The Pu isotopes, Pu-239 and Pu-240, were determined in annually-banded skeletons of an accurately dated (1943-1999) modern coral (Porites lobata) from Guam Island to identify historical Pu sources to the tropical Northwest Pacific Ocean. Activity concentrations of 239+240PU and Pu-240/Pu-239 atom ratios were determined in the dated coral bands using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Close-in fallout from the former US Pacific Proving Grounds (PPG) in the Marshall Islands and global fallout were identified as the two main sources. The Guam site was dominated by PPG close-in fallout in the 1950s, with an average Pu-240/Pu-239 atom ratio of 0.315 +/- 0.005. In addition, a higher Pu-240/Pu-239 atom ratio (0.456 +/- 0.020) was observed that could be attributed to fallout from the "Ivy Mike" thermonuclear detonation in 1952. The atom ratio decreased in the 1960s and 1970s due to increase in the global fallout with a low Pu-240/Pu-239 atom ratio (similar to 0.18). Recent coral bands (1981-1999) are dominated by the transport of remobilised Pu, with high Pu-240/Pu-239 atom ratios, from the Marshall Islands to Guam Island along the North Equatorial Current (NEC). This remobilised Pu was estimated to comprise 69% of the total Pu in the recent coral bands, although its contribution was variable over time. (C) 2010 Elsevier Ltd. All rights reserved.