Spatial variations of 226Ra, 228Ra, 137Cs, and 228Th activities in the southwestern Okhotsk Sea

We collected 14 water column seawater samples in the southwestern Okhotsk Sea and 7 surface samples around the northern area of Hokkaido Island, Northern Japan, and employed low-background γ-spectrometry with convenient minimal radiochemical processing to determine the activities of 226Ra (half-life t1/2 = 1600 y), 228Ra (5.75 y), 137Cs (30.2 y), and 228Th (1.91 y) in the samples. Activities of 226Ra (∼2.3 mBq/L), 228Ra (∼0.7 mBq/L), and 137Cs (∼1 mBq/L) of surface waters on the Okhotsk Sea side show notable differences from those on the Japan Sea side (Soya Warm Current Water; SWCW) (∼1.5 mBq/L; 1.5–2 mBq/L; 1.4–1.6 mBq/L), indicating their different origins and lateral mixing patterns. All of these nuclides exhibit unique vertical profiles; activities of soluble 226Ra, 228Ra, 137Cs, and reactive 228Th exhibit small variations from 50 to 500 m depth (226Ra, ∼2.2 mBq/L; 228Ra, ∼0.4 mBq/L; 137Cs, ∼1 mBq/L; 228Th, ∼0.13 mBq/L). These profiles can be explained by the convective mixing of surface water such as the East Sakhalin Current Water (ESCW) to this layer.

► 226Ra, 228Ra, 137Cs, and 228Th activities in the southwestern Okhotsk Sea were studied.
► Nuclide activities in the East Sakhalin Current are different from those of the Soya Warm Current.
► The intermediate layer activities vary little due to convective mixing of surface water.
► 228Th is in disequilibrium with 228Ra due to its short residency in the intermediate layer.