[M-IS08] Global climate change driven by the Southern Ocean and the Antarctic Ice Sheet

[MIS08-P09] Diatom turnover during the Pliocene Warm Period in the Ross Sea in relation to paleooceanographic change

*Saki Ishino¹, Osamu Seki², Masanobu Yamamoto³, IODP Expedition 374 Scientists (1.National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan, 2.Institute of Low Temperature Science, Hokkaido University, 3.Faculty of Environmental Earth Science, Hokkaido University)

Keywords:Diatom, the Southern Ocean, Paleoceanography, Pliocene, Micropaleontology

In the Southern Ocean, major peaks of diatom species turnover (origination and extinction rate) are known to occur in association with million-year-scale climate change. Changes in open-ocean habitat restricted by the ice shelf and sea ice extent are interpreted to have affected the diatom macroevolutionary processes. However, few studies have focused on the comparison of diatom evolutionary events and detailed marine paleoenvironmental changes on continental shelves and slopes, where ice shelf and sea ice coverage change dramatically, because most cores preserve discontinuous records and precise constraints of sedimentation ages are difficult.
In the Ross Sea, the first occurrences of Fragilariopsis bohatyi group concentrated in 3.0–3.5 Ma and plays a part in the significant peak of diatom turnover however, their paleoecological characteristics have been poorly understood. The International Ocean Drilling Program (IODP) Exp. 374 drilled Hole U1524A core, which contains continuous Plio-Pleistocene records in super high resolution on the continental slope. In this study, diatom assemblage analysis was conducted to clarify the occurrence process of Fragilariopsis bohatyi group. The fluctuation of Fragilariopsis bohatyi group abundance was compared with paleoceanographic interpretation based on fossil diatom indicators and biomarkers.
As a result of diatom valve counting and assemblage analysis, “in-situ” species valves occupied assemblages between 233 and 245 m CSF-A (ca. 3.21–3.16 Ma), and fluctuation of environmental indicator indicated rapid warming followed by gradual cooling environment. At the short interval at 245 m CSF-A, a significant increase in SST calibrated by TEX86 paleothermometry and the sharp decrease in hydrogen isotope values of C₁₈FA are recognized suggesting rapid warming and substantial input of ice-sheet meltwater. The relative abundance of the Fragilariopsis bohatyi group increased dramatically at 245 m CSF-A. These results infer that the existence of an open water environment accompanying rapid warming triggered the occurrence of Fragilariopsis bohatyi group.