LettersGlacial Southern Ocean freshening at the onset of the Middle Pleistocene Climate Transition

Changes in Southern Ocean hydrography may have played an important role in the Middle Pleistocene Transition (MPT), particularly through their impact on ocean circulation and atmospheric CO2 concentrations. Here we present foraminiferal Mg/Ca and δ18O results for the subsurface dwelling planktonic foraminifer Neogloboquadrina pachyderma (sinistral) at the Ocean Drilling Program (ODP) Site 1090. Results are used to reconstruct upper ocean temperatures and derive seawater δ18O in the Subantarctic Atlantic Ocean during the MPT. The new records indicate that, starting at ∼1250 ka, glacial temperatures and local (ice volume corrected) seawater δ18O in the upper water column of the Subantarctic Atlantic Ocean decreased, pointing to cooler (∼2 °C) and fresher (∼0.4‰) conditions than in the preceding glacial stages. These upper ocean hydrographic changes broadly coincide with the increase in the power of the 100 ky glacial-interglacial cycle in both records and with a shift towards decreased deep ventilation in the glacial Southern Ocean. Our finding suggests that an increase in Southern Ocean stratification, driven by the observed freshening of the upper water column, may have reduced the exchange of carbon between the deep Southern Ocean and the atmosphere during glacial stages. This process may have contributed, in combination with other mechanisms, to lower glacial atmospheric CO2 concentrations during the MPT.

► Cooling and freshening of the Subantarctic upper ocean during the MPT. ► Glacial Southern Ocean (SO) stratification during the MPT. ► SO physical and biological interactions, with implications on G/I pCO2 cycling.