OSOcean ScienceOSOcean Sci.1812-0792Copernicus PublicationsGöttingen, Germany10.5194/os-14-813-2018Transport of FNPP1-derived radiocaesium from subtropical mode water in the
western North Pacific Ocean to the Sea of JapanTransport of FNPP1-derived radiocaesium from subtropical mode waterInomataYayoiyinomata@se.kanazawa-u.ac.jpAoyamaMichiohttps://orcid.org/0000-0003-0362-7602HamajimaYasunoriYamadaMasatoshiInstitute of Nature and Environmental Technology, Kanazawa University,
Kanazawa, 920-1156, JapanInstitute of Environmental Radioactivity, Fukushima University,
Fukushima, 960-1296, JapanInstitute of Radiation Emergency Medicine, Hirosaki University,
Hirosaki, 036-8564, JapanYayoi Inomata (yinomata@se.kanazawa-u.ac.jp)24August201814481382627October201729November201719July20183August2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://os.copernicus.org/articles/14/813/2018/os-14-813-2018.htmlThe full text article is available as a PDF file from https://os.copernicus.org/articles/14/813/2018/os-14-813-2018.pdf
This study investigated the spatio-temporal variations in activity
concentrations in the Sea of Japan (SOJ) of 137Cs and these transport
process from the North Pacific Ocean to the SOJ through the East China Sea
(ECS) during 2012–2016. The 137Cs activity concentrations in the SOJ
have been increasing since 2012–2013 and reached a maximum in 2015–2016 of
approximately 3.4 Bq m-3, more than twice the pre-Fukushima accident
137Cs activity concentration of ∼1.5 Bq m-3. The
134Cs /137Cs activity ratios ranged from 0.36 to 0.51 in 2016.
After taking into account radioactive decay and ocean mixing, we concluded
that these 134Cs /137Cs activity ratios were evidence that the
Fukushima accident caused the increase in the 137Cs activity
concentrations. In the North Pacific south of Japan (NPSJ), the highest
137Cs activities in 2012–2013 were observed in water from a depth of
300 m, the potential water density anomaly (σθ) of which
corresponded to subtropical mode water (STMW). In the ECS, a clear increase
in the 137Cs activity concentration started at a depth of 140 m
(σθ= 25.2 kg m-3) in April 2013, propagated to the
surface layers at depths of roughly 0–50 m, reached a maximum in 2015 and
decreased in subsequent years. In the ECS, the Fukushima-derived radiocaesium
activity concentration in surface water reached a maximum in 2014–2015,
whereas the concentration in the SOJ reached a maximum in 2015–2016. The
propagation of Fukushima-derived radiocaesium in surface seawater from the
ECS into the SOJ therefore required approximately 1 year. These temporal
changes in 137Cs activity concentrations and 134Cs /137Cs
activity ratios indicated that part of the 137Cs and 134Cs derived
from the Fukushima accident (FNPP1-derived 137Cs and134Cs) was
transported within several years to the ECS and then to the SOJ via STMW from
the NPSJ. The integrated amount of FNPP1-derived 137Cs that entered the
SOJ before 2016 was estimated to be 0.21±0.01 PBq, 5.0 % of the
estimated total amount of FNPP1-derived 137Cs in the STMW. The
integrated amount of FNPP1-derived 137Cs that returned to the North
Pacific Ocean through the Tsugaru Strait was estimated to be 0.09±0.01 Bq, 43 % of the total amount of FNPP1-derived 137Cs
transported to the SOJ and 2.1 % of the estimated total amount of
FNPP1-derived 137Cs in the STMW.
Introduction
The Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011
released radiocaesium (137Cs, half-life 30.2 years, and 134 Cs,
half-life 2.06 years) directly into the air and also directly discharged
contaminated water to the ocean, primarily in March and April 2011. Eighty
per cent of the radiocaesium released to the atmosphere was deposited on the
surface of the ocean (Aoyama et al., 2016a; Buesseler et al., 2016; Hirose,
2016; Tsumune et al., 2013). The 137Cs activity concentration in the
surface seawater of the North Pacific Ocean after the FNPP1 accident ranged
from a few Bq m-3 to about 1 kBq m-3 (e.g. Aoyama et al., 2013a,
b; Honda et al., 2012; Kaeriyama et al., 2013, 2014). A basin-scale
assessment indicated that the FNPP1-derived radiocaesium was confined to a
region from 25 to 50∘ N and from 135∘ E to 135∘ W
in April and May 2011 (e.g. Aoyama et al., 2013b, 2016a; Inomata et al.,
2016; Tsubono et al., 2016). At latitudes as far north as ∼40∘ N, the radiocaesium was transported eastwards at a rate of
approximately 8 cm s-1 by North Pacific currents such as the Kuroshio
and Kuroshio Extension. It reached 165∘ E in July–September 2011
and the 180th meridian in January–March 2012 (Aoyama et al., 2013b).
FNPP1-derived radiocaesium was also found further south in the North Pacific
Ocean, where there was a subsurface maximum of 137Cs at a depth of ∼300 m (Kaeriyama et al., 2013, 2014, 2016; Kumamoto et al., 2013, 2017).
Similar subsurface maxima of 137Cs were found in subtropical mode water
(STMW; potential water density anomaly (σθ)
25.0–25.6 kg m-3; Masuzawa, 1969) and central mode water (CMW;
σθ= 26.0–26.5 kg m-3) (Aoyama et al., 2016a;
Kumamoto et al., 2014, 2015). Aeolian 137Cs deposited south of the
Kuroshio Extension was apparently subducted into the STMW and transported
southwards within 10 months after the FNPP1 accident. Rossi et al. (2013)
have reported that simulated FNPP1-derived 137Cs penetrated into the
ocean interior via intense subduction and vertical mixing during the winter
in the region of mode water formation as described by Oka and Qiu (2012).
