736 Noshkin et al. Table 5. ™Sr and *’Po (**Pb) concentrations, decay corrected to year of growth. Unsupported and supported ''Am concentrations. StL Unsup. 1971-1972 9.09451 1970 _ Ar Sup. 0.00 0.00 G.01 G.02 0,02 1963 0.0742 O.11 1960 1961 1960 1959 195€ O, 30461 0.05453 9.40433 1.85417 1957 0. 93425 1956 4b +f 1954-1955 1.4 413 *Unsup: lost — 0,09+37 oO.L7#hd 0.13439 G. 12436 ©.12+52 O.52413 99. % 9.00 1969 1968 1967 1HE 1965 1964 "4 ky pCi/e dry weight + 7 error Estimated year of growth Pacific (North equaOcean torial current) 0.45412 69. 78412 0o.T71+4 0.6946 0.7246 0,.89+h O.10 2.1545 1.0345 lost O.19+5 0.1546 0.1945 0.2045 0.22412 9.2149 0.20 0.25425 9.19410 1.44 16.042 o.k8+th O.45 a.28 2.3623 8.6843 O.42 4.6 16.1 33.341 32.241 39.341 Pu decay. Sup: n Ko Atoll reservoirs Bikini Lagoon i Ny 0.1945 1.2144 1.7543 unsupgpyted 2blamn. supported by 2 a4 00 F 0, 2449 0.29422 0.2517 1.04211 1.3559 | ky Ocean Fig. 6. Box model diagram of Bikini Lagoon describing the general flow and sources of radionuclides into and out of the lagoon. see 24lam tbetermined by beta counting 7°y - daughter of 90sr on low-level proportional counters. #210py activity determined by counting <l°Po using alpha spectrometry. the concentration (pCi g-!) in the annual growth bandis proportional to the concentration in the adjacent environment. On this basis, we find that all detected radio- side and east of the lagoon. Radivelements 6Co, *°TBi, *41Am, and !°Eu were not de- ' tected by gammaspectrometry in any water samples collected outside the atoll but were prominent in biota and sediment sam- ples (Fig. 1, Table 1) from the lagoon. We conclude that the atoll is the principal source of radionuclides to the lagoon en- vironment. Our investigation differs from others at- tempting to use corals to determine the trace element composition of adjacent waters. We are not comparing concentrations from different species or concentrations in whole specimens from different areas. We expect coral samples from different lagoon locations to have significantly different absolute radionuclide concentrations, and preliminary data have verified this assumption. Having only a single sample from one location, however, we need only assume for each element or radionuclide that each vearly increment of coral growth concentrates the same available fraction from seawater per unit weight of coral; then, nuclides (except 7!°Po) have decreased in the lagoon by twoto three orders of magnitude since the test years. The rate at which the radionuclide concentration changes with time is not constant. The rate of change was most rapid after the test series. In some years (e.g. 1964 compared to 1963 and 1965), the concentrations of some radionuclides changed abruptly: for these small but real changes we have no explanation. Wetried to correlate the 1964 increase with the peak in fallout deposition in the Northern Hemisphere, but the computed amount depos- ited in the lagoon in this period wasinsignificant compared to the observed change. Although these data are derived from only one coral collection, they suggest that unanticipated processes in the atoll may lead to abrupt changes in the concentrations of specific radionuclides in the lagoon environment. However since 1965 the concentrations of many specific radionuclides have decreased only slowly (after correction for decay}; this indicates that recycling from sedimentary processes, biological activity, 2 ys a muy