Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 2, pp. 135-138, 2005

ArticlesTemporal Variation of Carbon-14 Concentration in Tree-ring Cellulose for the Recent
50 Years
Yoshimune Yamada,*, a Kaeko Yasuike,a and Kazuhisa KomurabaFaculty of Pharmaceutical Sciences, Hokuriku University, Kanagawa-machi, Kanazawa-City 920-1181, Japan
Low Level Radioactivity Laboratory, Kanazawa University, Nomi-shi, Ishikawa 923-1224, Japanb

Received: June 6, 2005; In Final Form: August 7, 2005
Concentration of organically-bound 14C in the tree-ring cellulose of a pine tree grown in Shika-machi, Ishikawa
prefecture, Japan (37.1˚N, 136.5˚E), was measured for the ring-years from 1949 to 1999 and compared with thosein several trees from East Asia region reported by other researchers. The difference of ∆14C values was foundduring the period of 1970–1981 between our data and those of other reports, in addition to the difference during
the period of 1963–1967 caused by the so-called latitude dependence of the 14C variations in the northern troposphere. These results suggest that the 14C concentration in atmospheric CO2 was considerably disturbed during theperiod of 1970–1981, especially in 1970, 1976, and 1978–1981, in the East Asia region. This phenomenon may
be interpreted by the possibility of the several times of injections of 14C originated from a series of Chinese thermonuclear bomb tests.

1. Introduction
Most of environmental 14C originates from the continuous
production by cosmic rays in the upper atmosphere and fromthermonuclear bomb tests carried out during the 1950s andearly 1960s1−3 as well as the case of environmental tritium.4
The long-term variation in environmental 14C concentration
at the specific site is preserved in the 14C records in a series ofannual rings of the tree grown there.5−13 Measurement of 14C
in the cellulose fraction of tree rings is a preferred method forthe analysis of annual variation of 14C concentration, sinceorganically-bound carbon in the cellulose undergoes no change
following the time of tree-ring formation.
In the previous paper,9 the 14C concentration in the tree ringcellulose of an 80-year-old pine tree grown in Tatsunokuchimachi, Ishikawa prefecture, Japan (36.4˚N, 136.5˚E) (hereafter
the Tatsunokuchi-sample) was reported for the period of
1941–1987. An interesting fact was that there was a difference in the amplitude of the 14C concentrations between our
result and those of other researchers during the period of
1963–1967, though the year of two peaks in our result coincided with those in the known 14C concentration patterns in thetree rings.5−7 The difference may be interpreted by the socalled latitude dependence of the 14C variations in the northern
troposphere,1−3 which was due to the 14C released to the atmosphere at higher northern latitudes by a series of thermonuclear
bomb tests carried out during the 1950s and early 1960s.
In the present work, the 14C concentration in the tree-ringcellulose of a pine tree grown in Shika-machi, Ishikawa prefecture, Japan (37.1˚N, 136.5˚E) (hereafter the Shika-sample), wasmeasured for the ring-years of 1949 to 1999, and the results
were compared with those of the Tatsunokuchi-sample to study
regional differences in environmental 14C concentration. Shikamachi is located at little higher latitude than Tatsunokuchimachi, as shown in Figure 1.
Furthermore, the annual variation of 14 C concentration
obtained from the Shika-sample was compared with those of
14
C concentration in tree rings from East Asia region reported
by other researchers.
*Corresponding author. E-mail: yo-yamada@hokuriku-u.ac.jp.
FAX: +81-76-229-2781.

Figure 1. Sampling sites of the pine trees for 14C measurement, Shikamachi, Ishikawa prefecture, Japan (37.1˚N, 136.5˚E) and Tatsunokuchimachi, Ishikawa prefecture, Japan (36.4˚N, 136.5˚E).
Fine lines in the figure represent rivers in Ishikawa prefecture, Japan.

