Concentration and vertical flux of Fukushima-derived radiocesium in sinking particles from two sites in the Northwestern Pacific Ocean

[摘要] At two stations in the western North Pacific, K2 in the subarctic gyre and S1in the subtropical gyre, time-series sediment traps were collecting sinkingparticles when the Fukushima Daiichi Nuclear Power Plant (FNPP1) accidentoccurred on 11 March 2011. Radiocesium (¹³⁴Cs and ¹³⁷Cs) derivedfrom the FNPP1 accident was detected in sinking particles collected at 500 min late March 2011 and at 4810 m in early April 2011 at both stations. Thesinking velocity of ¹³⁴Cs and ¹³⁷Cs was estimated to be 22 to71 m day⁻¹ between the surface and 500 m and >180 m day⁻¹between 500 m and 4810 m. ¹³⁷Cs concentrations varied from 0.14 to0.25 Bq g⁻¹ dry weight. These values are higher than those of surfaceseawater, suspended particles, and zooplankton collected in April 2011.Although the radiocesium may have been adsorbed onto or incorporated intoclay minerals, correlations between ¹³⁴Cs and lithogenic material werenot always significant; therefore, the form of the cesium associated with thesinking particles is still an open question. The total ¹³⁷Cs inventoryby late June at K2 and by late July at S1 was 0.5 to 1.7 Bq m⁻² atboth depths. Compared with ¹³⁷Cs input from both stations by April 2011,estimated from the surface ¹³⁷Cs concentration and mixed-layer depth andby assuming that the observed ¹³⁷Cs flux was constant throughout theyear, 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 to312 yr). The estimated removal rates and residence times are comparable topreviously reported values after the Chernobyl accident (removal rate:0.2–1%, residence time: 130–390 yr).

[学科分类] 地球化学与岩石