Retention of 2011 Fukushima Dai-ichi NPP accident-derived 137Cs in the northern North Pacific-Bering Sea.

Tracing soluble radionuclides is critical for understanding the mechanisms underlying the transport of soluble materials in marine environments. In the present study, we investigated the surface distribution of the concentrations (activities) of 134Cs, 137Cs, 226Ra, and 228Ra in the northern North Pacific, Bering Sea, and Arctic regions from 2017 to 2024. The 226Ra and 228Ra concentrations distinctively separated the surface into the low-226Ra subtropical-transition and high-226Ra subarctic current areas in the northern North Pacific and the 228Ra-rich eastern shelf and 228Ra-poor western basin areas in the Bering Sea. In 2022-2024, the surface concentrations of the Fukushima Dai-ichi Nuclear Power Plant accident-derived 137Cs (FD-137Cs) ranged between 0.4 and 1.5 mBq/L, with the highest values in the eastern shelf of the Bering Sea. The lateral distribution of 137Cs concentrations is predominantly ascribed to the preferential inflow of FD-137Cs to the Bering Sea along with the upstream Alaskan Coastal Current. After the steep increase from 2015 to 2017, the temporal variation in FD-137Cs concentrations in the Bering Sea was small (0.8-1.5 mBq/L from 2018 to 2024). This could be attributed to the continual transport of FD-137Cs from the Alaskan Gyre area via the Aleutian Islands.