2. Experimental
Cellulose samples of tree rings. The trunk of a 67-yearold pine tree (Japanese Black Pine; Pinus thunbergii), 0.5 m
in diameter, and 1.2 m in length, was cut at the position of
0.5–1.7 m above the ground surface on August 10, 1999, andwas analyzed in the present work. The pine tree is the same
tree used for tritium measurement in the annual growth ringsby the present authors, previously.14© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/07/2005

136 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
All of the tree-ring layers had a thickness of more than 1.5mm, which was enough to separate without cross-contamination from subsequent years. Each cellulose fraction corresponding to the 1949–1999 ring-years was isolated from woodchips according to the same method without pre-treatment by
the steam explosion of wood chips15 which was applied to the
Tatsunokuchi-machi tree (Japanese Red Pine; Pinus densiflora).
Combustion of cellulose samples. The cellulose fractionof 100 g corresponding to a single year was gently burned toobtain the combustion water and CO2 by the method described
previously.9, 14 The combustion water was condensed in two
dry-ice traps connected in tandem. The carbon dioxide gas wascollected as dry-ice form by an EtOH trap cooled with liquid
nitrogen located behind the dry-ice traps. The resulting dry-ice
was immediately converted to calcium carbonate according to
the following procedure. A stream of CO2 generated from thedry-ice was trapped by bubbling in a 7M NH4OH solution, andthe absorbed CO2 was precipitated quantitatively as CaCO3 byadding an excess of CaCl2 to the solution followed by heating.
Approximately 280 g of dried CaCO3 obtained from 100 g ofcellulose was stored in an airtight vessel.
Benzene synthesis for the preparation of 14C counting
sources. The original CO2 was regenerated by adding a solution of perchloric acid dropwise to the 40 g of stored CaCO3.
It was then converted to benzene in vacuum through thefollowing reactions.
600 ˚C
2CO2 + 10Li → Li2C2 + 4Li2O
Li2C2 + 2H2O → 2LiOH + C2H2
3C2H2 → C6H6
with V2O5 catalyst
The counting source for liquid scintillation spectrometer
was prepared by adding a toluene based butyl-PBD scintillation solution to 3.5–4.0 g of synthesized benzene to a total
volume of 20 mL. The synthesized benzene prepared fromNIST oxalic acid (SRM 4990C) was used as a standard reference of 14C.
Measurement of 14C. The 14C activity was measured by a
low background liquid scintillation counter, Aloka LB-1, undertemperature-stabilized conditions at 12 ˚C at least for 100 min
× 40 times. The counting background of the 20 mL low potassium glass vial was 2.64 cpm at a rate of 58% efficiency. The
results are presented as ∆14C, defined in the previous paper.9
Isotopic fluctuation of a sample was corrected using the δ13C
value of –26.4 ‰, which was the average of the δ13C values
determined for several tree-ring samples with the Finnigan
MAT Stable Isotope Ratio Mass Spectrometer, MAT 252.
3. Results and Discussion
Concentration of organically-bound 14C in the Shika-sample(37.1˚N, 136.5˚E) was measured for the ring-years of 1949 to1999 and compared with those of the Tatsunokuchi-sample(36.4˚N, 136.5˚E)9 for the 1941–1987 period to investigate theregional differences of environmental 14C concentration.
The Shika-sample was grown in a small pine forest at theseaside of Shika-machi situated on a lowland plain area about
20 m above sea level and lies along the coast of the Sea ofJapan; high mountains are not present in the surroundings. On
the other hand, Tatsunokuchi-sample was grown at a site
halfway down a hill of about 50 m above sea level near the rimof the high mountain region of 1500–2000 m, located about 70
km south of Shika-machi. The two sets of sampling sites areshown in Figure 1.
The ∆14C values measured for the Shika-sample are given in
Table 1. The ∆14C values in the table were reproducible at ±10‰ or less, as described in the previous paper.9

YamadaTABLE 1: Carbon-14 Concentration in Tree-ring Celluloseof a Pine Tree in Shika-machi, Ishikwa Prefecture, Japan(37.1˚N, 136.5˚E)
Year∆14C*/ ‰Total
counting time/min

1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
1980
1979
1978
1977
1976
1975
1974
1973
1972
1971
1970
1969
1968
1967
1966
1965
1964
1963
1962
1961
1960
1959
1958
1957
1956
1955
1954
1953
1952
1951
1950
1949

