Radionuclides In Marine Sediments From The Beibu Gulf:Spatial Patterns,Transport Pathway,and Ionized Radiation Assessment

In the context of national strategies of"carbon peaking"and"carbon neutrality",nuclear power plant will enter a new stage of development.The Beibu Gulf is located at the key connection for the Belt and Road Initiative and behaves as the shortest distance and convenient pathway to the sea transportation for the southwest regions of China.There are several typical marine ecosystems(e.g.,mangroves,seagrass,coral reefs,etc.)with high quality habitats for many marine organisms in the Beibu Gulf.Additiaonlly,marine oil and gas resources are being widely explored in the Beibu Gulf.Although nuclear power plants have been operated in the region of the Beibu Gulf,marine radioactivity is rarely studied in the Beibu Gulf.As the archive to record source-to-sink and transport pathway information of radionuclides in marine sediments,the study of radionuclides in marine sediments is very important.In this study,surface sediments and sediment cores were comprehensively investigated in the Beibu Gulf.Naturally occurring radionuclides(²³⁸U,²²⁶Ra,²¹⁰Pb,²²⁸Ra,and ⁴⁰K)were analysed using high-purity germanium?spectrometry in order to reveal the horizontal and vertical distributions of radionuclides in the Beibu Gulf.The influencing factors of sediment grain size,total organic carbon,redox state,biological/physical disturbance and ocean circulation were discussed.The radionuclide fingerprints(²²⁶Ra/²³⁸U and ²³²Th/²³⁸U)were used as novel geochemical proxies in the regional marine framework of Guangdong-Qiongzhou Strait-Guangxi by constructing three-endmember model to quantify the distinct sources of sediment in the Beibu Gulf.In addition,radiological harzard(e.g.,radium equivalent activity,external hazard indice,internal hazard indice,absorbed gamma dose rate,ERICA,etc.)was quantified for assessing the environmental radiation quality and ionizing radiation on marine biotas(mollusc-bivalve,crustacean,polychaete worm,benthic fish,pelagic fish,zooplankton,phytoplankton,and macroalgae).This study will fill the gap of the radioactivity level and radiological hazard assessment in the Beibu Gulf,explore the radionuclide fingerprint for tracking the sediment source-sink,provide the field data for the verification of numerical simulation for marine radioactivity in the future,and propose potential tools for marine nuclear emergency and the safety of marine environment and ecology.The key conclusions are as follows:(1)The activities of ²³⁸U,²²⁶Ra,²¹⁰Pb,²²⁸Ra,and ⁴⁰K in surface sediments from the Beibu Gulf display a unique horizontal distribution of the"butterfly"pattern,mainly attributable to the controlling factors of the non-linear negative regulation of sediment grain size and positive role of total organic carbon in sediment.(2)The activity of ²³⁸U in sediment cores at the southwest of Weizhou Island station significantly increased with depth,while the activity of ²¹⁰Pb in other sediment cores indicated mixing effect in the surface coastal sediment.The redox state and biological/physical disturbance could affect the vertical distributions of radionuclides.(3)The activity ratios of ²²⁶Ra/²³⁸U and ²³²Th/²³⁸U in terrigenous sediments of the Beibu Gulf were obviously different from that in biogenic sediments of coral reefs,providing as potential geochemical proxies.(4)The ²²⁶Ra-²³⁸U endmember model and the ²³²Th-²³⁸U endmember model were indicated that 85%local input of sediment from the Guangxi and 15% sediment derived from the trans-region input of sediments in the Beibu Gulf.Guangdong Province,which has the largest number of nuclear power plants in China,was the potential source of radiological risk in the Beibu Gulf.(5)The radiological indices and ERICA tool showed that radionuclides in sediments from the Beibu Gulf should not cause significant ionizing radiation damage to the environment and marine organisms.