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

摘要

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).