Reconstructing ocean conditions and latitudinal migration of oceanic fronts offshore Tasmania during the mid-Pliocene using high-resolution dinoflagellate cyst assemblages from IODP Site 1168A.
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The mid-Pliocene was characterised by a warm climate with similar CO2 concentrations to today but higher sea levels which suggests a contribution from the Antarctic Ice Sheet. This warm climate was disrupted by the transient M2 glaciation at 3.3 Ma before returning to warmer conditions during the mPWP (3.264 Ma). This glaciation has generally been seen as a Northern Hemisphere glaciation, but recent studies have shown evidence that the Antarctic Ice Sheet must have been more affected by warming and cooling during the Pliocene. Uncertainties remain on how large that glaciation was and what the ocean conditions around Antarctica were before 3.3 Ma. The Southern Ocean is strongly latitudinally banded, which is indicated by the 3 Southern Ocean fronts. Glacial-interglacial climate variability strongly moves those latitudinal bands north and south. Changes in the configuration of the Antarctic Ice Sheet impact the latitudinal location of these fronts, as they are linked through sea ice season. To better understand how ocean conditions in the Southern Ocean changed in association with the mPWP and the M2 glaciation, we use dinoflagellate cyst assemblages from Site 1168, on the western margin of Tasmania, to reconstruct latitudinal Subtropical Front movement. We found a significant dinocyst assemblage change across the M2 glaciation. Before and after the M2, the dinocyst assemblage was dominated by O. centrocarpum which is indicative of a position north of the STF, while during the M2, there was a transient N. labyrinthus assemblage dominance which is indicative for a position south of the STF. Such an assemblage change indicates a migration of the STF, of around 3-4° latitude. Thus, we infer that the Antarctic Ice Sheet must have been more dynamic in the mid-Pliocene than previously assumed. Based on the observed latitudinal migration of the Subtropical Front, the Antarctic ice sheet must have extended and decreased across the Pliocene. In addition, by comparing with biomarker-based SST, we found an offset between changing SST and the maximum response of the dinocyst assemblage which can be linked to influxes of freshwater during the deglaciation of the M2 affecting the maximum relative abundance of certain species. Overall, our data proved that the Antarctic ice sheet was more sensitive to the variable Pliocene climate.