Radioactive level of coral reefs in the South China Sea.

In this study, we examined radioactivity simultaneously in surface marine sediments and coral skeletons collected from 12 locations of the fringing and atoll reefs in the South China Sea. Radioactive level declined from the fringing reefs to atoll reefs because of input of terrigenous minerals in the fringing reefs. Radioactivity was higher in coral skeletons than in marine sediments because of the high 228Ra activity in coral skeletons. Additionally, an abnormally low 226Ra/238U activity ratio (<0.1) of marine sediments in coral reefs was attributed to the biological process of active uptake of 226Ra and 238U from seawater by coral polyps rather than the ingrowth process in the 238U-230Th-226Ra decay chain. Several radiological indices were evaluated in coral reefs and significantly lower than recommended values. Particularly, the average Raeq in the atoll reefs was <5% of the world's average of Raeq. Our results displayed typically radioactive status in coral reefs without close-in fallout of anthropogenic radionuclides.