- Email: [email protected]
Radiocesium in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2013 and 2014
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radiocesium in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2013 and 2014 ⁎
Yuichiro Kumamotoa, , Michio Aoyamab, Yasunori Hamajimac, Shigeto Nishinod, Akihiko Murataa, Takashi Kikuchid a Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan b Institute of Environmental Radioactivity, Fukushima University, 1-1 Kanayagawa, Fukushima, Fukushima 960-1296, Japan c Low Level Radioactivity Laboratory, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan d Institute of Arctic Climate and Environment Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Fukushima Dai-ichi nuclear power plant Radiocesium Western subarctic area of the North Pacific Arctic Ocean Bering Sea
We measured radiocesium (134Cs and 137Cs) in seawater from the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2013 and 2014. Fukushima-derived 134Cs in surface seawater was observed in the western subarctic area and Bering Sea but not in the Arctic Ocean. Vertical profile of 134Cs in the Canada Basin of the Arctic Ocean implies that Fukushima-derived 134Cs intruded into the basin from the Bering Sea through subsurface (150 m depth) in 2014.
1. Introduction
of radiocesium in the North Pacific Ocean. The total amount of 137Cs (or Cs) derived from direct discharge of contaminated water and atmospheric deposition in the basin were estimated to be about 3.5 (Tsumune et al., 2012) and 12–15 (Aoyama et al., 2016a) PBq (1015 Bq), respectively. In addition, radiocesium measurements along cross-sectional line across the basin revealed horizontal and vertical spreading directions of Fukushima-derived radiocesium. The directlydischarged radiocesium has been transported eastward along surface current in surface mixing layer in the north of the Kuroshio Front (Aoyama et al., 2013; Kumamoto et al., 2016). In the south of the front, subtropical area, Fukushima-derived radiocesium deposited has been transported southward through subsurface layer due to subduction of the subtropical mode water (Kaeriyama et al., 2014; Kumamoto et al., 2014; Yoshida et al., 2015; Aoyama et al., 2016a). In the Bering Sea, a northern marginal sea adjacent to the North Pacific Ocean, Fukushima-derived 134Cs was observed in surface seawater in summer 2012 (Kumamoto et al., 2016). In the Arctic Ocean, however, Fukushima-derived 134Cs was not detected in 2012 (Smith et al., 2015; Kumamoto et al., 2016). These results suggest that Fukushima-derived 134Cs deposited on the Bering Sea but not on the Arctic Ocean and had not been transported from the Bering Sea to Arctic Ocean by summer 2012, about 1.5 years after the accident. Here we present results of radiocesium measurements in seawater from the western subarctic area of the North Pacific Ocean, Bering Sea, and 134
The massive Tohoku earthquake and consequent giant tsunami on March 11, 2011 resulted in release of radiocesium (134Cs and 137Cs) into the North Pacific Ocean from the Fukushima Dai-ichi nuclear power plant, FNPP1 (Yoshida and Kanda, 2012). Evaluation of Fukushima-derived 137Cs (half-life =30.04 y) in the marine environment is necessary to address risks to marine ecosystem and public health because of its relative long residence time in the ocean. In fact 137Cs derived from the nuclear weapon tests mostly in the 1950s and 1960s (bomb-produced 137Cs) was still observed in surface seawater of the North Pacific Ocean to be 1 – 2 Bq m−3 just before the FNPP1 accident (Aoyama et al., 2012), which is now masked by Fukushima-derived 137 Cs. On the other hand, the bomb-produced 134Cs had decayed to undetectable levels due to its short half-life (2.07 y) by March 2011. Therefore 134Cs is a unique tracer for Fukushima-derived radiocesium. Most of 134Cs/137Cs ratios, which were corrected to the date of the FNPP1 accident for radioactive decay, in soil collected near FNPP1 were about 1 (Saito et al., 2015), suggesting that total amounts of 134Cs and 137Cs released from FNPP1 were equivalent. During the past five years Fukushima-derived radiocesium was measured in seawater samples collected in the whole area of North Pacific Ocean. Efforts for the measurements just after the accident, in April and May 2011, achieved success in evaluation of the total amount
⁎
Corresponding author. E-mail address: [email protected] (Y. Kumamoto).
