THE ISOTOPIC SIGNATURE OF FALLOUT PLUTONIUM IN THE NORTH PACIFIC

Plutonium analyses of a dated coral record from the French Frigate Sho als in the central North Pacific indicate that there are two major sou rces of Pu in this basin: close-in (tropospheric) fallout from nuclear weapons testing at the Pacific Proving Grounds in the Marshall Island s in the 1950s and global (stratospheric) fallout which peaked in 1962 . Furthermore, the Pu-240/Pu-239 atom ratio of fallout from the Pacifi c Proving Grounds is characteristically higher (0.24) than that of glo bal fallout Pu (0.18-0.19). Seawater and sediment samples from the Nor th Pacific exhibit a wide range of Pu-240/Pu-239 values (0.19-0.34), w ith a trend towards higher ratios in the subsurface waters and sedimen t. Deep water Pu-240/Pu-239 ration are higher in the vicinity of the M arshall Islands relative to stations further from this close-in fallou t source. These preliminary data suggest that fallout Pu from the Paci fic Proving Grounds is more rapidly removed from the surface waters th an is global fallout Pu. Plutonium geochemistry appears to be related to the physical/chemical form of Pu-bearing particles generated by dif ferent fallout sources. (C) 1997 Elsevier Science Limited.