R/V SHINSEI MARU Cruise Report KS-21-13

調査海域: 日本海 / Area: Japan Sea ; 期間: 2021年7月4日～2021年7月14日 / Operation Period: July 4, 2021～July 14, 2021http://www.godac.jamstec.go.jp/darwin/cruise/shinsei_maru/ks-21-13/

RV Shinsei maru "Cruise Report" KS-20-17

調査海域: 常磐沖 / Area: Off Joban ; 期間: 2020年10月19日～2020年10月29日 / Operation Period: October 19, 2020～October 29, 2020http://www.godac.jamstec.go.jp/darwin/cruise/shinsei_maru/ks-20-17/

Behavior of Radiocesium in Sediments in Fukushima Coastal Waters: Verification of Desorption Potential through Pore Water

Concentrations of 137Cs in seawater, seabed sedi-ment, and pore water collected from the area around Fukushimawere investigated from 2015 to 2018, and the potential of coastalsediments to supply radiocesium to the bottom environment wasevaluated. The 137Cs concentration in the pore water ranged from33 to 1934 mBq L−1 and was 10−40 times higher than that in theoverlying water (seawater overlying within 30 cm on the seabed).At most stations, the 137Cs concentrations in the overlying waterand the pore water were approximately proportional to those in thesediment. The conditional partition coefficient between pore waterand sediment was [0.9−14] × 102 L kg−1, independent of the yearof sampling. These results indicated that an equilibrium of 137Cs between pore water and sediment has been established in a relatively short period, and 137Cs in the pore water is gradually exported to seawater near the seabed. A simple box model estimation based on these results showed that 137Cs in the sediment decreased by about 6% per year by desorption/diffusion of 137Cs from the seabed

Temporal distribution of plutonium isotopes in marine sediment cores off Fukushima and Ibaraki for the last three years after the Fukushima Dai-ichi Nuclear Power Plant accident

Large amounts of radionuclides were released into the atmosphere as well as discharged into the sea as a consequence of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident caused by the earthquake and subsequent tsunami on March 11, 2011. The radionuclide contamination in the marine environment due to the FDNPP accident is of great public and scientific concern. Radiocesium (137Cs and 134Cs) were detected in the sediment traps in the deep sea in Pacific Ocean one month after the FDNPP accident, indicating the quick incorporating of radiocesium in seawater with the sediments. In the coastal area of Fukushima and Ibaraki, spatial and temporal distributions of radiocesium in the marine sediments from June 2011 to February 2012 were determined. However, information about the temporal distribution of plutonium isotopes in the sediments off Fukushima and Ibaraki after the FDNPP accident is limited.Plutonium isotopes derived from the FDNPP accident have been detected in the soil and liter samples in the 20-30 km zone around the FDNPP, revealing the release of Pu from the accident. Although no detectable plutonium isotopes from the FNDPP accident in the marine environment 30 km outside the FDNPP till 2012 was reported, Pu characterization in the marine environment inside the 30 km zone remains unknown. The possible Pu contamination from the inside 30 km zone could be transported by ocean currents and thus extend the contamination area by time.Several sediment core samples were collected from the coastal areas off Fukushima and Ibaraki from July 2011 to January 2013. Vertical distribution of Pu activities and Pu atom ratios (240Pu/239Pu and 241Pu/239Pu) in the sediment cores were determined. Pu inventories in the sediment cores and temporal variation of Pu activities and isotopic composition are discussed to understand the source and transport of Pu in the sediments in the coastal areas off Fukushima and Ibaraki after the FDNPP accident.For the analysis of Pu isotopes in the sediment sample, ca. 2.0 g dried sample was weighted out and spiked with 1 pg 242Pu as yield monitor. The extraction of Pu was performed in a Teflon tube with 20 mL concentrated HNO3 on a hot plate at 180-200ºC for at least 4 h. A two-stage anion-exchange chromatographic method was employed for the separation of Pu and U and for the further purification of Pu. A double focusing SF-ICP-MS (Elemental Scientific Inc., Omaha, NE, USA) with membrane dosolvation unit (ACM) and a conical concentric nebulizer was used for the analysis of Pu concentration and Pu isotopic ratios.５th Asia-Pacific Symposium on Radiochemistry (APSORC 13

