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