17:15 〜 19:15
[MIS15-P14] ウェッデル海（ODP Site 689）で得られた前期〜中期中新世の珪藻化石記録
キーワード：珪藻、黄金色藻シスト、中新世、南極氷床
Since the Antarctic region has a significant influence on global climate, paleoenvironmental reconstruction in the Southern Ocean back to the geological past is essential for understanding recent climate change. In addition, several time intervals characterized by warmer temperature and higher atmospheric CO2 than present are of interest in discussing future climate, and the Middle Miocene climatic optimum (ca. 17–15 Ma) is one example. In this study, to contribute to our understanding of how the Southern Ocean and Antarctic ice sheet changed from the earliest Miocene to the middle Miocene, I analyzed siliceous microfossil assemblages from a sediment core, Ocean Drilling Program Leg 113 Hole 689B, collected from the Weddell Sea, Atlantic Sector of the Southern Ocean. As a result, a subtropical species Thalassionema nitzschioides var. parva, which is now distributed in lower latitudes than the Antarctic Circumpolar Current (subtropical region), was abundant with fluctuations between about 20 Ma and 15 Ma. This indicates that the Antarctic Circumpolar Current at that time behaved as a weaker thermal barrier than at present, or that the latitudinal thermal gradient was weaker. At about 14 Ma, the occurrence of cold-water taxa and an increase in the diversity of the diatom assemblage were observed. In addition, fossil chrysophyte cysts (fossil remains of freshwater algae), which may indicate inflow of glacial melt water, also became abundant at the same time. These results suggest that the West Antarctic ice sheet began to develop at about 14 Ma.