Latitudinal distribution of 137Cs in the western Indian Ocean

Abstract

The horizontal and vertical distributions of artificial radionuclide 137Cs were determined in the water column of the west Indian Ocean along the 67˚ E in 2017 (3˚ N to 22˚ S). The 137Cs activities in the surface water ranged from 0.33 ± 0.03 to 0.95 ± 0.06 mBq kg-1 (average: 0.67 ± 0.22 mBq kg-1), and the maximum activities were observed in the northernmost region (3˚ N). The vertical profiles showed higher activities in surface layer and decreased with depth. Overall, the activities of 137Cs showed the minimum detectable activities (MDA) in below 1,000 m depths. The variations of 137Cs activities in the upper layer (<200 m) might reflect the origin of water mass in this region. For examples, the input of Arabian Sea Surface Water (ASSW) in around 3˚S tend to decrease the activities of 137Cs below 50 m, and the input of South Indian Subtropical Underwater (STUW) in around 20˚ S could be result in decrease of the activities of 137Cs in 200-300 m. Chronologically, 137Cs activities had decreased in the surface layer upto 2 times relative to those in 1980s and 1990s (137Cs activities of average 2.41 ± 0.88 mBq kg-1 and 1.19 ± 0.32 mBq kg-1 , respectively) based on compiled dataset from IAEA (International Atomic Energy Agency) Marine Radioactivity Information System (MARIS). In this presentation, more details about the sources and trends of the spatial and temporal variation of 137Cs will be discussed.