Spatial variations of ²²⁶Ra, ²²⁸Ra, and ²²⁸Th activities in seawater from the eastern East China Sea

Abstract

A total of 74 surface and subsurface water samples at depths shallower than 200 m were collected in the eastern East China Sea (ECS), and spatial and seasonal variations in ²²⁶Ra, ²²⁸Ra, and ²²⁸Th activities were examined by low-background γ-spectrometry. The ²²⁸Ra and ²²⁸Th activities (as well as ²²⁸Ra/²²⁶Ra and ²²⁸Th/²²⁸Ra activity ratios) on the surface exhibited notable seasonal and lateral variations (²²⁸Ra/²²⁶Ra = 0.2-3.4; ²²⁸Th/²²⁸Ra = 0.1-0.4). From the ²²⁸Ra activity (0.2-6 mBq/L) and salinity (27.2-34.5), fractions of the Kuroshio, shelf, and Yangtze River waters on the surface of the ECS were estimated to be 50-95%, 3-50%, and 2-8% in July, and 95-100%, 1-5%, and 0-1% in April, respectively, exhibiting a lower fraction of shelf water in winter-spring and on the Japan side of the ECS. While the surface waters had high ²²⁸Th activity (~0.1-0.6 mBq/L) in July, resulting from the high activity of the parent ²²⁸Ra, their ²²⁸Th/²²⁸Ra ratio (~0.1) was lower compared with that of waters in February-April (0.15-0.35). ²²⁸Th is considered to be preferentially removed from the surface in summer, reflecting a high flux of biogenic particles.