2003; Menezes et al., 2003; Biondi et al., 2007). This provides a useful
complement to the standard techniques of dendrochronology in species where annual rings are
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not always clearly defined (Hua et al., 2003). Precise dating of tree rings for the pre-bomb period
may be achieved by obtaining a series of
Bomb
14
14
C dates and applying the wiggle-matching method.
C has also been used to estimate the ages of modern marine materials. It has been
employed, for example, to verify fish birth dates (Kalish, 1995; Kalish et al., 1996; Campana et
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D
al., 2002; Ardizzone et al., 2006) and to determine the growth rates of young rhodoliths (Frantz et
al., 2000). For young speleothems, which cannot be precisely dated by the Th/U method due to
low uranium concentration (<1 μg g–1) and insufficient 230Th, the timing of the onset of bomb 14C
EP
may be used as a time marker (Hodge et al., 2007; Mattey et al., 2008). In addition, if the DCF of
a speleothem is almost constant over time, which can be indicated by its δ13C values (Genty et al.,
AC
C
2001), a series of 14C dates from a sequence can potentially be employed to construct a reasonable
age-depth model for the speleothem for the pre-bomb period.
Acknowledgements
The author would like to thank Dan Yeloff and Tomasz Goslar for their generous provision
of Figs 7 and 8 respectively. Stephen Gale and two anonymous reviewers provided critical and
constructive comments, which greatly improved the manuscript.
23
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Editorial handling by: S.J. Gale
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Figure captions
Fig. 1. Atmospheric
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Fig. 2. Calibration of a single radiocarbon date of 150±40 years BP using the IntCal04 calibration
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ranges from AD 1660 to 1950 (indicated by the grey boxes).
Fig. 3. Radiocarbon dating by the wiggle-matching method. The solid dots represent a series of
radiocarbon dates separately by known time intervals (ti, with i = 1, 2, 3 and 4). The error bars
are 1σ. The dates form a block of wiggles (dashed grey lines) that may be moved along the xaxis to achieve the best fit with the 1σ range of the IntCal04 curve (solid black lines).
TE
D
Fig. 4. Radiocarbon calibration curves for the period AD 1400–1950. IntCal04 and SHCal04 are
representative of northern and southern temperate regions respectively. In the inset diagram
these records are compared with decadal 14C data from tropical northern Thailand. The error
EP
bars for the Thai data are 1σ.
Fig. 5. Atmospheric 14C in Northern Hemisphere zone 1 for the period AD 1955–2000 (Hua and
AC
C
Barbetti, 2004). For an F value measured in a terrestrial sample S (Fs), bomb 14C delivers two
possible calendar dates (T1 and T2), indicated by the grey boxes.
Fig. 6. (a) Regional tropospheric 14C curves for the period AD 1955–2001 for four different zones
(Northern Hemispheric zones 1–3 and Southern Hemispheric zone). (b) The four zones into
which the tropospheric 14C data have been grouped (Hua and Barbetti, 2004).
Fig. 7. Age-depth models for the top part (30–50 cm depth) of a peat core from Lille Vildmose in
Denmark. (a) The age-depth model based on Bpeat wiggle-matching. The dark solid line
39
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indicates the best fit with the IntCal04 curve. The grey bars indicate 95% confidence
intervals. (b) The section of the IntCal04 calibration curve corresponding to the period during
which the peat was deposited. (c) The age-depth model based on BCal (solid line). The
RI
PT
hollow histograms represent the probability distributions of individual calibrated ages
calculated using CALIB 5.0. The solid histograms depict the possible calibrated age ranges of
individual samples determined by BCal. (d) The main plant macrofossil components in the
peat sequence expressed as percentages. The reduction in the accumulation rate above 40 cm
SC
inferred by the BCal age model coincides with a change in peat composition from Sphagnum
section Cuspidata/Sphagnum cupsidatum to Sphagnum magellanicum (Yeloff et al., 2006).
