Reconstruction of the temporal distribution of 236U/238U in the Northwest Pacific Ocean using a coral core sample from the Kuroshio Current area

The temporal distribution of 236U in the Northwest Pacific Ocean was reconstructed through measurements of the annual changes in 236U/238U atom ratios in a coral core collected from Kume Island located within the Kuroshio Current region. In total, 74 years (1940-2014) of temporal trend of 236U/238U atom ratios were reconstructed using Accelerator Mass Spectrometry (AMS). The annually resolved 236U/238U ratios showed a strong increase from 1952, and two prominent peaks were found in the annual bands for 1954 (11.0 ± 1.2 × 10− 9) and 1958 (8.55 ± 1.17 × 10− 9); also an enhanced peak was observed for the band of 1963 (3.51 ± 0.20 × 10− 9). These peaks coincided with the occurrence of large nuclear tests in the Pacific Proving Grounds and/or the highest global fallout rate, suggesting a clear influence of these nuclear events on 236U/238U atom ratios. Furthermore, the coral results showed some disagreement with the annual changes for 236U in East Asian corals collected at the entrance to the Sea of Japan. Based on these results, the mixing ratios of the surface seawater currents in the Northwest Pacific Ocean have been estimated.