extracted ions from the graphite by a Cesium gun, were
accelerated to a voltage difference of 2 MeV while the
acceleration voltage is nowadays reduced at 0.5 MeV. The
reader is referred to various publications and web descriptions on the equipment and measurement methods (Taylor
et al. 1992).
Physical-Chemical Treatment
The samples are first cleaned by physicochemical treatments
to eliminate contaminants. Vegetation samples collected in
soils may include old carbonates and living rootlets, and the
14 C ages would thus increase or decrease respectively. After
a visual examination, the organic remains (seeds, coal,
plants) undergo the classic Acid-Alkali-Acid chemical
treatment (noted AAA). The aim is to eliminate contaminants from bacterial decomposition of organic matter since
the burial of the sample. Finally, CO 2 is obtained by burning
the sample into an evacuated quartz sealed tube filled with
copper oxide at about 800 °C. Carbonate samples (foraminifera, pteropods, corals, speleothems, earthworm granules) are examined under the microscope to check their
homogeneity. Corals and mollusk shells are pre-cleaned by
sand blasting to eliminate secondary calcite precipitation,
which may lower the
14 C ages. All the carbonates are leached in a weak acid to remove surface contaminants before
hydrolysis in a vacuum device. For AMS
14 C measurement,
the obtained CO 2 is converted into graphite by metal catalysis (Tisnérat-Laborde et al. 2001; Hatté et al. 2003).
Determination of a
14
C Age
The
14 C age is obtained by comparing the activity of a
sample to that of a standard reference, representative of the
atmospheric
14 C content in 1950 (NBS-I; NBS-II; sucrose)
regardless of the dating techniques. Chemicals as well as
vacuum lines and counting devices are contaminated by
modern
14 CO 2 which has a
14 C/
12 C ratio of 1.2 Â 10
−12 in
1950 AD. The effect of such a contamination on the
14 C ages
of a 1 mg carbon sample may be estimated using a simple
mixing equation. A
14 C age of 40,000 years
(
14 C/C = 6.88 Â 10
−15 ) would be lowered by 4300 years to
7200 years, due to a modern contamination of 5 µg to
10 µg, respectively. This emphasizes the importance of a
careful cleaning of the sample. The same amounts of
14 C-free contaminants will have little effect: about 40 years
and 80 years respectively regardless of the
14 C age of the
sample.
The first
14 C dating of cave paintings revealed an age of
about 31,000 years BP that deeply modified our understanding of how human art evolved (Valladas et al. 1992;
Cuzange et al. 2007). Clearly, such old ages cannot be due to
a contamination by the carbonates precipitate on cave walls,
as was frequently hypothesized. To increase the
14 C age of a
painting of 1 mg carbon and 15,000 years old, for example,
to an age of about 33,000 years would require some 900 µg
of
14 C-free contaminants, which is almost the entire sample
and this would have been seen through visual examination.
The internal contamination of samples, due to the
cleaning treatments, is carefully assessed by measuring the
14 C activity of an old (
14 C-free) sample (blank), cleaned in a
similar manner to samples of unknown ages. During a
14 C-AMS run of measurements, standard references, blank
samples and the samples to date are inserted. The “blank” or
“background” activity is subtracted from the activity of the
sample of unknown age. The smaller the sample, the greater
the effect of the internal contamination on the
14 C age. This
contamination varies depending on the nature of the samples. Even if the blank activity is very low, its variability
determines the accuracy of the
14 C age and the
14 C age limit.
Consider two samples of carbonate and charcoal, with a
14 C/C activity of 6.88 Â 10
−15 (an age of 40,000 years BP)
and an absolute error of 5%. The blank variability accounts
for 25% and the subtracted blank value for the carbonate and
charcoal is 5 Â 10
−16 and 19 Â 10
−16 , respectively. If the
blank variability increases by a factor of 2, the
14 C age
uncertainties will increase by 100 years for the carbonate
and by 700 years for the charcoal, respectively. The precision of a
14 C age older than 30,000 years BP may be
appreciably affected by a few thousand years when the blank
activity is not well-estimated.
Some Examples of Post-depositional
Disturbances of the
14
C Ages
The validity of the
14 C dating of climatic, archaeological or
geological events depends, in many cases, of mechanical or
biological disturbances of sedimentary deposits that may
occur after the death and the burial of organisms. In
archaeological sites, stratigraphic inversions of
14 C ages may
be due to soils disturbances linked either to successive
occupations by human and animals, or to wind and water
effects, or both. The increasingly small size of
14 C-dated
samples, which tend to migrate through the sedimentary
deposits, favors such
14 C age inversions.
Archaeological sites contain numerous
14 C datable
remains, such as mollusk shells, charcoals and bones.
Researchers give priority to the
14 C dating of charcoals,
because the
14
C dating of marine shells requires a correction
of the reservoir ages to be compared with those of bone and
vegetation samples. In a seaside Peruvian site, paired marine
mollusk shells and charcoals were associated in several
sediment layers to allow the sea surface reservoirs ages
66
M. Paterne et al.
accelerated to a voltage difference of 2 MeV while the
acceleration voltage is nowadays reduced at 0.5 MeV. The
reader is referred to various publications and web descriptions on the equipment and measurement methods (Taylor
et al. 1992).
