Journal of Environmental Radioactivity 146 (2015) 67e72
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Radiocarbon concentration in modern tree rings from Fukushima, Japan Sheng Xu a, *, Gordon T. Cook a, Alan J. Cresswell a, Elaine Dunbar a, Stewart P.H.T. Freeman a, Helen Hastie a, Xiaolin Hou b, Piotr Jacobsson a, Philip Naysmith a, David C.W. Sanderson a a b
Scottish Universities Environmental Research Center (SUERC), East Kilbride, G75 0QF, UK Center for Nuclear Technologies, Technical University of Denmark, 4000 Roskilde, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 December 2014 Received in revised form 20 March 2015 Accepted 6 April 2015 Available online
A 30-year-old Japanese cedar (Cryptomeria japonica), collected from Iwaki, Fukushima in 2014, was analyzed for the long-lived radionuclide 14C. Values of D14C varied from 211.7‰ in 1984 to 16.9‰ in 2013. The temporal D14C variation can be described as an exponential decline, indistinguishable from the general Northern Hemisphere Zone 2 (NH Zone 2) values in the atmosphere, until at least 1994. Values of D14C for 1999 and 2004 are slightly depleted compared with NH Zone 2 values, while from 1999 to 2013 the data suggest a clear depletion with a 2e8 ppmV additional CO2 contribution from a 14C-free (i.e. fossil carbon) source. This change coincides with local traffic increases since two nearby expressways were opened in the 1990's. In addition, the small but visible 14C pulse observed in the 2011 tree-ring might be caused by release from the damaged reactors during the Fukushima nuclear accident. © 2015 Published by Elsevier Ltd.
Keywords: Cryptomeria japonica tree ring Cellulose 14C Fossil fuel Traffic effect Fukushima nuclear accident
1. Introduction In addition to natural cosmogenic 14C that is mostly formed in the stratosphere, several anthropogenic sources influence atmospheric 14C activities. These include dilution produced by fossil fuel combustion (Suess, 1955), nuclear weapon tests (Nydal and € vseth, 1983), nuclear reactor and nuclear fuel reprocessing Lo plant discharges (McCartney et al., 1988a, b), and nuclear system failures/accidents (Buzinny et al., 1998). The effect of fossil fuel combustion (Suess Effect) was observed in radiocarbon concentrations within annual tree rings by Suess (1955). The combustion caused a significant decrease in 14C concentrations in the atmosphere from around 1890 onwards. After 1945, atmospheric nuclear weapons testing caused a worldwide increase in atmospheric 14C such that by 1955 the activity was in excess of the pre-Suess Effect, natural activity. A total of 213 PBq 14C from the testing were estimated to have been released into the environment (UNSCEAR, 2000). As a consequence, D14C was enhanced in the atmosphere to around 1000‰ by 1963 and gradually decreased to less than
* Corresponding author. Fax: þ44 1355 229898. E-mail address: [email protected] (S. Xu). http://dx.doi.org/10.1016/j.jenvrad.2015.04.004 0265-931X/© 2015 Published by Elsevier Ltd.
