Transuranics and other radionuclides in Bikini Lagoon: Concentration data retrieved from aged coral sections’ V. E. Noshkin, K. M. Wong, R. J. Eagle, and C. Gatrousis Bio-Medical Division, Lawrence Livermore Laboratory, University of California, Livermore 94550 Abstract X-radiography and autoradiography of thin vertical sections were used to estimate the growth rate of a specimen of Favites virens from Bikini Lagoon. Discrete bands of radioactivity were identifiable with specific nuclear test series. The coral growth rate of 8.0 mm year" determined by autoradiography is in good agreement with the rate of 8.1 + 2.2 mm year? derived from the “seasonal” alternating light and dark bands on X-radiographs. With these bands as growth rate indicators, the coral was sectioned into yearly increments and analyzed by low-level, nondestructive gamma spectrometry, radiochemical techniques, and mass spectrometry to reconstruct the variations in the concentration of transuranics and other radionuclides in the marine environment at Bikini since 1954. From the concentration data retained in this indicator species, the exchange rate of radionuclides between the lagoon and the open ocean is computed to be longer than exchange rates based on physical circulation data. There is no constant ratio of plutonium isotopes in the coral growth sections, suggesting that the redistributions of the several plutonium isotopes in the environment may be governed by different biogeochemical processes. Increased levels of 7°Po (*°Pb) were found in test-year growth sections, contradicting previous arguments that no “°Pb has resulted from weapons testing. Knutson et al. (1972), Knutson and Buddemeier (1973), and Buddemeier et al. (1974) demonstrated that massive coral colonies from Enewetak Atoll contain radioactive growth inclusions, detectable by autoradiography, that can be correlated with the annual density-banding evident in X-rayed sample sections. Results from these and other samples (Buddemeier et al. 1974) show periodicity in growth, interpreted as density variations in the CaCO, skeleton, that can be used to estimate growth rates of massive coral colonies. Other recent applications, using natural occurring radionuclides (Dodge and Thomson 1974; Moore and Krishnaswami 1972), have also used radiometric techniques to determine coral growth. Radiometric studies (Dodge and Thomson 1974) have substantiated the relationship between annual srowth-band composition and the changing environment in which the coral grows. Previous investigations of the growth rates of fossil and reef coral have been adequately 2Work was performed under the auspices of the U.S. Energy Research and Development Administration. LIMNOLOGY AND OCEANOGRAPHY reviewed (Buddemeier et al. 1974; Dodge and Thomson 1974). Our studies began in an attempt to reconstruct changes in transuranium concentrations in the marine environment of the Pacific test sites through observations of the yearly response of the coral to environmental changes. Although several radiological surveys have been made at the Pacific test sites, no chronological information is available on the behavior of specific radionuclides in these environments. The radiological records retained in the closed aragonite structure of coral couldcircumvent the need for years of real-time data and provide insight into the behavior of these radioelements in the lagoon. Knutson and Buddemeier (1973) found ®°Sr in coral from Enewetak and concluded, on the basis of the observed differential changes in concentration, that the lagoon community acts as a long term source of Sr, probably by exchange with and solution of carbonate materials deposited during the period of nuclear testing. It is not clear that a single mechanism can explain all environmental radiological information retained by coral colonies; we need infor- 729 SEPTEMBER 1975, V. 20(5)