M
AN
U
Fig. 8. Age-depth modelling of peat core Mauntschass-03 from southeast Switzerland showing
the effect of the increase in the density of 14C dating (Goslar et al., 2005). The probability
distributions of the calibrated ages of individual samples down the sequence are shown with
grey silhouettes. The small rectangle in the upper right-hand corner of each figure represents
TE
D
the date of collection of the section (depth = 0 cm). The arrow in the lower left-hand corner
of each figure represents the depth of a sample whose calendar age lies beyond the range of
the diagram. The smooth lines passing through the maxima of the probability distributions
EP
represent the most probable age-depth curves. The dashed lines represent the uncertainties of
each age-depth model. The vertical bars on the right-hand side show changes in peat
AC
C
accumulation rate, as inferred from the lithology of the sequence.
Stage 1 (top panel): seven samples were dated. These yielded two possible age-depth curves.
Stage 2 (middle panel): 10 samples were dated. These produced a single age-depth curve.
Stage 3 (bottom panel): 12 samples were dated. These generated an age-depth model with much
lower uncertainty.
Fig. 9. (a) The downcore variation in the
14
C concentration in wood fragments, fine organic
debris and twigs from Core 3, Sky Lake, Mississippi, USA. (b) The
14
C concentration in
twigs from Cores 3 and 4, Sky Lake, Mississippi, USA compared with that in nearby tree
40
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rings of known age. The error bars (1σ) associated with the 14C data are equal to or smaller
than the size of the data symbols (Davidson et al., 2004).
Fig. 10. The 14C concentration of ring-count dated growth rings of Triplochiton scleroxylon from
RI
PT
Cameroon (Worbes et al., 2003) plotted on the atmospheric 14C curve from Wellington, New
Zealand for the period AD 1957–1992 (Manning and Melhuish, 1994).
Fig. 11. The concentration of 14C in the surface waters of the Pacific Ocean for the period AD
SC
1950–1984 recorded in corals. The data are from Konishi et al. (1982) for Okinawa, Druffel
(1987) and Druffel et al. (2001) for Hawaii and Panama, Toggweiler et al. (1991) for Tarawa
M
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U
and Fiji, and Druffel and Griffin (1995) for Heron Island. The error bars are 1σ. The
chronologies of these corals are based on counting growth bands backwards from the dates of
collection.
Fig. 12. The 14C concentrations of the first annual increment of Centroberyx affinis otoliths from
New South Wales, Australia plotted against their otolith-count derived birth dates (Kalish,
TE
D
1995). These are compared with the 14C concentrations of Pagrus auratus otoliths of known
age from New Zealand (Kalish, 1993). The horizontal error bars associated with the
presumed birth dates are 1σ. The vertical error bars associated with the 14C data are 1σ.
EP
Fig. 13. The pattern of bomb 14C in annually laminated stalagmites from the caves of Han-surLesse, Belgium (Genty et al., 1998) and La Faurie, France (Genty and Massault, 1999). The
AC
C
stalagmites were dated by counting annual layers (represented by a couplet composed of a
white porous lamination and a dark compact lamination). The horizontal error bars
encompass the one to two years of laminae sampled for each 14C analysis. The vertical error
bars associated with the 14C data are 1σ.
