Abstract
Abstract. At two stations in the western North Pacific, K2 in the subarctic gyre and S1 in the subtropical gyre, time-series sediment traps were collecting sinking particles when the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident occurred on 11 March 2011. Radiocesium (134Cs and 137Cs) derived from the FNPP1 accident was detected in sinking particles collected at 500 m in late March 2011 and at 4810 m in early April 2011 at both stations. The sinking velocity of 134Cs and 137Cs was estimated to be 22 to 71 m day−1 between the surface and 500 m and >180 m day−1 between 500 m and 4810 m. 137Cs concentrations varied from 0.14 to 0.25 Bq g−1 dry weight. These values are higher than those of surface seawater, suspended particles, and zooplankton collected in April 2011. Although the radiocesium may have been adsorbed onto or incorporated into clay minerals, correlations between 134Cs and lithogenic material were not always significant; therefore, the form of the cesium associated with the sinking particles is still an open question. The total 137Cs inventory by late June at K2 and by late July at S1 was 0.5 to 1.7 Bq m−2 at both depths. Compared with 137Cs input from both stations by April 2011, estimated from the surface 137Cs concentration and mixed-layer depth and by assuming that the observed 137Cs flux was constant throughout the year, the estimated removal rate of 137Cs from the upper layer (residence time in the upper layer) was 0.3 to 1.5% yr−1 (68 to 312 yr). The estimated removal rates and residence times are comparable to previously reported values after the Chernobyl accident (removal rate: 0.2–1%, residence time: 130–390 yr).
Highlights
After the FNPP1 accident in March 2011, total mass flux (TMF) increased and, it dipped in early May, the highest value was reached in June
Between November and February, CaCO3 was the dominant component of the samples, whereas biogenic opal was dominant after March
Sediment trap data revealed that FNPP1-derived radiocesium was quickly transported to the deep sea in the western North Pacific
Summary
In November 2010, time-series sediment traps (McLane Mark7G-21) were deployed at 500 m and 4810 m at stations K2 (47◦ N, 160◦ E; water depth, ∼ 5200 m), which is in the subarctic gyre, and S1 (30◦ N, 145◦ E; water depth, ∼ 5800 m), which is in the subtropical gyre (Fig. 1). The horizontal distances from FNPP1 to K2 and S1 are about. MR11-02 cruise track in April 2011 (Honda et al, 2012). Before the traps were deployed, their collecting cups were filled with a seawaterbased 10 % buffered formalin solution as a preservative. The sediment traps at K2 and S1 were recovered in June and July 2011, respectively, by R/V Mirai
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