Kamidaira et al. (2018) have also reported that their model simulation
reproduced relatively high 137Cs activity concentration below the mixed
layer. Their simulation indicated that about 43 % of FNPP1-derived
137Cs was transported below the mixed layer by eddy processes.
Location of the sampling points after the FNPP1 accident. Large
black circles are sites monitored by the Japanese government. Blue circles
are sites monitored by the Korean government. Black triangles are sites with
measured vertical profiles. The colours of other circles correspond to the
137Cs activity concentrations measured in 2011. Open circles are sites
measured after 2012. The area around Japan was divided into three regions: the SOJ, ECS and NPSJ
(< 141.5∘ E). The locations of monitoring stations are also
plotted in (a).
Before the FNPP1 accident, 137Cs was injected into the environment as a
result of the large-scale atmospheric nuclear weapons tests that occurred in
the late 1950s and early 1960s and the Chernobyl accident in 1986.
Caesium-137 was therefore present in the North Pacific Ocean and its marginal
seas (Aoyama et al., 2006; Aoyama, 2010; Inomata et al., 2009, 2012). Russia
and the former USSR dumped radioactive waste into the northern part of the
Sea of Japan (SOJ), although this dumping caused no significant increase in
the activity concentrations of 137Cs (Miyao et al., 1998). During the
2000s, the distribution of 137Cs activity concentrations was almost
homogeneous in the Pacific Ocean, although the activities were relatively
high in the western part of the subtropical gyre in the North Pacific Ocean
(>2 Bq m-3) and the South Pacific Ocean (>1.5 Bq m-3) (Aoyama et al., 2013a). There have hence been two sources
of 137Cs in the North Pacific Ocean and its marginal seas: the
137Cs released by global fallout (global fallout-derived 137Cs) and
FNPP1-derived 137Cs. In contrast, the 134Cs derived from global
fallout and the Chernobyl accident had decayed to below the limit of
detection by 1993 because of its 2-year half-life (Miyao et al., 1998).
Therefore, 134Cs has been a satisfactory chemical tracer of the
radiocaesium derived from the FNPP1 accident.
Atmospheric deposition of radiocaesium after the FNPP1 accident caused
activity concentrations of radiocaesium in the north-eastern SOJ in May 2011
(1.5–2.8 Bq m-3) to be approximately 1–2 times the activity
concentrations before the FNPP1 accident (∼1.5 Bq m-3) (Fig. 1).
By the end of 2011, the 137Cs activity concentrations in the
north-eastern part of the SOJ had decreased rapidly to almost the same levels
as before the FNPP1 accident (Inoue et al., 2012).
The SOJ, the main region of interest in this study, is located between the
Eurasia and the Japanese archipelago. It has an area of 1.013×106 km2 and a mean depth of 1.67 km (Menard
and Smith, 1966). The south-western SOJ is connected to the East China Sea
(ECS) through Tsushima-kaikyō, and the north-eastern SOJ is connected to the North Pacific Ocean
through the Tsugaru Strait. Warm, saline seawater passes through Tsushima-kaikyō via the Tsushima Warm Current (TWC). This current splits into two
separate currents. One is the nearshore current that flows along the west
coast of Honshū Island, Japan. Some of this water passes through the
Tsugaru Strait and into the Pacific Ocean. The remainder is transported to
the north and passes through Söya-kaikyö into the Sea of Okhotsk.
The other current flows north of the Korean Peninsula. This current meets the
North Korean Cold Current, which is an extension of the Liman Cold Current.
This northward-flowing warm, subtropical water meets southward-flowing cold,
subarctic SOJ waters to form the polar front at approximately 40∘ N.
The SOJ is therefore divided largely into two regions (Prants et al., 2015).
The current system in and around the SOJ should be taken into consideration
when assessing the spatio-temporal variations in radiocaesium derived from
global fallout and the FNPP1 accident.
The purposes of this study were to (1) investigate the spatio-temporal
variations in activity concentrations in the SOJ of 137Cs released as a
result of the FNPP1 accident, (2) investigate the processes responsible for
transport of radiocaesium from the North Pacific Ocean to the SOJ through the
East China Sea, and (3) estimate the amount of FNPP1-derived 137Cs
transported into the SOJ through Tsushima-kaikyō as well as the amount of
FNPP1-derived 137Cs returned into the North Pacific Ocean through the
Tsugaru Strait during 2012–2016 and the uncertainties of these estimates.
Sampling, measurements and methodsData sources
After the FNPP1 accident, many radiocaesium measurements were taken in the
SOJ and western North Pacific Ocean (Fig. 1). To elucidate the temporal and
spatial distributions of the radiocaesium activity concentrations, we used as
many data points as possible. We therefore compiled all available data from
the literature and reported studies. Most of the data before the FNPP1
accident are included in the database “Historical Artificial Radionuclides
in the Pacific Ocean and its Marginal Seas” (HAM database) (Aoyama and
Hirose, 2004, and their updated version). The data recorded after the FNPP1
accident have been presented by Aoyama et al. (2016a). The term “surface
seawater” as used in this study refers to a sample collected at a depth of
less than 10 m.
