IAEA’97 expedition to the NW Pacific Ocean—results of oceanographic and radionuclide investigations of the water column

An international sampling cruise, the IAEA’97 NW Pacific Expedition, was carried out in 1997 with the primary objective of contributing to better understanding of the present distributions of radionuclides in the open ocean and of the processes affecting their distributions in the water column, to study the sources which have introduced radionuclides to the Pacific Ocean, and to compare present results with historical data sets obtained from the previous expeditions (e.g., the Geochemical Ocean Sections Programme, GEOSECS). The observed temperature, salinity, dissolved oxygen, and silicate levels in water samples collected showed differences in the upper 1000 m layer in all studied profiles. Transuranics water profiles showed typical sub-surface maxima, with concentrations less by about a factor of 4 than the GEOSECS results, at depths greater by about a factor of 2 and a decrease in the water column inventory by about 20% over 24 years, which were less a function of particle-driven vertical processes and more in response to water mass circulation in the region. Increases in concentrations were observed in deep water near the sea floor. Transuranics inventories in the water column substantially exceeded those expected from global fallout. 90Sr and 137Cs data confirm that the observed changes in concentration profiles have resulted from physical circulation of the regional water masses and transport of radionuclides to deep waters. The IAEA’97 results present the most comprehensive recent study on the distribution of 239,240Pu, 90Sr and 137Cs in the NW Pacific Ocean with a description of the behaviour of these radionuclides in the water column and the effects of the physical forcing of water mass circulation over the last 24 years.