Calibration of the
14
C Ages
It is now well-established that the
14 C ages are not equal to
the absolute ages because (i) the
14 C ages are calculated with
the 5568 year half-life, (ii) the
14 C concentration in the
atmosphere varies as a function of time due to changes in the
production rate by cosmic rays which are modulated by the
solar and earth magnetic fields and (iii) of the changes in the
carbon cycle. The
14
C calibration consists of the precise
measurement of the difference between an absolute (calendar) age and a
14 C age. An international group of scientists
led by Stuiver and Reimer joins efforts to iterate the calibration datasets. The last product is the IntCal13 calibration,
which extends over the past 50,000 years (Reimer et al.
2013).
Methods and Results
The calibration procedure consists in measuring the
14 C age of
a sample while the absolute ages are determined by three
methods with the best possible accuracy. The first is based
either on the counting of annual tree-ring growth (dendrochronology) or on the counting of the annual laminae
(varves) deposited in marine or lake sediments. The second
consists in the precise U-Th dating of carbonates (warm water
corals or speleothems) by mass spectrometry (Chap. 6). The
third consists in synchronizing the variations of climate
proxies in marine sediments and in speleothems, the last two
being dated by the U-Th method.
In the eighties, the
14 C and U-Th dating benefitted from
new techniques in mass spectrometry coupled to an accelerator or by thermal ionization (TIMS), respectively—which
allowed their precision to be greatly improved and the size of
samples to be reduced by a factor of 1000 (from one gram to a
few milligrams). Bard had first proposed to extend the calibration from 10,000 to 22,000 cal BP through paired
14 C and
Th-U dating of corals from the Barbados islands (Fig. 4.3)
(Bard et al. 1990; Reimer et al. 2013). The calibration record
IntCal13 is based on the
14 C dating of the terrestrial and
marine samples referenced onto an ‘absolute’ timescale. It
extends up to 13,900 cal BP by means of dendrochronology
and up to *50,000 cal BP from terrestrial plant species
deposited in the varved sediments of Lake Suigetsu. It is
completed by U-Th-dated samples and has corrected
14 C ages
(see below) of the Hulu speleothem and of Atlantic and
Pacific corals. Finally, corrected
14 C ages of foraminifera in
the varved and unvarved sediments from the Cariaco basin
linked to the Hulu speleothem by coeval climatic fluctuations
are also included. The marine calibration record similarly
covers the past 50,000 cal BP and it is based on the dating of
corals from the Atlantic and Pacific oceans and of planktonic
foraminifera in the Cariaco sediments in the Caribbean Sea
(Fig. 4.4). The sea surface
14 C is not in equilibrium with that
of the atmosphere. The comparison of the atmospheric and
marine
14 C dating at a same calendar age between 0 and
13,900 cal BP allowed the atmosphere-ocean
14 C difference
(the marine reservoir age-see below) to be quantified and the
marine calibration to be anchored to the atmospheric record
by subtracting the reservoir age to the marine
14 C ages. In
IntCal13, the variability of the reservoir ages during the
glacial period were taken into account by augmenting the
value by 200 years prior to 13,900 cal BP. Similarly, the
14 C
in speleothems is not equal to that of the atmosphere, and the
difference (dead carbon fraction—see below) is measured
between the time interval 0–13,900 cal BP and then considered constant prior to 13,900 cal BP.
As initiated with IntCal04, the mathematical approach
that defines the envelope of IntCal13 takes into account both
uncertainties on
14 C dating and those associated with the
absolute ages. In addition, it also takes into account the
diversity of records and the representativeness of atmospheric
14 C by assigning different statistical weights to them
before returning the most likely fit between measured
14 C
activity and absolute age. The IntCal13 error envelope represents the best fit between the included datasets and it is
much smoother than those of the previous calibration
records. It does not take into account the scattering of the
14 C ages in the different datasets, which do not permit ‘real
14 C variations’ to be discriminated from measurement noise.
Those real variations are likely to be attributable to the
complex history of each carbon reservoir in either the terrestrial (atmospheric) or marine environments over the past
50,000 cal year due to climate and oceanic circulation
fluctuations, and to production changes through the variations of the Earth and solar magnetic field.
Examples of Precise Calibration of
14
C Ages
The Dating of the Eruption of Santorini
During the eruption of Santorini in Cyclades in the Aegean
Sea, huge amounts of volcanic ash were emitted into the
atmosphere and spread eastward and southward, covering
much of the Middle East. The ash bed is a useful chronological marker for the whole region, allowing the chronology
of human connections to be refined throughout the eastern
basin of the Mediterranean Sea during the second millennium BC. Archaeologists, for example, linked the apogee of
the Minoan civilization to that of the New Kingdom in Egypt
in the sixteenth century BC, on the basis of the elegance of
the decorations of objects found in the ash layer in Santorini.
