-26The variation in the ratios Co>7/zn65 and Co°! /Fe5> in plankton samples with change in collection site may also be due to differences in levels of the three isotopes in the fallout material. However, the possibilities should not be overlooked that it may be due to preferential scavenging of zine by calcite and calcium hydroxide in the area of intense fallout or to biological effects. Data that would resolve these points are not evailable and an effort should be made to obtain the necessary information in any future operation. Evidence that the differences in Co5’/zn®5 ratios are real and not due to techniques of separation and analysis is found in the gamma spectrum curves on whole plankton samples (Fig. 2). Inspection of the curves from the three samples shows clearly the differences between the levels of gn©5 and both of the cobalt isotopes, Co57 and Co, in the whole sample before separations and analyses were performed. DOE ARCHIVES 31