The extremely high 137Cs inventory in the Sulu Sea: a possible mechanism

Abstract

Large-volume seawater samples were collected in the Sulu and South China Seas and their 137Cs activities were determined by γ-ray spectrometry using a low background type high-purity Ge detector. Vertical distributions of 137Cs activity showed an exponential decrease in the South China Sea, whereas a subsurface maximum at 200 m depth and monotonic decrease below 300 m were observed in the Sulu Sea. A significant difference in intermediate water 137Cs activities in the 500–2000 m depth was observed between the Sulu and South China Seas, i.e., the 137Cs activities in the Sulu Sea were remarkably higher than those in the South China Sea. The difference in the 137Cs inventory below 500 m was ∼1200 Bq m −2 between the Sulu and South China Seas. The 137Cs total inventory of 3200 Bq m −2 in the Sulu Sea was 5.7 times higher than that expected from global fallout. A possible mechanism controlling this extremely high 137Cs total inventory may be inflows of the 137Cs rich water masses through the Luzon Strait, lateral transport across the Mindoro Strait into the Sulu Sea, and then subduction into the deep layer in the basin.