Takafumi Aramaki

Anthropogenic radionuclides in the Japan Sea: their distributions and transport processes

The anthropogenic radionuclides, (90)Sr, (137)Cs and (239+240)Pu, were measured in the water column of the Japan Sea/East Sea during 1997-2000. The vertical profiles of radionuclide concentrations showed: exponential decrease with depth for (90)Sr and (137)Cs, and surface minimum/subsurface maximum for (239+240)Pu. These results do not differ substantially from results reported previously. The area-averaged concentrations of radionuclides in the Japan Sea are higher than those found in the Northwest Pacific Ocean below surface layer showing the accumulation of the radionuclides in the deep waters in the Japan Sea. Concerning spatial distributions, the area of high (137)Cs inventory extends from the Japan Basin into the Yamato Basin. It is suggested that wintertime convection of water, occurring mainly in the Japan Basin, causes the radionuclides to sink. The nuclides then advect into the Yamato Basin after detouring around the Yamato Rise.

The AMS facility at the Japan Atomic Energy Research Institute (JAERI)

A new AMS facility has been set up at the Japan Atomic Energy Research Institute (JAERI) in 1997 and 14C as well as 129I will be used for marine environmental studies. The 3 MV AMS system is able to perform high-precision 14C as well as heavy element AMS. The 14C-AMS section of the system has been accepted and its precision is comparable to other high-precision 14C-AMS systems. The heavy element section is under preparation and close to acceptance. A sample preparation system for the extraction of dissolved inorganic carbon (DIC) from seawater has been built up and tested. An accuracy check of the sample preparation system and the 14C-AMS system has been carried out with internationally accepted 14C standard material.

Distribution Of Radiocarbon In The Southwestern North Pacific

Radiocarbon measurements in seawater samples taken at six stations in the southwestern North Pacific visited during the International Atomic Energy Agency (IAEA) 1997 Pacific Ocean Expedition were carried out at the accelerator mass spectrometry (AMS) facility of the Japan Atomic Energy Research Institute (JAERI). Three stations were located close to GEOSECS stations, and three were in the vicinity of Bikini and Enewetak Atolls, which may be influenced by former nuclear weapons testing. Compared with the GEOSECS data (1973), our results show an increase of 14 C in intermediate waters. Furthermore, it is estimated that bomb-produced 14 C inventories in the water column have increased by more than 20% during the last 24 years. However, vertical profiles of Δ 14 C at the stations near Bikini and Enewetak Atolls show a similar general trend to those found in other stations.

Anthropogenic Radionuclides in Seawater of the Japan Sea The Results of Recent Observations and the Temporal Change of Concentrations

Between 1996 and 2002, a wide-area research project on anthropogenic radionuclides was carried out in an area covering the Japanese and Russian Exclusive Economic Zones of the Japan Sea, through a collaboration of Japanese and Russian institutes. The aim was to investigate the migration behavior of anthropogenic radionuclides (90Sr, 137Cs, and 239+240Pu) in the sea. Four expeditions conducted in the Japan Sea between 2001 and 2002 found that the observed concentrations and distributions of radionuclides were similar to those found in previous investigations. Inventories estimated from the concentration data indicate that larger amounts of these radionuclides accumulate in the Japan Sea seawater (by a factor of 1.5–2.1) than are supplied by global fallout in the same latitude belt. Further, we found that the 90Sr and 137Cs concentrations in the intermediate layer show temporal variations with time scales of 1 to several years. The results of cross-analysis using the data of 137Cs and dissolved oxygen suggest that the distribution and variation of radionuclide concentrations in the intermediate layer may reflect water mass movement in the upper part of the Japan Sea.

RADIOCARBON IN THE WATER COLUMN OF THE SOUTHWESTERN NORTH PACIFIC OCEAN—24 YEARS AFTER GEOSECS

In the framework of the Worldwide Marine Radioactivity Studies (WOMARS) project, water profile samples for radiocarbon measurements were taken during the IAEA97 cruise at 10 stations in the southwestern North Pacific Ocean. While 14C concentrations were rapidly decreasing from the surface (∆14C about 100‰) down to about 800 m at all visited stations (∆14C about -200‰), the concentrations below 1000 m were almost constant. Some stations were in proximity to the GEOSECS stations sampled in 1973; thus, 14C profiles could be compared after a 24-yr interval. Generally, 14C concentrations had decreased in surface waters (by 50-80‰) and increased (by about the same amount) in intermediate waters when compared with GEOSECS data. In deep waters (below 1000 m), the observed 14C concentrations were similar to GEOSECS values. The bomb-produced 14C inventory had increased by more than 20% over the 24 yr from 1973 to 1997 and was estimated to be about (32 ± 5) 1012 atom m^(-2), with an annual 14C flux of (1.3 ± 0.3) 1012 atom m^(-2) yr^(-1). The results suggest that bomb-produced 14C has been advected northwards by the Kuroshio Current and the Kuroshio Extension and stored in the intermediate layer as North Pacific Intermediate Water.

