The modern δ¹³C_DIC distribution in southwest subtropical Pacific deep waters is consistent with a regional mixing regime between water masses of open Pacific Ocean and Tasman Sea origin. This mixing regime is reconstructed across the middle-late Pleistocene using a record of benthic foraminiferal δ¹³C in a sediment core from the New Caledonia Trough. The relative influence on the mixing regime from open Pacific Ocean deep waters is seen to be significantly reduced during glacial in comparison to interglacial stages over the past 1.1 Ma. The spatial δ¹³C gradient in the Southern Ocean between deep waters entering the Tasman Sea and the open Pacific Ocean is shown to be consequently greater during glacial than interglacial stages but was generally reduced across the period of the Middle Pleistocene Transition. The existence of strong spatial chemical gradients in the glacial Southern Ocean limits its capacity to act as an enhanced sink for atmospheric carbon.