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We investigated lateral profiles of 228 Ra (half-life; 5.75 years) activity and 228 Ra/ 226 Ra (1600 years) activity ratio using 241 surface water samples collected in/around the Sea of Japan and the East China Sea (ECS) during June–October of 2009–2014. In the ECS, the 228 Ra/ 226 Ra ratio in the surface waters exhibited markedly wide variation (<0.05–3.5) in June, predominantly reflecting the mixing between the 228 Ra-rich continental shelf water and the 228 Ra-depleted Kuroshio Current water. In July, the surface waters of the central Sea of Japan (135–138°E) became separated into three currents: the Offshore Branch of the Tsushima Warm Current (OBTWC) ( 228 Ra/ 226 Ra =0.7–1.2) at 39–41°N, the Coastal Branch of the TWC (CBTWC) (~0.7) on the southern side, and sub-Arctic Current (~0.7) on the northern side. From the central to northeastern Sea of Japan, the 228 Ra/ 226 Ra ratio at the surface (0.8–1.0) was within a range between that of the CBTWC and OBTWC. The fraction of continental shelf water in the CBTWC, OBTWC, and in their combined current was estimated to be 11–16%, ~8%, and 10–11%, respectively. [ABSTRACT FROM AUTHOR]