Long term variation of 137Cs inventory in the global ocean from 1957 to 2018

The spatial and temporal variations in ¹³⁷Cs concentrations in the surface seawater in the global ocean from 1957 to 2018 were analyzed by using the ''HAM database - global 2018'' and “IAEA-MARIS database” in order to understand the behaviors of ¹³⁷Cs originated atmospheric weapons tests, nuclear fuel reprocessing plants, and nuclear power plant accidents at Chernobyl and Fukushima. The global ocean was divided into 37 boxes. The 0.5yr average value of ¹³⁷Cs, apparent half residence times (Tap), and ¹³⁷Cs inventory in each box was estimated. The 0.5yr average value of ¹³⁷Cs in each oceanic region (box) indicate that ¹³⁷Cs decreased exponentially from 1970 to 2010 in the Pacific Ocean (PO), Indian Ocean (IO), and Atlantic Ocean (AO), except for the Arctic Ocean, North Atlantic Ocean and its marginal sea due to the discharge of ¹³⁷Cs from the nuclear fuel reprocessing plants. The geographical difference of ¹³⁷Cs activity concentrations in the global ocean become to be small in the year of 2010. The temporal variation of ¹³⁷Cs column inventory suggests that ¹³⁷Cs derived from the large scale atmospheric weapon tests exist largely in the subtropical NPO, equatorial PO, and subtropical SPO (25°N-25°S). ¹³⁷Cs transport from the PO to the IO occurs in the region from 0°-15°S via Indonesian through flow. The signature of ¹³⁷Cs transport from the IO to the AO is also detected. The ¹³⁷Cs inventory in the surface seawater in the year 2010 is estimated to be 57±17 PBq. Considering that the radioactive decay ¹³⁷Cs are estimated to be 347 PBq, the ¹³⁷Cs existed into the ocean interior is estimated to 173±52 PBq. These indicate that about 30% of ¹³⁷Cs released into the surface seawater have been transported into the ocean interior in 2010. The ¹³⁷Cs inventory in 2011 in the surface seawater in the global ocean were 69±15 PBq. The ¹³⁷Cs released by the Fukushima Nuclear Power Plant 1 accident increased to 16.5±4.8 PBq and this value is in good agreement with previous studies.