Analysis of 50-y record of surface 137Cs concentrations in the global ocean using the HAM-global database

Abstract

We investigated spatial and temporal variations in ¹³⁷Cs concentrations in the surface waters of the global ocean for the period from 1957 to 2005 using the “HAM database – a global version”. Based on the 0.5-y average value of ¹³⁷Cs concentrations in the surface water in each sea area, we classified the temporal variations into four types. (1) 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. (2) In the equatorial Pacific and Indian Oceans, the ¹³⁷Cs concentrations varied within a constant range in the 1970s and 1980s, suggesting the advection of ¹³⁷Cs from areas of high global fallout in the mid-latitudes of the North Pacific Ocean. (3) In the eastern South Pacific and Atlantic Oceans (south of 40 °S), the concentrations decreased exponentially over the five decades. (4) 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 apparent half-residence times of ¹³⁷Cs in the surface waters of the global ocean from 1970 to 2005 ranged from 4.5 to 36.8 years. The apparent half-residence times were longer in the equatorial region and shorter in the higher latitudes. There was no notable difference between the latitudinal distributions of the apparent half-residence times in the Pacific and Indian Oceans. These results suggest that ¹³⁷Cs in the North Pacific Ocean is transported to the equatorial, South Pacific, and Indian Oceans by the oceanic circulation.