http://dx.doi.org/10.1016/j.apradiso.2017.02.036 Received 10 August 2016; Received in revised form 20 February 2017; Accepted 21 February 2017 0969-8043/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Kumamoto, Y., Applied Radiation and Isotopes (2017), http://dx.doi.org/10.1016/j.apradiso.2017.02.036
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
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Fig. 1. (a) Sampling stations of surface seawater for radiocesium measurement during MR13-06 (triangles, September-October 2013) and MR14-05 (diamonds, September-October 2014) cruises. To compare with these data, sampling stations and radiocesium data obtained during MR12-E03 (Kumamoto et al., 2016) are also shown in this figure (circles, September-October 2012). Symbols with a small dot in the Canada Basin of the Arctic Ocean indicate stations where seawater samples were also collected from 25 to 800 m depth. A map in this figure was drawn using Ocean Data View software (Schlitzer, 2016). (b) Activity concentration of 134Cs decay-corrected to the date of FNPP1 accident, 11 March 2011 (Bq m−3) in surface seawater. Symbols are same as those in (a). (c) Same as (b) but for 137Cs.
ground Ge-detectors, which were calibrated with gamma-ray volume sources (Eckert & Ziegler Isotope Products) certificated by Deutscher Kalibrierdienst (DKD). 134Cs and 137Cs activities were evaluated from gamma-ray peaks at 605/796 and 661 keV, respectively. Averaged detection limits of the 134Cs and 137Cs measurements at MIO/JAMSTEC for the MR13-06 samples were about 0.5 and 0.2 Bq m−3, respectively. Because the Ge-detectors of LLRL/KU were placed in an underground laboratory (Hamajima and Komura, 2004), averaged detection limit of the 134Cs and 137Cs at LLRL/KU for the MR14-05 samples were about 0.1 and 0.05 Bq m−3, respectively. In order to compare these data directly all the radiocesium activity concentration were decay-corrected to the date of FNPP1 accident, 11 March 2011. Averaged detection limits of the decay-corrected 134Cs and 137Cs measured at MIO/ JAMSTEC (LLRL/KU) were about 1.2 and 0.5 Bq m−3 (0.4 and 0.2 Bq m−3), respectively.
Arctic Ocean in 2013 and 2014 and discuss transportation of Fukushima-derived radiocesium from the Bering Sea to Arctic Ocean during about 3.5 years after the accident. 2. Methods Seawater samples for radiocesium measurement were collected during cruises of research vessel "MIRAI", MR13-06 (SeptemberOctober 2013) and MR14-05 (September-October 2014). Fig. 1a shows location of sampling stations. Surface seawater were sampled using a 10-L bucket (0 m depth) or pump (about 4 m depth). Temperature and salinity in surface water in the bucket were measured using a calibrated mercurial thermometer and the salinometer (Autosal model 8400, Guildline Instruments), respectively. Water temperature and salinity of the pumped-up surface waters were estimated from data from a CTD system (SBE-11plus, Sea-Bird Electronics, Inc.). The salinity sensor on the CTD was calibrated using results of the bottle salinity measurement. Deeper seawater from 25 m to 800 m depths were collected using 12Liter Niskin-X bottles (General Oceanics Inc.) equipped with another C TD system (SBE-11plus, Sea-Bird Electronics, Inc.) calibrated using the salinometer. We collected about 20 L of seawater from each depth. Unfiltered seawater was acidified to pH 1.6 by 40 cm3 of concentrated nitric acid after the sampling. After the cruises, in laboratories of Mutsu Institute for Oceanography / Japan Agency for Marine-Earth Science and Technology (MIO/JAMSTEC) or Japan Marine Science Foundation, radiocesium in the seawater sample was concentrated using an improved ammonium phosphomolybdate (AMP) method (Aoyama and Hirose, 2008). Radiocesium was quantitatively separated from seawater by coprecipitation with AMP/Cs compound by adding 4 g of AMP and a pipetted aliquot of CsCl solution containing 0.26 g cesium as carrier. Yield of AMP/Cs compound was more than 98%. Radiocesium activity in AMP/Cs samples from the MR13-06 and MR14-05 cruises were measured at MIO/JAMSTEC and Low Level Radioactivity Laboratory / Kanazawa University (LLRL/KU), respectively, using low back-
3. Results All the radiocesium, temperature, and salinity data obtained in 2013 and 2014 are listed in a supplementary table. In order to discuss temporal change we also show radiocesium data obtained in 2012 (Kumamoto et al., 2016) in areas where we measured radiocesium activity concentration in 2013 and 2014 (Fig. 1). Fukushima-derived 134Cs in surface water was observed in the subarctic area and Bering Sea in 2013 and 2014 (Fig. 1b). The decaycorrected activity concentration at 42.5°N in the subarctic area in 2013 (2.6 Bq m−3) was close to those observed in 2012. The activity concentrations at the other two stations in the subarctic area in 2013 were below the detection limit (about 1.2 Bq m−3 in decay-corrected activity concentration). In contrast, decay-corrected activity concentrations of 137Cs at these stations in 2013 were in same range as those at nearby stations in 2014 (2.0 – 2.5 Bq m−3), where activity concentration of 134Cs was about 1.0 Bq m−3 in 2014 (Fig. 1c). Therefore decaycorrected activity concentration of 134Cs in the subarctic area in 2013 and 2014 was probably higher than 1.0 Bq m−3. 134Cs was also 2
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below detection limit (about 0.2 Bq m−3). Because the decay-corrected detection limit of the 134Cs measurement for samples collected in 2013 was about 1.2 Bq m−3, temporal changes of 134Cs between 2012 and 2013, or 2013 and 2014 are uncertain. In 2014, we found a barely significant activity concentration of 134Cs at 150 m depth. The detection limits of decay-corrected 134Cs above 200 m depth in 2014 (about 0.3 Bq m−3) were lower than those from the deeper layer (about 0.6 Bq m−3) because measurement times for the formers (about 600,000 s) were longer than those for the latters (about 300,000 s). We recognized small counting peaks at both 605 and 796 keV only in gamma-ray spectrum of the sample from 150 m depth and the measurement time was extended to 757,000 s, which resulted in the detection of significant 134Cs, 0.24 Bq m−3.
