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Abstract: The anthropogenic radionuclides such as 137Cs, 90Sr, 99Tc, 129I and some transuranics are important tracers of transport and biogeochemical processes in the ocean. 137Cs, with a half-life of 30years, a major fission product present in a dissolved form in seawater, is a good tracer of oceanic circulation at a time scale of several decades. At WOCE P6 line along 30°S during the BEAGLE cruise in 2003, surface seawater (around 80L) was collected a few meters below the ocean surface by a pumping system. Water column samples (from 5 to 20L) were collected using a Rosette multisampling system and Niskin bottles. 137Cs was separated from seawater samples using ammonium phosphomolybdate (AMP) and analysed for 137Cs in low-level HPGe gamma-ray spectrometers. Results allowed to draw a detailed picture of the distribution of 137Cs in the South Pacific Ocean along P6 line. A 137Cs depth section was depicted from about 160 samples. 137Cs concentrations in the subsurface layers ranged from 0.07±0.04Bqm−3 to 1.85±0.145Bqm−3, high in the Tasman Sea and very low in the eastern region where upwelling occurs. Water column inventories of 137Cs from surface to 1000dbar depth ranged from 270±104 to 1048±127Bqm−2. It was concluded that the source of higher 137Cs concentration and inventories in the Tasman Sea was 137Cs deposited in the mid latitude of the North Pacific Ocean and transported across the equator during four decades. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

The present plutonium and 137Cs concentrations in South Pacific Ocean surface waters were determined. The water samples were collected in the South Pacific mid-latitude region (32.5 °S) during the BEAGLE expedition conducted in 2003–04 by JAMSTEC. 239,240Pu concentrations in surface seawater of the South Pacific were in the range of 0.5 to 4.1 mBq m−3, whereas 137Cs concentrations ranged from 0.07 to 1.7 Bq m−3. The observed 239,240Pu and 137Cs concentrations in the South Pacific were almost of the same level as those in the North Pacific subtropical gyre. The surface 239,240Pu in the South Pacific subtropical gyre showed larger spatial variations than 137Cs, as it may be affected by physical and biogeochemical processes. The 239,240Pu/137Cs activity ratios, which reflect biogeochemical processes in seawater, were generally smaller than that observed in global fallout, except for the most eastern station. The 239,240Pu/137Cs 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 137Cs 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 137Cs 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. [Copyright &y& Elsevier]