Cesium-134 and 137 activities in the central North Pacific Ocean after the Fukushima Dai-ichi Nuclear Power Plant accident

[摘要] Surface seawater ¹³⁴Cs and ¹³⁷Cs samples werecollected in the central and western North Pacific Ocean during the2 yr after the Fukushima Dai-ichi Nuclear Power Plantaccident to monitor dispersion patterns of these radioisotopes towardsthe Hawaiian Islands. In the absence of other recent sources and dueto its short half-life, only those parts of the Pacific Ocean wouldhave detectable ¹³⁴Cs values that were impacted by Fukushimareleases. Between March and May 2011, ¹³⁴Cs was not detectedaround the Hawaiian Islands and Guam. Here, most ¹³⁷Csactivities (1.2–1.5 Bq m^–3) were in the range of expectedpreexisting levels. Some samples north of the Hawaiian Islands(1.6–1.8 Bq m^–3) were elevated above the 23-monthbaseline established in surface seawater in Hawaii indicating thatthose might carry atmospheric fallout. The 23-month time-seriesanalysis of surface seawater from Hawaii did not reveal any seasonalvariability or trends, with an average activity of1.46 ± 0.06 Bq m^–3 (Station Aloha, 18 values). Incontrast, samples collected between Japan and Hawaii contained¹³⁴Cs activities in the range of 1–4 Bq m^–3, and ¹³⁷Cs levels were about 2–3 times above the preexistingactivities. We found that the southern boundary of the Kuroshio andKuroshio extension currents represented a boundary for radiationdispersion with higher activities detected within and north of themajor currents. The radiation plume has not been detected over thepast 2 yr at the main Hawaiian Islands due to the transportpatterns across the Kuroshio and Kuroshio extension currents.

[学科分类] 地球化学与岩石