We also focused on the Japanese government's monitoring data collected at
Tomari (42.98–43.17∘ N, 140.21–140.30∘ E), Aomori
(41.13–41.22∘ N, 141.50–141.67∘ E), Niigata
(37.62–38.10∘ N, 138.38–138.84∘ E), Ishikawa
(36.87–37.29∘ N, 136.43–136.47∘ E), Fukui
(35.75–36.09∘ N, 135.50–135.83∘ E), Shimane
(35.67–35.80∘ N, 132.87–133.2∘ E), Saga
(33.57–33.62∘ N, 129.73–129.98∘ E) and Kagoshima
(31.58–31.93∘ N, 130.02–130.15∘ E) (Marine Ecology
Research Institute, 2011, 2012, 2013, 2014, 2015, 2016) (Fig. 1). These
measurements, except for the Aomori sites, were taken once a year
(from the middle of May to early June). Near the Aomori sites, offshore
monitoring was also conducted twice a year (May and October) at the Rokkasho
Reprocessing Plant (39.5–41.4∘ N, 141.5–142.3∘ E).
Seawater was sampled at various depths, from the surface to 664 m, at each
monitoring site. Monitoring data (site 304-01: 33.0∘ N,
127.7∘ E; site 105-11: 37.3∘ N, 131.3∘ E) from the
Korean government were also used in this analysis (Korea Institute of Nuclear
Safety, 2011, 2012, 2013, 2014, 2015, 2016) (Fig. 1). At these monitoring
sites, surface seawater (0 m depth) measurements were taken four times
(February, April, August and October) each year.
Temporal variations in the 137Cs activity concentrations in
surface seawater in the Sea of Japan during the period from 1960 to 2016.
Circles indicate measured values, and red circles indicate data measured
after the accident (11 March 2011). Black circles indicate the 0.5-year
average value. Standard deviations of data were removed to clearly show the
temporal variation.
Measurement of 134Cs activity concentrations
The 134Cs activity concentrations in most of the monitoring datasets
were below the limit of detection because of the low sensitivity of the
measurement methods. We also collected seawater samples to investigate the
spatial distribution of 134Cs activity concentrations. The sample
volumes ranged from a few litres to 10 L. The radiocaesium in the sampled
seawater was extracted using the ammonium phosphomolybdate (AMP)–Cs
compound method as improved by Aoyama and Hirose (2008). The 134Cs and
137Cs activity concentrations in the AMP–Cs compound were determined
with the ultra-low-background gamma-ray detectors in the Low Level
Radioactivity Concentration Laboratory of Kanazawa University (Hamajima and
Komura, 2004). Aoyama and Hirose (2008) and Aoyama et al. (2016a) have
described the measurement procedure in detail. All radioactivity
concentrations shown in this research were decay-corrected to the time of
sample collection. In addition, we also determined
134Cs /137Cs activity ratios decay-corrected to 11 March 2011,
the time of the FNPP1 accident.
Estimation of FNPP1-derived 137Cs activity concentrations
In this study, we estimated the increase in 137Cs activity
concentrations due to the FNPP1 accident. The 137Cs activity
concentrations in the SOJ decreased exponentially after 1965. The decrease in
the 137Cs activity concentrations slowed down from 2000 to 2010 (Fig. 2). By
assuming that the apparent half-residence time of global fallout-derived
137Cs was approximately the same before and after the FNPP1 by using the
negative exponential fitted curve, we estimated the global fallout-derived
137Cs activity concentrations after the accident (Fig. S1 in the
Supplement). The FNPP1-derived 137Cs was then estimated from the
difference between the measured activity concentrations and the values
extrapolated from the exponential fitted curve during the period 2000–2010
(Fig. S1).
Estimation of amounts of 137Cs transported during
2012–2016
We estimated the amounts of FNPP1-derived 137Cs transported in each
region during 2012–2016 with Eq. (1):
Transport amount=∑i=2012n(FNPP1-derived137Csactivity concentration in yeari)×(annual
averageseawater transport volume in yeari),
where n= 2016. We used the annual average FNPP1-derived 137Cs
activity concentrations at stations Saga and 304-11 for the ECS, station
Shimane for the eastern TWC and station 105-01 for the western TWC. We used
station Aomori to estimate the amount of FNPP1-derived 137Cs transported
to the North Pacific Ocean via the Tsugaru Strait. The amount of
FNPP1-derived 137Cs transported northwards along the western coast of
Hokkaidō was estimated from the difference between the total inflow of
FNPP1-derived 137Cs at the entrance of the Tsugaru Strait its outflow
from the strait. The annual average volume of seawater transported through
the TWC was estimated from the data of Fukudome et al. (2010) and Han et
al. (2016). The average volumes of TWC transported into the SOJ annually
ranged from 2.59±0.50×106 to 2.82±0.58×106 m3 s-1, of which the average volumes of seawater
transported through the east and west channels of Tsushima-kaikyō annually were, respectively, 1.08±0.26–1.26±0.27×106 m3 s-1 and 1.43±0.38–1.56±0.47×106 m3 s-1 during 2012–2016. Nishida et al. (2003) estimated
the volume transported through the Tsugaru Strait to be 1.0–1.3×106 m3 s-1; we used this value because we did not have in
situ monitoring data for this value. The evaluation of seasonal variations
was difficult in this study because the monitoring data were limited;
relevant fluxes were measured once a year at the Japanese monitoring site and
four times a year at the Korean monitoring sites.
Temporal variations in the FNPP1-derived 137Cs activity concentrations in the surface seawater at
the monitoring sites in Japan after 2011. FNPP1-derived 137Cs:
(a) Tomari, Aomori, (b) Niigata, Ishikawa, Fukui, Shimane,
(c) Saga, Kagoshima, 314-01 and (d) 105-11.
