Extremely low radioactivity in marine sediment of coral reefs and its mechanism

Abstract

Radionuclides are hazardous materials which impose ionizing radiation on humans and are valuable proxies for tracking environmental processes. Radionuclides are widely reported in global marine sediments, which are recognized as the final destination of radionuclides. As one of the typical marine ecosystems, coral reefs are “hotspots” of high biodiversity providing rich biological resources and significant tourist sites for humans. Seafood consumption and SCUBA diving would induce internal and external radiation on humans, respectively. However, radioactivity is rarely studied in coral reef ecosystems. Here, we exhibited radionuclides in marine sediments collected from the fringing and atoll reefs with a latitudinal coverage of 14° (~1500 km) in the South China Sea (SCS). Naturally occurring radionuclides (40K, 226Ra, 228Ra, and, 238U) were quantified by Standard River Sediment (GBW08304a) and were validated by Irish Sea Sediment (IAEA-385) using High Purity Germanium (HPGe) γ spectrometry (Canberra BE6530). The energy resolution of HPGe γ-spectrometry (~2 keV) is much better than that of NaI γ-spectrometry (~50 keV). Overall, radioactivity in marine sediments was in the order of 238U (28.96 Bq/kg) >40K (24.36 Bq/kg) >228Ra (5.08 Bq/kg) >226Ra (3.34 Bq/kg). Radioactivity was higher in marine sediments collected from the fringing reefs than the atoll reefs due to the influence of terrigenous minerals in the fringing reefs. The occurrence of terrigenous minerals was confirmed by the mineral components (K-feldspar, illite, and quartz) using X-ray diffraction analysis. Additionally, the rare earth elements (REE) are generally used to trace terrigenous inputs. In the present study, the ranking pattern of the 40K activity was consistent with the concentration of REE in marine sediments (sediments outside the coral reef regions > sediments from fringing reefs > sediments from atoll reefs), giving supporting evidence of the influence of terrigenous minerals. A comparison of radionuclides in marine sediments from coral reefs and other sea regions indicated that the radioactive level (Radium equivalent activity, Ra eq) of marine sediment in coral reefs was only 1%−10% of that of other marine sediments and global soil. The value of Ra eq in marine sediments collected from coral reefs (3−42 Bq/kg) was also lower than that in marine sediments collected from the pristine environment of the Arctic Ocean (70.62 Bq/kg) and Southern Ocean (76.92 Bq/kg). Particularly, the mean value of Ra eq in marine sediments of the atoll reefs in the SCS (3.97 Bq/kg) was less than 5% of that in global average soil (108.70 Bq/kg). Therefore, radioactivity in marine sediment was extremely low in coral reefs relative to other sea regions. Marine sediments in coral reefs had three unique characteristics of low 40K activity, low 226Ra and 228Ra activity, and low 226Ra/238U ratio. It was worth noting that the abnormally low 226Ra/238U activity ratio (<0.1) of marine sediments was observed in the atoll reefs and was attributed to the biological process of active uptake of 226Ra and 238U from seawater by coral polyps rather than the ingrowth process of the 238U-230Th-226Ra decay chain. The mechanism of radioactive characteristics in marine sediments was attributed to the fragmentation and weathering of biominerals in coral reefs in contrast to the weathering and subsequent transport of soil and rocks in other marine sediments outside the coral reef region. These three characteristics have potential to be novel geochemical proxies for tracking particle dynamics in coral reefs in addition to other traditional approaches including the Al, Ti, REE, and mineral components.