observed in the southern Bering Sea in 2014 (no sampling in 2013) and the decay-corrected activity concentration was close to those observed in 2012 (about 0.5 Bq m−3). The decay-corrected activity concentration of 134Cs in the northern Bering Sea in 2014 was below the detection limit (about 0.3 Bq m−3). Decay-corrected activity concentrations of 137 Cs in 2012 and 2014 were in same range (about 1.0–1.2 Bq m−3, Fig. 1c). Therefore activity concentration of 134Cs in surface water of the northern Bering Sea in 2014 was probably close to the activity concentration observed in 2012, about 0.2 Bq m−3. Decay-corrected activity concentration of 137Cs was higher than that 134 of Cs in the subarctic area and Bering Sea due to the pre-existing 137 Cs derived from the nuclear weapon tests before the FNPP1 accident. The difference between them was about 1 Bq m−3, which is equivalent to the bomb-produced 137Cs and agrees with the activity concentration of 137Cs observed before the FNPP1 accident (Aoyama et al., 2012). In the Arctic Ocean activity concentration of 134Cs was below detection limit (Fig. 1b). The detection limits of decay-corrected 134Cs in 2012 and 2014 in surface seawater were about 0.2 and 0.3 Bq m−3, respectively, although that in 2013 was about 1.2 Bq m−3. Thereby activity concentration of 134Cs in surface water in the Arctic Ocean could be lower than 0.3 Bq m−3 between 2012 and 2014. Activity concentration of 137Cs in the Arctic Ocean was about 1.2 – 1.7 Bq m−3, which is higher than those in the northern Bering Sea by 0.3 – 0.5 Bq m−3 (Fig. 1c). Decay-corrected activity concentrations of 137Cs in the Canada Basin in the Arctic Ocean were about 1.5–2 Bq m−3 in surface layer above 200 m depth and those in deeper layer than that depth were 3 – 4 Bq m−3 (Fig. 2). The higher concentration in the deeper layers is explained by polarward transportation of 137Cs released from nuclear fuel reprocessing plants in the United Kingdom and France into the North Atlantic Ocean mostly in the 1980s (Smith et al., 2011). The 137 Cs fallout due to the nuclear weapon tests in the Arctic Ocean was smaller than that in the North Pacific Ocean (Aoyama et al., 2006). Therefore the higher activity concentrations of 137Cs in the surface seawater in the Canada Basin than those in the northern Bering Sea was probably due to entrainment of 137Cs from the deeper layer to the surface mixing layer in the Canada Basin during the past thirty years. Decay-corrected activity concentrations of 134Cs in the Canada Basin were below detection limit from surface to about 800 m depth except one data from 150 m depth in 2014 (Fig. 2). In 2012, the decaycorrected activity concentrations of 134Cs above 200 m depth were
4. Discussion In surface seawater of the Arctic Ocean, Fukushima-derived 134Cs decay-corrected was below the detection limit (about 0.3 Bq m−3) between 2012 and 2014. Activity concentrations of 137Cs in the surface waters of the Arctic Ocean were higher than those in the northern Bering Sea between 2012 and 2014 (Fig. 1c). These relatively higher concentration in the Arctic Ocean were not derived from the FNPP1 accident but from the nuclear fuel reprocessing plants and observed before the FNPP1 accident in March 2011 (Smith et al., 2011). Therefore, the radiocesium data suggest that the Fukushima-derived radiocesium had not been deposited on and transported into the Arctic Ocean by 2014 in the surface seawater. In contrast, Fukushima-derived 134 Cs was observed in 2012 and remained in 2014 in surface seawater of the Bering Sea, which implies that 134Cs was deposited onto the Bering Sea and circulated within the sea. This difference in the atmospheric deposition between the Bering Sea and Arctic Ocean agrees with those in atmospheric model simulations (e.g. Aoyama et al., 2016b). The water depth of the Bering Strait between the Bering Sea and Arctic Ocean (Chukchi Sea) is shallower than about 50 m depth and the Bering Sea water is transported into the Arctic Ocean through the Bering Strait (Walsh et al., 1989; McRoy, 1993; Springer and McRoy, 1993). Water mass from the Bering Sea spreads northward and observed as low-salinity waters above 200 m depth in the Canada Basin (Fig. 3b). The low-salinity water originated from the Bering Sea can be divided into two waters above/below about 100 m depth, low-salinity /
