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 1mBqm⁻³. Concentrations reached a maximum (6mBqm⁻³) at 600–800m, 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 1500m depth, whereas in deep waters they were almost constant. The estimated ²³⁹,²⁴⁰Pu inventories in the South Pacific water column (0–2300m) 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.