Changes in radiocarbon concentration in modern wood from Nagoya, central Japan

Nuclear Instruments and Methods in Physics Research B 223–224 (2004) 507–510 www.elsevier.com/locate/nimb Changes in radiocarbon concentration in modern wood from Nagoya, central Japan Andrzej Rakowski b a,b,* , Toshio Nakamura a, Anna Pazdur b a Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan Radiocarbon Laboratory, Institute of Physics, Silesian University of Technology, Gliwice, Poland Abstract Recently radiocarbon (14 C) concentration in the atmospheric CO2 has decreased over time due to the exchange of CO2 between atmosphere and ocean, but also due to emission of 14 C free-CO2 from burning fossil fuels. The second contribution, known as the Suess effect, can be observed in the highly industrialized and/or urban area. 14 C concentrations in annual rings of a pine tree from the urban area in Nagoya, Japan, that grew over the last 24 years were measured with AMS to be remarkably lower than those observed in ‘‘clear air’’ of corresponding years at Schauinsland station in Germany. The measured data were used to estimate the rate of the fossil component in atmospheric CO2 that was derived from fossil fuel burning. Exponential and linear functions were fitted to the secular variations of 14 C concentration in annual rings to calculate the decay constant and an average decreasing rate of 14 C concentration. The result suggests that the use of annual rings of trees to obtain the secular variations of 14 C concentration of atmospheric CO2 can be useful and efficient for environmental monitoring and modeling of the carbon distribution in local scale. Ó 2004 Elsevier B.V. All rights reserved. PACS: 89.60.)k Keywords: Radiocarbon concentration; Tree ring; Secular variation; Carbon dioxide; Fossil fuel component 1. Introduction Atmospheric CO2 is increasing due to the emission of gases from burning fossil fuels, such as coal, petroleum and natural gas. Recently the CO2 concentration has exceeded 370 ppm [1,2]. This process causes changes in the proportion of carbon * Corresponding author. Address: Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan. Tel.:+81-52-789-2579; fax:+81-52-789-3092. E-mail address: andrzej_rakowski@hotmail.com (A. Rakowski). isotopes in the atmosphere and subsequently in the biosphere and ocean. Radiocarbon concentration in atmosphere rapidly increased in late 1950s and early 1960s due to nuclear bomb tests, reaching a maximum in 1963, when the level was almost double that of natural 14 C concentration [3]. After the test ban treaty was enforced, the 14 C concentration in the atmosphere slowly decreased, due to carbon exchange mainly against the oceanic carbon reservoir and the input of ‘‘dead’’ carbon from fossil fuels. In the 1990s, the level was still about 10% higher than pre-bomb concentration [4,5]. Several laboratories around the world are continuously monitoring atmospheric CO2 levels and 14 C 0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.04.095 508 A. Rakowski et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 507–510 concentrations, either directly from the atmosphere [4,6], or in plants [5,7,8]. Results from such studies show that large local emissions of fossil-fuel CO2 will cause changes in the carbon isotope composition of the atmosphere and local biosphere. This effect is strongest in heavily industrialized areas, but can also be observed in highly urbanized areas, where CO2 is emitted from industrial facilities, vehicles and other common sources. The magnitude of changes in 14 C concentration depends on the distance to sources of CO2 emissions [9]. To investigate the secular changes in 14 C concentration in the urban area of Nagoya city, Japan, and to estimate the CO2 contribution by a local consumption of fossil fuels, we have analyzed carbon isotope composition of annual rings of a pine tree grown in the main campus of Nagoya University. 