Observed eastward progression of the Fukushima ¹³⁴Cs signal across the North Pacific

Abstract

Radionuclide samples taken as part of hydrographic surveys at 30°N in the North Pacific reveal that the easternmost edge of Fukushima‐derived 134Cs observed at 174.3°W in 2012 had progressed eastward across the basin to 160.6°W by 2013. The 2013 30°N observations indicate surface 134Cs concentrations of 3–5 Bq/m3 between 160°E and 160°W, slightly lower concentrations west of 160°E and no detectable signal east of 160.6°W. Profile samples show 134Cs penetration to 500 m west of 180° with shoaling penetration depth toward to the east. The near‐uniform vertical distribution of 137Cs between 152°W and 121.3°W in the top 500 m is indicative of trace amounts of radionuclides remaining from weapons testing. The physical processes responsible for the deep 134Cs penetration in the western Pacific appear to be related to distinct water mass subduction pathways; however, the timing and rapidity of deep penetration over the broad scales observed has yet to be clarified.

Oceanic dispersion of Fukushima‐derived radioactive cesium: a review

“…The first recording of Fukushima-derived radioactive Cs in the eastern side of the North Pacific shows good agreement with the eastward speed of Fukushima-derived radioactive Cs as 5 cm sec À1 in the central and eastern North Pacific estimated by Aoyama et al (2015b). Some of the global circulation models (Behrens et al, 2012;Rypina et al, 2014) also closely simulated the results of Smith et al (2014) and Yoshida et al (2015). Although Fukushima-derived 137 Cs in seawater has dispersed across the North Pacific, the concentrations of 137 Cs including the background 137 Cs on the eastern side of the North Pacific were at a comparatively low level (~2.1 Bq m À3 ) because of the dilution process over the entire North Pacific.…”

“…() and Yoshida et al . (). Although Fukushima‐derived 137 Cs in seawater has dispersed across the North Pacific, the concentrations of 137 Cs including the background 137 Cs on the eastern side of the North Pacific were at a comparatively low level (~ 2.1 Bq m −3 ) because of the dilution process over the entire North Pacific.…”

“…Yoshida et al . () reported the Fukushima‐derived radioactive Cs in the vicinity of the Californian coast in 2014. The first recording of Fukushima‐derived radioactive Cs in the eastern side of the North Pacific shows good agreement with the eastward speed of Fukushima‐derived radioactive Cs as 5 cm sec −1 in the central and eastern North Pacific estimated by Aoyama et al .…”

“…On the eastern side of North Pacific, Smith et al (2014) reported the intrusion of Fukushima-derived 137 Cs offshore of the Canadian coast firstly in June 2012 at a trace level (0.2-0.4 Bq m À3 ) which was estimated from the data of 134 Cs and 134 Cs/ 137 Cs ratio of Fukushima-derived radioactive Cs, and in February 2014 at the maximum level of 137 Cs as 2.1 Bq m À3 , which includes background 137 Cs and Fukushima-derived 137 Cs. Yoshida et al (2015) reported the Fukushima-derived radioactive Cs in the vicinity of the Californian coast in 2014. The first recording of Fukushima-derived radioactive Cs in the eastern side of the North Pacific shows good agreement with the eastward speed of Fukushima-derived radioactive Cs as 5 cm sec À1 in the central and eastern North Pacific estimated by Aoyama et al (2015b).…”

Fukushima Daiichi–Derived Radionuclides in the Ocean: Transport, Fate, and Impacts

“…By February 2014, the 137 Cs signal had increased to 2 Bq m −3 throughout the upper 150 m, doubling the signal associated with fallout background from atmospheric nuclear weapons tests (Smith et al 2015). These activities have continued to increase over time, with offshore 137 Throughout much of the Pacific Ocean, the FDNPP plume was rapidly mixed throughout the upper 100-150 m of the water column and has only more slowly begun to penetrate to intermediate water depths , Smith et al 2015, Yoshida et al 2015). An important exception is the subsurface transport of FDNPP Cs in various Pacific mode waters that are formed through winter cooling and buoyancy loss, leading to mixing and homogenization of hydrographic properties deep in the water column .…”