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[MIS15-15] Planktic foraminiferal assemblages track fronts' migrations in the Indian sector of the Southern Ocean over the past 1.1 million years
Keywords:Southern Ocean, Polar Front, Foraminifera
Oceanic fronts such as the Subantarctic Front (SAF), Polar Front (PF), and Southern Antarctic Circumpolar Current Front (SACCF) separate different oceanic realms of the Southern Ocean. Of these, the PF is a distinct boundary between cold, fresh waters to the south and warm, saltier waters to the north with important impact on ocean circulation, ecosystems, and ocean-atmosphere CO2 fluxes. It is therefore of prime importance to document past PF migrations. The past PF position can be tracked from ocean temperature reconstructions, laminated diatom mat presence, and major planktic foraminifera species' abundance (i.e., Neogloboquadrina pachyderma, Globigerina bulloides). In this study, we investigated planktic foraminiferal assemblages in core MD19-3576 (46°S, 44°E, water depth 2419 m, core length 57.6 m) collected on Del Caño Rise in the Subantarctic Zone of the Indian sector of the Southern Ocean to estimate past changes in the relative position of the PF over the past 1.1 million years (1100 ka).
A total of 201 samples were examined, and 19 species in 10 genera of planktic foraminifera were identified. From 1100 to 430 ka, the number of species ranged from 2 to 9, and the polar species N. pachyderma dominated (43.5%–97.6%), with lower abundances observed during the interglacial stages. Concomitantly, the subpolar species Turborotalita quinqueloba (0%–22.1%) and temperate species G. bulloides (0%–38.8%) increased. From 430 ka to the present, the number of species ranged from 5 to 12, and abundant species alternated. Neogloboquadrina pachyderma (8.9%–93.8%) dominated glacial periods while G. bulloides (1.8%–58.4%) was more abundant during interglacial periods. The subtropical species Globorotalia crassaformis abundances peaked during Marine Isotope Stage (MIS) 11 showing warmest conditions through the studied interval.
Based on the foraminiferal assemblage, the PF migrated northward (southward) during glacials (interglacials) but with a mean position north of the studied site between 1100 and 430 ka. No transitional change was apparent during the Mid-Pleistocene Transition but the frontal system shifted south rapidly at the Mid-Brunhes Event around 430 ka. From 430 ka to the present, the PF was alternatively positioned north and south of the studied site during glacials and interglacials, with a southernmost position during MIS 11.
A total of 201 samples were examined, and 19 species in 10 genera of planktic foraminifera were identified. From 1100 to 430 ka, the number of species ranged from 2 to 9, and the polar species N. pachyderma dominated (43.5%–97.6%), with lower abundances observed during the interglacial stages. Concomitantly, the subpolar species Turborotalita quinqueloba (0%–22.1%) and temperate species G. bulloides (0%–38.8%) increased. From 430 ka to the present, the number of species ranged from 5 to 12, and abundant species alternated. Neogloboquadrina pachyderma (8.9%–93.8%) dominated glacial periods while G. bulloides (1.8%–58.4%) was more abundant during interglacial periods. The subtropical species Globorotalia crassaformis abundances peaked during Marine Isotope Stage (MIS) 11 showing warmest conditions through the studied interval.
Based on the foraminiferal assemblage, the PF migrated northward (southward) during glacials (interglacials) but with a mean position north of the studied site between 1100 and 430 ka. No transitional change was apparent during the Mid-Pleistocene Transition but the frontal system shifted south rapidly at the Mid-Brunhes Event around 430 ka. From 430 ka to the present, the PF was alternatively positioned north and south of the studied site during glacials and interglacials, with a southernmost position during MIS 11.