Fig. 2. Vertical profiles of activity concentrations of 134Cs (closed circles) and 137Cs (open circles) decay-corrected to the date of FNPP1 accident, 11 March 2011 (Bq m−3) at stations 32 of MR13-06 (b, 9 September 2013) and 9 of MR14-05 (c, 11 September 2014) in the Canada Basin. To compare with these data, the vertical profile obtained during at station 39 of MR12E03 (Kumamoto et al., 2016) are also shown in this figure (a, 21 September 2012). Thick broken lines indicate the detection limit of 134Cs measurements.
3
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Fig. 3. Same as Fig. 2 but for potential temperature (a, °C), salinity (b, PSS-78), and potential density anomaly, σθ (c, kg m−3). Symbols are same as those in Fig. 1.
online version at doi:10.1016/j.apradiso.2017.02.036.
high-temperature surface and high-salinity / low-temperature subsurface waters (Fig. 3a and b). The former surface water is called Bering summer water (BSW) because it flows into the Arctic Ocean from Bering Sea in summer season (Shimada et al., 2001; Steele et al., 2004). The latter subsurface water is called Bering winter water (BWW) because it flows into the Arctic Ocean in winter season. Its high-salinity (S =32.4 – 34.0) and low-temperature (T≤–1.6 °C) are due to brine rejection during sea ice formation (Coachman and Barnes, 1961). A barely significant activity concentration of Fukushima-derived 134 Cs (0.24 Bq m−3) was observed at 150 m depth in the Canada Basin in summer 2014 for the first time (Fig. 2c). We also found that activity concentration of 137Cs at that depth in 2014 was lower than those observed in 2012 and 2013 (Fig. 2a and b). Although temporal changes in salinity and water density (anomaly) at 150 m depth were small, water temperature at that depth in 2014 was lower than those observed in 2012 and 2013 (Fig. 3). These data imply that BWW at 150 m depth in the Canada Basin in 2014 was less modified (or not mixed well with other water masses, BSW and water from the North Atlantic) than those in 2012 and 2013 and transported Fukushima-derived 134Cs from the Bering Sea. It is not clear why Fukushima-derived 134Cs was observed in BWW but not in shallower BSW in 2014. As suggested by the lower salinity (Fig. 3b), ice melt and river waters probably diluted BSW, which resulted in dilution of the Fukushima-derived 134Cs below the detection limit. Horizontal and vertical water mixing in the surface layer in the Canada Basin is controlled by meso-scale eddies (Nishino et al., 2011). Difference in the meso-scale eddy activity between 2012/2013 and 2014 could explain the active transportation of BWW and Fukushimaderived 134Cs into the Canada Basin in 2014.
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Acknowledgments We thank the crew of the R/V "MIRAI" and the staff of Marine Works Japan, Ltd. for their help during sample collection on board. This work partially supported by Grant-in-Aid for Scientific Research on Innovative Areas, the Ministry of Education, Culture, Sports, Science and Technology Japan (KAKENHI), Interdisciplinary Study on Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP Accident (#24110005). Appendix A. Supporting information Supplementary data associated with this article can be found in the 4
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Shimada, K., Carmack, E.C., Hatakeyama, K., Takizawa, T., 2001. Varieties of shallow temperature maximum waters in the western Canada Basin of the Arctic Ocean. Geophys. Res. Lett. 28, 3441–3444. Smith, J.N., Brown, R.M., Williams, W.J., Robert, M., Nelson, R., Moran, S.B., 2015 Arrival of the Fukushima radioactivity plume in North American continental waters. Proc. Natl. Acad. Sci. USA 112, 1310–1315. Smith, J.N., McLaughlin, F.A., Smethie Jr., W.M., Moran, S.B., Lepore, K., 2011. Iodine– 129, 137Cs, and CFC–11 tracer transit time distributions in the Arctic Ocean. J. Geophys. Res. http://dx.doi.org/10.1029/2010JC006471. Springer, A.M., McRoy, C.P., 1993. The paradox of pelagic food webs in the northern Bering Sea-III. Patterns of primary productivity. Cont. Shelf Res. 13, 575–599. Steele, M., Morison, J., Ermold, W., Rigor, I., Ortmeyer, M., Shimada, K., 2004. Circulation of summer Pacific halocline water in the Arctic Ocean. J. Geophys. Res. 109, C02027. http://dx.doi.org/10.1029/2003JC002009.
