Plutonium in seawaters of the western North Pacific

Plutonium concentrations in seawater of the western North Pacific observed over the period from 1985 to 1997 are reported. In the 1990's, surface Pu-239,Pu-240 concentrations in the western North Pacific were relatively homogeneous (around 3 mBq/m(3)), although surface Pu-239,Pu-240 before the 1980's showed a latitudinal distribution with highs in mid-latitudes and lows in the Equatorial region. Temporal variations of surface Pu-239,Pu-240 in three different sea areas were examined. In mid-latitudes (21 degrees -35 degreesN), surface Pu-239,Pu-240 exponentially decreased with an apparent residence time of 9.9 years, whereas in the subtropical and Equatorial areas, decrease rates of surface Pu-239,Pu-240 were slower than in mid-latitudes. The vertical profiles of Pu-239,Pu-240 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 Pu-239,Pu-240 at the station (27 degrees 48 'N,130 degrees 44 'E) was calculated to be 113 Bq/m(2).