Abstract. 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.
DRAFT: A COMPUTER CODE FOR THE CALCULATION OF FISSION PRODUCT ACTIVITY RATIOS.
