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.