Distributions of 224, 223ra and 137cs in the Luzon Strait and Its Adjacent Waters and Their Response to Typhoon Rainbow

This paper reports the distribution of natural radionuclides of 224, 223 Ra in the surface water and stratified waters of the Luzon Strait and its adjacent waters during the cruises of September 2015 and May 2016. To understand the impact of the Fukushima nuclear accident, the artificial radionuclide 137 Cs in the waters was also analyzed. The results showed that the activities of 224, 223 Ra and 137 Cs were all within the natural radioactive background levels of the marine environment in the South China Sea. 224 Ra had a higher activity level in the water of the north South China Sea to the west of the Luzon Strait, and a lower activity level in the oceanic Philippine Sea to the east. The 137 Cs activity had no obvious spatial trends. Based on the vertical trends of 224 Ra, 137 Cs, and water temperature and salinity at three stations (LS3, LS5 and LS8), the distinct characteristics of the activity levels and gradients of 224 Ra and 137 Cs among the tropical surface water, subsurface water and mid-deep water were revealed. Typhoon Rainbow event reversed the overall circulation of the Luzon Strait and its adjacent area. A huge amount of western Pacific water characterized by low 224 Ra activities flooded into the South China Sea, reducing the activity level of 224 Ra in the waters. However, there were no significant differences of 137 Cs activity between the West Pacific and the north South China Sea, and ocean current changes had no effect on the 137 Cs activity levels of the water bodies.