ResultsIncreased 137Cs activity concentrations in the ECS and the
SOJ
Figure 2 shows the temporal variation in the 137Cs activity
concentrations in the surface seawater of the SOJ after the 1960s as well as
the half-year average values. The 137Cs activity concentrations
decreased exponentially from 1960 to 1970; these concentrations originated
from atmospheric nuclear weapons testing (Aoyama, 2010). The increase in the
137Cs activity concentrations in 1986 was the result of the Chernobyl
accident (Miyao et al., 1998). The apparent half-residence time during the
period 1970–1990 was estimated to be 16.3±0.5 years (Inomata et al.,
2009). In the 2000s, the magnitude of the rate of decrease in 137Cs
activity concentrations was small, and the half-year average 137Cs
activity concentrations in the surface seawater reached a low of
1.5–2.4 Bq m-3. Within several months after the FNPP1 accident, high
activity concentrations of 137Cs, up to 3.3 Bq m-3, were
observed. The 137Cs activity concentrations were higher in the
north-eastern part of the SOJ than in the south-western part of the SOJ
(Fig. 1). These high 137Cs activity concentrations were associated with
wet deposition of 137Cs released into the atmosphere during the FNPP1
accident in March–April 2011. In addition to the abrupt increase in the
137Cs activity concentrations just after the FNPP1 accident in 2011, the
137Cs activity concentrations subsequently increased gradually in the
SOJ (Aoyama et al., 2017). The 137Cs activity concentrations in 2016
reached 2.6 Bq m-3, which was the same as the concentrations in 1998.
Temporal variations in the 134Cs /137Cs activity
concentration ratio in the surface seawater at the monitoring stations in
Japan from January 2016 to June 2016. (a) Tomari, Aomori, (b) Niigata, Ishikawa,
Fukui, Shimane, (c) Saga and Kagoshima.
Figure 3 shows the FNPP1-derived 137Cs activity concentrations measured
at monitoring stations in the ECS and SOJ by the Japanese and Korean
governments. These monitoring stations are located along a branch of the
Kuroshio in the ECS and along the western and eastern TWC in the SOJ. The
long-term variations in the 137Cs activity concentrations and
FNPP1-derived 137Cs at each monitoring station are also shown in
Figs. S2–9. With the exception of the high activity concentrations at Aomori
in 2011, caused by the wet deposition of FNPP1-derived 137Cs released to
the atmosphere, an increase in the FNPP1-derived 137Cs in the SOJ
started in 2012 and 2013. The FNPP1-derived 137Cs gradually increased up
to 1.3 Bq m-3 in 2016 (Fig. 3a, b). The lower activity concentrations
at Niigata (Fig. 3b) were because the measurements were made at a station
near Sado Island, off Niigata, rather than at the usual government monitoring
site. At these monitoring stations, there was no significant trend of
decreasing FNPP1-derived 137Cs activity concentrations in 2015 and 2016.
In contrast, the FNPP1-derived 137Cs activity concentrations at stations
upstream from the SOJ in the ECS (Fig. 3c; Kagoshima, Saga and station
314-01) increased beginning in 2012–2013. The increase was clearly apparent
after 2014 in the ECS. The FNPP1-derived 137Cs activity concentrations
were slightly lower in 2016 than in 2015. At station 105-11, which was
located off South Korea (Fig. 1), marked increases in FNPP1-derived
137Cs activity concentrations were observed in 2014, and FNPP1-derived
137Cs activity concentrations reached approximately 1.4 Bq m-3 in
2015 (Fig. 3d). The FNPP1-derived 137Cs concentrations were lower in
2016 than in preceding years.
Temporal variations in the 137Cs activity concentrations at
stations (a) 314-01 and (b) 105-11.
The 134Cs /137Cs activity ratios in 2016 ranged from 0.27 to
0.51 (Fig. 4). There was no significant difference in the
134Cs /137Cs activity ratios among the stations in the ECS and
the SOJ.
Vertical distributions of the 137Cs activity concentrations
over the (a) depth profile in the NPSJ, (b) potential water density anomaly
profile in the NPSJ, (c) depth profile in the ECS, (d) potential water density
profile in the ECS, (e) depth profile at 105-01 along the WTWC and (f)
potential water density anomaly profile at 105-01 along the western TWC. Colour
indicates the collection time (year).
Figure 5 shows the temporal variations in the 137Cs activity
concentrations at different depths at station 314-01 in the ECS (Fig. 5a;
0–140 m) and at station 105-11 in the SOJ (Fig. 5b; 0–2000 m). Increases
in the 137Cs activity concentrations in the subsurface layer (at 140 m
and 200 m at stations 314-01 and 105-11, respectively) occurred in 2013,
approximately 1 year earlier than in the shallower layers. At station
314-01, the 137Cs activity concentrations at a depth of 140 m were
higher than those at the surface beginning in 2013. They increased up to 3.2±0.28 Bq m-3 in 2014 and then tended to decrease after 2015. At
shallower depths (0–50 m), 137Cs activity concentrations increased in
2014 and did not significantly decrease in 2015 and 2016. At station 105-11,
the increase in the 137Cs activity concentrations started earlier in the
subsurface seawater (200 m depth) in 2013. In 2014, the 137Cs activity
concentrations in surface seawater also increased and reached 2.8±0.2
Bq m-3 in 2015. Decreases in the 137Cs activity concentrations at
200 m and at the surface were observed after 2015, and a subsurface peak in
the 137Cs activity concentration at 200 m was not observed after 2015.
Similar variations characterised by an increase in the 137Cs activity
concentration that started from the ocean interior were observed at the
Kagoshima and Saga monitoring stations (Fig. S10).
