penetrate into the upper atmosphere. As a result, the
14 C
production decreases as the solar activity increases, and vice
versa. Similarly, the cosmic ray intensity in the upper
atmosphere is modulated by the changes in the Earth’s
magnetic field, which acts as a shield against the
electrically-charged cosmic protons. The greater the intensity
of the geomagnetic field, the fewer the number of cosmic
protons arriving in the upper atmosphere, and therefore the
production of
14 C is lower in the atmosphere (Damon et al.
1978; Stuiver et al. 1991). Changes in the Earth’s magnetic
field could explain about 50% of the
14 C variation in the
atmosphere between the last glacial and the Holocene, and
the
14 C aging of the atmosphere during the last glacial
related to a high magnetic field intensity (Lal and Charles
2007).
The dendrochronological record of the atmospheric
14 C
content over the last 10,000 years has, in turn, given the
opportunity to evaluate the changes of solar activity beyond
the first observations in the seventeenth century. Spectral
analyses of the atmospheric
14
C emphasize long cycles of
88 years, 208 years and 2050 years attributed to changes in
solar activity (Damon and Peristykh 2000). During the last
70 years, the solar activity was at an exceptionally high level
and many other periods of high activity, although shorter,
occurred in the past, such as, for example, at the beginning
of the Holocene. In addition, some authors have thought that
the unusual solar activity may have contributed, in a small
part, to the recent climate change observed in the late
twentieth century (Muscheler et al. 2005).
Climate changes modify the size of the carbon reservoirs
and the CO 2 fluxes between them and therefore the atmospheric
14 CO 2 as noted by de Vries (Stuiver et al. 1991).
Libby (1952) had already estimated the impact of climate
changes on the
14 C ages. He assumed that the sea level
lowering (*100 m) and the temperature decrease during the
last ice age reduced the oceanic carbon inventory, resulting
in an increase of the specific activity of
14 C (Fig. 4.1).
Assuming that the
14 C activity in the reservoir exchange
changed by 10%, Libby calculated that the glacial
14 C ages
would be too young by some 800 years. We know now that
the
14 C ages are too young by about 2000 years with respect
to ‘true’ ages during the last glacial maximum (Reimer et al.
2013). The ocean circulation and the carbon cycle were
deeply modified during the last ice age that modulated the
concentration of atmospheric CO 2 . Between the last glacial
maximum and the Holocene, the atmospheric CO 2 concentration increased from 190 to 280 ppm as recorded in the
Antarctic ice cores.
The growing evidence of the atmospheric
14 C variations
through time led to two consequences: the establishment of a
standard, i.e. a reference value of
14 C for the atmosphere
(N 0 ), and the calibration process which precisely quantifies
the difference between the true or ‘absolute’ ages and the
14 C
ages.
As requested by Arnold in 1956, the National Bureau of
Standards (Washington) prepared a standard (NBS-I) of
450 kg of oxalic acid (HOOC-COOH), an organic compound extracted from a French crop of sugar beet in 1955
(Arnold and Libby 1949). It provided the reference activity
of year zero, from which the
14 C age of a sample is calculated. The reference activity was taken at 95% that of NBS-I
to account for the Suess and nuclear bomb effects and the
reference year has been set arbitrarily at 1950. Since that
time, other standards were prepared (NBS-II, sucrose). The
14 C ages are expressed in years BP (Before Present, present
being equal to 1950 AD). In archeology, the terms Anno
Domini (AD) and Before Christ (BC) are used. Lately
appears the term Common Era (CE) which is equivalent to
AD and BCE to BC.
Before measuring the
14 C activity in a sample and calculating its
14 C age, the fractionation of the stable isotopes
of carbon must be considered. Craig in 1953 demonstrated
that the
13 C/
12 C ratios (d
13 C) vary in contemporary materials
as a function of the reservoir’s d
13 C in which they form. The
amount of enrichment/depletion of
14 C due to biological and
physicochemical fractionation processes is approximately
two times that measured by d
13 C in the same sample. The
d
13 C value, expressed in ‰ compared to the standard PDB
(Pee Dee Belemnite), is about −6.5‰ for the pre-industrial
atmospheric CO 2 . It varies between −2‰ and +3‰ in the
carbonates of seashells and between −20 and +3‰ for those
of lake shells. The d
13 C values of plants vary between −27
and −14‰ due to photosynthesis processes. Processes of
assimilation of atmospheric CO 2 by plants are carried out
mainly according to two cycles of transformation of organic
compounds, called the Calvin cycle and the Hatch and Slack
cycle, or also as C3 and C4, the second with a lower isotope
discrimination than the first. As a result of the great variability of the d
13 C values, the
14 C activities have been
normalized to a common reference of d
13 C set at −25‰ (see
below). The complete procedure for calculation of age is
explained in Box 1.
