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  • 1 Meteorological Research Institute Geochemical Research Department Nagamine 1-1 Tsukuba, Ibaraki 305-0052 Japan
  • | 2 Hakodate Marine Observatory Mihara 3-4-4 Hakodate, Hokkaido 041-0806 Japan
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Plutonium concentrations in seawater of the western North Pacific observed over the period from 1985 to 1997 are reported. In the 1990's, surface 239,240Pu concentrations in the western North Pacific were relatively homogeneous (around 3 mBq/m3), although surface 239,240Pu before the 1980's showed a latitudinal distribution with highs in mid-latitudes and lows in the Equatorial region. Temporal variations of surface 239,240Pu in three different sea areas were examined. In mid-latitudes (21°–35°N), surface 239,240Pu exponentially decreased with an apparent residence time of 9.9 years, whereas in the subtropical and Equatorial areas, decrease rates of surface 239,240Pu were slower than in mid-latitudes. The vertical profiles of 239,240Pu in mid-latitudes of the western North Pacific showed a surface minimum, a subsurface maximum and decreased with increasing depth. The water column inventory of 239,240Pu at the station (27°48'N, 130°44'E) was calculated to be 113 Bq/m2.