Propagation of radiocaesium from the upstream region (NPSJ and ECS) to
the downstream region (the SOJ)
Figure 6 shows the distributions of the 137Cs activity concentrations
versus depth and σθ in the North Pacific South of Japan
(NPSJ), in the ECS and at station 105-11. In the NPSJ, the 137Cs
activity concentrations reached subsurface maxima of 8.2–12.3 Bq m-3
at approximately 300 m depth in 2012–2013 (Fig. 6a). Those high 137Cs
activity concentrations were measured in the region bounded by
136–138∘ E and 26–30∘ N. After 2014, a subsurface peak in
the 137Cs activity concentration was not observed. The location of these
subsurface peaks in 137Cs activity concentrations in the layer
corresponding to a σθ of 25.2 kg m-3 (Fig. 6b) is
consistent with previous findings of a 137Cs activity maximum in STMW
with a σθ of 25.0–25.6 kg m-3 (Kaeriyama et al.,
2014; Kumamoto et al., 2014). In the ECS, the 137Cs activity
concentrations gradually increased beginning in 2012 and reached a maximum
(2.9±0.24 Bq m-3) in 2015. The concentrations decreased in 2016
(Fig. 6c). High 137Cs activity concentrations (>2 Bq m-3) in the ECS were found in the layer corresponding to σθ values of 23.6–25.2 kg m-3 (Fig. 6d). In contrast,
137Cs activity concentrations at station 105-11 in the western SOJ
decreased with increasing depth until 500 m (Fig. 6e). The highest
137Cs activity concentrations at station 105-11 were measured in 2014
and 2015; they decreased in 2016. The 137Cs activity concentrations that
exceeded 2 Bq m-3 at station 105-11, except in one sample measured in
2015, were found at σθ values of 25.8–27.1 kg m-3
(Fig. 6f).
Hovmöller diagrams of the 137Cs activity
concentrations at a potential water density anomaly of
25.2±0.5 kg m-3 along with an eastern TWC. The ECS stations described on the x axis are
Kagoshima and Saga stations. The south-western SOJ includes the monitoring
stations Shimane, Fukui, Ishikawa and Niigata. The north-western SOJ includes
the monitoring stations Aomori and Tomari. Colour indicates the 137Cs
activity concentrations (Bq m-3).
Figure 7 shows Hovmöller diagrams of 137Cs activity concentrations
at σθ values of 25.2±0.5 kg m-3 along the TWC in
the ECS and at coastal sites in the eastern SOJ. These σθ
surfaces were selected to show the maximum 137Cs activity concentrations
in the ECS. Figure S11 shows the vertical distributions of the 137Cs
activity concentrations with depth and σθ at each monitoring
station. Note that in the SOJ, the vertical distributions of the 137Cs
activity concentrations below 250 m were almost constant, and a subsurface
peak of 137Cs was not found at the monitoring stations along the eastern
TWC (Fig. S11). The 137Cs activity concentrations before the FNPP1
accident were approximately 1.5 Bq m-3. In the ECS, the 137Cs
activity concentrations gradually increased and attained maxima during
2014–2015 (Fig. 7). The 137Cs activity concentrations in the ECS tended
to decrease in 2016 in a layer with a σθ of 25.2±0.5 kg m-3. In the south-western part of the SOJ (Shimane, Fukui,
Ishikawa and Niigata), the 137Cs activity concentrations gradually
increased beginning in 2012 and reached a maximum of 2.5 Bq m-3 during
2015–2016; those trends were almost the same as the trends at the monitoring
stations in the ECS. In the north-western SOJ (Aomori and Tomari), the
137Cs activity concentrations increased slightly and exceeded
2 Bq m-3 in 2016. These results indicate that the propagation of
FNPP1-derived 137Cs from the ECS (32–34∘ N) to the SOJ
(35–38∘ N) along the TWC (Fig. 7) occurred within 1 year.
Schematic diagram of the FNPP1-derived 137Cs transport in the
North Pacific Ocean. The bold line indicates the Kuroshio pathway. The thin
black lines indicate the flow pathway of the Tsushima Warm Current. The pink
region is the STMW formation area. The green region is the lighter variety of the CMW (L-CMW) formation area, and the blue region indicates the Transition Region Mode Water (TRMW) and the denser variety of the CMW (D-CMW) formation area. The estimated
accumulated flux during the period from 2012 to 2016 at each section is
shown. The inventory in the STMW was deduced by Kaeriyama et al. (2016).
Transport process and total amount of FNPP1-derived 137Cs
transported into the SOJ during 2012–2016
Here we describe the possible pathway of FNPP1-derived 137Cs from the
Pacific Ocean to the SOJ via the ECS. The westward-flowing undercurrent would
have transported FNPP1-derived 137Cs in the STMW south-westwards, as
reported by Oka (2009). The 137Cs entrained in the STMW would have
reached the western boundary of the SOJ at lower latitudes and become
entrained into the Kuroshio (Oka and Qiu, 2012) before being transported
northwards to the west of Kyushu by the TWC bifurcation from the Kuroshio.