Box 1
As in most definition of isotopes, the
14 C content is
expressed by a d in ‰, which defines the difference
between a sample and a standard, which may be the
NBS-I standard, the d
13 C of which is equal to −19‰
relative to the PDB (Broecker and Olson 1959; Olsson
4 Carbon-14
55
14 C
production decreases as the solar activity increases, and vice
versa. Similarly, the cosmic ray intensity in the upper
atmosphere is modulated by the changes in the Earth’s
magnetic field, which acts as a shield against the
electrically-charged cosmic protons. The greater the intensity
of the geomagnetic field, the fewer the number of cosmic
protons arriving in the upper atmosphere, and therefore the
production of
14 C is lower in the atmosphere (Damon et al.
1978; Stuiver et al. 1991). Changes in the Earth’s magnetic
field could explain about 50% of the
14 C variation in the
atmosphere between the last glacial and the Holocene, and
the
14 C aging of the atmosphere during the last glacial
related to a high magnetic field intensity (Lal and Charles
2007).
The dendrochronological record of the atmospheric
14 C
content over the last 10,000 years has, in turn, given the
opportunity to evaluate the changes of solar activity beyond
the first observations in the seventeenth century. Spectral
analyses of the atmospheric
14
C emphasize long cycles of
88 years, 208 years and 2050 years attributed to changes in
solar activity (Damon and Peristykh 2000). During the last
70 years, the solar activity was at an exceptionally high level
and many other periods of high activity, although shorter,
occurred in the past, such as, for example, at the beginning
of the Holocene. In addition, some authors have thought that
the unusual solar activity may have contributed, in a small
part, to the recent climate change observed in the late
twentieth century (Muscheler et al. 2005).
Climate changes modify the size of the carbon reservoirs
and the CO 2 fluxes between them and therefore the atmospheric
14 CO 2 as noted by de Vries (Stuiver et al. 1991).
Libby (1952) had already estimated the impact of climate
changes on the
14 C ages. He assumed that the sea level
lowering (*100 m) and the temperature decrease during the
last ice age reduced the oceanic carbon inventory, resulting
in an increase of the specific activity of
14 C (Fig. 4.1).
Assuming that the
14 C activity in the reservoir exchange
changed by 10%, Libby calculated that the glacial
14 C ages
would be too young by some 800 years. We know now that
the
14 C ages are too young by about 2000 years with respect
to ‘true’ ages during the last glacial maximum (Reimer et al.
2013). The ocean circulation and the carbon cycle were
deeply modified during the last ice age that modulated the
concentration of atmospheric CO 2 . Between the last glacial
maximum and the Holocene, the atmospheric CO 2 concentration increased from 190 to 280 ppm as recorded in the
Antarctic ice cores.
The growing evidence of the atmospheric
14 C variations
through time led to two consequences: the establishment of a
standard, i.e. a reference value of
14 C for the atmosphere
(N 0 ), and the calibration process which precisely quantifies
the difference between the true or ‘absolute’ ages and the
14 C
ages.
As requested by Arnold in 1956, the National Bureau of
Standards (Washington) prepared a standard (NBS-I) of
450 kg of oxalic acid (HOOC-COOH), an organic compound extracted from a French crop of sugar beet in 1955
(Arnold and Libby 1949). It provided the reference activity
of year zero, from which the
14 C age of a sample is calculated. The reference activity was taken at 95% that of NBS-I
to account for the Suess and nuclear bomb effects and the
reference year has been set arbitrarily at 1950. Since that
time, other standards were prepared (NBS-II, sucrose). The
14 C ages are expressed in years BP (Before Present, present
being equal to 1950 AD). In archeology, the terms Anno
Domini (AD) and Before Christ (BC) are used. Lately
appears the term Common Era (CE) which is equivalent to
AD and BCE to BC.
Before measuring the
14 C activity in a sample and calculating its
14 C age, the fractionation of the stable isotopes
of carbon must be considered. Craig in 1953 demonstrated
that the
13 C/
12 C ratios (d
13 C) vary in contemporary materials
as a function of the reservoir’s d
13 C in which they form. The
amount of enrichment/depletion of
14 C due to biological and
physicochemical fractionation processes is approximately
two times that measured by d
13 C in the same sample. The
d
13 C value, expressed in ‰ compared to the standard PDB
(Pee Dee Belemnite), is about −6.5‰ for the pre-industrial
atmospheric CO 2 . It varies between −2‰ and +3‰ in the
carbonates of seashells and between −20 and +3‰ for those
of lake shells. The d
13 C values of plants vary between −27
and −14‰ due to photosynthesis processes. Processes of
assimilation of atmospheric CO 2 by plants are carried out
mainly according to two cycles of transformation of organic
compounds, called the Calvin cycle and the Hatch and Slack
cycle, or also as C3 and C4, the second with a lower isotope
discrimination than the first. As a result of the great variability of the d
13 C values, the
14 C activities have been
normalized to a common reference of d
13 C set at −25‰ (see
below). The complete procedure for calculation of age is
explained in Box 1.
Box 1
As in most definition of isotopes, the
14 C content is
expressed by a d in ‰, which defines the difference
between a sample and a standard, which may be the
NBS-I standard, the d
13 C of which is equal to −19‰
relative to the PDB (Broecker and Olson 1959; Olsson
4 Carbon-14
55