van dad. aa gh St 150-480 miles. Since the major part of the radioactivity was deposited in the vicinity of Eniwetok Test Site, the samples collected at increased distances represented older contamination carried to the west by the water currents. With increased distance from the test site, the ratios of Co57/Zn® gradually changed from a value of 0.1 at 150 miles to a value of 5.5 at 480 miles, and the ratio of Co5’/Fe5> followed a parallel course. Thus. with increased time after contamination, the level of radioactive cobalt in the plankton increased by a factor of approximately 50 over that of Zn® and Fe. Grazing fishes feed upon the plankton and in turn are eaten by the pelagic carnivores such as tuna, bonitos and sharks. The rate of movement of these + fishes is great in comparison with that of the plankton, and thus they would tend to disperse the radioactive material accumulated by feeding upon the contaminated plankton. If the pelagic fishes have a high turnover rate for cobalt and a low turnover rate for iron and zinc, they would tend to excrete the radioactive cobalt back into the water during their migrations through the contaminated area and would retain the radioactive zinc and iron. Thus the radioactive cobalt would again become available to the plankton, whereas the radiozine and iron would beeffectively tied up in a reservoir. The ratio of Co5?/Zn® and Co7/Fe®> in plankton would therefore increase with time after initial contamination. In the marine plankton and fish exposed to fallout contamination, the principal radioelements concentrated after a few weeks’ interval belonged to the transition series. In terrestrial organisms these radicelements are either absent or present in very low amounts. Sr®°°—Y* and Cs1!3’7Ba13?m contributed a major part of the activity in the land plants. Thus either a concentrating mechanism operates in the sea for the transition elements. or an exclusion mechanism operates on land. Conversely, in the case of Sr*° and Cs18? concentration occurs on land and exclusion occurs in the sea. Strom et al in 1958 reported the Co®/Sr® ratios in fallout samples from the 1956 test series. The samples were collected on large trays which had been exposed at various locations in the fallout pattern. The average ratio of abundance of Co®/Sr* at zero time was approximately 1/4, and the Co®/Cs197 ratio would thus be approximately 1/6. Because Co® has a half-life approximately one-fifth that of Sr®, the ratio in favour of the presence of Sr®° would increase with time. If there were no exclusion against Sr® in the sea, one would expect it to be present in the samples at a level four times or more that of Co®. The same ratios should hold for land plants and land animals. However. although Sr® and Cs!3’ are found in plants at Eniwetok at levels of approximately 7% and 85%, of the total radioactivity, Co® is absent, or present in only trace amounts. In the case of radioactive cobalt, zinc and manganese, the previously-discussed factors that limit uptake of Sr®® and Cs!37 in marine organisms, except the scavenging action by CaO, Ca(OH,), and calcite. would tend to cause increased uptake of the non-fission-product elements. Thus the cobalt, manganese, and zine that occur in fallout in the sea in the parti- culate form are not subject to chemical competition by similar elements nor to appreciable isotope dilution by their stable counterparts, neither should coprecipitation occur. However, cobalt, manganese and zinc would tend to precipitate with Fe*® and stable iron into a finely-divided form with only a limited tendency to sink through the thermocline, and thus these elements along with radioiron would remain available to the plankton in the mixed layer. 128