Cs-137 activity in surface water in the western North Pacific

Temporal and spatial variations of Cs-137 activity in surface waters in the western North Pacific are examined along the 165 degreesE transect. Cs-137 i, seawater of the western North Pacific has been introduced by global fal lout originating from atmospheric nuclear weapons tests, which caused major fallout in the early 1960's. At this time Cs-137 activities in the surface waters in mid latitudes of the North Pacific were 10-20 Bq(.)m(-3). South of 30 degreesN, Cs-137 activity decreased gradually towards the south. The surface Cs-137 activity was about 5 Bq(.)m(-3) in the Equatorial region. In the 1970's, the difference in Cs-137 activities in surface water between m id latitudes and the Equatorial region became smaller. The Cs-137 activity in surface water at 40 degreesN - Equatorial region was almost constant at the level of 1.7-3.7 Bq(.)m(-3) in the late 1990's. In the Equatorial regio n, the Cs-137 activity in surface water showed no temporal change except fo r radioactive decay over these four decades. The surface Cs-137 level was 1 .4-1.8 Bq(.)m(-3) in the north subarctic region around 50 degreesN in the l ate 1990's. The lower Cs-137 activity may have been caused by deeper convec tion in this sea area and dilution by fresh water flux.