107.0 ± 4.6
116.7 ± 4.6
115.7 ± 5.4
121.4 ± 5.8
124.3 ± 4.2
132.2 ± 3.2
137.7 ± 3.0
159.0 ± 4.3
134.3 ± 4.3
166.6 ± 4.4
176.7 ± 4.4
187.1 ± 4.4
175.9 ± 4.8
195.9 ± 3.5
222.7 ± 4.8
224.6 ± 5.5
242.4 ± 5.3
275.7 ± 5.4
259.3 ± 5.2
307.2 ± 6.2
293.2 ± 6.1
313.1 ± 5.3
358.1 ± 4.8
351.0 ± 4.8
393.1 ± 5.5
402.1 ± 5.5434.2 ± 5.5
483.2 ± 5.2
480.5 ± 3.7
499.4 ± 3.1
538.7 ± 3.8
573.6 ± 6.1
654.2 ± 6.5
691.9 ± 6.8
760.7 ± 6.4
814.0 ± 6.7
726.9 ± 6.5
327.2 ± 4.4
218.0 ± 3.6
215.7 ± 4.9
233.0 ± 5.1
128.1 ± 2.8
235.9 ± 3.7
38.0 ± 4.6- 2.6 ± 4.4- 17.6 ± 4.3- 28.7 ± 4.6- 42.3 ± 2.8- 35.9 ± 3.8- 33.9 ± 3.6- 44.4 ± 4.3

4000
4000
4000
5000
5000
10000
11000
5000
8000
5000
5000
5000
4000
8000
5000
4000
4000
4000
4000
4000
4000
8000
5000
5000
6000
4000
4000
5000
10000
14000
10000
4000
4000
4000
4000
4000
4000
8000
8000
4000
4000
12000
8000
4000
4000
4000
4000
9000
5000
4000
4000

*The error denotes a counting error of 1σ.
All samples were measured using a low background liquid scintillation counter, Aloka LB-1.

J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005 137

Temporal Variation of Carbon-14 Concentration

900

800

800

700

700

600

600
∆ C/‰

1000

900

500

14

∆14 C / ‰

1000

400

6 8 10 11

15 16

21

26

500
400
300

300

200
200

100
100

0

0
-100
1940

-100
1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

Year

Figure 2. Annual variation of the ∆14C values of the Shika-sample andTatsunokuchi-sample.
The bar graph: Tatsunokuchi-sample○: Shika-sample

The annual variation of ∆14C values of the Shika- and the
Tatsunokuchi-sample are plotted as a function of the ring-year
in Figure 2. Temporal variation of organically-bound 14 C
concentration in the Shika-sample showed essentially similarvariations to that of the Tatsunokuchi-sample. The ∆14C valueof both the Shika- and the Tatsunokuchi-sample began to increase
from 1954 to a small broad peak of 250 ‰ in 1959 – 1961,
followed by a rapid increase to the highest value of approximately
800 ‰ in 1964. After the maximum, it diminished gradually
year by year during past two or three decades. However, it
demonstrated a small difference in the amplitude of the ∆14C
value during the period of 1949–1967, 1970, and 1978–1981.
After 1982, significant difference was not observed between
the two sets of samples. These differences suggest that the 14C
concentration in atmospheric CO2 fluctuated even in the narrow
region only by about 70 km of distance for the period before
1982. It is assumed that the 14C records in a series of annual
rings of the tree reflect the variation of annual average levelsof 14C concentration in atmospheric CO2 at the growth site of
the tree, since the carbon necessary for synthesizing cellulose isderived essentially by photosynthesis from the CO2 in the air.
In Figure 3, the annual variation of ∆14C values obtained
from the Shika-sample is compared with those of ∆14C values intree rings from East Asia region reported by other researchers,
for a spruce from Dailing, China (47˚N, 129˚E),7 for red pinesfrom Mt. Chiak (37˚23’N, 128˚3’E and 37°20’N, 128˚4’E ) and
Mt. Kyeryong (36˚20’N, 127˚13’E), Korea,13 and for Japanesecypress from Kiso mountainous district, Gifu prefecture, Japan(35.6˚N, 137.5˚E).6
An interesting result in comparing our data with those ofother researchers is that there existed significant difference inthe amplitude of the ∆14C value during the period of 1963–1967.
The disagreement of the ∆14C value among trees during theperiod of 1963–1967 can be interpreted by the so-called latitude dependence of the 14C variations in the northern troposphere,6−7 which was due to the 14C released to the atmosphereat higher northern latitudes by a series of thermonuclear detonations carried out during the early 1960s.
The most important result in this work is that the differenceof ∆ 14 C values was found during the period of 1970 – 1981
between our data and those of other reports. The spruce from
Dailing, China, located in higher latitude than Shika-machi,
showed higher ∆14C values than our pine tree. The red pinesfrom Mt. Chiak and Mt. Kyeryong, Korea, located at almost
the same latitude of Shika-machi, showed slightly lower ∆14C
values than our pine tree. On the contrary, the ∆14C values ofJapanese cypress from Kiso mountainous district, Gifu prefecture, Japan, coincided with those of our pine tree, except forthe period of 1976 and 1980–1981.
On the basis of these results, it was found that the 14C concen-

