The deep ocean is the major store of heat and carbon in Earth's surface environment and thus has a major impact on climate. Waters that fill the deep ocean come from the North Atlantic and the Southern Ocean, but there is debate on their relative importance. Here we reconcile previous estimates using deep sea phosphate and oxygen data. We show that although a large volume of deep water comes from the south, this does not spend enough time in the southern surface to fully exchange heat and CO₂.