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.