Paleo-Tsushima Water and its effect on surface water properties in the East Sea during the last glacial maximum: Revisited

摘要

The semi-enclosed deep marginal East Sea is known by limited sill flow and low sea-surface salinity during the last glacial maximum (LGM) when sea level was about 130 m lower than the present level. Although three straits (the Tsugaru, the Soya and the Tartar) with shallower than 130 m sills were completely closed, the Korea Strait with a maximum sill depth of 140 m seems to have persisted as a partial connection to the East Sea, allowing a sill flow. The volume transport at the Korea Strait during the LGM is estimated at approximately 0.3-1.1 × 10~(12)m~3/yr, by using bathymetry, seismic reflection profiles and current data. The low sea-surface salinity has been explained by the East China Sea Coast Water (ECSCW) and high precipitation. However, the existing geological observations indicate that precipitation was reduced in the glacial East Sea. The high-resolution numerical simulation results predict that evaporation (2.16 mm/day) exceeded precipitation (1.43 mm/day), further suggesting net evaporation (evaporation minus precipitation) rates (0.2 × 10~(12)m~3/yr) over the LGM East Sea. This signifies that the precipitation was not the factor lowering surface paleosalinity and that the paleo-Tsushima Water carried a huge amount of freshwater from the ECSCW than previously expected. The calculated surplus evaporation (0.2 × 10~(12)m~3/yr) and sill flow (0.3-1.1 × 10~(12)m~3/yr) are not identical, but they could be oceanographically considered as similar. The comparison between both values implies that most of the throughflow ultimately escaped the East Sea through the evaporation process, during the LGM. The regional sea level in the almost isolated East Sea might be largely maintained by a rough balance between incoming throughflow and outgoing evaporation during the LGM. The geographic restriction due to lowered sea level and lower surface salinity by limited vertical mixing in the glacial East Sea are analogous to modern oceanographic features in the Black Sea.