Year

Figure 3. Comparison of annual variation of ∆14C values in tree rings
from different sites. Arrows and numerals at the top of the figure
represent the number and time of year of the Chinese thermonuclear
bomb test.
The solid line: pine tree in this work (37.1˚N, 136.5˚E)
7▲: spruce from Dailing, China (47˚N, 129˚E), by D. Kaimei et al.
□: red pines from Mt. Chiak (37˚23’N, 128˚3’E and 37˚20’N,
128˚4’E ) and Mt. Kyeryong (36˚20’N, 127°13’E), Korea, by J.
H. Park et al.13○: Japanese cypress from Kiso mountainous district, Gifu prefecture, Japan (35.6˚N, 137.5˚E), by T. Nakamura et al.6
The dotted line: compiled atmospheric 14C curve for the northernhemisphere temperate region by Q. Hua and M. Barbetti3

tration in atmospheric CO2 was considerably disturbed during
the period of 1970–1981, especially in 1970, 1976, and 1978–
1981, in the East Asia region. However, for the period from1970 onwards, it is known that there has not been much difference between locations in terms of the 14C concentration inatmospheric CO2 in the Northern Hemisphere, since the bomb
14
C reached a global equilibrium in the late 1960s.1−3 Compiled
atmospheric 14C curve for the northern hemisphere temperateregion by Hua and Barbetti 3 is shown by the dotted line in
Figure 3. Their average yearly atmospheric ∆ 14 C values
agreed well with our data except the period of 1970 and 1978–
1981, whereas, they disagreed with the data of 1976–1982 ringsof the spruce from Dailing, China. It was pointed out by
Kaimei et al.7 that excessive 14C in the 1976–1982 rings of the
spruce from Dailing, China, was likely due to the effect of 1976
and 1980 Chinese bomb tests.
After the Partial Test Ban Treaty of 1963, nuclear tests in theatmosphere declined greatly, however, the People’s of Republicof China continued atmospheric nuclear test until 1980. Bomb
14
C is produced in the nuclear reaction between thermal neutrons
delivered in the explosion and nitrogen in the air. About 7 times
larger amounts of thermal neutron are released by the thermonuclear explosion than by the atomic explosion. Thus, the time ofyear of the Chinese thermonuclear bomb tests are shown byarrows in Figure 3. The Chinese thermonuclear bomb tests wereconducted 8 times, namely the 6th (June, 1967), 8th (December,1968), 10th (September, 1969), 11th (October, 1970), 15th
(June, 1973), 16th (June, 1974), 21st (November, 1976), and26th (October, 1980).4
It is apparent from these considerations that the disturbanceof the 14C concentration in atmospheric CO2 occurred during
the period of 1970–1981 in the East Asia region may be interpreted by the possibility of the several times of injections of
14
C originated from a series of Chinese thermonuclear bomb
tests. This assumption was supported by the fact that the ∆14C
values from three distinct sites in the East Asia region agreed
well with each other after 1982, when the Chinese thermonuclear bomb tests were suspended.
Acknowledgments. The authors wish to express their
thanks to Dr. Hikaru Amano and Dr. Mariko Andoh, Japan
Atomic Energy Research Institute, for their helpful advice in138 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
our mass spectrometer analyses. This study was supported inpart by a Grant from the Ishikawa Prefectural Institute of Public
Health and Environmental Science.
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