High Cs-137 inventory in the Sulu and South China Seas

Large-volume seawater samples (250 L each) were collected from the surface to the bottom at the Sulu and South China Seas during the Piscis Austrinus Expedition of the R/V Hakuho-Maru. The activity of Cs-137 on ammonium molybdophosphate was determined by gamma-spectrometry using a low background type high-purity Ge detector. Cs-137 activities in surface waters ranged from 2.4 to 2.8 mBq/L, showing the typical current activity level. The most striking feature is that a difference of Cs-137 activity, about a factor of 2-6, can be observed in the intermediate water (750-1250 m depth) between the Sulu Sea and South China Sea, i.e., the Cs-137 activities below 750-m depth in the Sulu Sea were significantly higher than those in the South China Sea. Furthermore, total inventory of 2930 Bq/m2 at the Sulu Sea was remarkably higher than that of 1740 Bq/m2 at the South China Sea. A possible mechanism controlling the high Cs-137 inventory in the Sulu Sea may be the inflow of the Cs-137 rich North Pacific Tropical Water and upper North Pacific Intermediate Water through the Mindoro Strait into the Sulu Sea, then conveyance into the deep layer in the Sulu Sea basin.American Geophysical Union 2006 Western Pacific Geophysics Meetin