Present background levels of surface 137Cs and 239,240Pu concentrations in the Pacific

The present levels of anthropogenic radionuclides, 137Cs and 239,240Pu, in surface waters of the Pacific are estimated using the HAM database, which is a comprehensive data set of 137Cs, 239,240Pu and other anthropogenic radionuclides. The time-series data of surface 137Cs and 239,240Pu suggests that surface 137Cs and 239,240Pu concentrations in the Pacific since 1971 decrease exponentially in each sea area, although the temporal changes of surface 137Cs and 239,240Pu differed between sea areas of the Pacific. The calculated 137Cs concentrations in the Pacific surface waters in 2000 are homogeneous in the Pacific, comparing with that in the 1960s and 1970s, although they are still high in the North Pacific and low in the South Pacific. The biogeochemical and physical processes are significant factors to control present levels of surface 239,240Pu concentrations, which are high in the Subarctic and equatorial Pacific and low in the mid-latitude region of the eastern North Pacific.