Vertical distribution of fallout radionuclides in Enewetak lagoon sediments: effects of burial and bioturbation on the radionuclide inventory
The lagoon sediments off Runit Island at Enewetak Atoll contain elevated concentrations of fallout radionuclides from nuclear device testing in the 1950's. Contrary to previous assumptions about the lagoon sediment radionuclide distribution, we have found large concentrations of 60Co, 137Cs, 125Sb, 155Eu, 207Bi, 238PU, 239+240PU and 241Am at depth in sediment cores up to 2 m in length. Transuranic isotope distributions determined by alpha track autoradiography reveal concentrations within the topmost 40 cm of most cores, whereas the other radionuclides are concentrated deeper in the cores. In the two most heavily contaminated cores, radionuclide concentrations correlate with a distinct layer of fine-grained (<38 μm) carbonate that probably represents blast-pulverized reef and sediments from a nearby barge event. Over 90% of the radioactivity within this layer is in an iron-rich noncarbonate residue that contains magnetite formed from the condensation of metallic apparatus vaporized by the fireball. Associated pyrite has formed in situ from the alteration of magnetite under anoxic conditions. The sampling area is heavily colonized by several species of burrowing callianassid ghost shrimp (Crustacea: Thalassinidea), and some of the cores show evidence of burrowing into the radioactive layers. The burrowing activities of these shrimp facilitate radionuclide remobilization within the lagoon by advection of sediment and porewater during sorting and pumping and by oxidation of suboxic sediments at depth.
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Document Type: Research Article
Publication date: 1986-01-01
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