Masayuki Minakawa

228Ra/226Ra ratio and 7Be concentration in the Sea of Japan as indicators for water transport: comparison with migration pattern of Fukushima Dai-ichi NPP-derived 134Cs and 137Cs

To assess the migration patterns of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we analyzed (228)Ra/(226)Ra ratios and (7)Be concentrations and compared them with (134)Cs and (137)Cs concentrations in seawater samples collected within the Sea of Japan before and after the FDNPP accident (i.e., during the period 2007-2012) using low-background γ-spectrometry. The (228)Ra/(226)Ra ratios in surface waters exhibited lateral and seasonal variations, reflecting the flow patterns of surface water. This indicates the transport patterns of the FDNPP-derived radiocesium by surface water. Cosmogenic (7)Be (half-life: 53.3 d) exhibited markedly high concentrations (5-10 mBq/L) at depths shallower than 50 m, with concentrations decreasing steeply (0.2-2 mBq/L) at depths of 50-250 m. The distribution of (7)Be concentrations suggests that the downward delivery of the FDNPP-derived radiocesium to below 50 m depth was negligible for a few months prior to its removal from the Sea of Japan.

Spatial variations of 226Ra, 228Ra, 137Cs, and 228Th activities in the southwestern Okhotsk Sea

We collected 14 water column seawater samples in the southwestern Okhotsk Sea and 7 surface samples around the northern area of Hokkaido Island, Northern Japan, and employed low-background γ-spectrometry with convenient minimal radiochemical processing to determine the activities of (226)Ra (half-life t(1/2)=1600 y), (228)Ra (5.75 y), (137)Cs (30.2 y), and (228)Th (1.91 y) in the samples. Activities of (226)Ra (~2.3 mBq/L), (228)Ra (~0.7 mBq/L), and (137)Cs (~1 mBq/L) of surface waters on the Okhotsk Sea side show notable differences from those on the Japan Sea side (Soya Warm Current Water; SWCW) (~1.5 mBq/L; 1.5-2 mBq/L; 1.4-1.6 mBq/L), indicating their different origins and lateral mixing patterns. All of these nuclides exhibit unique vertical profiles; activities of soluble (226)Ra, (228)Ra, (137)Cs, and reactive (228)Th exhibit small variations from 50 to 500 m depth ((226)Ra, ~2.2 mBq/L; (228)Ra, ~0.4 mBq/L; (137)Cs, ~1 mBq/L; (228)Th, ~0.13 mBq/L). These profiles can be explained by the convective mixing of surface water such as the East Sakhalin Current Water (ESCW) to this layer.

Vertical cycling of Cu and Ni in the western North and Equatorial Pacific

Abstract Vertical profiles of Cu and Ni were determined at 5 stations in the western North Pacific and in the western equatorial Pacific. Cu concentrations are 1–4 nmol/kg at a depth of 1 km, and increase gradually with depth to 4–5 nmol/kg at 4 km. The Cu vertical profiles indicate that Cu is scavenged in the intermediate water. Ni concentrations are 7–10 nmol/kg at a depth of 1 km and about 10 nmol/kg at 2–3 km depth, and then decrease gradually toward the bottom. The convex profiles indicate that Ni is regenerated in the intermediate-deep water. By applying a one-dimensional advection and diffusion model, the mean Cu removal rate is calculated to be 8 μ mol-Cu/m 2 yr between 1–4 km water in the North Pacific. The mean renewal time of dissolved Cu in the deep North Pacific water is about 500 yr. The mean regeneration rate of Ni in the deep North Pacific water is 6 μ mol-Ni/m 2 yr. The mean renewal time is about 600 yr.

Detection and temporal variation of 60Co in the digestive glands of the common octopus, Octopus vulgaris, in the East China Sea

(60)Co were detected in common octopus specimens collected in the East China Sea in 1996-2005. The source of (60)Co has remained unclear yet. Stable isotope analyses showed that there was no difference in stable Co concentrations between octopus samples with (60)Co and without (60)Co. This result showed that the stable Co in the digestive gland of octopus potentially did not include a trace amount of (60)Co and the source of (60)Co existed independently. Furthermore, investigations of octopus in other area and other species indicated that the origin of the source of (60)Co occurred locally in the restricted area in the East China Sea and not in the coastal area of Japan. Concentrations of (60)Co have annually decreased with shorter half-life than the physical half-life. This decrease tendency suggests that the sources of (60)Co were identical and were temporary dumped into the East China Sea as a solid waste.