Physical-Chemical Treatment
The samples are first cleaned by physicochemical treatments
to eliminate contaminants. Vegetation samples collected in
soils may include old carbonates and living rootlets, and the
14 C ages would thus increase or decrease respectively. After
a visual examination, the organic remains (seeds, coal,
plants) undergo the classic Acid-Alkali-Acid chemical
treatment (noted AAA). The aim is to eliminate contaminants from bacterial decomposition of organic matter since
the burial of the sample. Finally, CO 2 is obtained by burning
the sample into an evacuated quartz sealed tube filled with
copper oxide at about 800 °C. Carbonate samples (foraminifera, pteropods, corals, speleothems, earthworm granules) are examined under the microscope to check their
homogeneity. Corals and mollusk shells are pre-cleaned by
sand blasting to eliminate secondary calcite precipitation,
which may lower the
14 C ages. All the carbonates are leached in a weak acid to remove surface contaminants before
hydrolysis in a vacuum device. For AMS
14 C measurement,
the obtained CO 2 is converted into graphite by metal catalysis (Tisnérat-Laborde et al. 2001; Hatté et al. 2003).
Determination of a
14
C Age
The
14 C age is obtained by comparing the activity of a
sample to that of a standard reference, representative of the
atmospheric
14 C content in 1950 (NBS-I; NBS-II; sucrose)
regardless of the dating techniques. Chemicals as well as
vacuum lines and counting devices are contaminated by
modern
14 CO 2 which has a
14 C/
12 C ratio of 1.2 Â 10
−12 in
1950 AD. The effect of such a contamination on the
14 C ages
of a 1 mg carbon sample may be estimated using a simple
mixing equation. A
14 C age of 40,000 years
(
14 C/C = 6.88 Â 10
−15 ) would be lowered by 4300 years to
7200 years, due to a modern contamination of 5 µg to
10 µg, respectively. This emphasizes the importance of a
careful cleaning of the sample. The same amounts of
14 C-free contaminants will have little effect: about 40 years
and 80 years respectively regardless of the
14 C age of the
sample.
The first
14 C dating of cave paintings revealed an age of
about 31,000 years BP that deeply modified our understanding of how human art evolved (Valladas et al. 1992;
Cuzange et al. 2007). Clearly, such old ages cannot be due to
a contamination by the carbonates precipitate on cave walls,
as was frequently hypothesized. To increase the
14 C age of a
painting of 1 mg carbon and 15,000 years old, for example,
to an age of about 33,000 years would require some 900 µg
of
14 C-free contaminants, which is almost the entire sample
and this would have been seen through visual examination.
The internal contamination of samples, due to the
cleaning treatments, is carefully assessed by measuring the
14 C activity of an old (
14 C-free) sample (blank), cleaned in a
similar manner to samples of unknown ages. During a
14 C-AMS run of measurements, standard references, blank
samples and the samples to date are inserted. The “blank” or
“background” activity is subtracted from the activity of the
sample of unknown age. The smaller the sample, the greater
the effect of the internal contamination on the
14 C age. This
contamination varies depending on the nature of the samples. Even if the blank activity is very low, its variability
determines the accuracy of the
14 C age and the
14 C age limit.
Consider two samples of carbonate and charcoal, with a
14 C/C activity of 6.88 Â 10
−15 (an age of 40,000 years BP)
and an absolute error of 5%. The blank variability accounts
for 25% and the subtracted blank value for the carbonate and
charcoal is 5 Â 10
−16 and 19 Â 10
−16 , respectively. If the
blank variability increases by a factor of 2, the
14 C age
uncertainties will increase by 100 years for the carbonate
and by 700 years for the charcoal, respectively. The precision of a
14 C age older than 30,000 years BP may be
appreciably affected by a few thousand years when the blank
activity is not well-estimated.
Some Examples of Post-depositional
Disturbances of the
14
C Ages
The validity of the
14 C dating of climatic, archaeological or
geological events depends, in many cases, of mechanical or
biological disturbances of sedimentary deposits that may
occur after the death and the burial of organisms. In
archaeological sites, stratigraphic inversions of
14 C ages may
be due to soils disturbances linked either to successive
occupations by human and animals, or to wind and water
effects, or both. The increasingly small size of
14 C-dated
samples, which tend to migrate through the sedimentary
deposits, favors such
14 C age inversions.
Archaeological sites contain numerous
14 C datable
remains, such as mollusk shells, charcoals and bones.
Researchers give priority to the
14 C dating of charcoals,
because the
14
C dating of marine shells requires a correction
of the reservoir ages to be compared with those of bone and
vegetation samples. In a seaside Peruvian site, paired marine
mollusk shells and charcoals were associated in several
sediment layers to allow the sea surface reservoirs ages
66
M. Paterne et al.