358 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Central and Eastern Lagoon Cores 



The four sediment cores collected from the central and eastern regions of the Atoll 

 (stations B-27, B-16, B-15, and B-30) were similar in three respects: First, there was no 

 significant net increase or decrease in the concentration of radionuclides measured 

 between the upper and lower sections in any of these four cores; second, the distribution 

 profiles of 2^9'2^0pu, ^'^'Am, '^^Eu, ^^^Cs, ^^'^Bi, and ^'^Co concentrations were 

 roughly similar with depth in the individual cores; and third, the 2 3 8p|j^2 3 9,2 4 0p^ ratios 

 measured in all but the lower section of the cores from station B-27 were similar to that 

 in the surface-sediment section. Because of the very short (6 cm) length of the cores at 

 stations B-16 and B-30, no further interpretation of the observed radionuclide profiles 

 was warranted. Except for the ordering of '^^'Am in one section and the 2 3 9,2 4 op^^ 

 concent! ation in the 10- to 12-cm section immediately below, the ordering sequence of 

 radionuclide concentrations in the 16-cm core from station B-15 was the same as that in 

 the surface sediments (Fig. 9). In the 10-cm core collected from station B-27, the 

 sequence of ^^Co, ^°^Bi, and ^^^Cs concentrations measured did not change with depth 

 from that shown in Fig. 9. However, in the 4- to 8-cm region, '^^ ' Am concentration was 

 higher than ^^^'^^*^Pu, and in the 8- to 10-cm section, the ordering sequence was the 

 same as that at lagoon stations B-16, B-26, and B-22 to the west. 



The constancy of the concentrations of all radionuclides measured to depths of 10 cm 

 (core B-27) and 16 cm (core B-15, Table 3) showed that a considerable penetration of 

 radionuclides occurred in these sediments, which appeared physically to be normal lagoon 

 deposits. Assuming a negligible natural sedimentation rate, the penetration of radio- 

 nuchdes into these sediments is significantly greater than that observed by Held (E. Held, 

 University of Washington, unpubUshed results) in Rongelap Atoll sediments. However, 

 these two sediment cores were the longest obtained from any station in the Atoll having 

 unpulverized sediments, which suggests that these sediments may have been significantly 

 less consolidated than average. This could explain both the length of the core collected 

 and the radionuclide concentrations measured with depth. 



Sedimentation Rates at Station B-2 



Measurements of the concentrations of ^^°Pb and ^^^Ra with depth in core B-2 were 

 used to determine the effective sedimentation rates based on tlie ^"'Pb age dating 

 teclinique (Goldberg, 1963; Koide, Soutar, and Goldberg, 1971). The average ^^^Ra 

 concentration of 0.131 pCi/g, which was measured by gamma counting, was used to 

 determine the concentrations of unsupported ^'°Pb. Tlie unsupported ^^°Pb concentra-- 

 tions measured in the 0- to 2-, 4- to 6-, 6- to 8-, and 10- to 12-cm sections decreased 

 logarithmically with depth, which indicates a constant sedimentation rate for the upper 

 layers. Below 12 cm the ^^°Pb concentrations were not significantly different from the 

 ^^^Ra concentrations measured, which indicates no unsupported '^^^Pb. The effective 

 sedimentation rate was determined by calculating a Hnear regression of the unsupported 

 ^^*^Pb concentrations in the upper 1 1 cm of sediment. A sedimentation rate of 0.58 

 cm/yr (correlation 0.98) was calculated for the upper 11 cm of sediment. Thus the 

 approximate date calculated for the deposition of the 11-cm section was 1953, a date 

 consistent with the period of nuclear testing at Bikini. 



These data indicate that two different processes were responsible for the deposition 

 of the 40 cm of sediment sampled at this station (B-2): (1) slow accumulation of 

 sediment occurred in the upper layers (1 1 cm) and (2) at some point below 1 1 cm in the 



