-23- 
Three of the radioactive non-fission-product elements, 
zinc, cobalt, and iron, contributed an average of 71 per 
cent of the total activity in the plankton samples. This 
observation is in contrast to those on terrestrial samples 
in which these isotopes were absent or, at most, present in 
trace amounts only (16). Thus, either a concentrating 
mechanism must function in the sea for the non-fission- 
product elements, making them available to the organisms, 
or an exclusion mechanism must operate on land. The ex- 
tremely low levels at which these isotopes occur in dirt 
samples near the target area suggest that concentration oc- 
curs in the sea. 
In the case of these radioelements the previously dis- 
cussed factors that control uptake by marine organisms, with 
the possible exception of scavenging action by calcium hy- 
droxide or calcite, would tend to cause increased uptake. 
Thus, these elements in fallout probably occur in the sea in 
particulate form, are not subject to chemical competition by 
similar elements or to appreciable isotope dilution by their 
stable counterparts, nor would co-precipitation occur in the 
strict sense of the word. However, negatively charged cobalt, 
menganese, and zinc would tend to precipitate with iron into 
a finely divided form, exhibiting only a limited tendency to 
sink below the thermocline and thus remain available to the 
plankton in the mixing layer. 
The isotopes Mn54, Fe55, Co57, Co58, co, ana zn®5 are 
transition elements and have characteristics of variable 
