Fig. 9-4 Distribution of 239+240Pu inventory in seabed sediment in the Japan Sea (Bq/m2) Larger inventories were observed in the southeastern margin of the Japan Sea.

Fig. 9-5 Sediment trap A "Sediment trap" is deployed at mid-depth in the ocean. It can collect settling particles for multiple periods (from several days to a month for each period) automatically and continuously.

Fig. 9-6 Settling flux and the origin of aluminum in three regions in the Japan Sea (off Vladivostok, off the Noto Peninsula, and off Okushiri Island). "Settling flux" is defined as the quantity of particles collected per unit area and collection time Using the elemental composition of settling particles, lithogenic particles in seawater were classified into three origins, particles from the Asian Continent mainly through the atmosphere (yellow column in Fig. 9-6), particles transported from the East China Sea by the Tsushima Warm Current (red), and particles originating from the Island Arc such as the islands of Japan (black). The quantity of lithogenic particles of each origin was calculated by this study. Settling fluxes were determined at a depth of 1 km.

Understanding the radioecological processes in the Japan Sea (East Sea) is important for Far Eastern Countries because many nuclear facilities are located in the region surrounding the Japan Sea. Furthermore, the Japan Sea is regarded as a "miniature" of the ocean because of its bowl-like topography and unique seawater circulation. From 1997 to 2002, we conducted oceanographic observations in the Japanese and Russian exclusive economic zones in the Japan Sea. This clarified that the inventory of anthropogenic radionuclides in seabed sediment was greatest in the southeastern margin of the Japan Sea (Fig. 9-4). To clarify the transport process of particulate material, a sampling device called a "Sediment trap" (Fig. 9-5) was employed. Settling particles were collected in three regions in the Japan Sea. Using elemental composition of the settling particles, lithogenic aluminosilicates in the water column were classified into three transport processes (Fig. 9-6). In the western Japan Sea, more than 80% of lithogenic particles were estimated to originate through the atmosphere from the Asian Continent, partly as yellow dust (KOSA). In the eastern Japan Sea, it was suggested that a significant amount of lithogenic particles are supplied horizontally from the East China Sea and/or the islands of Japan. These findings enable us to understand that the supply of anthropogenic radionuclides to the deep parts of the Japan Sea is controlled not only by the fallout to the ocean surface but also by seawater movement and ocean topography.

Reference S. Otosaka et al., Lithogenic Flux in the Japan Sea Measured with Sediment Traps., Mar. Chem., 91 (1-4), 143 (2004).

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