The date of this eruption could not be defined to within
60 years, despite hundreds of
14 C dating of charcoals found
4 Carbon-14
57
14
C Ages
It is now well-established that the
14 C ages are not equal to
the absolute ages because (i) the
14 C ages are calculated with
the 5568 year half-life, (ii) the
14 C concentration in the
atmosphere varies as a function of time due to changes in the
production rate by cosmic rays which are modulated by the
solar and earth magnetic fields and (iii) of the changes in the
carbon cycle. The
14
C calibration consists of the precise
measurement of the difference between an absolute (calendar) age and a
14 C age. An international group of scientists
led by Stuiver and Reimer joins efforts to iterate the calibration datasets. The last product is the IntCal13 calibration,
which extends over the past 50,000 years (Reimer et al.
2013).
Methods and Results
The calibration procedure consists in measuring the
14 C age of
a sample while the absolute ages are determined by three
methods with the best possible accuracy. The first is based
either on the counting of annual tree-ring growth (dendrochronology) or on the counting of the annual laminae
(varves) deposited in marine or lake sediments. The second
consists in the precise U-Th dating of carbonates (warm water
corals or speleothems) by mass spectrometry (Chap. 6). The
third consists in synchronizing the variations of climate
proxies in marine sediments and in speleothems, the last two
being dated by the U-Th method.
In the eighties, the
14 C and U-Th dating benefitted from
new techniques in mass spectrometry coupled to an accelerator or by thermal ionization (TIMS), respectively—which
allowed their precision to be greatly improved and the size of
samples to be reduced by a factor of 1000 (from one gram to a
few milligrams). Bard had first proposed to extend the calibration from 10,000 to 22,000 cal BP through paired
14 C and
Th-U dating of corals from the Barbados islands (Fig. 4.3)
(Bard et al. 1990; Reimer et al. 2013). The calibration record
IntCal13 is based on the
14 C dating of the terrestrial and
marine samples referenced onto an ‘absolute’ timescale. It
extends up to 13,900 cal BP by means of dendrochronology
and up to *50,000 cal BP from terrestrial plant species
deposited in the varved sediments of Lake Suigetsu. It is
completed by U-Th-dated samples and has corrected
14 C ages
(see below) of the Hulu speleothem and of Atlantic and
Pacific corals. Finally, corrected
14 C ages of foraminifera in
the varved and unvarved sediments from the Cariaco basin
linked to the Hulu speleothem by coeval climatic fluctuations
are also included. The marine calibration record similarly
covers the past 50,000 cal BP and it is based on the dating of
corals from the Atlantic and Pacific oceans and of planktonic
foraminifera in the Cariaco sediments in the Caribbean Sea
(Fig. 4.4). The sea surface
14 C is not in equilibrium with that
of the atmosphere. The comparison of the atmospheric and
marine
14 C dating at a same calendar age between 0 and
13,900 cal BP allowed the atmosphere-ocean
14 C difference
(the marine reservoir age-see below) to be quantified and the
marine calibration to be anchored to the atmospheric record
by subtracting the reservoir age to the marine
14 C ages. In
IntCal13, the variability of the reservoir ages during the
glacial period were taken into account by augmenting the
value by 200 years prior to 13,900 cal BP. Similarly, the
14 C
in speleothems is not equal to that of the atmosphere, and the
difference (dead carbon fraction—see below) is measured
between the time interval 0–13,900 cal BP and then considered constant prior to 13,900 cal BP.
As initiated with IntCal04, the mathematical approach
that defines the envelope of IntCal13 takes into account both
uncertainties on
14 C dating and those associated with the
absolute ages. In addition, it also takes into account the
diversity of records and the representativeness of atmospheric
14 C by assigning different statistical weights to them
before returning the most likely fit between measured
14 C
activity and absolute age. The IntCal13 error envelope represents the best fit between the included datasets and it is
much smoother than those of the previous calibration
records. It does not take into account the scattering of the
14 C ages in the different datasets, which do not permit ‘real
14 C variations’ to be discriminated from measurement noise.
Those real variations are likely to be attributable to the
complex history of each carbon reservoir in either the terrestrial (atmospheric) or marine environments over the past
50,000 cal year due to climate and oceanic circulation
fluctuations, and to production changes through the variations of the Earth and solar magnetic field.
Examples of Precise Calibration of
14
C Ages
The Dating of the Eruption of Santorini
During the eruption of Santorini in Cyclades in the Aegean
Sea, huge amounts of volcanic ash were emitted into the
atmosphere and spread eastward and southward, covering
much of the Middle East. The ash bed is a useful chronological marker for the whole region, allowing the chronology
of human connections to be refined throughout the eastern
basin of the Mediterranean Sea during the second millennium BC. Archaeologists, for example, linked the apogee of
the Minoan civilization to that of the New Kingdom in Egypt
in the sixteenth century BC, on the basis of the elegance of
the decorations of objects found in the ash layer in Santorini.
The date of this eruption could not be defined to within
60 years, despite hundreds of
14 C dating of charcoals found
4 Carbon-14
57