Reconstruction of the thickness of the Tsushima current in the Sea of Japan during the Quaternary from molluscan fossils

The present distribution of the benthic fauna in the Sea of Japan reveals that the thickness of the warm Tsushima Current is about 150 m, because cold-water elements are found in bottom faunas in the underlying cold intermediate water below this depth. Identification of such cold-water taxa from Quaternary molluscan assemblages makes it possible to estimate the thickness of the Tsushima Current in the past. In order to determine the depth and nature of the cold-water fauna living below the warm surface current, molluscan and planktonic faunas were analyzed from the Omma Formation, which comprises at least 14 superposed cyclothems corresponding to oxygen isotope stages 50 to 22. on faunal analyses, one molluscan association that lived below the warm Tsushima Current was identified from the horizon equivalent to stage 47 (about 1.45 Ma). The depth of the upper limit of the association, that is, the thickness of the Tsushima Current, was estimated using sedimentary structures and oxygen isotope records from deep-sea cores. The result show that the thickness of the current was less than two-thirds that of today.