Plutonium isotopes in seawater of the North Pacific: Effects of close-in fallout

Abstract

Abstract The behavior of 239,240 Pu in seawater was studied using recent 239,240 Pu data from the North Pacific Ocean. 239,240 Pu activity concentrations in surface waters of the North Pacific in 2001 and 2002 ranged from 1 to 10 mBq m −3 . 239,240 Pu water profiles at the mid-latitude of the eastern North Pacific and at the Equatorial western North Pacific showed typical patterns consisting of a surface minimum, a mid-depth maximum and a gradual decrease in concentration with increasing water depth, although some temporal changes in the distribution of 239,240 Pu were observed. 240 Pu/ 239 Pu atom ratios in seawater samples, ranging from 0.17 to 0.28, suggest that major sources of plutonium in the western North Pacific were both global fallout and close-in fallout from nuclear weapons tests carried out at the Marshall Islands. Plutonium concentrations in shallower water layers (<1000 m) of the North Pacific varied temporally due to physical and biogeochemical processes, whereas its behavior has been rather conservative in deep layers (>2000 m).