The FNPP1-derived 137Cs would then have entered the SOJ through Tsushima-kaikyō via the TWC. FNPP1-derived 137Cs transported by the TWC
would have been divided into western (coastal stations in Korea) and eastern
(eastern coast of the Japanese Islands) portions. The remaining FNPP1-derived
137Cs in the eastern TWC would have been transported northwards along
the western coast of Hokkaidō (Fig. 8). The integrated FNPP1-derived
137Cs in the ECS during 2012–2016 was estimated to be 0.21±0.01
PBq, which corresponds to 5.0 % of the FNPP1-derived 137Cs in the
STMW (4.2±1.1 PBq) estimated by Kaeriyama et al. (2016) (Table 1). The
bifurcation of the TWC resulted in transport of 0.11±0.01 PBq of
FNPP1-derived 137Cs in the eastern channel, which corresponds to
2.6 % of the FNPP1-derived 137Cs in the STMW. The amount of
FNPP1-derived 137Cs transported in the western channel was estimated to
be 0.09±0.01 PBq, which corresponds to 2.1 % of the FNPP1-derived
137Cs injected into the STMW. The amounts of FNPP1-derived 137Cs in
the eastern and western outflows are consistent with the range of uncertainty
for the inflow amounts: 0.21±0.01 PBq. A small part of the transported
FNPP1-derived 137Cs (0.09±0.01 PBq; 2.1 % of FNPP1-derived
137Cs in STMW) was returned to the North Pacific Ocean via transport
through the Tsugaru Strait. In other words, the amount of FNPP1-derived
137Cs that was returned to the North Pacific Ocean corresponded to
43 % of the total amount of FNPP1-derived 137Cs transported to the
SOJ and 2.1 % of the estimated total amount of FNPP1-derived 137Cs
in the STMW. The remaining 0.03±0.002 PBq (14 % of the transported
FNPP1-derived 137Cs in the SOJ; 0.7 % of the transported
FNPP1-derived 137Cs in STMW) would have been transported to the northern
part of the SOJ (west of Hokkaidō) or, in part, to the interior of the
ocean via deep convection and surface mixing.
Latitudinal and horizontal distributions of the
134Cs /137Cs activity ratios measured at the coastal sites of
the SOJ and ECS in 2015–2016. The values were radioactive decay-corrected to
11 March 2011. The data measured in the Ogasawara area (red circle in
b) were also added. (a) Latitudinal distribution;
(b) horizontal distribution.
DiscussionSignature of FNPP1-derived 137Cs inflow in the SOJ by using
the 134Cs /137Cs activity ratio
According to atmospheric model simulations, atmospheric deposition of
FNPP1-derived 137Cs occurred over a wide region in the western North
Pacific Ocean (30–55∘ N, 140–180∘ E) (e.g. Aoyama et al.,
2016b). One of the regions where 137Cs was deposited south of the
Kuroshio and Kuroshio Extension corresponded to the region of STMW formation
(Aoyama et al., 2016b; Oka et al., 2012). The release of FNPP1-derived
radiocaesium into the atmosphere occurred at the end of March and in early
April 2011. Because the deposition of FNPP1-derived 137Cs into the North
Pacific Ocean was therefore a point source with respect to time,
FNPP1-derived radiocaesium is a very useful tracer for investigating the
transport of FNPP1-derived radiocaesium in the North Pacific Ocean. The
vertical distributions of the 137Cs activity concentrations in the NPSJ
indicated that FNPP1-derived 137Cs was entrained in STMW (Fig. 6).
Estimated transport amount of FNPP1-derived 137Cs in the
monitoring site in the SOJ along with the Tsushima Warm Current.
Transport amount Ratio againstRatio against to inflowto STMW (%)into the SOJ (%)FlowStation(PBq)*Inflow to SOJ ECS to SOJSaga/304-010.21±0.015.0±2.3At Tsushima-kaikyō Western TWC105-110.09±0.012.1±1.243±10Eastern TWCShimane0.11±0.012.6±1.450±7Two branches of eastern TWC Outflow to Pacific OceanAomori off0.09±0.012.1±1.243±10Northward transportTomari0.03±0.0020.7±0.314±2
* The value was decay-corrected to 11 March 2011. ECS were
estimated by using the average value of the 314-01 and Saga monitoring sites.
The transport amount (*) was estimated by sum of the duration from 2012 to
2016.
There were several indicators of the transport process: (i) maximum
137Cs activity concentrations were observed in the subsurface layer
rather than the surface seawater; (ii) σθ data indicated that
the 137Cs activity concentrations were highest in STMW; (iii) in the
ECS, the similarity of the σθ of 137Cs-contaminated
seawater to the σθ of STMW is an indication that
FNPP1-derived 137Cs was transported into the ECS via STMW from the NPSJ;
and (iv) the 137Cs activity concentrations in the northern ECS
(> 30∘ N) were higher than those in the southern ECS
(< 30∘ N) (Fig. S12). In this study, however, there were
not enough data to elucidate the transport route in more detail.
Figure 9 shows the latitudinal distribution of the
134Cs /137Cs activity ratio, which was decay-corrected to
11 March 2011. The 134Cs /137Cs activity ratio ranged from 0.1
to 0.72, and the ratios in the Ogasawara region and in the ECS were almost
the same as those in the SOJ (Fig. 9a). Considering that the
134Cs /137Cs activity ratio in the radiocaesium that
originated from the FNPP1 accident was almost 1 (Buesseler et al., 2011),
variations in the ratio indicated that seawater contaminated with
FNPP1-derived radiocaesium had mixed with seawater contaminated with global
fallout-derived radiocaesium during transport. The highest activity
concentration ratio (0.72), which was found near Kagoshima, is an indication
of the highest contribution of FNPP1-derived radiocaesium. In contrast, the
relatively low 134Cs /137Cs activity ratio in the region
bounded by 30–32∘ N implies a small contribution of FNPP1-derived
radiocaesium to that region of the Pacific Ocean. Within the ECS, the
137Cs activity concentrations tended to increase northwards as suggested
by Aoyama et al. (2017) (Fig. S12). The 137Cs activity concentrations
measured at Okinawa in the southern ECS
(http://search.kankyo-hoshano.go.jp/servlet/search.top?pageSID=113836570;
last access: 17 August 2018) averaged approximately 1.7±0.47 Bq m-3, slightly higher than those measured before the FNPP1
accident (Fig. S13). The implication is that less FNPP1-derived radiocaesium
was transported to the southern ECS than to the northern ECS, as shown in
Fig. S12.
