14
13
divided into a grid consisting of a series of 6-km? regions; at least
one sediment sample was obtained from each region to provide
vadiological data for areal distributions.
eastern reef.
All sediments are composed
of different quantities of fine and coarse-grained carbonate material,
shells, coral fragments, and Halimeda debris.
To assess the sediment
inventory, no attempt was made to distinguish concentration levels in
specific sedimentary components.
Figs, 2 through 5 illustrate the main
features of the transuranic distributions in the surface layer of the
lagoon sediment.
Isolated regions of relatively high concentrations of
23942405, are evident in some lesser contaminated areas of the lagoon;
other small regions of high surface radioactivity might have escaped
detection.
The areal distributions are based on available data from the
samples that were collected and analyzed,
TMumerous nuclear tests.
the locations of larger or more
Highest plutonium concentrations are associated
with the sediments from the northwest quadrant in a north- and
Alice and Belle and several km southwest of Mike and Koa craters.
A
second region of relatively high concentration is in sediments off the
The activity in this region is lower than the
concentrations in sediments in the northwest,
surface concentrations were less than ? pCi/g.
All surface sediment
samples obtained during and since 1972 contained
2394240,
Pu.
The
inventory in only the top 2.5-cm layer (mC4/ km) of sedinent exceeds the
activity deposited to the earth's surface as worldwide fallout in any
latitude band in the northern or southern hemisphere (Ha73).
Although the surface distribution of 241 Am in the sediments appears
similar to that of
2394240,
the ratio of
39424
241,2394240,
activities
(Fig. 5) shows that the radionuclides are not well mixed throughout the
surface deposits.
The ratio in the sediments ranges from 0,06 to 0.93,
Most of the transuranic
inventory in the surface sediments can be separated roughly from the
lesser contaminated deposits by a line extending from the Southwest
is 0.29 + 0.17, and the
ratio determined from the mean surface concertrations
+ 0.06,
(lelle 1)
is
+,36
The average ratio is similar te that found in central Pacific
and northeast Atlantic sediments (Li76), which receive only worldwide
fallout deposition but have, in contrast, one-half the average
south-oriented elliptical area, roughly 2 to 3 km east of the islands of
shore of Yvonne Island.
range between 2 and 170 pCi/g dry weight, while south of this line the
activities from all surface sediment samples,
Highest surface concentrations are associated with the sediments
near, but not necessarily adjacent to,
42 4
The surface 239 2405, concentrations north of this line
The mean ratio, however, determined by averaging PAT Ay A342 40,
The transuranics are distributed nonuniformly over the lagoon
floor.
Passage to the island of Tom (Munjor), which is south of Yvonne on the
concentration ratio
of
surface
sédiments
at
PRikini
(Ne75)
2394240
.
Cin in the vurfuce sedivcunts,
the distribution ratio of 2385 23
shown in Fig. 4, demonstrates the nonunifornuity amon, plutonium isotopes
in components of the sediment in the Atoll environment.
There are,
however, large geographical regions of the lagoon with ¢lialar {sot :pic
ratios in the sediment.
On the other band, small areas of
the day.
oa,
such as a 600-m strip on the lagoon side of Yvonne Island, contain
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