The distribution of radiocesium in the Indian ocean and its relation to the exit passage of the Indonesian Throughflow

The objective of this study was to determine the presence of radiocesium (¹³⁴Cs and ¹³⁷Cs) 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 ¹³⁷Cs 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 ¹³⁴Cs concentration was below the detection limit, which indicates that the ¹³⁷Cs 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 ¹³⁷Cs data to determine the distribution pattern of ¹³⁷Cs both horizontally and vertically in the exit passage of the ITF.