Advection and vertical mixing of FNPP1-derived 137Cs in the
SOJ
In this study, we revealed that FNPP1-derived 137Cs entered the SOJ via
the ECS. FNPP1-derived 137Cs was then transported northward with the
TWC. In the SOJ, there was a time lag of approximately 1 year in the
propagation of FNPP1-derived radiocaesium (Fig. 7). Based on measurements of
phosphate, one of the dominant seawater nutrients, Kodama et al. (2016) have
revealed that the phosphate concentrations in surface seawater of the SOJ
during winter are positively correlated (p<0.05) with the
phosphate concentrations in the saline ECS seawater during the preceding
summer, and the surface water of the southern SOJ is almost entirely replaced
by ECS seawater during May–October. They have also suggested that the
transport of water-soluble constituents from the ECS to the SOJ takes at
least ∼0.5 years. The propagation of FNPP1-derived radiocaesium into
the SOJ is consistent with the timescale of propagation of changes of
nutrient concentrations from the ECS to the SOJ (Kodama et al., 2016).
The 137Cs activity concentrations at station 105-11, located along the
western side of the TWC in the SOJ, were highest at the surface and gradually
decreased with increasing depth (Fig. 6e). This vertical distribution
differed from those in the NPSJ (Fig. 6a) and ECS (Fig. 6c). In particular,
the subsurface peak observed in the NPSJ and ECS did not appear at station
105-11. At station 105-11, most of the 137Cs was in seawater with a
σθ of 25.7–27.3 kg m-3 (Fig. 6f), higher than the
σθ in the NPSJ and ECS. A similar vertical distribution was
also observed along the western coast of the Japanese Islands on the eastern
side of the TWC (Fig. S11). These distributions were due to the cooling of
the surface layer after water was transported through Tsushima-kaikyō.
Physical processes such as the convergence and subduction of surface water
inside eddies are important mechanisms of the downward transport of
radiocaesium (Miyao et al., 1998; Budyansky et al., 2015). There is a
possibility that the downward transport of radiocaesium is seasonal. The
137Cs derived from global fallout should have already penetrated below
the surface layer and have accumulated in the deeper layers of the SOJ.
Mass balance of FNPP1-derived 137Cs in the SOJ
The most reliable estimates of the amount of FNPP1-derived 137Cs
deposited in the North Pacific Ocean via atmospheric release and direct
release to the Ocean are 11.7–14.8 PBq (Aoyama et al., 2016b; Tsubono et
al., 2016) and 3.5±0.7 PBq (Tsumune et al., 2012, 2013), respectively.
The FNPP1-derived 137Cs inventory in the North Pacific Ocean has been
estimated to be 15.2–18.3 PBq by comparing the observed inventory with
model simulation results (Aoyama et al., 2016b), 15.3±2.6 PBq by
optimum interpolation analysis (Inomata et al., 2016) and 16.1±1.4 PBq
by model simulation (Tsubono et al., 2016). According to the estimation by
Kaeriyama et al. (2014), the amount of 134Cs in the STMW was
approximately 4.2±1.1 PBq in 2012. The estimation by Kaeriyama et
al. (2014) implies that the FNPP1-derived 137Cs transported into the SOJ
from 2012 to 2016 accounted for 5.0 % of the FNPP1-derived 137Cs
inventory in the STMW. Approximately 60 %–65 % of the total inflow
of FNPP1-derived 137Cs in the SOJ was transported in 2015 and 2016.
After entering the SOJ, the FNPP1-derived 137Cs followed different
paths. The amounts of FNPP1-derived 137Cs transported along the eastern
TWC (2.6 % of the FNPP1-derived 137Cs inventory in the STMW) and
western TWC (2.1 % of the FNPP1-derived 137Cs inventory in the STMW)
were similar or slightly larger along the former path. Of the FNPP1-derived
137Cs transported in the SOJ, approximately 43 % returned to the
North Pacific Ocean through the Tsugaru Strait during 2012–2016.
Because 5 % of the FNPP1-derived 137Cs in the STMW was transported
into the SOJ, 95 % may remain in the STMW. On the basis of long-term
measurements of the 137Cs activity concentration adjacent to the FNPP1,
Tsumune et al. (2017) have estimated that the rate of direct release of
FNPP1-derived 137Cs was 2.2×1014 Bq day-1 until
November 2011, after which it decreased exponentially with time to 3.9×109 Bq day-1 on 26 October 2015. Assuming that the rate of
decrease in the 137Cs activity concentrations remained the same from
26 October 2015 to 31 December 2016, the total amount of 137Cs directly
released from the FNPP1 site during the period 2012–2016 has been estimated
to be 0.03 PBq (Tsumune et al., 2017). However, not all of the directly
released 137Cs was subducted into the STMW. An amount equal to 0.03 PBq
corresponds to 0.7 % of 137Cs inventory in the STMW. We can conclude
that the FNPP1-derived 137Cs in STMW was decreasing during the study
period.
An analysis of historical data from 1950 to 2000 indicates that the
137Cs activity concentrations in the western North Pacific Ocean were
decreasing exponentially during that time. However, the 137Cs activity
concentrations during 2000–2010 were almost constant at 1.5–2 Bq m-3
(Inomata et al., 2009). Considering that there was scarcely any global
fallout-derived 137Cs deposition after the 1970s, the 137Cs
activity concentrations in the seawater of the North Pacific Ocean were
controlled by physical oceanographic processes such as advection and
diffusion from roughly 1980 until the FNPP1 accident (Inomata et al., 2009).