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Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radiocesium in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2013 and 2014 ⁎
Yuichiro Kumamotoa, , Michio Aoyamab, Yasunori Hamajimac, Shigeto Nishinod, Akihiko Murataa, Takashi Kikuchid a Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan b Institute of Environmental Radioactivity, Fukushima University, 1-1 Kanayagawa, Fukushima, Fukushima 960-1296, Japan c Low Level Radioactivity Laboratory, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan d Institute of Arctic Climate and Environment Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Fukushima Dai-ichi nuclear power plant Radiocesium Western subarctic area of the North Pacific Arctic Ocean Bering Sea
We measured radiocesium (134Cs and 137Cs) in seawater from the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2013 and 2014. Fukushima-derived 134Cs in surface seawater was observed in the western subarctic area and Bering Sea but not in the Arctic Ocean. Vertical profile of 134Cs in the Canada Basin of the Arctic Ocean implies that Fukushima-derived 134Cs intruded into the basin from the Bering Sea through subsurface (150 m depth) in 2014.
1. Introduction
of radiocesium in the North Pacific Ocean. The total amount of 137Cs (or Cs) derived from direct discharge of contaminated water and atmospheric deposition in the basin were estimated to be about 3.5 (Tsumune et al., 2012) and 12–15 (Aoyama et al., 2016a) PBq (1015 Bq), respectively. In addition, radiocesium measurements along cross-sectional line across the basin revealed horizontal and vertical spreading directions of Fukushima-derived radiocesium. The directlydischarged radiocesium has been transported eastward along surface current in surface mixing layer in the north of the Kuroshio Front (Aoyama et al., 2013; Kumamoto et al., 2016). In the south of the front, subtropical area, Fukushima-derived radiocesium deposited has been transported southward through subsurface layer due to subduction of the subtropical mode water (Kaeriyama et al., 2014; Kumamoto et al., 2014; Yoshida et al., 2015; Aoyama et al., 2016a). In the Bering Sea, a northern marginal sea adjacent to the North Pacific Ocean, Fukushima-derived 134Cs was observed in surface seawater in summer 2012 (Kumamoto et al., 2016). In the Arctic Ocean, however, Fukushima-derived 134Cs was not detected in 2012 (Smith et al., 2015; Kumamoto et al., 2016). These results suggest that Fukushima-derived 134Cs deposited on the Bering Sea but not on the Arctic Ocean and had not been transported from the Bering Sea to Arctic Ocean by summer 2012, about 1.5 years after the accident. Here we present results of radiocesium measurements in seawater from the western subarctic area of the North Pacific Ocean, Bering Sea, and 134
The massive Tohoku earthquake and consequent giant tsunami on March 11, 2011 resulted in release of radiocesium (134Cs and 137Cs) into the North Pacific Ocean from the Fukushima Dai-ichi nuclear power plant, FNPP1 (Yoshida and Kanda, 2012). Evaluation of Fukushima-derived 137Cs (half-life =30.04 y) in the marine environment is necessary to address risks to marine ecosystem and public health because of its relative long residence time in the ocean. In fact 137Cs derived from the nuclear weapon tests mostly in the 1950s and 1960s (bomb-produced 137Cs) was still observed in surface seawater of the North Pacific Ocean to be 1 – 2 Bq m−3 just before the FNPP1 accident (Aoyama et al., 2012), which is now masked by Fukushima-derived 137 Cs. On the other hand, the bomb-produced 134Cs had decayed to undetectable levels due to its short half-life (2.07 y) by March 2011. Therefore 134Cs is a unique tracer for Fukushima-derived radiocesium. Most of 134Cs/137Cs ratios, which were corrected to the date of the FNPP1 accident for radioactive decay, in soil collected near FNPP1 were about 1 (Saito et al., 2015), suggesting that total amounts of 134Cs and 137Cs released from FNPP1 were equivalent. During the past five years Fukushima-derived radiocesium was measured in seawater samples collected in the whole area of North Pacific Ocean. Efforts for the measurements just after the accident, in April and May 2011, achieved success in evaluation of the total amount
⁎
Corresponding author. E-mail address: [email protected] (Y. Kumamoto).
