12
In contrast, the old soil has well-developed Ay and Ay
hexrizons and supports dense vegetation, which produces a heavy
litter fall during the dry season and which contributes to the
redistribution of radionuclides in the system.
Sea birds nest in the vegetation and land crabs, predominantly Birgus latro and Coenobita perlatus, burrow in the soil.
Earthworms are seldom found and terrestrial isopods, although
found in old soils on some islets,
are few.
There are also
few soil insects, which are mainly tenebrionid and carabid
beetles as well as ants.
Depth Gradient of Gamma Spectra
Old Soil
Sr teeneetenanEP
The gamma spectra, with background subtracted, of the 0 to
l-inch, 1 to 2-inch, and 9 to 10-inch depth increments from old
soil collected in March 1959 are given in Fig. 5.
The spectra
of the increments between 2 inches and 9 inches have been
omitted from the figure for clarity but show a gradual change
from the condition at 1 to 2 inches to that at 9 to 10 inches.
The radionuclides corresponding to the photopeaks are indicated
in the figure.
The photopeaks of co°°, zn°, wn?*, cet44_py 144
and pul?5 show significant counts in the surface increments but
are not detectable in samples from greater depths.
The amounts
of cst37 and spi2s decrease and the proportions of est37 and