LAUSR

Abstract Fossil diatom records from the Southern Ocean are needed to clarify the detailed conditions under warmer-than-present global climate in the Piacenzian Stage (3.60–2.588 Ma), and to provide direct evidence of simultaneity between changes in environmental conditions and diatom species turnovers. In this study, fossil diatom records between 3.5 and 2.8 Ma were investigated at two sites located along a latitudinal gradient within the Indian sector of the Southern Ocean (offshore from Prydz Bay and south-east of the Kerguelen Plateau). Paleoceanographic changes were assessed from assemblage changes of extant diatom indicators. Fluctuations of their relative percentages were compared with those of extinct diatom species. The diatom assemblage between 3.5 and 3.1 Ma consisted of open ocean species which indicate warmer-than-present SST. Notable increases in sea-ice associated species were identified at 3.3 Ma and 3.1 Ma at both sites. Our results suggest sea-ice existence during short intervals of 3.3 Ma and 3.1–2.8 Ma at the studied area. At both sites, relative abundances of species which have first occurrences during the Piacenzian, increased at 3.2–3.1 Ma, conversely, those of species which have last occurrences during the Piacenzian decreased at the same period. These diatom records show a correlation with cooling at 3.1 Ma suggested from extant indicators and compositional changes in fossil diatoms from the Southern Ocean. This comparison can provide insights into the environmental effects on diatom evolutionary acceleration, which can help identify ecologies of extinct species.