Vertical profiles of plutonium in the central South Pacific

Vertical profiles of dissolved plutonium activity concentration in South Pacific Ocean water columns were determined using ICP-SF-MS after Fe co-precipitation and radiochemical purification. The water samples were collected in the central South Pacific mid-latitude region (32.5°S) during the BEAGLE2003 expedition conducted in 2003–2004 by the Japan Agency for Marine-Earth Science and Technology. 239Pu concentrations in surface water were around 1 mBq m−3. Concentrations reached a maximum (6 mBq m−3) at 600–800 m, and then decreased with increasing depth. The vertical profile pattern was similar to that observed in the North Pacific subtropical gyre, although concentrations of 239Pu in the deep South Pacific were significantly lower than those in the North Pacific subtropical gyre. The 239Pu/137Cs activity ratios, which reflect biogeochemical processes in seawater, increased exponentially from the surface to 1500 m depth, whereas in deep waters they were almost constant. The estimated 239,240Pu inventories in the South Pacific water column (0–2300 m) were of the same order of magnitude as previous estimates in 1996 (15°S, 148°W); however they were about one order of magnitude lower than those in the western North Pacific.