Contribution of 137Cs-enriched particles to radiocesium concentrations in seafloor sediment: Reconnaissance experiment.

Autoradiography was used to detect 137Cs-enriched particles in sediment samples. The contributions of 137Cs-enriched particles to 137Cs concentrations in sediment samples ranged from 9% to 64%. These experiments revealed that the variability of 137Cs concentrations was due mainly to the heterogeneous distribution of 137Cs-enriched particles in the samples. Therefore, the heterogeneous distribution of 137Cs-enriched particles is probably one of the main factors responsible for the temporal and spatial variations of 137Cs concentrations in sediment samples

Contribution of 137Cs-enriched particles to radiocesium concentrations in seafloor sediment: reconnaissance experiment

Autoradiography was used to detect 137Cs-enriched particles in sediment samples. The contributions of 137Cs-enriched particles to 137Cs concentrations in sediment samples ranged from 9% to 64%. These experiments revealed that the variability of 137Cs concentrations was due mainly to the heterogeneous distribution of 137Cs enriched particles in the samples. Therefore, the heterogeneous distribution of 137Cs enriched particles is probably one of the main factors responsible for the temporal and spatial variations of 137Cs concentrations in sediment samples

Radiolaria and Phaeodaria (siliceous Rhizaria) in south-western and northern Norwegian fjords during late summer 2016: dominant species and biomass in shallow-water assemblages

To determine the present-day community composition of siliceous Rhizaria (Radiolaria and Phaeodaria) in Norwegian fjords, plankton tows were conducted in south-western and northern Norwegian fjords in September 2016. The mean total abundance of radiolarians was 306 m–3 in the Sognefjord complex, which was the southern research site, and, in the north, 945 m–3 in Malangen and 89 m–3 in Balsfjord, both above the Arctic Circle. Sticholonche zanclea was the most abundant radiolarian in the Sognefjord complex and Malangen, accounting for 78–100% (mean 89%) of radiolarian abundance. The mean total abundance of phaeodarians was 1554 m–3 in the Sognefjord complex, 51 m–3 in Malangen and 11 m–3 in Balsfjord. Medusetta arcifera was the most abundant phaeodaria in the Sognefjord complex, accounting for >99% of phaeodarian abundance, but was absent in Malangen and Balsfjord, where Protocystis tridens accounted for >96% of phaeodarian abundance. The carbon biomass of S. zanclea and M. arcifera was 188 and 438 µg C m–3, respectively, which is similar to and 8.6 times higher than, respectively, that of phaeodarians >1 mm in the western North Pacific, suggesting that M. arcifera contributes to organic carbon transport in the Sognefjord complex. Amphimelissa setosa (Nassellaria, Radiolaria), which was a dominant species in the study area in 1982–83, was absent in the present study in all sampled fjords. This could have been caused by the approximately 2 °C increase in water temperature that has occurred since 1990 and can be taken as evidence of a climate-change-associated local temperature rise linked to the warming of advected Atlantic Water

Appearances of Fukushima Daiichi Nuclear Power Plant-Derived ¹³⁷Cs in Coastal Waters around Japan: Results from Marine Monitoring off Nuclear Power Plants and Facilities, 1983–2016

Monitoring of ¹³⁷Cs in seawater in coastal areas around Japan between 1983 and 2016 yielded new insights into the sources and transport of Fukushima Daiichi Nuclear Power Plant (FDNPP)-derived ¹³⁷Cs, particularly along the west coast of Japan. Before the FDNPP accident (1983–2010), the activity concentrations of ¹³⁷Cs, mainly from fallout, were decreasing exponentially. Effective ¹³⁷Cs half-lives in surface seawater ranged from 15.6 to 18.4 yr. After the FDNPP accident (March 2011) ¹³⁷Cs activity concentrations in seawater off Fukushima and neighboring prefectures immediately increased. Since May/June 2011, ¹³⁷Cs activity concentrations there have been declining, and they are now approaching preaccident levels. Along the west coast of Japan remote from FDNPP (i.e., the Japan Sea), however, radiocesium activity concentrations started increasing by 2013, with earlier (May/June 2011) increases at some sites due to airborne transport and fallout. The inventory of ¹³⁷Cs in the Japan Sea (in the main body of the Tsushima Warm Current) in 2016 was calculated to be 0.97 × 10¹⁴ Bq, meaning that 0.44 × 10¹⁴ Bq of FDNPP-derived ¹³⁷Cs was added to the estimated global fallout ¹³⁷Cs inventory in 2016 (0.53 × 10¹⁴ Bq). The net increase of ¹³⁷Cs inventory in the Japan Sea through the addition of FDNPP-derived ¹³⁷Cs accounts for approximately 0.2% of the total ¹³⁷Cs flux from the plant to the ocean from the accident

