107
using the best fit concentrations of 226p4 for each section, a rate of 0.84
eyr is indicated.
With the constant rate of sedimentation indicated for the
upper 11 cm of sediment, the time calculated for the deposition is 20 years
using the rate determined by subtraction of the average 226p, concentration, and
13 years uSing the best fit 226n, concentrations measured in each sediment core
sectiori.
The “age” of the sediment deposited at the 11 cm depth in the core is
thus 1952 or 1959, using the average or best fit
226
Ra concentrations, respec-
tively.
The implication of these data is that two different processes are responsible for deposition of the 40 cm of sediment collected in this core.
These
data indicate that slow accumulation of sediment of a very smal] particle size
has occurred in the upper layers, whereas at some point below 11 cm in the
core, rapid accumulation of both fine and natural size components predominated.
Inspection of Figure 23 and
Appendix table II
shows that both 606, and
20755 have markedly different concentration profiles from other radionuclides
in the core above and below about 1] cm.
This may indicate that not only the
process but also the source or radionuclide composition of the contaminated
debris may have differed for the two depth regions in the sediment column,
Station B-20 and C-12
The distribution of uranium concentrations measured in the sediment cores
collected from stations B-20 and C-12 are shown in Table 15.
The few concen-
trations measured give no indication of systematic change with depth in the
cores.
In the station B-20 core, the
238,,.235
U ratios found are not appreci-
ably different from the ratio of 21.8, characteristic of the relative uranium —
isotopic abundance of sea water.
Th
2 234.238
U:"~"U
ratios found are significantly
lower than the ratio (1.15) characteristic of sea water and are interpreted as
indicating, as discussed in Sections 5.1-7
and 5.3-4, that these coralline