Comparison of δ18O of Dissolved Oxygen (O2) in Three Basins of the East/Japan Sea

The East/Japan Sea is a marginal sea enclosed by Korea, Japan, and Russia, but it has some oceanic characteristics, therefore, it is called "Miniature Ocean". It has three basins deeper than 2000 m, which are Japan Basin (JB), Ulleung Basin (UB), and Yamato Basin (YB). The concentration and δ18O of O2 in seawater is controlled by photosynthesis, respiration, and air-sea gas exchange near the surface. Respiration reduces O2 concentration and enriches 18O, while photosynthesis brings the opposite result. For this reason, δ18O of O2 is used a tracer for biological and chemical processes in the ocean interior. To compare the distribution of δ18O of O2 in three basins of the East/Japan Sea, the vertical samples were collected through three expeditions in May and October, 2007. Seawater was injected to the pre-evacuated glass bottles, and the dissolved gases were extracted instantly. The molecular oxygen was separated, and the 18O/16O ratio was determined by GC-MS. The concentration of O2 was 220-340 μmol/kg in the surface, and reached the maximum at about 50 m, and decreased as the depth increased, 200 μmol/kg in the bottom. The δ18O values in the surface were 22-24 ‰ (vs. SMOW), which was slightly lower than the atmospheric equilibrium value (24.2 ‰) at sea surface temperature, and the values in the bottom were 29-31 ‰, strongly enriched in 18O. The δ18O values increased from JB to UB and YB below 1000 m. Possible reasons for higher δ18O in UB and YB may be 1) higher oxygen utilization rate (OUR) or 2) older water mass with the same OUR. As considerable differences in OUR among three basins could hardly be found in previous studies, the same OUR can be applied to all basins. The age difference between basins in the East/Japan Sea estimated from the distribution of δ18O will be discussed.