2006 Volume 40 Issue 3 Pages 167-176
To obtain implications for the vertical water mixing within the Sea of Japan, we collected 29 water samples from 2 sites around the Yamato Ridge (3,685 m and 2,610 m depth), and 24 samples from coastal areas (12 sites) along the Japan Island (85-475 m depth) in May-June 2004 and 2005. 226Ra (t1/2=1,600 y), 228Ra (5.75 y) and 137Cs (30.2 y) were quantitatively coprecipitated by BaSO4 and AMP using 20-60 L of water samples, and their activities were measured by low-background y-spectrometry. From surface to the Upper Proper water (1-1,000 m), the 226Ra and 137Cs activities exhibit gradual decrease from 1.8-0.2 mBq/L and 1.7-0.2 mBq/L, respectively. From the gradient of 228Ra activity, the vertical eddy diffusivity was calculated to be 〜6 cm2/s around the Yamato Ridge. In the Deep Proper Water (>1,000 m), the 228Ra activity keeps approximately constant (〜0.2 mBq/L). These vertical distributions are reflecting the intense vertical mixing compared with those in other oceans. For deep sites of coastal areas (>〜250 m depth), the 228Ra activities of bottom waters are notably lower (〜0.5 mBq/L) than those of shallow sites (<〜250 m) (0.7-2.2 mBq/L). The exchange of radium between shallow and deep water is inhibited by the boundary layer (thermocline at 〜250 m depth).