Occurrence of highly radioactive microparticles in the seafloor sediment from the Pacific coast 35 km northeast of the Fukushima Daiichi Nuclear Power Plant

To understand the properties and significance of highly radioactive particles in the marine environment, we have examined seafloor sediment with a radioactivity of ∼1200 Bq/kg (dry weight; after decay correction to March 2011) collected 35 km northeast of the Fukushima Daiichi Nuclear Power Plant (FDNPP). Among the 697 highly radioactive particles separated from the sediment, two particles, D1-MAX and D1-MID, had a total Cs radioactivity of ∼56 and 0.67 Bq (after decay correction to March 2011), respectively. These particles were characterized with a variety of electron microscopic techniques, including transmission electron microscopy. The 134Cs/137Cs radioactivity ratio of D1-MAX, 1.04, was comparable to that calculated for Unit 2 or 3. D1-MAX consisted mainly of a Cs-rich microparticle (CsMP) with a silica glass matrix. The data clearly suggested that D1-MAX resulted from a molten core–concrete interaction during meltdowns. In contrast, D1-MID was an aggregate of plagioclase, quartz, anatase, and Fe-oxide nanoparticles as well as clay minerals, which had adsorbed soluble Cs. D1-MID was likely a terrestrial particle that had been transported by wind and/or ocean currents to a site 35 km from the FDNPP. The radioactive fractions of D1-MAX and D1-MID were 15% and 0.36%, respectively, of the total radioactivity in the bulk sediment. These highly radioactive particles have a great impact on the movement of radioactive Cs in the marine environment by carrying condensed Cs radioactivity with various colloidal and desorption properties depending on the host phase

Appearances of Fukushima Daiichi Nuclear Power Plant-Derived ¹³⁷Cs in Coastal Waters around Japan: Results from Marine Monitoring off Nuclear Power Plants and Facilities, 1983–2016

Monitoring of ¹³⁷Cs in seawater in coastal areas around Japan between 1983 and 2016 yielded new insights into the sources and transport of Fukushima Daiichi Nuclear Power Plant (FDNPP)-derived ¹³⁷Cs, particularly along the west coast of Japan. Before the FDNPP accident (1983–2010), the activity concentrations of ¹³⁷Cs, mainly from fallout, were decreasing exponentially. Effective ¹³⁷Cs half-lives in surface seawater ranged from 15.6 to 18.4 yr. After the FDNPP accident (March 2011) ¹³⁷Cs activity concentrations in seawater off Fukushima and neighboring prefectures immediately increased. Since May/June 2011, ¹³⁷Cs activity concentrations there have been declining, and they are now approaching preaccident levels. Along the west coast of Japan remote from FDNPP (i.e., the Japan Sea), however, radiocesium activity concentrations started increasing by 2013, with earlier (May/June 2011) increases at some sites due to airborne transport and fallout. The inventory of ¹³⁷Cs in the Japan Sea (in the main body of the Tsushima Warm Current) in 2016 was calculated to be 0.97 × 10¹⁴ Bq, meaning that 0.44 × 10¹⁴ Bq of FDNPP-derived ¹³⁷Cs was added to the estimated global fallout ¹³⁷Cs inventory in 2016 (0.53 × 10¹⁴ Bq). The net increase of ¹³⁷Cs inventory in the Japan Sea through the addition of FDNPP-derived ¹³⁷Cs accounts for approximately 0.2% of the total ¹³⁷Cs flux from the plant to the ocean from the accident

The Contribution of Sources to the Sustained Elevated Inventory of ¹³⁷Cs in Offshore Waters East of Japan after the Fukushima Dai-ichi Nuclear Power Station Accident

We have evaluated the contribution of sources of ¹³⁷Cs to the inventory of radiocesium in waters (surface area: 6160 km², water volume: 753 km³) off Fukushima Prefecture and neighboring prefectures from May 2011 to February 2015. A time-series of the inventory of ¹³⁷Cs in the offshore waters revealed a clearly decreasing trend from May 2011 (283.4 TBq) to February 2015 (1.89 TBq). The ¹³⁷Cs inventory about four years after the accident was approximately twice the background inventory of 1.1 TBq. The magnitudes of the ¹³⁷Cs influxes from sources into offshore waters for periods of 182–183 days were estimated from the first period (1 October 2011 to 31 March 2012: 15.3 TBq) to the last period (1 October 2014 to 31 March 2015: 0.41 TBq). We assumed that three sources contributed ¹³⁷Cs: continuous direct discharge from the Fukushima Dai-ichi Nuclear Power Station (FNPS) even after the massive discharge in late March 2011, desorption/dissolution from sediments, and fluvial input. Quantification of these sources indicated that the direct discharge from the FNPS is the principal source of ¹³⁷Cs to maintain the relatively high inventory in the offshore area