Cesium: Global Ocean Distribution

Abstract

Since the 1950s Cesium‐137( 137 Cs) concentrations in the surface waters of the global ocean have been extensively measured in numerous studies. This article, based on the analysis of the 137 Cs concentrations in the HAM‐global database, summarizes the results of spatial and temporal variations in 137 Cs concentrations in the surface waters (0–10 m) of the global ocean for the period 1957 to 2005. 137 Cs in the surface waters of the North Pacific Ocean and North Atlantic Ocean has mainly originated from the nuclear weapon testing global fallout in the late 1950s and early 1960s. In the 1970s and 1980s, the discharge from the nuclear fuel reprocessing plants was the dominant source for the North Atlantic Ocean and its northeast marginal seas. The seas that were largely affected by the fallout of the Chernobyl accident in 1986 were the Baltic, the Black, and the Mediterranean Seas. In the 1990s and 2000s, the 137 Cs concentrations were decreasing in the global ocean. During this time, the difference in concentrations in the various seas reduced when compared with those in the 1960s and 1970s. In 2005, the most anthropogenic 137 Cs contaminated area of the global ocean was Baltic Sea, whereas the most undisturbed area was the Antarctic Ocean. 137 Cs in the surface water had an almost homogeneous distribution in the North Pacific, the equatorial region in the Pacific, and the Indian Ocean. The fact that the apparent‐half residence time (the decreasing rate of 137 Cs concentration) is almost equal in these regions suggests that the 137 Cs injected into the western North Pacific Ocean by global fallout was transported to the subtropics and tropics through oceanographic processes. Part of 137 Cs was recirculated in the Pacific Ocean, and the rest was transported into the Indian Ocean via the Indonesian through flow.