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. © 2012, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.