Development of the Compartment Model POSEIDON-G for Seamless Cross-Scale Transport and Food-Chain Transfer of Radioactivity in the Global Ocean

A compartmental model POSEIDON-G of the transport and fate of radionuclides in the Global Ocean has been developed. The marine environment is a system of 3D compartments (boxes) representing the water column, bottom sediment, and marine organisms forming food chains. Water exchange between boxes was calculated using the monthly circulation data from the ocean general circulation model. Model equations are solved numerically using the high-accuracy matrix exponential method. The efficiency of the numerical algorithm was substantially improved relative to the previous version (POSEIDON-R). We account for all important global sources of 137 Cs to reproduce the 137 Cs concentration for the period 1945–2030. The simulation agrees well with measurements of 137 Cs concentration in the ocean basins. Across the Global Ocean, before 2011 the geometric mean and geometric standard deviation for simulated-to-observed ratios were 1.10 and 1.79, respectively, whereas after 2011, they were 0.97 and 1.73, respectively. The 137 Cs inventory in the World Ocean reached its maximum of 491 PBq in 1971, and then decreased to 181 PBq in 2026. Inventory in bottom sediments increased from 1% of the ocean inventory in 1975 to 12% in 2025. The primary source in the Western Pacific during 1950–1960 was close-in and regional fallout. The technology for embedding a higher-resolution regional model within a global model was tested for the Fukushima Daiichi accident. Such an approach improves the assessment of the regional and global impacts of the accident on the marine environment by accounting for background concentrations and fluxes of activity from the global model.