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

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)

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

“…The atmospheric transport models (e.g., Stohl et al, 2012) predicted that some radionuclides from the atmospheric source were spread south of the Kuroshio current. This was later confirmed by seawater (Aoyama et al, 2013a; and sinking particle analysis (Honda et al, 2013). The samples T1-01 to T1-08 from 2011 were on the southern side of the Kuroshio and Kuroshio extension currents with seawater salinity in the range of 34.0-34.5.…”

Fukushima Daiichi–Derived Radionuclides in the Ocean: Transport, Fate, and Impacts

“…• E) and S1 in the subtropics (30 (Honda et al 2013) (Figure 3a). Both locations had traps deployed at 500 and 4,810 m and were quite distant from the FDNPP site: 1,875 km northeast for K2 and 950 km southeast for S1, in water depths of 5,200 m and 5,800 m for K2 and S1, respectively.…”

Wind-driven interannual variability of sea ice algal production in the western Arctic Chukchi Borderland

“…3e). The sinking speed is maintained at 200 m d −1 below a depth of 1000 m. These profiles are reasonable, because sinking of particulate organic materials generally accelerates with depth due to particle densification processes (e.g., aggregation in shallow depths and elimination of light/fragile organic materials at middle depths) (Honda et al, 2013). The modeled OPL is treated in the same manner.…”