(4)
Except for Bikini and Eneu all the other islands comprising
Bikini Atoll show evidence of some degree of shoreline erosion and wave overwash.
Because of their low elevation, exposure to wave action and small size
all would appear to be too hazardous for permanent habitation. All of the
southern islands are situated very close to the outer edge of the ocean reef
flat (in most cases 100-200 m), increasing their vulnerability to storm waves.
Even the northern islands show recent evidence of shoreline erosion from the
southern lagoon side, possibly the result of large waves entering the lagoon
via the wide southeastern passage.
The reef platform that comprises the uppermost visible perimeter of the
Bikini Atoll forms a shallow terrace to depths of 20 m'to widths of 2-3, km.
Seaward of the shallow terrace, however, the ocean bottom generally drops
precipitously, and at a distance of 5 km from Bikini. Island ocean depths are
approximately 2000 m and within 8 km are as great as 3000 m (see Figure 3).
The Bikini lagoon, which covers some 632 km2, has an average depth of
45 m and a maximum depth of 58m. The lagoon floor generally is quite flat
and consists mainly of loose sandy and silty carbonate sediments except for
the occurrence of numerous coral pinnacles and patch reefs, some of which may
exceed a km in diameter and stand several tens of meters high; very few,
however, are located near Bikini and Eneu Islands.
The sediments that make up the lagoon bottom essentially are of 5 types:
fine debris, corals, Foraminifera, Halimeda, and mollusk shells (Emery, Tracey,
and Ladd, 1954). Generally the shallowest parts of the lagoon bottom near the
reef flats are covered with fine debris with a particle size averaging less
than about 0.5 mm is, diameter, which consists primarily of skeletons of reef
organisms. Throughout the rest of the lagoon, the calcareous remains of the
alga Halimeda up to about a centimeter across are the most abundant constituent
of the bottom sediments, except in a few deeper areas where Foraminifera are
abundant.
Island.
Figure 5 shows the distribution of bottom material near Bikini
Of special interest for this Committee is the suitability of lagoon bottom
sediments for use as topping material should existing soil be removed from one
or more islands. In this regard several characteristics of the bottom material
are of importance: + their ease of dredging, (2) their radioactivity, and
(3) their fertility (with respect to plant growth).
As can be seen from Figure 4, large quantities of loose easily dredgeable
sediments are available at shallow depths near Bikini and Eneu Islands.
Studies on the radionuclides of the top layer of sediment (0-12 cm) have shown
low levels of radioactivity in the entire area within 15 km of Bikini and Eneu
Islands (Figure 5); however, the depth profile of specific activity is not well
known for the lagoon sediment. Recent work by McMurtry, et al, (in press) in
Enewetak Atoll shows no consistent decrease in activity within the upper 200 cm
of lagoon sediment, and in fact, in some cases the radioactivity increases ~
dramatically at depth. They attribute these results primarily to bioturbation
from benthic invertebrates and possibly to constant natural sedimentation since
the testing era, resulting in burial of the more radioactive layer. The
results from bottom samples collected in November, 1983 in Bikini Lagoon
—
should provide additional information when analyses are completed by Lawrence
Livermore Laboratory.
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