The temporal and spatial variation of 137Cs concentration in the Western North Pacific and its marginal seas during the period from 1979 to 1988

Observations to determine the time-dependent geographical distributions of caesium isotopes in the surface water were carried out in the western North Pacific during the period from 1979 to 1988. After the Chernobyl accident, the enhanced observation was carried out to study the geographical distribution of the Chernobyl radioactivity. Large volume samples (40–2000 liter) were collected and filtered, then both dissolved and paniculate caesium isotopes were determined by gamma-ray spectrometry. The 137Cs concentrations in the surface water along the 137 °E transect during the period from 1979 to 1988 were 5.0–10.6 mBq l−1 between 30 °N and 24 °N, 3.4–7.8m Bq l−1 between 24 °N and 7 °N, and 2.1–6.9 mBq l−1 between 7°N and the Equator. The 137Cs concentrations at stations between 30 °N and 7 °N did not show clear temporal variation, while those at stations South of 7 °N increased from 1986 to 1988. The 137Cs concentration south of 7 °N becomes the same magnitude in two latitude bands north of 7 °N in 1987 and 1988. The Chernobyl derived 134Cs was detected in the surface water at the marginal seas around Japan north of about latitude 30 °N, which reflects the meridional distribution of the Chernobyl derived 134Cs in the surface air over the western North Pacific. The paniculate caesium isotope concentration ranged from 0.1 to 1.0% of the total caesium isotope concentration in 1986 and 1987.