石灰質ナンノ化石からみた北大西洋海域の古海洋環境変動－IODP Site U1314の調査から－

Calcareous nannofossils were observed to reconstruct surface water conditions during the late Pliocene sediments (2.55 to 2.88 Ma) from the southern Gardar Drift, in the subpolar North Atlantic IODP Site U1314 (56°21.9’1N, 27°53.3’W) at an average resolution of 3 kyr. Based on quantitative analyses twenty-four species belonging to fourteen genera are identified and are rare to very abundant with moderate to good preservation. One calcareous nannofossil datum event is found within the samples interval, which is the Last Occurrence (LO) of Discoaster tamalis in sample B28H-2A, B at 272.81 mcd. It correlates with Datum A discussed by Sato et al. (2004) which is considered strongly related to the heavy glaciation in the Northern Hemisphere. The datum event dates to an age 2.756 Ma, situated within upper Gauss Chron or just below MIS G7. Based on the nannofossil event that is found at IODP Site U1314, the studied interval belongs to Zone NN16 (Martini, 1971). However, referring to Okada and Bukry (1980) the studied interval is included within two Subzones, which are Subzones CN12b and CN12a as defined by the LO of D. tamalis. In general, the coccolith assemblages are dominated by species belonging to genus Reticulofenestra with different (sized-defined) morphotype. Thus the significant changes in paleoceanographic condition are mainly shown by the ratio of Reticulofenstra morphometry.Calcareous nannofossil assemblage, magnetic susceptibility (MS) and natural gamma radiation (NGR) records in the IODP Site U1314 indicate that the surface ocean condition was very stable with a warm and oligotrophic condition prior to ~2.76 Ma. It is characterized by the dominance of medium and large Reticulofenestra with low detrital input to the site area (low NGR values). The size variations of Reticulofenestra are uni-modal indicated by coccolith size of 3 to 4μm. This suggests an extension of Mid-Pliocene warm influence and the final closure of Central American Seaway (CAS) was incomplete. The warm current originates in eastern equatorial Atlantic flowed to the Pacific Ocean and the northward surface currents were weak with no ice sheets covered the northern hemisphere when the CAS was still opened. The main source of sediment in the Gardar Drift is pelagic sediment that has been eroded and redeposited by the southward flowing Iceland‐Scotland Overflow Water (ISOW) and Northeast Atlantic Deep Water (NEADW) (Grützner and Higgins, 2010). It is reflected with variations of MS values compare with the isotope oxygen stratigraphy. Before ~2.76 Ma, no climatic changes are found in the Arctic Ocean. Therefore the changes of oxygen isotope were only controlled by Antartic ice sheet volume changes. The increased sea ice extent in Antarctic or the Southern Ocean resulted in increased northward influx of Antarctic Bottom Water (AABW) formation and intensified Lower Deep Water (LDW).At around 2.76 Ma the studied site was marked by the collapse of sea surface stability with strong mixing condition and increased non-magnetic minerals (possibly IRD; Hayashi et al., 2010). It reflects the intensification of Northern Hemisphere Glaciation (NHG) and correlates with the final closure of CAS. This condition is characterized by an abrupt decrease of the maximum coccolith of Reticulofenestra specimens and short-term decrease and increase of MS and NGR values (Hayashi et al., 2010). Afterward as the Quaternary climate style established, the relative abundance of Reticulofenestra coccolith, especially specimens with coccolith size <2 μm and 3-4 μm show a positive and negative correlation to the isotope stratigraphy of Lisiecki and Raymo (2005). The small Reticulofenestra specimens (<2 μm) seem to increase in several colder phases or glacial stages (positive correlation), whereas the Reticulofenestra group of 3-4 μm shows decreasing in abundance during the glacial stages (negative correlation). The orrespondence between those two types of specimens is might be having different preference ecology or different species. The size variations of Reticulofenestra during this period are cyclic changes from uni-modal to bimodal. During interglacial stages, larger Reticulofenestra specimens characterize late Pliocene of subpolar North Atlantic Ocean with warmer and stable ocean condition. It ends with shifting dominance larger Reticulofenestra specimens to smaller size during glacial stages. It is represented by unstable sea surface and mixing condition. The smaller specimens of Reticulofenestra specimens are likely more adaptable to unfavorable conditions, relative to larger specimens, in exploiting limited resources. In the study area, the mixing condition or higher nutrient input might be due to increase of ice sheets that lead to higher velocity of sea surface circulation causes a collapse of sea surface waters, IRD transport and deposition that are triggering eutrophic conditions, the southward migration of the subpolar front, or combination of all factors. Some delays are observed in the peak of smaller Reticulofenestra (