The plutonium content of Pacific Ocean waters

The content of plutonium in seawater collected during the cruises of 1968 to 1973 in the entire Pacific Ocean, mainly along the line of 170 degree W and 146 degree W longitude extending from 50 degree N to 68 degree S, not only in the surface layer but also in the deep layer down to 3000m depth was determined. Results obtained in the eastern South Pacific and some of the western North Pacific were also reported. The plutonium content in the North Pacific surface water ranged from 2.2 to 9.4X10-4pCi/l, while it is lower in the South Pacific, ranging from 1.3 to 3.4X10-4pCi/l. Intimate correlation was observed between the plutonium content in surface water and 90Sr deposition rate. The ratio of 238Pu/sup(239,240)Pu in surface water ranged from 0.1 to 0.81 in the North Pacific and 0.35 to 2.4 in the South Pacific. These values are in the same order of magnitude as seen in the land fall-out during the same period. sup(239,240)Pu/137Cs in surface water ranged from 0.0009 to 0.0028 and it is lower than the average ratio of 0.006 in fall-out in Tokyo. In the intermediate or deep layer relatively higher ratios were observed as compared with those in surface water. This suggests a faster downward transport of plutonium in the marine environment than of strontium or caesium. (author)