A dynamic framework for marine radioactivity baselines: Climate-driven variability of 137Cs in the Pacific Ocean

Radiocesium-137 ( 137 Cs) is a useful tracer of ocean circulation because of its long physical half-life and relatively conservative behavior in seawater. We investigated variability in Pacific 137 Cs concentrations and apparent half-lives using observational datasets and numerical ocean model simulations. Two experiments were compared: one forced by repeating normal-year forcing (NYF) and the other by interannually varying atmospheric forcing (GIAF). Because the source terms and model configuration were identical, the GIAF–NYF difference was used as an indicator of the forcing-related model difference to interannual atmospheric forcing. During 1990–2007, observations in the Western and Subarctic North Pacific Ocean (NPO) showed large scatter but overall decreasing trends. Observational apparent half-lives were 13.4 years in the Western NPO and 15.7 years in the Subarctic NPO, compared with model estimates of 16.3–16.8 years and 14.2–15.0 years, respectively. The model reproduced the regional-scale long-term decrease. In subtropical boxes, GIAF–NYF differences varied regionally. Lag-correlation analyses also showed region-dependent patterns with the PDO and Niño-3 indices, but none remained statistically significant after accounting for temporal autocorrelation and multiple comparisons across lags. These patterns were therefore regarded as exploratory associations rather than statistically confirmed climate-driven responses. The results indicate that the apparent half-life of 137 Cs is an integrated diagnostic reflecting radioactive decay, source history, redistribution and dilution, atmospheric-forcing variability, and observational sampling. Marine radioactivity baselines should therefore be treated as dynamic quantities rather than static reference values.