http://dx.doi.org/10.1016/j.apradiso.2017.02.036 Received 10 August 2016; Received in revised form 20 February 2017; Accepted 21 February 2017 0969-8043/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Kumamoto, Y., Applied Radiation and Isotopes (2017), http://dx.doi.org/10.1016/j.apradiso.2017.02.036
Applied Radiation and Isotopes xxx (xxxx) xxx–xxx
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Fig. 1. (a) Sampling stations of surface seawater for radiocesium measurement during MR13-06 (triangles, September-October 2013) and MR14-05 (diamonds, September-October 2014) cruises. To compare with these data, sampling stations and radiocesium data obtained during MR12-E03 (Kumamoto et al., 2016) are also shown in this figure (circles, September-October 2012). Symbols with a small dot in the Canada Basin of the Arctic Ocean indicate stations where seawater samples were also collected from 25 to 800 m depth. A map in this figure was drawn using Ocean Data View software (Schlitzer, 2016). (b) Activity concentration of 134Cs decay-corrected to the date of FNPP1 accident, 11 March 2011 (Bq m−3) in surface seawater. Symbols are same as those in (a). (c) Same as (b) but for 137Cs.
ground Ge-detectors, which were calibrated with gamma-ray volume sources (Eckert & Ziegler Isotope Products) certificated by Deutscher Kalibrierdienst (DKD). 134Cs and 137Cs activities were evaluated from gamma-ray peaks at 605/796 and 661 keV, respectively. Averaged detection limits of the 134Cs and 137Cs measurements at MIO/JAMSTEC for the MR13-06 samples were about 0.5 and 0.2 Bq m−3, respectively. Because the Ge-detectors of LLRL/KU were placed in an underground laboratory (Hamajima and Komura, 2004), averaged detection limit of the 134Cs and 137Cs at LLRL/KU for the MR14-05 samples were about 0.1 and 0.05 Bq m−3, respectively. In order to compare these data directly all the radiocesium activity concentration were decay-corrected to the date of FNPP1 accident, 11 March 2011. Averaged detection limits of the decay-corrected 134Cs and 137Cs measured at MIO/ JAMSTEC (LLRL/KU) were about 1.2 and 0.5 Bq m−3 (0.4 and 0.2 Bq m−3), respectively.
Arctic Ocean in 2013 and 2014 and discuss transportation of Fukushima-derived radiocesium from the Bering Sea to Arctic Ocean during about 3.5 years after the accident. 2. Methods Seawater samples for radiocesium measurement were collected during cruises of research vessel "MIRAI", MR13-06 (SeptemberOctober 2013) and MR14-05 (September-October 2014). Fig. 1a shows location of sampling stations. Surface seawater were sampled using a 10-L bucket (0 m depth) or pump (about 4 m depth). Temperature and salinity in surface water in the bucket were measured using a calibrated mercurial thermometer and the salinometer (Autosal model 8400, Guildline Instruments), respectively. Water temperature and salinity of the pumped-up surface waters were estimated from data from a CTD system (SBE-11plus, Sea-Bird Electronics, Inc.). The salinity sensor on the CTD was calibrated using results of the bottle salinity measurement. Deeper seawater from 25 m to 800 m depths were collected using 12Liter Niskin-X bottles (General Oceanics Inc.) equipped with another C TD system (SBE-11plus, Sea-Bird Electronics, Inc.) calibrated using the salinometer. We collected about 20 L of seawater from each depth. Unfiltered seawater was acidified to pH 1.6 by 40 cm3 of concentrated nitric acid after the sampling. After the cruises, in laboratories of Mutsu Institute for Oceanography / Japan Agency for Marine-Earth Science and Technology (MIO/JAMSTEC) or Japan Marine Science Foundation, radiocesium in the seawater sample was concentrated using an improved ammonium phosphomolybdate (AMP) method (Aoyama and Hirose, 2008). Radiocesium was quantitatively separated from seawater by coprecipitation with AMP/Cs compound by adding 4 g of AMP and a pipetted aliquot of CsCl solution containing 0.26 g cesium as carrier. Yield of AMP/Cs compound was more than 98%. Radiocesium activity in AMP/Cs samples from the MR13-06 and MR14-05 cruises were measured at MIO/JAMSTEC and Low Level Radioactivity Laboratory / Kanazawa University (LLRL/KU), respectively, using low back-
