Plutonium and 137Cs in surface water of the South Pacific Ocean

The present plutonium and (CS)-C-137 concentrations in South Pacific Ocean surface waters were determined. The water samples were collected in the South Pacific mid-latitude region (32.5 degrees S) during the BEAGLE expedition conducted in 2003-04 by JAMSTEC. (239),(PU)-P-240 concentrations in surface seawater of the South Pacific were in the range of 0.5 to 4.1 mBq m(-3), whereas (CS)-C-137 concentrations ranged from 0.07 to 1.7 mBq m(-3). The observed Pu-239,Pu-240 and (CS)-C-137 concentrations in the South Pacific were almost of the same level as those in the North Pacific subtropical gyre. The surface (219,240)pu in the South Pacific subtropical gyre showed larger spatial variations than Cs-137, as it may be affected by physical and biogeochemical processes. The Pu-239,Pu-240/(137)cS activity ratios, which reflect biogeochemical processes in seawater, were generally smaller than that observed in global fallout, except for the most eastern station. The Pu-239,Pu-240/(CS)-C-137 ratios in the South Pacific tend to be higher than that in the North Pacific. The relationships between anthropogenic radionuclides and oceanographic parameters such as salinity and nutrients were examined. The (CS)-C-137 concentrations in the western South Pacific (the Tasman Sea) and the eastern South Pacific were negatively correlated with the phosphate concentration, whereas there is no correlation between the Cs-137 and nutrients concentrations in the South Pacific subtropical gyre. The mutual relationships between anthropogenic radionuclides and oceanographic parameters are important for better understanding of transport processes of anthropogenic radionuclides and their fate in the South Pacific. (c) 2007 Elsevier B.V All rights reserved.