Dissolved selenium species in the Sulu Sea, the South China Sea and the Celebes Sea

We have measured the vertical profiles of selenite, selenate and organic selenide in the Southeast Asian Basins (the Celebes Sea, the Sulu Sea and the South China Sea) for the two years, 1996 and 2002. Total selenium, selenite and selenate showed nutrient-like profiles. In particular, the vertical profiles of selenate and organic selenide in the Sulu Sea differed greatly between 1996 and 2002. The salinity and selenite/selenate ratio in the surface layer showed that the Sulu Sea surface water in these two years was derived from different sources. The vertical profiles of selenite, selenate and organic selenide in the South China Sea in 1996 and 2002 were almost the same. Both selenite and selenate were slightly elevated at the surface in the South China Sea. These inorganic selenium species seemed to be derived from atmospheric input, admixture of the East China Sea surface water, West Philippine Sea surface water, and fluvial, coastal and shelf waters of the South China Sea. The relationship between the concentration of organic selenide and the relative fluorescent intensity of fluorescent organic matter in the South China Sea suggested the organic selenide exists in the refractory DOC fraction possibility as humic-like substances. The total dissolved selenium concentration and the selenite/selenate ratio in deep water indicated that the water mass age of the Celebes Sea deep water was similar to that of the western North Pacific deep water. On the other hand, the water mass age of the Sulu Sea is younger than that of the western North Pacific. The selenite/selenate ratio in the South China Sea deep water was two times higher than that of the western North Pacific. This result suggested that a large amount of organic detritus was supplied to the South China Sea, and then the organic selenide in the organic detritus rapidly oxidized to selenite.