Several studies have reported that there were subsurface maxima of 137Cs
derived from global fallout in layers corresponding to surfaces of constant
σθ in the STMW and CMW (e.g. Aoyama et al., 2008). A
possible explanation for the constancy of 137Cs activity concentrations
during 2000–2010 is that the 137Cs subducted into the STMW and CMW in
the 1960s might have returned to the surface after a few decades. However,
the increase in FNPP1-derived 137Cs activity concentrations in the SOJ
began in 2012–2013. This increase implies the existence of a faster
transport route from the North Pacific Ocean to the SOJ via the subtropical
gyre.
Conclusions
The 137Cs activity concentrations in the Sea of Japan (SOJ) increased
beginning in 2012–2013 and reached a maximum in 2015–2016 of approximately
3 Bq m-3, which is above the pre-Fukushima accident level of
1.5 Bq m-3. The 134Cs /137Cs activity ratios of
0.36–0.51 in the ECS and the SOJ are evidence that the increase in the
137Cs activity concentration was derived from the Fukushima accident.
An increase in FNPP1-derived 137Cs activity concentrations was first
observed in the subsurface layer with a σθ of ∼25.2 kg m-3 (corresponding to STMW) in the southern part of the
Kuroshio Current in the NPSJ in 2012–2013. The 137Cs activity
concentrations in the NPSJ have been decreasing since 2014. In the ECS, an
increase in FNPP1-derived 137Cs was observed in 2014; decreases have
been observed since 2016. Vertical distributions of the 137Cs activity
concentrations in the ECS were almost constant or contained a subsurface
peak. These results indicated that there was a 1–2-year time lag between the
maxima of 137Cs activity concentrations in the NPSJ and ECS. The
increase in the 137Cs activity concentrations in the SOJ began in 2012
and continued until 2016. A time lag in the propagation of FNPP1-derived
137Cs of approximately 1 year was observed in the SOJ. The similarity
of 134Cs /137Cs activity ratios in the surface layers of the
ECS, SOJ and Ogasawara region suggest that the source of radiocaesium was
the same in all three of these regions.
The total amount of recirculated FNPP1-derived 137Cs from 2012 to 2016
was estimated to be 0.21±0.01 PBq, which corresponds to 5.0 % of
the FNPP1-derived 137Cs in the STMW. Of this total, the amounts of
FNPP1-derived 137Cs transported along the western and eastern TWC were
almost the same or slightly larger along the eastern TWC. Of the FNPP1-derived 137Cs transported into the SOJ,
approximately 43 % (2.1 % of the total amount in STMW) passed through
the Tsugaru Strait and back into the North Pacific Ocean. The FNPP1-derived
137Cs transported northwards was estimated to be 14 % of the amount
transported into the SOJ (0.7 % of the total amount in STMW).
The most important result of this study was the determination that
FNPP1-derived 137Cs was rapidly transported into the SOJ within several
years after the FNPP1 accident.
Most of the data before the FNPP1
accident are included in the database Historical Artificial Radionuclides
in the Pacific Ocean and its Marginal Seas (HAM database) (Aoyama and
Hirose, 2004, and their updated version). The data recorded after the FNPP1
accident have been presented by Aoyama et al. (2016a). The Japanese
government's monitoring data are reported by the Marine Ecology Research
Institute (http://www.kaiseiken.or.jp/publish/itaku/itakuseika.html, in
Japanese). The Korean government monitoring data are reported by the Korea
Institute of Nuclear Safety (http://clean.kins.re.kr/info/in03_rept_list.jsp, in Korean).
The supplement related to this article is available online at: https://doi.org/10.5194/os-14-813-2018-supplement.
YI (corresponding author) conducted data analysis and the preparation of the manuscript with contributions
from all co-authors. MA developed the database of radioactivity and
pretreatment of seawater samples for measurement of radiocaesium. YH measured
the radiocaesium activity concentrations. MY organized the seawater sampling
for radiocaesium measurement.
The authors declare that they have no conflict of
interest.
Acknowledgements
For collection of seawater samples, the authors thank Miyuki Takahashi and
Shun-pei Tomita of the Oga Aquarium, Akita, Japan; the staff at Kinosaki
Aquarium, Hyogo, Japan; Hajime Chiba at Toyama Kosen, Toyama, Japan;
Shigeo Takeda and the captain and crew of the Nagasaki-maru,
Nagasaki Univ.; Mitsuru Hayashi and the captain and crew of the
Fukae-maru Kobe Univ.; Yuki Nikaido and the crew of the Sado Kisen,
Niigata, Japan; Akira Wada and the captain and crew of the ferry Queen Coral Plus of the Marix Line, Kagoshima,
Japan; Kenichi Sasaki and the captain and crew of the Ushio-maru,
Hokkaido Univ., Hakodate, Japan; and Keiri Imai and the captain and crew of
the Oshoro-maru, Hokkaido Univ., Hakodate, Japan. We thank
Eitarou Oka of the University of Tokyo and an anonymous reviewer for valuable
comments. We also thank Naoki Hirose of Kyusyu University for providing us
with seawater volume transport data. We also thank Rika Hozumi for her work
extracting radiocaesium from seawater samples. This research was financially
supported by a Grant-in-Aid for Scientific Research on Innovative Areas,
“Interdisciplinary study on environmental transfer of radionuclides from the
Fukushima Dai-ichi NPP Accident” (project no. 25110511) from the Japanese
Ministry of Education, Culture, Sports, Science and Technology (MEXT). This
research was also supported with funds provided by the cooperation program of
the Institute of Nature and Environmental Technology, Kanazawa University
(JFY2016, 2017), and the cooperation program of the Institute of Radiation
Emergency Medicine, Hirosaki University (JFY2016, 2017). Edited by: Matthew Hecht Reviewed by: Eitarou
Oka and one anonymous referee
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