84 MIYAKE [CHAP. 3 



and Kameda (1955) initially observed the direct evidence for the large-scale 

 advection and diffusion of fission products in sea-water from the Bikini-Eniwetok 

 proving ground four months after nuclear tests in March, 1954. Their observa- 

 tions showed significant levels of radioactivity at a distance of 2000 km from 

 Bikini, suggesting a westward drift of about 20 cm/sec along the North Equa- 

 torial Current. Little activity was found in the North Equatorial Counter 

 Current. Nine months later, in the spring of 1955, the Taney survey (U.S.A.E.C, 

 1956) confirmed the same magnitude of westward drift of the radioactivity, 

 showing a significant level of activity at least 7000 km downstream from the 

 source of contamination. The higher activity was located at that time off the 

 coast of Luzon Island, in the Philippines. 



In the summer of 1955, the maximum activity was observed in the Kuroshio 

 region off the coast of Japan. This established the direct connection between 

 the North Equatorial Current and the Kuroshio. 



The results of the calculation of diffusion coefficients from the time change 

 of radioactivity distribution in the North Pacific revealed values between 

 0.8 and 1.4 x 10 9 where the lateral distance from the center of diffusion was 

 from 890 km to 3200 km (Miyake and Saruhashi, 1960). 



Folsom (see Revelle et al., 1955) investigated the vertical mixing in the upper 

 layer with the aid of fission products. Artifically radioactive substances were 

 introduced at the surface in an area where the mixed layer was about 100 m 

 thick. The rate of downward motion of the lower boundary of the radioactive 

 water was approximately 10 -1 cm sec -1 . This motion ceased abruptly at the 

 bottom of the mixed layer. 



The vertical distribution of artificial radioactivity in the open ocean was first 

 studied by the Japanese Bikini Expedition in June, 1954. The results showed 

 most of the activity to be confined to the mixed layer above the thermocline. 

 However, depth profiles showed that activity was present at some locations a 

 few hundred meters deep. Using such data Miyoshi (1956) calculated the diffu- 

 sion coefficient in the vertical direction to be 5-10 g cm -1 sec -1 in the tropical 

 area of the North Pacific. In March, 1955, the activity extended to about 

 600 m below the surface (Harley, 1956). 



Horizontal diffusion in the relatively shallow waters of the Bikini lagoon 

 was studied by Munk, Ewing and Revelle (1949). The average value of the 

 horizontal component of the effective diffusion coefficient, Ah, during three 

 days in which the radius of the radioactive area increased to about 4 km was 

 found to be 1.5 x 10 5 g cm -1 sec -1 and Ah/r was close to 0.5 g cm -2 sec -1 . This 

 latter value is in good agreement with estimates of this ratio obtained by other 

 means. 



The radioisotopes useful for tracer experiments in the ocean must have a 

 half-life compatible with the mixing rate of oceanic waters and yet short 

 enough not to bring about a permanent hazard. Revelle et al. (1955) drew 

 attention to 86 Rb (19.5 d), 131 I (8.0 d) and 140 Ba (12.8 d). Craig (1957a) discussed 

 the usefulness of tritium and 14 C as tracers of physical processes in the sea. 



