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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-