| Budget Amount *help |
¥3,800,000 (Direct Cost: ¥3,800,000)
Fiscal Year 2005: ¥200,000 (Direct Cost: ¥200,000)
Fiscal Year 2004: ¥1,700,000 (Direct Cost: ¥1,700,000)
Fiscal Year 2003: ¥1,900,000 (Direct Cost: ¥1,900,000)
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| Research Abstract |
All materials treated in this study were collected from deep-sea floor sediments in the North Pacific Ocean via the dill ship JOIDES resolution assigned to the Ocean Drilling Program. Three areas along the middle latitudes are chosen from west to east ; Sanriku-oki representing the western boundary current area, California margin from the eastern area, and Shatsky Rise for connecting with the others. I analyzed marine deposits consisting of microfossils and clastic particles from Miocene through Pleistocene Epoch, and I compared the stratigraphic distributions of diatom frequency based on morphology among three areas.
Round shape diatoms became dominant species in the Sanriku-oki offing, but there were many upwelling current types with round shape in Shatsky Rise. Three taxon groups involving round shape, upwelling current type and boat-shaped type took turns at occupying the position of dominant species off California margin. Stable continuation of environments brought round shapes to
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a level of regular excellence in Sanriku-oki offing. On the other hand, changeable environments in which coastal upwelling current happened sporadically were given off California margin. Although composition and frequency of fossil assemblages are different from each other, the outward appearance of change is similar in increase or decrease among three areas. There was a considerable distance between areas, however, it seems that their areas monitored influence of the same current in the North Pacific Ocean through time.
On both of Shatsky Rise and California margin, diatoms replaced radiolarians as the summit of individual number at 8 Ma. Siliceous microfossil assemblages consist mainly of diatoms and radiolarians, and those found before 8 Ma are named Assemblage-1, those after 8 Ma are called Assemblage-2, respectively. It is supposed that the microfossil transformation at 8 Ma corresponds with a strict change in ocean current system supervising from the surface to the bottom of the North Pacific due to closing of the Indonesian Gateway. In the cycle of increase or decrease of both diatoms and radiolarians, each period shortens from about 3 Ma and the amplitude got violent. The boundary between cycle changes subdivides siliceous assemblages into the early Assemblage-2a and the late Assemblage-2b. It is thought that a succession from Assemblage-2a through Assemblage-2b reflects reinforcement for the Northern Hemisphere glaciation becoming stronger from 3 Ma. Less
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