Temporal variation of 137Cs inventory in the western North Pacific

Abstract

The temporal variation of water column inventories of 137Cs in the western North Pacific was examined. 137Cs in seawater of the western North Pacific has originated mainly from global fallout from atmospheric nuclear weapons tests, which occurred in the early 1960's. In the western North Pacific, the water column inventories of 137Cs were 5200 Bq·m-2 in 1973 (GEOSECS station GX225) and 3400 Bq·m-2 in 1983 (KH-84-3 station AN-1). In 1997, the 137Cs inventory was 2600 Bq·m-2 at station 1 of the IAEA'97 cruise. The global fallout at Tsukuba since 1945 was estimated to be 5600 Bq·m-2 in 1973, 4600 Bq·m-2 in 1983 and 3500 Bq·m-2 in 1997. The water column inventories of 137Cs were 93–74% of the estimated global fallout for the same latitude. The typical features of the temporal variation for three vertical profiles taken in 1973, 1983 and 1997 are the decrease of 137Cs activity in the mixed layer and the gradual increase of 137Cs activity in deeper layers. The proportion of the 137Cs inventory in waters down to 1000 meters was 100% in 1973, 88% in 1983 and 75% in 1997. These findings suggest that large amount of the 137Cs entering through sea surface would remain in the mixed layer for more than a few decades.