Toshiaki Takayama

Paleoenvironmental Changes in the Japan Sea During the Last 85,000 Years

Five distinct changes in the paleoenvironment of the Japan Sea within the last 85,000 years are revealed from the sedimentary record of a piston core recovered from the Oki Ridge. Changes in both surface and deepwater conditions are registered by changes in lithology, calcium carbonate content, organic carbon content, oxygen and carbon isotope ratios, and microfossil assemblages including calcareous nannoplankton, diatoms, radiolaria, and foraminifera. Between 85 and 27 ka the warm Tsushima Current did not flow into the Japan Sea, and cold surface water conditions prevailed. Environments at the seafloor fluctuated between dysaerobic to weakly oxic conditions. Between 27 and 20 ka, freshwater input to the Japan Sea, probably from the Huang Ho River in China, stratified the water column, and the severe anoxic conditions eliminated most benthic fauna. Between 20 and 10 ka the cold Oyashio Current flowed into the Japan Sea through the Tsugaru Strait, reestablishing deepwater ventilation. Shallow water benthic assemblages of the North Pacific Ocean subsequently colonized the Japan Sea and occupied the vacant niches of the deep basins. Between 10 and 8 ka the foraminifer compensation level (FCL) gradually rose to a depth shallower than 1000 m, and bottom conditions changed from dysaerobic to oxic. At 10 ka the warm Tsushima Current started to flow into the Japan Sea through the Tsushima Strait to establish the modern oceanographic regime which has existed since 8 ka. The eustatic sea level during the last glacial maximum was above the sill depths (130 m) of the Tsushima and Tsugaru straits, assuming that tectonic movements at these straits were negligible for the last 20 ka.

Quaternary time scale for the Ontong Java Plateau: Milankovitch template for Ocean Drilling Program Site 806

A 2 m.y. oxygen isotope record of Globigerinoides sacculifer from the Ontong Java Plateau, based on cores from Ocean Drilling Program Leg 130, is dated by matching variations to an orbital template. The procedure allows us to present the most complete Quaternary record available for the western equatorial Pacific. The template-generating algorithm describes a balance between growth and melting of ice. Following basic Milankovitch theory, ice growth is taken as constant, while melting is taken to depend on summer insolation, current ice mass, and average past ice mass. Template settings must be changed once, between 1 and 1.2 Ma, to reflect a major shift in climate. Template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances. One result of template dating is an exact age for the Brunhes-Matuyama magnetic reversal boundary, at 790 ±5 ka, as well as several other precise dates (900 ka for the middle Pleistocene climate shift; 1070, 1240, and 1450 ka for isotope stages 31, 37, and 47, respectively). Sedi- mentation rates fluctuate between 18 and 28 m/m.y., a ca. 400 ka cycle being the most prominent. Major anomalies arise within the transitional regime (1.2 to 1 Ma). The origin of the cycles is unknown; we propose productivity variations in the western equatorial Pacific.

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

Abstract 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. Based 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.