The Sea of Japan and Its Unique Chemistry Revealed by Time-Series Observations over the Last 30 Years

Abstract

Chemical tracers in seawater, as well as physical parameters such as temperature and salinity, have been measured to better characterize the dynamics of water convection and its spatiotemporal changes in the Sea of Japan (also called the Japan Sea), a semi-closed, hyperoxic marginal sea (maximum depth: ∼3,800 m) in the northwestern corner of the Pacific Ocean. Repeated conductivity, temperature, and depth (CTD) observations and measurements of dissolved oxygen, for more than 30 years, have confirmed that the bottom layer of the Japan Sea, with a thickness of ∼1 km below the boundary at a depth of ∼2,500 m, is characterized by vertical homogeneity with fluctuations of potential temperature and dissolved oxygen of <0.001• C and <0.5 µmol kg −1 , respectively. The timescale of the abyssal circulation in the Japan Sea has been estimated to be 100-300 years, using 14 C and other chemical tracers. Stable isotope analyses for dissolved He, O 2 and CH 4 have given us information on their unique geochemical cycles in the Japan Sea. Profiles of the short-lived radioisotope 222 Rn just above the sea bottom have brought new insights into the short-term lateral water movement with a timescale of several days in the Japan Sea bottom water. It is of special concern that the gradual deoxygenation and warming of the bottom water over the last 30 years have resulted in an ∼10% decrease in dissolved oxygen and ∼0.04• C increase in potential temperature, suggesting a change of the deep convection system in the Japan Sea. The temporal changes in the vertical profiles of tritium from 1984 to 1998 have suggested a shift of the abyssal circulation pattern from a "total (overall) convection mode" to a "shallow (partial) convection mode". It is likely that the global warming since the last century has hindered the formation of dense surface seawater and its ability to sink down to the bottom, isolating the bottom layer from the deep convection loop that is indispensable as the source of cold and oxygen-rich water. However, the decreasing trend of bottom dissolved oxygen between 1977 and 2010 was not monotonous; rather, it was interrupted by an occasional break in the winter of 2000-2001, when severely cold weather may have resulted in especially dense surface water to sink down to the bottom layer for its ventilation.

Construction of perfectly continuous records of physical properties for dark-light sediment sequences collected from the Japan Sea during Integrated Ocean Drilling Program Expedition 346 and their potential utilities as paleoceanographic studies

“…Salinity and nutrient in the TWC is controlled by the circulation in the East China Sea and the Yangtze discharge which could be controlled by precipitation over South China during the East Asian summer monsoon season. The deep water circulation in the Japan Sea is characterized by intense ventilation due to severe cooling during winter monsoon season, which promotes the hyper-oxic deep water condition and shallow carbonate compensation depth of the sea (Gamo et al 2014). The Japan Sea is also located at downwind of the Asian dust sources and receives a significant amount of dust transported by westerlies in spring or northwestern monsoon wind in winter Tada 2000, 2002).…”

Temporal and vertical distributions of anthropogenic 236U in the Japan Sea using a coral core and seawater samples

“…Convection and formation of deepwater caused by dry and cold winds are reported for the Japan Sea [e.g., Nitani , ; Gamo and Horibe , ; Sudo , ; Gamo , ], which is similar to that observed for the global conveyor belt in the world's oceans [ Broecker , ]. For these reasons, the Japan Sea, although being a marginal body of water, has been referred as a “Miniature Ocean” [e.g., Gamo et al ., ], that can serve as a model for the world's oceans.…”

“…In the Japan Sea, very homogeneous water exists at depth of below a few-hundred meters. This water mass, which is called the ''Japan Sea Proper Water,'' has extremely narrow range of water temperature (0.0-0.68C) and salinity (34.06-34.08), and is highly oxygenated (0.2-0.23 mM) [e.g., Uda, 1934;Yasui et al, 1967;Gamo et al, 2014]. Convection and formation of deepwater caused by dry and cold winds are reported for the Japan Sea [e.g., Nitani, 1972;Gamo and Horibe, 1983;Sudo, 1986;Gamo, 1999], which is similar to that observed for the global conveyor belt in the world's oceans [Broecker, 1990].…”

Manganese and iron reduction dominate organic carbon oxidation in surface sediments of the deep Ulleung Basin, East Sea

“…Recent oceanographic observations revealed that the gradual deoxygenation and warming of the bottom water of the East Sea over the last 30 years have resulted in an ∼ 10 % decrease in dissolved oxygen and ∼ 0.04 • C increase in potential temperature (Kim et al, 2001;Gamo, 2011;Gamo et al, 2014). Benthic metabolism and respiratory C org oxidation coupled to various terminal electron-accepting processes in the sediments are largely controlled by the combination of O 2 content, temperature, and biological production overlying the water column (Canfield et al, 2005).…”