100‰ after 2000. In particular, a significant increase in the specific activity of 14C of up to 282 Bq kg1 14C was reported in a tree ring sample, located about 20 km northwest of the Chernobyl Nuclear Power Plant, in 1986 (Buzinny et al., 1998). It is therefore important to quantify both the natural and anthropogenic 14C sources in the modern atmospheric environment to establish whether there was any significant release from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. As an alternative to direct measurements of 14 C concentrations in atmospheric CO2, measurements from tree rings are considered to be an ideal proxy for atmospheric 14C. After the FDNPP accident on 11 March 2011, significant quantities of radioactive substances were released into the atmosphere, causing serious radioactive contamination of a large area of eastern Japan. Numerous measurements of Fukushima-derived radionuclides in the worldwide environment have been reported recently. However, these studies mainly focused on short-lived gammaemitting nuclides such as radiocaesium and radioiodine. Measurements of long-lived nuclides, in particular beta-emitting 14C and 129I, are still relatively limited (i.e., Povinec et al., 2013; Steinhauser, 2014). To our best knowledge, no report on 14C in the local environment has been reported, so that 14C remains one of Fukushima's most understudied radionuclides. Thus, this work aims to investigate 14C levels before and after the accident, to
68
S. Xu et al. / Journal of Environmental Radioactivity 146 (2015) 67e72
identify any release by measuring 14C in annual tree rings of a Japanese cedar (Cryptomeria japonica) from the Iwaki region and comparing the values with the global atmospheric 14CO2 datasets. If there was a 14C release from the damaged reactors during the accident, assessment of 14C would provide important information not only for the application of 14C in environmental and climate change research, but also for the estimation of radiation exposure due to the 14C emission. 2. Material and methods A block of Japanese cedar (Cryptomeria japonica) was obtained by cutting down a ca. 3 m high, living tree in Iwaki, Fukushima, Japan (37.01 N and 140.81 E) on 14 February 2014. The sampling site is ca. 220 m west of the Joban Expressway and ca. 10 km west of Iwaki city, which had a population of 330,000e360,000 between 1995 and 2013 (Fig. 1). Being ca. 50 km southwest of the FDNPP, this area was contaminated by radionuclides during the 2011 accident (Saito et al., 2015). The alpha cellulose fraction of wood is the most reliable for determining the 14C incorporated at the time of growth (Hopper et al., 1998). Thus, from a 1.5-cm-thick partial disc cut from the trunk, each ring was cut out as units of earlywood and latewood for years from 1984 to 1989 and from 2009 to 2013 and whole ring for years 1994 and 1999. Each isolated wood subdivision was pretreated using a slightly modified version of the alpha cellulose preparation described by Hopper et al. (1998). The process includes sequential solvent extraction steps to remove waxes, oils and resins, etc., as well as other potential organic contaminants present. For the three-step soxhlet extraction process, the wood sample is weighed, cut into thin slivers and placed in a glass-fibre extraction thimble plugged with glass wool. An appropriate volume of chloroform/ethanol (C2H5OH/CHCl3) (1:2 by volume) is added and the sample refluxed for 8 h. The solvent is allowed to cool and then discarded. This process is repeated using ethanol only and finally with distilled water. The sample is dried overnight at
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Radiocarbon concentration in modern tree rings from Fukushima, Japan Sheng Xu a, *, Gordon T. Cook a, Alan J. Cresswell a, Elaine Dunbar a, Stewart P.H.T. Freeman a, Helen Hastie a, Xiaolin Hou b, Piotr Jacobsson a, Philip Naysmith a, David C.W. Sanderson a a b
Scottish Universities Environmental Research Center (SUERC), East Kilbride, G75 0QF, UK Center for Nuclear Technologies, Technical University of Denmark, 4000 Roskilde, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 December 2014 Received in revised form 20 March 2015 Accepted 6 April 2015 Available online
A 30-year-old Japanese cedar (Cryptomeria japonica), collected from Iwaki, Fukushima in 2014, was analyzed for the long-lived radionuclide 14C. Values of D14C varied from 211.7‰ in 1984 to 16.9‰ in 2013. The temporal D14C variation can be described as an exponential decline, indistinguishable from the general Northern Hemisphere Zone 2 (NH Zone 2) values in the atmosphere, until at least 1994. Values of D14C for 1999 and 2004 are slightly depleted compared with NH Zone 2 values, while from 1999 to 2013 the data suggest a clear depletion with a 2e8 ppmV additional CO2 contribution from a 14C-free (i.e. fossil carbon) source. This change coincides with local traffic increases since two nearby expressways were opened in the 1990's. In addition, the small but visible 14C pulse observed in the 2011 tree-ring might be caused by release from the damaged reactors during the Fukushima nuclear accident. © 2015 Published by Elsevier Ltd.