3. Results All the radiocesium, temperature, and salinity data obtained in 2013 and 2014 are listed in a supplementary table. In order to discuss temporal change we also show radiocesium data obtained in 2012 (Kumamoto et al., 2016) in areas where we measured radiocesium activity concentration in 2013 and 2014 (Fig. 1). Fukushima-derived 134Cs in surface water was observed in the subarctic area and Bering Sea in 2013 and 2014 (Fig. 1b). The decaycorrected activity concentration at 42.5°N in the subarctic area in 2013 (2.6 Bq m−3) was close to those observed in 2012. The activity concentrations at the other two stations in the subarctic area in 2013 were below the detection limit (about 1.2 Bq m−3 in decay-corrected activity concentration). In contrast, decay-corrected activity concentrations of 137Cs at these stations in 2013 were in same range as those at nearby stations in 2014 (2.0 – 2.5 Bq m−3), where activity concentration of 134Cs was about 1.0 Bq m−3 in 2014 (Fig. 1c). Therefore decaycorrected activity concentration of 134Cs in the subarctic area in 2013 and 2014 was probably higher than 1.0 Bq m−3. 134Cs was also 2
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below detection limit (about 0.2 Bq m−3). Because the decay-corrected detection limit of the 134Cs measurement for samples collected in 2013 was about 1.2 Bq m−3, temporal changes of 134Cs between 2012 and 2013, or 2013 and 2014 are uncertain. In 2014, we found a barely significant activity concentration of 134Cs at 150 m depth. The detection limits of decay-corrected 134Cs above 200 m depth in 2014 (about 0.3 Bq m−3) were lower than those from the deeper layer (about 0.6 Bq m−3) because measurement times for the formers (about 600,000 s) were longer than those for the latters (about 300,000 s). We recognized small counting peaks at both 605 and 796 keV only in gamma-ray spectrum of the sample from 150 m depth and the measurement time was extended to 757,000 s, which resulted in the detection of significant 134Cs, 0.24 Bq m−3.
observed in the southern Bering Sea in 2014 (no sampling in 2013) and the decay-corrected activity concentration was close to those observed in 2012 (about 0.5 Bq m−3). The decay-corrected activity concentration of 134Cs in the northern Bering Sea in 2014 was below the detection limit (about 0.3 Bq m−3). Decay-corrected activity concentrations of 137 Cs in 2012 and 2014 were in same range (about 1.0–1.2 Bq m−3, Fig. 1c). Therefore activity concentration of 134Cs in surface water of the northern Bering Sea in 2014 was probably close to the activity concentration observed in 2012, about 0.2 Bq m−3. Decay-corrected activity concentration of 137Cs was higher than that 134 of Cs in the subarctic area and Bering Sea due to the pre-existing 137 Cs derived from the nuclear weapon tests before the FNPP1 accident. The difference between them was about 1 Bq m−3, which is equivalent to the bomb-produced 137Cs and agrees with the activity concentration of 137Cs observed before the FNPP1 accident (Aoyama et al., 2012). In the Arctic Ocean activity concentration of 134Cs was below detection limit (Fig. 1b). The detection limits of decay-corrected 134Cs in 2012 and 2014 in surface seawater were about 0.2 and 0.3 Bq m−3, respectively, although that in 2013 was about 1.2 Bq m−3. Thereby activity concentration of 134Cs in surface water in the Arctic Ocean could be lower than 0.3 Bq m−3 between 2012 and 2014. Activity concentration of 137Cs in the Arctic Ocean was about 1.2 – 1.7 Bq m−3, which is higher than those in the northern Bering Sea by 0.3 – 0.5 Bq m−3 (Fig. 1c). Decay-corrected activity concentrations of 137Cs in the Canada Basin in the Arctic Ocean were about 1.5–2 Bq m−3 in surface layer above 200 m depth and those in deeper layer than that depth were 3 – 4 Bq m−3 (Fig. 2). The higher concentration in the deeper layers is explained by polarward transportation of 137Cs released from nuclear fuel reprocessing plants in the United Kingdom and France into the North Atlantic Ocean mostly in the 1980s (Smith et al., 2011). The 137 Cs fallout due to the nuclear weapon tests in the Arctic Ocean was smaller than that in the North Pacific Ocean (Aoyama et al., 2006). Therefore the higher activity concentrations of 137Cs in the surface seawater in the Canada Basin than those in the northern Bering Sea was probably due to entrainment of 137Cs from the deeper layer to the surface mixing layer in the Canada Basin during the past thirty years. Decay-corrected activity concentrations of 134Cs in the Canada Basin were below detection limit from surface to about 800 m depth except one data from 150 m depth in 2014 (Fig. 2). In 2012, the decaycorrected activity concentrations of 134Cs above 200 m depth were
