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

利用从太平洋、大西洋和印度洋采集的DSDO-ODP样品，分析了上新世至更新世钙质超微化石的地理分布，以重建过去200万年的古海洋条件。赤道太平洋和加勒比海的水鞘分布模式被解释为新的洋流系统的证据，这种新的洋流系统是由2.75 Ma巴拿马地峡的出现而产生的。该事件与加勒比海巴哈马海平面的下降和北冰洋海冰的增加密切相关，并据此对太平洋低、高纬度样品中的钙质超微化石进行了分析。以冷水物种为特征的组合在高纬度地区较为有限。近海Iwaki、宫古岛和中低纬地区的ODP洞以Discoaster等温水超微化石为特征。这些古生物地理的分界线位于2.75 Ma之前的40°。然而，巨型球藻丰度的证据表明，这一界线在2.75 Ma之后发生了南移。

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.