2. Samples and methods Nagoya city (35° 570 N, 136° 580 E) is located in Aichi Prefecture, central Japan, and with a population of over two million is the fourth largest city in Japan. Along with pollution related to urban infrastructure and vehicles, local industrial facilities in Nagoya and the surrounding area are the main source of CO2 emissions. Sampling was done from a pine tree (Pinus densiflora) growing near the intersection of two main roads with heavy traffic on the Higashiyama campus of Nagoya University and in an area 8 km east of the city center. Samples of annual growth rings, as a radial section of 5 mm in diameter and about 30 cm in length, in the pine tree were collected using a hollow drill. To obtain sufficient material for AMS analysis, three core samples were collected from each tree and the annual growth rings were separated. A mixture of early wood and late wood was used to represent a single year sample. Samples were washed in distilled water and prepared using the A–A–A method. The treated residues were combined with cupric oxide and sealed into glass tubes evacuated with a rotary pump. The tubes were then placed in an electric furnace for 2 h at 850 °C. Carbon dioxide produced from the samples was purified in a glass cryogenic vacuum line system and then reduced to graphite using iron powder as a catalyst [10]. The resulting mixture of graphite and iron powder was dried and pressed into a target holder for AMS 14 C measurements. The stable carbon isotope ratio was measured using a Finnigan MAT 252 isotope-ratio mass spectrometer at the Center for Chronological Research, Nagoya University. Radiocarbon contents are reported as D14 C in permil (‰) deviations from the standard sample, 95% activity of NBS oxalic acid (SRM-4990) [11]. The stable carbon isotope ratio is expressed in d 13 CPDB notation on the PDB scale [12]. 3. Results and discussion Fig. 1 shows the variation of 14 C concentrations in tree rings for the 24 years from 1979 to 2002. The 14 C data for the period 1977–1996 from Schauinsland [4] are included to represent yearly average values of 14 C concentration in ‘‘clear air’’. The yearly average values were used, instead of seasonal averages, according to mild and maritime climate in Nagoya, which extends the vegetation period for pine tree to almost whole year. An exponential function was fitted to the data from Schauinsland to estimate the D14 C values for the missing period of 1997–2002 by extrapolation. Results show a gradual decrease in 14 C activity over time. Radiocarbon concentrations in annual rings of the pine tree from Nagoya are lower than those observed in ‘‘clean air’’ of relevant years from the Schauinsland station. This can be attributed to a local Suess effect. The D14 C values of 55.4‰, 48.7‰ and 49.5‰ obtained for 1997, 1998 and 1999 annual rings of the pine tree, respectively, are systematically lower than those for leaves of a tree (Quercus variabilis) collected nearby (64.1‰, 54.3‰ and 63.2‰ [14] for respective years), possibly due to different vegetation location as well as period between two kinds of trees used for the experiments. Radiocarbon concentrations 79‰, 69‰ and 62‰, in tree rings during the years 1994, 1995 and 1996, respectively, are also significantly lower than yearly mean values of D14 C in atmospheric CO2 at Schauinsland (117‰, 111‰ and 102‰ [4]), but are similar to respective values for A. Rakowski et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 507–510 509 380 CO2 [ppmV] 800 (a) 700 600 (b) 370 360 350 Delta 14C [per mil] 340 500 330 1980 1985 1990 1995 2000 Year 400 300 200 100 Nagoya - tree rings Schauinsland - CO2 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year Fig. 1. (a) Radiocarbon concentration in tree rings from Nagoya, central Japan. The intermittent line represents the changes of radiocarbon concentration in tree rings in Nagoya since 1951 (unpublished data) and solid line represents annual average values of 14 C concentration measured in ‘‘clear air’’ at Schauinsland station [4]. (b) Atmospheric concentration of CO2 measured in Schauinsland [2] for the period of 1979–2003. atmospheric CO2 at Zagreb (81‰, 88‰ and 56‰ [7]), where decrease of 14 C concentration can be also expected due to Suess effect. To estimate long-term changes in 14 C concentration, an exponential function was fitted to the results for tree rings from Nagoya, giving the correlation factor R to be 0.99. The function yields an inverse value of the decay constant to be 13.1yr, comparable to the estimates of around 16yr reported in the literatures [4,7,15]. Additionally, a linear function was fitted to the D14 C data for the periods 1979–1983 and 1984–2002, separately, and the D14 C value was estimated to decrease at a rate of 15.6‰/yr and 7.1‰/yr, respectively. Atmospheric D14 C values measured