Mid-Pliocene subtropical front variability in the Southern Ocean

The mid-Pliocene (3.3-3.0 Ma) is a time when the Earth's climate fluctuated between cold glacial conditions (marine isotope stage M2; 3.3 Ma) and periods when global temperatures were ~3°C warmer than the pre-industrial (Mid-Pliocene warm period; 3.3-3.025 Ma) when CO₂ concentrations reached ~400 ppm. Thus, the paleoclimate reconstruction of this time interval provides an analogue of the present-day and near-future climate change in a moderate pCO₂ increase scenario. Although fluctuations in benthic δ¹⁸O in the mid-Pliocene were predominantly associated with Northern Hemisphere glacial dynamics, the contribution of Antarctic ice to mid-Pliocene glacial-interglacial cyclicity is unknown. Moreover, the surface oceanographic response of the Southern Ocean to Pliocene glacial-interglacial climate change is poorly documented

We studied 2 sedimentary records from offshore west Tasmania (ODP Site 1168) and the Agulhas Plateau (IODP Site U1475), both located close to the modern position of the subtropical front (STF) in the Southern Ocean and encompassing the mid-Pliocene. The STF is a crucial surface water mass boundary separating cold, fresher subantarctic waters and warm, more saline subtropical waters and plays a key role in global ocean circulation, ocean-atmosphere CO₂ exchange and meridional heat transport.

We use lipid biomarkers, dinoflagellate cyst assemblages and benthic foraminiferal clumped isotopes to reconstruct surface and bottom oceanographic conditions over the mid-Pliocene including the M2 glaciation. We identify similar sea surface temperature (SST) changes at the two sites. Site 1168 SST cools from 18°C to 12°C and at Site U1475 from 21°C to 18°C across the M2 glaciation. Dinoflagellate cyst assemblages suggest strong latitudinal shifts of the subtropical front associated to Pliocene glacial-interglacial climate changes. However, the most profound assemblage shift occurs at the M2 deglaciation stage at both sites, suggesting strong and unprecedented surface water freshening. Preliminary clumped isotope results suggest bottom water temperatures at Site 1168 are stable around 9°C between M2 and mid-Piacenzian warm period, indicating that the enrichment in δ¹⁸O across the M2 is mainly contributed by large ice volume changes. We interpret the surface water freshening of the subantarctic zone as signaling major iceberg calving following the M2 glaciation, suggesting that the Antarctic contribution to the M2 glaciation was profound.