Multidecadal oceanographic changes in the western Pacific detected through high-resolution bomb-derived radiocarbon measurements on corals

High-resolution measurements of radiocarbon (C-14) in corals can be used to reconstruct past variability in ocean conditions. Here we report seasonal Delta C-14 changes in coral from Ishigaki Island, Japan, and compare with previously reported data from Palau and Guam. Our data clearly indicate a significant increase in Delta C-14 from 1947 to 1998 related to atmospheric nuclear bomb testing. The three early Delta C-14 spikes related to the atmospheric nuclear bomb tests in the US Proving Grounds at Bikini and Enewerak atoll conducted in 1954, 1956, and 1958 were detected from the Ishigaki coral. After 1976, variability in the Mindanao Dome region related to North Equatorial Current (NEC) bifurcation latitude migration affected the Delta C-14 difference between Palau and Guam, whereas the difference between Ishigaki and Guam was not correlated with the bifurcation latitude. The Delta C-14 difference between Ishigaki and Guam may be due to mesoscale eddies in the Kuroshio area. On the decadal scale, the northward shift of NEC bifurcation latitude after 1976, the year as known as Pacific Decadal Oscillation regime shift from negative to positive, was concurrent with the abundant westward-propagating mesoscale eddies in the Subtropical Countercurrent region and stronger Kuroshio transport off the east Taiwan, which may be represented by a smaller Delta C-14 difference between Ishigaki and Guam after 1976.