41
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Year (AD)
1400
600
1500
1600
1700
1800
1900
a
RI
PT
400
SC
300
200
M
AN
U
Radiocarbon Age (BP)
500
100
EP
0
∆
M
AC
C
S
14
C (o/oo)
10
-10
b
TE
D
20
D
-20
LIA
-30
1400
1500
1600
1700
Year (AD)
1800
1900
Figure 1
IntCal04 calibration curve
Gaussian distribution of uncalibrated 14 C age
Probability distribution of calibrated ages
RI
PT
600
500
SC
400
M
AN
U
300
200
150 ± 40 BP
TE
D
100
EP
0
AC
C
Radiocarbon Age (BP)
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1400
1500
1600
1700
Year (AD)
1800
1900
Figure 2
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RI
PT
600
SC
M
AN
U
400
TE
D
300
EP
200
AC
C
Radiocarbon Age (BP)
500
100
0
1400
1500
1600
t1
1700
Year (AD)
t2
t3
1800
t4
1900
Figure 3
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700
RI
PT
400
350
200
SC
250
M
AN
U
C Age (BP)
14
500
300
150
400
100
200
1400
1700
1750
1800
Year (AD)
EP
300
100
1650
TE
D
1600
AC
C
Radiocarbon Age (BP)
600
IntCal04
SHCal04
N. Thailand (17o N, 102o E)
1500
1600
1700
Year (AD)
1800
1900
Figure 4
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RI
PT
Bomb curve for Northern Hemisphere zone 1
F value measured in sample S
Probability distribution of calibrated dates
M
AN
U
SC
1.8
1.6
Fs
EP
TE
D
1.4
1.2
1.0
1950
AC
C
Fraction modern carbon (F)
2.0
T1
1960
T2
1970
1980
Year (AD)
1990
2000
Figure 5
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a
RI
PT
NH
NH
NH
SH
M
AN
U
SC
1.8
zone 1
zone 2
zone 3
zone
1.6
EP
TE
D
1.4
1.2
AC
C
Fraction modern carbon (F)
2.0
1.0
1950
1955
1960
1965
1970
1975
1980
Year (AD)
1985
1990
1995
2000
Figure 6a
b
AC
C
EP
TE
D
M
AN
U
SC
RI
PT
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Figure 6b
45
50
EP
40
TE
D
C Age (BP)
14
500
30
a
400
300
200
100
400
300
200
c
200
100
100
b
d
35
500
0
RI
PT
cal BP
100
80
60
Sphagnum tenellum
Sphagnum
Cuspidatum
Sphagnum section
Cuspidata
Sphagnum
magellanicum
SC
Monocots undiff.
cal BP
300
M
AN
U
400
AC
C
Depth (cm)
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20
Figure 7
AC
C
EP
TE
D
M
AN
U
SC
RI
PT
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Figure 8
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0
a
20
40
RI
PT
Depth (cm)
60
80
100
SC
120
M
AN
U
140
160
180
0.8
1.0
1.2
Wood fragments
Fine organic debris
Twigs
1.4
1.6
1.8
b
20
EP
40
100
120
AC
C
Depth (cm)
60
80
2000
1990
1980
1970
1960
1950
TE
D
0
Year (AD)
Fraction modern carbon (F)
140
Core 3 twigs
Core 4 twigs
Tree rings
160
180
0.8
1.0
1.2
1.4
Fraction modern carbon (F)
1.6
1.8
Figure 9
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1.8
RI
PT
M
AN
U
SC
1.6
TE
D
1.4
EP
1.2
AC
C
Fraction modern carbon (F)
Wellington, New Zealand 42o S, 175o E
Triplochiton scleroxylon
1.0
1950
1960
1970
1980
Year (AD)
1990
2000
Figure 10
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RI
PT
SC
1.14
M
AN
U
1.10
TE
D
1.06
0.98
0.94
0.90
1950
Okinawa 26oN, 128oE
Hawaiian Is. 24oN, 166oW
Tarawa 1oN, 172oE
Panama 8oN, 82oW
Fiji 18oS, 179oE
Heron Is. 23oS, 152oE
EP
1.02
AC
C
Fraction modern carbon (F)
1.18
1955
1960
1965
1970
Year (AD)
1975
1980
1985
Figure 11
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RI
PT
M
AN
U
SC
1.10
TE
D
1.05
EP
1.00
AC
C
Fraction modern carbon (F)
1.15
0.95
0.90
1945
1955
1965
Pagrus auratus ototliths, New Zealand
Centroberyx affinis otoliths, NSW, Australia
1975
Birthdate (Year AD)
1985
1995
Figure 12
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1.16
TE
D
1.04
EP
1.00
0.96
0.92
0.88
1940
RI
PT
SC
M
AN
U
1.08
AC
C
Fraction modern carbon (F)
1.12
Han-stm5, Han-sur-Lesse cave
Fau-stm14, La Faurie
1950
1960
1970
Year (AD)
1980
1990
2000
Figure 13