Global Distributions of 137Cs, 239,240Pu and the Ratio of 239,240Pu/137Cs in an Ocean General Circulation Model

The spatial distributions and the temporal variations of 137Cs and 239,240Pu concentrations were simulated by using an ocean general circulation model (OGCM). These radionuclides are introduced into the ocean by global fallout originating from atmospheric nuclear weapons tests. 137Cs derived from global fallout is transported into the ocean interior by advection and diffusion, and the 137Cs concentration is reduced by radioactive decay. In contrast to 137Cs, 239,240Pu, which is a particle reactive radionuclide, is a biogeochemical tracer. The global distribution of the 239,240Pu/137Cs ratio was investigated in an OGCM with a biogeochemical process model. A half regeneration depth (HRD) of 239,240Pu was estimated from curve fitting of the vertical profile of the 239,240Pu/137Cs ratio. Simulated distribution of the HRD is in good agreement with observation, except in the subarctic gyre. The HRD is a good tool to improve the parameters in the biogeochemical process. (author)