Keywords: Cryptomeria japonica tree ring Cellulose 14C Fossil fuel Traffic effect Fukushima nuclear accident
1. Introduction In addition to natural cosmogenic 14C that is mostly formed in the stratosphere, several anthropogenic sources influence atmospheric 14C activities. These include dilution produced by fossil fuel combustion (Suess, 1955), nuclear weapon tests (Nydal and € vseth, 1983), nuclear reactor and nuclear fuel reprocessing Lo plant discharges (McCartney et al., 1988a, b), and nuclear system failures/accidents (Buzinny et al., 1998). The effect of fossil fuel combustion (Suess Effect) was observed in radiocarbon concentrations within annual tree rings by Suess (1955). The combustion caused a significant decrease in 14C concentrations in the atmosphere from around 1890 onwards. After 1945, atmospheric nuclear weapons testing caused a worldwide increase in atmospheric 14C such that by 1955 the activity was in excess of the pre-Suess Effect, natural activity. A total of 213 PBq 14C from the testing were estimated to have been released into the environment (UNSCEAR, 2000). As a consequence, D14C was enhanced in the atmosphere to around 1000‰ by 1963 and gradually decreased to less than
* Corresponding author. Fax: þ44 1355 229898. E-mail address: [email protected] (S. Xu). http://dx.doi.org/10.1016/j.jenvrad.2015.04.004 0265-931X/© 2015 Published by Elsevier Ltd.
100‰ after 2000. In particular, a significant increase in the specific activity of 14C of up to 282 Bq kg1 14C was reported in a tree ring sample, located about 20 km northwest of the Chernobyl Nuclear Power Plant, in 1986 (Buzinny et al., 1998). It is therefore important to quantify both the natural and anthropogenic 14C sources in the modern atmospheric environment to establish whether there was any significant release from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. As an alternative to direct measurements of 14 C concentrations in atmospheric CO2, measurements from tree rings are considered to be an ideal proxy for atmospheric 14C. After the FDNPP accident on 11 March 2011, significant quantities of radioactive substances were released into the atmosphere, causing serious radioactive contamination of a large area of eastern Japan. Numerous measurements of Fukushima-derived radionuclides in the worldwide environment have been reported recently. However, these studies mainly focused on short-lived gammaemitting nuclides such as radiocaesium and radioiodine. Measurements of long-lived nuclides, in particular beta-emitting 14C and 129I, are still relatively limited (i.e., Povinec et al., 2013; Steinhauser, 2014). To our best knowledge, no report on 14C in the local environment has been reported, so that 14C remains one of Fukushima's most understudied radionuclides. Thus, this work aims to investigate 14C levels before and after the accident, to
68
S. Xu et al. / Journal of Environmental Radioactivity 146 (2015) 67e72
identify any release by measuring 14C in annual tree rings of a Japanese cedar (Cryptomeria japonica) from the Iwaki region and comparing the values with the global atmospheric 14CO2 datasets. If there was a 14C release from the damaged reactors during the accident, assessment of 14C would provide important information not only for the application of 14C in environmental and climate change research, but also for the estimation of radiation exposure due to the 14C emission. 2. Material and methods A block of Japanese cedar (Cryptomeria japonica) was obtained by cutting down a ca. 3 m high, living tree in Iwaki, Fukushima, Japan (37.01 N and 140.81 E) on 14 February 2014. The sampling site is ca. 220 m west of the Joban Expressway and ca. 10 km west of Iwaki city, which had a population of 330,000e360,000 between 1995 and 2013 (Fig. 1). Being ca. 50 km southwest of the FDNPP, this area was contaminated by radionuclides during the 2011 accident (Saito et al., 2015). The alpha cellulose fraction of wood is the most reliable for determining the 14C incorporated at the time of growth (Hopper et al., 1998). Thus, from a 1.5-cm-thick partial disc cut from the trunk, each ring was cut out as units of earlywood and latewood for years from 1984 to 1989 and from 2009 to 2013 and whole ring for years 1994 and 1999. Each isolated wood subdivision was pretreated using a slightly modified version of the alpha cellulose preparation described by Hopper et al. (1998). The process includes sequential solvent extraction steps to remove waxes, oils and resins, etc., as well as other potential organic contaminants present. For the three-step soxhlet extraction process, the wood sample is weighed, cut into thin slivers and placed in a glass-fibre extraction thimble plugged with glass wool. An appropriate volume of chloroform/ethanol (C2H5OH/CHCl3) (1:2 by volume) is added and the sample refluxed for 8 h. The solvent is allowed to cool and then discarded. This process is repeated using ethanol only and finally with distilled water. The sample is dried overnight at