at two stations in Croatia (Zagreb and Plitovice) during a similar period yielded decreasing rates of 12‰ yr and 10.6‰/yr, respectively [7]. Similarly, for Schauinsland station, decreasing rates were obtained to be 14.1‰/yr and 9.7‰/yr for the periods 1983–1985 and 1985–1989, respectively [13]. The decreasing rates in D14 C at four different places show similar values for similar periods. This suggests that the process of diluting 14 C concentration in the atmospheric CO2 could have a similar character in different locations and environments. The linear regression predicts that D14 C values will be equal to 0‰ in 2006 for Nagoya, comparable with the estimates of 2007 for Schauinsland [4], 2004 for Groningen [16] and 2000 for Zagreb [7]. Radiocarbon concentrations in the atmosphere over large cities depend on emissions of CO2 from fossil fuels. Concentrations of CO2 can be divided into three components: a background component (Ca ); a biogenic component (Cb ); and a fossil fuel component related to the anthropogenic emission of CO2 (Cf ). Mathematical equations, widely described in [6] and [8], can be used to describe the relationships between each component and carbon isotopic composition, and were used to estimate the fossil fuel component. The fossil fuel component of CO2 concentration in the atmosphere at the sampling location in Nagoya was calculated using data obtained experimentally, along with additional data that represent yearly average values of D14 C and CO2 concentration in ‘‘clear air’’ at the Schauinsland station [2,4], which were assumed to include both background and biogenic components. Calculated results are shown in Fig. 2. The yearly average value of Cf for the analyzed 510 A. Rakowski et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 507–510 rings of trees to obtain the secular variations of 14 C concentration of atmospheric CO2 provides us a valuable environmental monitor. 18 Cf [ppmV] 16 Acknowledgements 14 12 10 1980 1985 1990 1995 2000 Year Fig. 2. Average annual values of the fossil component Cf for the period 1979–2002, calculated by using 14 C concentration of tree rings from Nagoya, Japan. Also used were the data from Schauinsland station, representing 14 C and CO2 concentrations in ‘‘clear air’’ [2,4]. period was 15.2 ppmv, which fell between the approximate seasonal values of 27.5 ppmv (winter) and 10 ppmv (summer), calculated for the period 1983–1994 in the urban area of Krak
ow [6]. The error of estimation for the anthropogenic emission of CO2 (Cf ) is around ±8% or less of the Cf value. 4. Conclusion Radiocarbon concentrations in annual rings of a pine tree from Nagoya are lower than those of ‘‘clear air’’ in corresponding years, as a result of the emission of CO2 from fossil fuel use. The 14 C data were fitted with exponential and linear functions to obtain estimates of the decay constant and the decrease rate, respectively, of 14 C concentrations. The linear functions were used also to predict the year when the D14 C level will be equal to 0‰ . From the 14 C records in annual rings, it was also possible to calculate the fossil component of CO2 in the area near the pine tree sample on the Higashiyama Campus, Nagoya University. The results show that, with the isotopic information recorded in tree rings, it is possible to estimate 14 C concentrations for the past. The use of annual This work was supported partly by the Ministry of Education, Sciences, Sport and Culture of Japan. We express our thanks to all staff of the Center for Chronological Research, Nagoya University, for their kind support, and many thanks are, in particular, to Tomoko Ohta and Akiko Ikeda for introducing us the procedure of sample preparation and to Dr. Etsuko Niu for her help in 14 C measurements. References [1] C.D. Keeling, T.P. Whorf, J. van der Plicht, Nature 375 (1995) 666. [2] GLOBALVIEW-CO2 : Cooperative Atmospheric Data Integration Project – Carbon Dioxide. CD-ROM, NOAA CMDL, Boulder, Colorado, 2003 (also available on Internet via anonymous FTP to ftp.cmdl.noaa.gov, path: ccg/co2/GLOBALVIEW). [3] R. Nydal, K. L€ ovseth, Oak Ridge National Laboratory NDP-057, 1996. [4] I. Levin, B. Kromer, Radiocarbon 39 (2) (1997) 205. [5] R. McNeely, Environ. Int. 20 (5) (1994) 675. [6] T. Kuc, M. Zimnoch, Radiocarbon 40 (1) (1998) 417. [7] I. Krajcar-Broni
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