Factors controlling 134Cs activity concentrations in sediment collected off the coast of Fukushima Prefecture in 2013-2015

To elucidate the activity concentration and behavior of radiocaesium, we observed the spatial and vertical distributions of radiocaesium in sediment collected at 12 monitoring stations off the coast of Fukushima Prefecture in 2013-2015. In bulk surface-layer sediment (core depth: 0-3 cm), high 134Cs activity concentrations were observed at stations around the water depth of 100 m, where the sediment was rich in silt to clay particles and organic matter. The activity concentrations generally decreased with increasing core depth and the extent of the decrease was different between the stations. The difference trend for temporal change of 134Cs activity concentrations between the two zones of off Onahama and within 30 km of the FDNPS was partly attributed to the mobility of sediment particles reflecting water content, porosity and permeability. At some stations, shaped peaks for activity concentrations were temporarily observed in upper-layer sediment (core depth: 0-1 cm) or sediment below that. The 134Cs activity concentrations in the surface-layer sediment were not always accompanied by an increase in the contribution from fine (i.e., silt to clay) particle-bound 134Cs in the sediment. In October 2014, sediment collected at a station about 1.5 km from FDNPS was found to have broad 134Cs peaks in the middle-layer sediment (core depth: 5-16 cm). In this middle-layer sediment, both sand and silt to clay fractions contributed to the increased 134Cs activity concentrations. On the other hand, such broad peaks were not found in October 2015. These results suggest that the activity concentrations in sediment had changed significantly by a complicated process of sediment mixing, erosion or re-sedimentation that affected the broad peak for the activity concentration in the middle-layer sediment

Dissolved radiocaesium in seawater off the coast of Fukushima during 2013-2015

To discuss the factors influencing radiocaesium distributions and its variation in the marine environment, dissolved radiocaesium was measured in seawater samples collected off the coast of Fukushima Prefecture during 2013-2015. The 137Cs activity concentrations in the surface-layer seawater at NPE2, NP0, NP2, NPE1, AN7, I01, E and A stations, which are located within about 10 km from the east coast of Japan, were higher than those at other stations. These high activity concentrations seem to be affected by several factors, including increased river input or contaminated water input from the Fukushima Dai-ichi Nuclear Power Station due to heavy rain events

Spatial Distribution of ∆14C Values of Organic Matter in Surface Sediments off Saru River in Northern Japan, One Year after a Flood Event in 2006

From the 20th International Radiocarbon Conference held in Kona, Hawaii, USA, May 31-June 3, 2009.Dispersion and deposition of terrestrial organic matter by flooding on the inner shelf were studied using C/N ratios, 13C, and ∆14C values of sedimentary organic matter. Surface sediment samples (top 2 cm) were collected from coastal areas near the Saru River in southwestern Hokkaido, northern Japan, 1 yr after a flood event in 2006. Riverine suspended solids were also collected at a fixed station downstream during 2006-2008. Sandy sediments were located at the front of the river mouth and the western part of the sampling area, with the 13C of organic matter ranging from -23.8 to -22.0, ∆14C of -655 to -388, and an organic carbon/total nitrogen (C/N) ratio of 5.9-7.7. On the other hand, silt and clay sediments were distributed in a restricted area 11-16 km from the river mouth, with lighter 13C (-26.7 to -24.1) and higher ∆14C (-240 to -77) of organic matter and C/N ratio (7.8-13.3). From end-member analysis, the apparently younger and less degraded organic matter in the silt and clay sediments consists mainly of terrestrial organic matter released by flood events. They remain in the depression, although most flood deposits were moved to deep-sea environments.The Radiocarbon archives are made available by Radiocarbon and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202