740 Noshkin et al. TTT Ff Pr?s?TtTfFTfPheT eT hh? hThrrhtlséh€6f 10.0[- ~y 2 ‘p T a - [— | 4 ma a a an 1 a i oo OS _ ti Ww w t 1.07— | r 2 T ] 4 i mo _ a, = Nl = & 1) L 2 fad 4 + E ¢ a 0.01 ol 4 L 7 qa a ae Qo . wi | a a a ge = 4 L~ _ bo 4 J 4 = [ 0.001 7 Pit = Wo nO oO uw n it ao wn on pede tp oS io an ™ ‘oO mn trip = wo nn wo Lea) nn pp tl oo wo an o Mm mn ol ™ on Year of coral growth section Fig. 7. ™Eu:™*Pu and unsupported “Am: *™1Puy ratios in the yearly coral growth sections. trations. At this time we can offer no explanation of this difference. Assessing the possible influence of *°8Pu from fallout and from SNAP-9A on lagoon concentrations, we found this input to have little or no effect on the concentration levels during any one year. Consider, for example, that the cumulative deposition of ***Pu up to 1971 has been only 0.009 mCi km? between 10° and 20° N latitude (Hardy et al. 1973). Comparison of this value with the 1972 average standing level in Bikini Lagoon of 0.033 mCi km~ {mean depth of the lagoon is 46 m) shows that the present lagoon level alone exceeds the maximum possible concentration derived from fallout. The mechanisms regulating the discrimination of these plutonium isotopes are meshed with biogeochemical processes within the lagoon and are as yet not under- stood. The failure of the data to conform to predictable patterns is a most important feature of this study and complicates the interpretation of the behavior of plutonium radionuclides in this environment. It does not appear possible, with these data, to predict unequivocally the behavior of all plutonium isotopes in the environment from an assessment of a single isotope of the element; a great deal of research is still required to unravel the biogeochemistry of this element. ; The *8°Pu activity in the coral sections seems to correlate better with Eu (Fig. 7) than with *°8Pu. The Eu : 78°Puratio in the coral sections decreases in value with a best-fit half-life of 5.1 years which is the radiological decay half-life of 5*Eu. Similar geochemical processes appear to govern the fate and behavior of this lanthanide and of *8°Pu in the lagoon, and the way in which the change in ratio correlates with time supports the age assessment of each section. We pointed outearlier that the 741Am in the coral growth sections originates from the decay of ?4'Pu in the coral and also di- rectly from the environment. If the envi- ronmental source of ?44Am is from *41Pu decay only, and the coral does not discrimi- nate between these two transuranics, the excess ““Am to *4!Pu ratio in the coral should follow a predictable growth curve with time. Plotted in Fig. 7 are the ratios of unsupported *41Amto 741Pu in each coral growth section and a calculated growth curve of 7Am from 7#!Pu (to = 1934). Al- though the errors are large we find that the ratios in the post-test years, with the exception of the 1960 value, are changing with time in a manner consistent with the predicted curve. However, all the values are displaced above the 1954 curve and would be further removed from a growth curve originating in 1956 or 1958. The “41Am in the 1954-1955 growth section falls on the predicted curve, showing there was essentially no 741Am directly produced during the 1954 test series. If we assumethis to be true for the 1956 and 1958test series, the coral must either have been preferen-