Regional Studies in Marine Science 25 (2019) 100496 Contents lists available at ScienceDirect Regional Studies in Marine Science journal homepage: www.elsevier.com/locate/rsma The distribution of radiocesium in the Indian ocean and its relation to the exit passage of the Indonesian Throughflow Ali Alkatiri a,b , Heny Suseno c ,∗ , Sumi Hudiyono d , Setyo Sarwanto Moersidik e a School of Environmental Science, Universitas Indonesia, Indonesia b Technology Center for Marine Survey, Agency for Assessment and Application of Technology, Indonesia c Marine Radioecology Group, Center for Radiation Safety Technology and Metrology, National Nuclear Energy Agency, Indonesia d Department of Chemistry, Faculty of Mathematics and Science, Universitas Indonesia, Indonesia e Department of Environmental Engineering, Faculty of Engineering, Universitas Indonesia, Indonesia highlights • Potential sources of anthropogenic radiocesium from North Pacific Sea. • Indonesian Throughflow (ITF) can brings water mass from the Pacific Ocean to the Indian Ocean. • The Southern Bali Waters and the Southern Lombok waters are exit passages of ITF. • Marine monitoring of radiocesium in the exit passage of Indonesian Throughflow (ITF). article info Article history: Received 2 May 2018 Received in revised form 27 December 2018 Accepted 27 December 2018 Available online 2 January 2019 Keywords: Fukushima Indonesian Throughflow Radiocesium abstract The objective of this study was to determine the presence of radiocesium ( 134 Cs and 137 Cs) at the monitoring sites and to link its presence to the characteristics and mass water dynamics at the exit of the Indonesian Throughflow (ITF). The main sources of radiocesium are from human activities in the North Pacific Sea, such as from global fallout and release from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) reactor, and are potentially brought to Indonesian waters through the Indonesian Throughflow mechanism. Sea water samples were collected from the surface, thermocline, and deep layers during the expedition. The concentration of 137 Cs on the surface was between 0.042–1.003 Bq m −3 , the concentration range in the thermocline layer was 0.008–0.795 Bq m −3 , and the concentration in the deep layer was 0.046–0.680 Bq m −3 . The 134 Cs concentration was below the detection limit, which indicates that the 137 Cs comes from global fallout. In this research, the measurement of oceanographic parameters was also conducted, and the results showed that temperatures were in the range of 4.982–27.45 ◦ C, salinity was in the range of 34.232–34.979 PSU, and the density was between 22.0979–27.4028 kg m −3 . The salinity profile indicates that the eastern part had a lower salinity level than the western part. The Pacific Equatorial Water Mass was found to be the most dominant in the ITF. Furthermore, these oceanographic data were combined with 137 Cs data to determine the distribution pattern of 137 Cs both horizontally and vertically in the exit passage of the ITF. © 2019 Elsevier B.V. All rights reserved. 1. Introduction On March 11, 2011, an accident occurred in the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) reactor, which was caused by an earthquake with a magnitude of 9.0 on the Richter scale (Honda et al., 2012; Inoue et al., 2012a; Yamamoto et al., 2014). Approximately 10 19 Bq of radionuclides were released into the environment (Sakaguchi et al., 2012), and approximately 10 17 Bqs of 137 Cs, 134 Cs and 131 I spread into the global aquatic environment ∗ Corresponding author. E-mail address: henis@batan.go.id (H. Suseno). (Chino et al., 2011). The total amount of 137 Cs released directly into marine waters was estimated to be 2.7 × 10 16 Bq (Bailly du Bois et al., 2012). Tsumune et al. (2011) estimated that 1.60 × 10 17 Bq 131 I and 1.5 × 10 16 Bq 137 Cs were released into the Pacific Ocean. Steinhauser et al. (2014) estimated that more than 80% of the 137 Cs released into the atmosphere is deposited in the Pacific Ocean. The potential for these radioactive elements to enter Indonesian waters is relatively high due to the Indonesian Throughflow system (ITF). The Indonesian Throughflow (ITF) is a system of water mass circulation from the Pacific Ocean to the Indian Ocean through In- donesian waters caused by the sea level difference of both oceans. The Pacific water mass consists of the North and South Pacific water masses (Gordon et al., 2010; Sprintall et al., 2009). The ITF https://doi.org/10.1016/j.rsma.2018.100496 2352-4855/© 2019 Elsevier B.V. All rights reserved.