Nuclear weapons testing conducted within the Bikini Atoll in the Marshall Islands resulted in local fallout contamination, including long lived isotopes of plutonium (Pu) and uranium (U), within the surrounding environment. The Bikini lagoon provides a continuous source of Pu (239+240 Pu) and U (236 U) to the Pacific Basin. The rate at which these isotopes are flushed from the lagoon to the ocean is balanced by inputs from remobilization of lagoon sediments. This research will provide a basis for predicting future change in the solubility and export potential of Pu and U in lagoon sediments. This is important to better understand long-term consequences of these isotopes transferred to the marine environment and/or local food chains. A chronological record of 239+240 Pu and 236 U concentrations and isotope ratios (240 Pu/ 239 Pu and 236 U/ 239 Pu) within the lagoon was developed by analyzing two banded corals retrieved from the Bikini Atoll in 2016. Chronology was determined using x-ray imaging and stable isotope analysis using isotope-ratio mass spectrometry (IRMS). Samples were removed along the growth axis of the slabbed corals. Extraction chromatography and anion exchange columns were utilized to separate and purify Pu and U from the coral matrix. 239+240 Pu and 236 U concentrations and isotope ratios (240 Pu/ 239 Pu and 236 U/ 239 Pu) were calculated from measurements of Pu isotopes and 236 U using accelerator mass spectrometry (AMS). 235 U and 238 U concentrations and 235 U/ 238 U ratios were assessed using inductively coupled plasma mass spectrometry (ICP-MS). This information was used to construct a model of the loss rate of 239+240 Pu and 236 U inside Bikini Atoll Lagoon. Isotopic ratio measurements suggest that the sourceterm of Pu and U inside the lagoon is slowly changing overtime.

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