Surface water mixing estimated from 228Ra and 226Ra in the northwestern North Pacific

We investigated the horizontal distributions of 228Ra and 226Ra in surface waters of the northwestern North Pacific Ocean and Okhotsk Sea. Ratios of 228Ra/226Ra were relatively large in the Tsugaru Current (0.6–0.8) and Okhotsk Sea (0.4–0.5), and small in the Western Subarctic Gyre (<0.2) and the Oyashio (0.25–0.3). 228Ra/226Ra ratios in western Subarctic Water (SAW) rose slightly upon mixing with Okhotsk Water (OKW), before becoming the Oyashio Water (OYW). Also, ratios in the OYW increased during mixing with Tsugaru Current Water (TCW). Estimating from 228Ra/226Ra ratios and 226Ra activities with a simple two-end members-mixing model, we assumed that approximately 23% of the OYW originated from the OKW and the coastal region off northern Honshu (Japan) was strongly influenced by the TCW. From a diagram of 228Ra activities against salinity, we could roughly divide surface seawater in the study area into the five water masses, which were SAW, OYW, OKW, TCW, and Subtropical Water (STW).