Convection of surface water in the northeastern Japan Sea: Implications from vertical profiles of 134Cs concentrations

Abstract

This study examined the spatial profiles of 134Cs concentrations in the Japan Sea (particularly in the northeastern area) in July 2017 and July 2018. We detected 134Cs in water samples above a depth of ~200 m (~0.3–1 mBq/L decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident), which predominantly reflected the mixing of 134Cs-contaminated Tsushima Warm Current (TWC) water within 4 years after 134Cs re-entry from the Pacific Ocean into the Japan Sea in 2013. Furthermore, 134Cs was only detected at depths of 300 m and 500 m (~0.2 mBq/L) in the northern Japan Basin, which corresponded to ~30% of the annual mean concentration levels in TWC water (0.75 mBq/L). Observed spatial variations in 134Cs concentrations were explained by the surface TWC circulation and subsequent convection processes.