4. Discussion In surface seawater of the Arctic Ocean, Fukushima-derived 134Cs decay-corrected was below the detection limit (about 0.3 Bq m−3) between 2012 and 2014. Activity concentrations of 137Cs in the surface waters of the Arctic Ocean were higher than those in the northern Bering Sea between 2012 and 2014 (Fig. 1c). These relatively higher concentration in the Arctic Ocean were not derived from the FNPP1 accident but from the nuclear fuel reprocessing plants and observed before the FNPP1 accident in March 2011 (Smith et al., 2011). Therefore, the radiocesium data suggest that the Fukushima-derived radiocesium had not been deposited on and transported into the Arctic Ocean by 2014 in the surface seawater. In contrast, Fukushima-derived 134 Cs was observed in 2012 and remained in 2014 in surface seawater of the Bering Sea, which implies that 134Cs was deposited onto the Bering Sea and circulated within the sea. This difference in the atmospheric deposition between the Bering Sea and Arctic Ocean agrees with those in atmospheric model simulations (e.g. Aoyama et al., 2016b). The water depth of the Bering Strait between the Bering Sea and Arctic Ocean (Chukchi Sea) is shallower than about 50 m depth and the Bering Sea water is transported into the Arctic Ocean through the Bering Strait (Walsh et al., 1989; McRoy, 1993; Springer and McRoy, 1993). Water mass from the Bering Sea spreads northward and observed as low-salinity waters above 200 m depth in the Canada Basin (Fig. 3b). The low-salinity water originated from the Bering Sea can be divided into two waters above/below about 100 m depth, low-salinity /
Fig. 2. Vertical profiles of activity concentrations of 134Cs (closed circles) and 137Cs (open circles) decay-corrected to the date of FNPP1 accident, 11 March 2011 (Bq m−3) at stations 32 of MR13-06 (b, 9 September 2013) and 9 of MR14-05 (c, 11 September 2014) in the Canada Basin. To compare with these data, the vertical profile obtained during at station 39 of MR12E03 (Kumamoto et al., 2016) are also shown in this figure (a, 21 September 2012). Thick broken lines indicate the detection limit of 134Cs measurements.
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Fig. 3. Same as Fig. 2 but for potential temperature (a, °C), salinity (b, PSS-78), and potential density anomaly, σθ (c, kg m−3). Symbols are same as those in Fig. 1.
online version at doi:10.1016/j.apradiso.2017.02.036.
high-temperature surface and high-salinity / low-temperature subsurface waters (Fig. 3a and b). The former surface water is called Bering summer water (BSW) because it flows into the Arctic Ocean from Bering Sea in summer season (Shimada et al., 2001; Steele et al., 2004). The latter subsurface water is called Bering winter water (BWW) because it flows into the Arctic Ocean in winter season. Its high-salinity (S =32.4 – 34.0) and low-temperature (T≤–1.6 °C) are due to brine rejection during sea ice formation (Coachman and Barnes, 1961). A barely significant activity concentration of Fukushima-derived 134 Cs (0.24 Bq m−3) was observed at 150 m depth in the Canada Basin in summer 2014 for the first time (Fig. 2c). We also found that activity concentration of 137Cs at that depth in 2014 was lower than those observed in 2012 and 2013 (Fig. 2a and b). Although temporal changes in salinity and water density (anomaly) at 150 m depth were small, water temperature at that depth in 2014 was lower than those observed in 2012 and 2013 (Fig. 3). These data imply that BWW at 150 m depth in the Canada Basin in 2014 was less modified (or not mixed well with other water masses, BSW and water from the North Atlantic) than those in 2012 and 2013 and transported Fukushima-derived 134Cs from the Bering Sea. It is not clear why Fukushima-derived 134Cs was observed in BWW but not in shallower BSW in 2014. As suggested by the lower salinity (Fig. 3b), ice melt and river waters probably diluted BSW, which resulted in dilution of the Fukushima-derived 134Cs below the detection limit. Horizontal and vertical water mixing in the surface layer in the Canada Basin is controlled by meso-scale eddies (Nishino et al., 2011). Difference in the meso-scale eddy activity between 2012/2013 and 2014 could explain the active transportation of BWW and Fukushimaderived 134Cs into the Canada Basin in 2014.
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Acknowledgments We thank the crew of the R/V "MIRAI" and the staff of Marine Works Japan, Ltd. for their help during sample collection on board. This work partially supported by Grant-in-Aid for Scientific Research on Innovative Areas, the Ministry of Education, Culture, Sports, Science and Technology Japan (KAKENHI), Interdisciplinary Study on Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP Accident (#24110005). Appendix A. Supporting information Supplementary data associated with this article can be found in the 4
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