Seasonal variation in the 228Ra/226Ra ratio of coastal water within the Sea of Japan: Implications for the origin and circulation patterns of the Tsushima Coastal Branch Current

In the current study, low-background γ-spectrometry was employed to determine the 228Ra/226Ra activity ratio and 137Cs activity of 84 coastal water samples collected at six sites along the main island of Japan (Honshu Island) within the Sea of Japan, including the Tsushima Strait, and two other representative sites on Honshu Island (a Pacific shore and the Tsugaru Strait) at 1-month intervals in 2006. The 228Ra/226Ra ratio of coastal waters in the Sea of Japan exhibited similar patterns of seasonal variation, with minimum values during early summer (228Ra/226Ra = 0.6–0.8), maximum values during autumn (228Ra/226Ra = 1.5–3), and a time lag in their temporal changes ( 2.5 months and over 1300 km distance). However, the 2 other sites represented no clear periodic variation. In contrast to the positive correlation between 137Cs activity (0.6–1.7 mBq/L) and salinity (15–35), the 228Ra/226Ra ratio of coastal water samples from the Sea of Japan was not observed to correlate with salinity, and the increase in the 228Ra/226Ra ratio was not as marked (0.5–1; May–June 2004 and 2005) during the migration along Honshu Island. The input of land-derived water and/or the diffusion of radium from coastal sediments is unlikely to have affected the wide seasonal variation in the 228Ra/226Ra ratio observed in these water samples. The seasonal variation in the 228Ra/226Ra ratio recorded for the coastal waters of the Sea of Japan is considered to be mainly controlled by the remarkable changes in the mixing ratio of the 228Ra-poor Kuroshio and the 228Ra-rich continental shelf waters within the East China Sea (ECS). After passing through the Tsushima Strait, this water mass moves northeast along the coastline of the Sea of Japan as the Tsushima Coastal Branch Current (TCBC).