Near the Eocene's close (∼34 million years ago), the climate system underwent one of the largest shifts in Earth's history: Antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed. Previous studies conjectured that poleward penetration of warm, subtropical currents, the East Australian Current (EAC) in particular, caused Eocene Antarctic warmth. Late Eocene opening of an ocean gateway between Australia and Antarctica was conjectured to have disrupted the EAC, cooled Antarctica, and allowed ice sheets to develop. Here we reconstruct Eocene paleoceanographic circulation in the Tasmanian region, using (1) biogeographical distributions of phytoplankton, including data from recently drilled Ocean Drilling Program Leg 189 sites and (2) fully coupled climate model simulations. We find that the EAC did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern. Our results do not support changes in “thermal isolation” as the primary driver of the Eocene‐Oligocene climatic transition.

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A new method for calculating water mass transport between different ocean basins from the velocity fields obtained by numerical models is presented. The method is applied to the velocity field of the Southern Ocean simulated by a primitive equation model (fine resolution Antarctic model). With this method it is possible to judge whether a water mass has been ventilated or not, to estimate how many times it has circled around Antarctica, and to calculate the time it has spent in the Southern Ocean. Calculations have also been undertaken revealing to what extent the changes of temperature, salinity, and density have been caused by mixing and by ventilation. Two major ways to redistribute the water through the Southern Ocean are identified. The first one redistributes 53% of the water and involves an un ventilated direct exchange between the oceans, the second one redistributes 33% by going around Antarctica. It is found that, on average, the water mass makes six circuits before the water is ventilated and subsequently driven to the north by the Ekman transport. A heat transport study is carried out for the Atlantic, showing that the northward heat transport into the Atlantic comes 85% from the Indian Ocean and the rest from the Drake Passage.