137Cs in the western South Pacific Ocean

The 137Cs activities were determined for seawater samples from the East Caroline, Coral Sea, New Hebrides, South Fiji and Tasman Sea (two stations) Basins of the western South Pacific Ocean by γ spectrometry using a low background Ge detector. The 137Cs activities ranged from 1.4 to 2.3 Bq m−3 over the depth interval 0–250 m and decreased exponentially from the subsurface to 1000 m depth. The distribution profiles of 137Cs activity at these six western South Pacific Ocean stations did not differ from each other significantly. There was a remarkable difference for the vertical profiles of 137Cs activity between the East Caroline Basin station in this study and the GEOSECS (Geochemical Ocean Sections Study) station at the same latitude in the Equatorial Pacific Ocean; the 137Cs inventory over the depth interval 100–1000 m increased from 400±30 Bq m−2 to 560±30 Bq m−2 during the period from 1973 to 1992. The total 137Cs inventories in the western South Pacific Ocean ranged from 850±70 Bq m−2 in the Coral Sea Basin to 1270±90 Bq m−2 in the South Fiji Basin. Higher 137Cs inventories were observed at middle latitude stations in the subtropical gyre than at low latitude stations. The 137Cs inventories were 1.9–4.5 times (2.9±0.7 on average) and 1.7–4.3 times (3.1±0.7 on average) higher than that of the expected deposition density of atmospheric global fallout at the same latitude and that of the estimated 137Cs deposition density in 10° latitude by 10° longitude grid data obtained by Aoyama et al. [Aoyama M, Hirose K, Igarashi Y. Re-construction and updating our understanding on the global weapons tests 137Cs fallout. J Environ Monit 2006;8:431–438], respectively. The possible processes for higher 137Cs inventories in the western South Pacific Ocean than that of the expected deposition density of atmospheric global fallout may be attributable to the inter-hemisphere dispersion of the atmospheric nuclear weapons testing 137Cs from the northern stratosphere to the southern one and its subsequent deposition, and water-bearing transport of 137Cs from the North Pacific Ocean to the western South Pacific.