Circulation in the Northern Japan Sea Studied Chiefly with Radiocarbon

Radiocarbon concentrations in the northernmost region of the Japan Sea were observed during the summer of 2002. The averaged surface ?14C (above 100 m depth) was 52 ± 8‰, which is significantly higher compared with the values of the Pacific Ocean and Okhotsk Sea. The Δ14C in the deep water decreased with density, and the minimum value was 70‰ By analyzing 14C and other hydrographic data, we found that i) the Tsushima Warm Current Water reaches to the surface layer in the southern Tatarskiy Strait; ii) deep convection did not occur in the northernmost region, at least not after the winter of 20012002; and iii) the bottom water that was previously formed in this region may step down southward along the bottom slope and mix with the Japan Sea Bottom Water. Furthermore, a new water mass characterized by high salinity (>34.09 psu) was found in the subsurface layer in the area north of 46°N.

Carbon isotope composition of dissolved humic and fulvic acids in the Tokachi River system.

This study reports carbon isotopic ratios (Δ(14)C and δ(13)C) of dissolved humic and fulvic acids in the Tokachi River system, northern Japan. These acids have a refractory feature and they represent the largest fraction of dissolved organic matter in aquatic environments. The acids were isolated using the XAD extraction method from river water samples collected at three sites (on the upper and lower Tokachi River, and from one of its tributaries) in June 2004 and 2005. δ(13)C values were -27.8 to -26.9 ‰ for humic and fulvic acids. On the other hand, the Δ(14)C values ranged from -247 to +26 ‰ and the average values were -170 ± 79 ‰ for humic acid and -44 ± 73 ‰ for fulvic acid. The difference was attributed to the residence time of fulvic acid in the watershed being shorter than that of humic acid. The large variation suggested that humic substances have a different pathway in each watershed environment.

A 3 MV heavy element AMS system using a unique TOF set-up

A heavy element AMS system, based on a 3 MV Tandetron, has been put into operation at JAERI, Mutsu, Japan. The system uses sequential injection, designed for cycle frequencies of up to 1000 Hz. The high-energy section is unique in that the identification of the isotopes of interest is done in two successive steps, each using a separate foil combined with energy discrimination. This method allows for the detection of elements that suffer from problematic isobar interference like 36Cl and 41Ca. In that case the foils are chosen to be relatively thick in order to achieve the required energy dispersion. In order to cope with the large scattering caused by the foils, the applied TOF has a unique design that features the acceptance of extremely high divergent beams of up to 80 mrad. During the acceptance tests the precision was shown to be ∼1.1% for 129I measurements. The background was found to be below 10−13.

Current status of the megabenthic community in coastal Fukushima Prefecture, Japan, in the wake of the Great East Japan Earthquake

We conducted fisheries-independent bottom-trawl surveys along the coast of Fukushima, Japan, from 2013 to 2017 to study the megabenthic community structure after the 2011 earthquake, tsunami, and nuclear disaster. Although we observed no substantial changes in biodiversity, total abundance and biomass fluctuated among years, primarily because of temporary increases in the abundance or biomass of small shrimp and squid, or variations in abundance or biomass of mid-sized fishes (i.e. puffers and flatfishes) and large elasmobranchs. Echinoderm abundance and biomass decreased in all areas. Crustacean abundance and biomass were extremely low in the central and southern offshore transects. Our results suggest that there has been no recognizable recovery in the megabenthic community, and megabenthic species off the coast of Fukushima might have been experiencing reproductive or recruitment failure. Further research is needed to reveal the causal factors behind changes in these megabenthic communities.

Spatial variations of 226 Ra, 228 Ra, 134 Cs, and 137 Cs concentrations in western and southern waters off the Korean Peninsula in July 2014

We examined the spatial distributions of 226Ra, 228Ra, 134Cs, and 137Cs concentrations (activities) in seawater off the western and southern Korean Peninsula in July 2014. Radium-228 (and 226Ra) concentrations in water samples varied widely from 5 to 14 mBq/L (2-4 mBq/L), showing a negative correlation with salinity, particularly at the surface off the western Korean Peninsula. This indicates that the seawaters in this area are fundamentally comprised of 228Ra-poor and high-saline Kuroshio Current water and 228Ra-rich and low-saline water (e.g., continental shelf water), with various mixing ratios. Although Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived 134Cs was below the detection limit (<0.08 mBq/L) in waters off the western Korean Peninsula, low level 134Cs (0.1-0.2 mBq/L) was detected in waters off the southern Korean Peninsula accompanied by higher 137Cs concentrations (1.6-1.9 mBq/L) relative to that off the western Korean Peninsula. Combined with the lower radium concentrations, the detection of 134Cs is explained by mixing of FDNPP-derived radiocesium-contaminated Kuroshio Current water.