Pronounced mid-Pleistocene southward shift of the Polar Front in the Atlantic sector of the Southern Ocean

Ocean Drilling Program (ODP) Site 1094, situated in the Atlantic sector of the Antarctic region below the Polar Front (PF), is dominated by silica-rich sediments during the Pleistocene, but does contain occasional calcium carbonate-rich intervals. The analysis of the coccolithophore assemblages of one of these intervals in the mid-Pleistocene allows us to interpret a prominent southward displacement of the PF. An age model constructed using paleomagnetic and biostratigraphic information places this event around 1 Ma (close to the Jaramillo subchron) in an interval of strong eccentricity and high amplitude in the insolation record.We distinguish five intervals based on coccolithophore assemblage and abundances. According to the age model used, this calcium carbonate-rich period should include Marine Isotope Stage 31, the most significant interglacial period recorded at this time; however, this model cannot decipher if adjacent isotope stages are partially represented. Interval V (the oldest) is interpreted as an episode with progressive abundance of coccolithophores with a characteristic assemblage from the Polar Front Zone (PFZ) but with dominance of siliceous micro-organisms. Interval IV contains the maximum abundance of coccolithophores with an assemblage characteristic of the Subantarctic Zone (SAZ), with peaks in abundance of Calcidiscus leptoporus, Coccolithus pelagicus, Helicosphaera carteri and Syracosphaera spp. related with the proximity of the Subantarctic Front (SAF). The southward displacement of the frontal system, linked to an increase in sea-surface temperature produced increases in diversity and in productivity. Conversely, we consider that the presence of some coccolithophore taxa, linked in other environments with anomalies in salinity, could reflect a pulse in the melting of West Antarctic Ice Sheet, as suggested previously for this interval. Interval III is characterized by a PFZ assemblage reflecting again a northward shift of the PF. Interval II is considered in this context as another southward pulse, but less important than Interval IV and with the absence of SAF and/or melting-related taxa. Interval I is considered a transition from a situation similar to the present-day PFZ to the Antarctic Zone conditions as consequence of a northward displacement of PF and Weddell Gyre/Antarctic Circumpolar Current boundary.