Study on Paleoceanography during last two million years based on the analysis of cold water nannofossil

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The Pliocene to Pleistocene geographical distributions of calcareous nannofossils is analyzed using DSDO-ODP samples collected from the Pacific, Atlantic, and Indian Ocean to reconstruct paleoceanographic condition during last two million years. The distribution patterns of Coccolithus pelagicus in the equatorial Pacific and Caribbean Sea is interpreted as evidence of new current system, which was created by the appearance of the Isthmus of Panama in 2.75Ma. This event is strongly related to both the drop of the sea level recognized in the Bahama region in the Caribbean Sea, and increasing of the sea ice in the Arctic region.Focusing to this event, calcareous nannofossils in the samples collected from the low to high latitude of the Pacific Ocean are anakyzed. The assemblages characterized by the cold water species are limited in the higher latitude region. Offshore Iwaki, Miyakojima Island, and ODP Holes in the middle to low latitude regions are characterized by the warm water nannofossils such as Discoasters. The boundary between these paleobiogeography is situsted in the 40° before 2.75Ma. However, the evidence of abundancy of Coccolithus pelagicus indicates the occurrence of southward movement of this boundary after 2.75Ma.