Biomagnification of 7Be, 234Th, and 228Ra in marine organisms near the northern Pacific coast of Japan

Enrichment of natural radionuclides of thorium, radium and beryllium in several kinds of marine organisms was investigated near the Pacific coast of Miyagi Pref., Japan. The radioactivity of 7Be, 210Pb, 234Th, 238U, 228Ra and 137Cs was measured using gamma spectrometry. High concentrations of 234Th were observed in ascidian livers (50-400 Bq/kg dry) and excrement (2000-2900 Bq/kg dry), although the parent 238U concentrations were less than 3 Bq/kg dry. Such extreme disequilibrium between 238U and 234Th activity was observed in other organisms (barnacles, mussels and brown algae). Relatively high concentrations of 228Ra were detected in ascidian livers and were observed to decrease according to its half-life (5.75 year), suggesting disequilibrium with its parent 232Th. High concentrations (about 1900-5000 Bq/kg dry) of 7Be were detected in ascidian liver. Possible mechanisms for the observed biomagnification and bioaccumulation of these radionuclides in the organisms analyzed were proposed.