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

We investigated the horizontal distributions of Ra-228 and Ra-226 in surface waters of the northwestern North Pacific Ocean and Okhotsk Sea, Ratios of Ra-228/Ra-226 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). Ra-228/Ra-226 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 Ra-228/Ra-226 ratios and Ra-226 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 Ra-228 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). (C) 2008 Elsevier Ltd. All rights reserved.