Abstract
Radioactivities of 7Be as well as 210Pb and radioactive Cs isotopes were measured in the core samples taken at the Mekong delta, Vietnam, in wet and dry seasons to understand the seasonal change of sedimentary environment. Activity of 7Be was detected in the surface samples (up to〜30cm in depth) of the core sediment taken 195m off from the shore in the wet season (October 2011), while it was under detection limit even at the very surface of the core sediment taken in the next dry season (February 2012). Activity of excess 210Pb was almost constant, suggesting the fast sedimentation. Furthermore activities of 134Cs and 137Cs that were released from the Fukushima Dai-ichi Nuclear Power Plant, Japan, in March 2011 by the extraordinary earthquake and tsunami accident were detected only in the upper samples (5-15cm in depth) taken in the wet season (October 2011). These facts imply that new surface sediment was deposited fast in the wet season and that it was removed in the next dry season (about 3 months later). The 7Be, 210Pb and radioactive Cs isotopes are proved to be good tools for the research on delta sediments.
