Fukushima-derived radiocesium in Discussions western North Pacific sediment traps.

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 FNPP1 accident was detected in sinking particles collected at 500m by late March 2011 and at 4810m by early April 2011 at both stations. The sinking velocity of ¹³⁴Cs and ¹³⁷Cs was estimated to be 8 to 36m day^-1 between the surface and 500m and > 180m day^-1 between 500m and 4810m. ¹³⁷Cs specific activity 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 in- corporated 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 flux 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 to both stations by April 2011, estimated from the surface ¹³⁷Cs activity 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% (68 to 312 yr). The estimated removal rates and residence times are comparable to previously reported values. [ABSTRACT FROM AUTHOR]