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 Ra-228 activity in coral skeletons. Additionally, an abnormally low Ra-226/U-238 activity ratio (< 0.1) of marine sediments in coral reefs was attributed to the biological process of active uptake of Ra-226 and U-238 from seawater by coral polyps rather than the ingrowth process in the U-238-Th-230-Ra-226 decay chain. Several radiological indices were evaluated in coral reefs and significantly lower than recommended values. Particularly, the average Ra-eq 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.