¹³⁷Cs activity in surface water in the western North Pacific

Abstract

Temporal and spatial variations of ¹³⁷Cs activity in surface waters in the western North Pacific are examined along the 165°E transect. ¹³⁷Cs in seawater of the western North Pacific has been introduced by global fallout originating from atmospheric nuclear weapons tests, which caused major fallout in the early 1960's. At this time ¹³⁷Cs activities in the surface waters in mid latitudes of the North Pacific were 10–20 Bq·m^-3. South of 30°N, ¹³⁷Cs activity decreased gradually towards the south. The surface ¹³⁷Cs activity was about 5 Bq·m^-3 in the Equatorial region. In the 1970's, the difference in ¹³⁷Cs activities in surface water between mid latitudes and the Equatorial region became smaller. The ¹³⁷Cs activity in surface water at 40°N – Equatorial region was almost constant at the level of 1.7–3.7 Bq·m^-3 in the late 1990's. In the Equatorial region, the ¹³⁷Cs activity in surface water showed no temporal change except for radioactive decay over these four decades. The surface ¹³⁷Cs level was 1.4–1.8 Bq·m^-3 in the north subarctic region around 50°N in the late 1990's. The lower ¹³⁷Cs activity may have been caused by deeper convection in this sea area and dilution by fresh water flux.