Analysis of 60-yr record of surface 137Cs concentrations in the global ocean

We investigated spatial and temporal variations in 137Cs concentrations in the surface waters of the global ocean for the period from 1957 to 2018. In order to study the distribution of 137Cs concentrations in surface seawater, we divided the global ocean into 37 latitudinal boxes on the basis of known ocean current systems, latitudinal and longitudinal distributions of 137Cs concentrations, the distribution of global fallout, locations of nuclear reprocessing plants, fallout from the Chernobyl accident, and release from Fukushima Nuclear Power Plant accident. Based on the 0.5-y average value of 137Cs concentrations in the surface water in each box, we classified the temporal variations into four types. In the North Pacific Ocean where there was high fallout from atmospheric nuclear weapons tests, the rates of decrease in the 137Cs concentrations changed over the five decades: the rate of decrease from the 1950s to the 1970s was much faster than that after the 1970s, and the 137Cs concentrations were almost constant after the 1990s. Latitudinal differences in 137Cs concentrations in the North Pacific Ocean became small with time. After March 2011, extremely high concentrations (3.26×107 Bq/m3) were observed in the western North Pacific Ocean based on the direct release and atmospheric deposition of FNPP1-derived 137Cs. In the equatorial Pacific and Indian Oceans, the 137Cs concentrations varied within a constant range in the 1970s and 1980s, due to the advection of 137Cs from areas of high global fallout in the mid-latitudes of the North Pacific Ocean. In the eastern South Pacific and Atlantic Oceans (south of 40°S), the concentrations decreased exponentially over the six decades. In the Arctic and North Atlantic Oceans, including marginal seas, 137Cs concentrations were strongly controlled by discharge from nuclear reprocessing plants after the late 1970s. The 137Cs concentrations were rapidly decreased after the early 1980s, and advected into the Arctic Ocean. 
The averaged 137Cs concentrations in each box in the year of 1970 were 1-716 Bq/m3, and those were decreased to 0.2-28 Bq/m3 in the year of 2010. The apparent half-residence times of 137Cs in the surface waters of the global ocean from 1970 to 2010 ranged from 4.2 to 48.1 years for each box. 

(Reference)
Inomata et al. (2009) Analysis of 50-y record of surface 137Cs concentrations in the global ocean using the HAM-global database. Journal of Environmental Monitoring, DOI: 10.1039/b811421h.