The explanation of the large atmospheric CO₂ fluctuations that accompanied the glacial-interglacial cycles of the late Pleistocene represents an important test of our understanding of carbon cycle dynamics and feedbacks. It may also represent an important step towards an explanation of glacial-interglacial climate variability itself. The patterns of atmospheric CO₂ and radiocarbon (?¹⁴C_atm) variability across the last deglaciation strongly suggest that the entire inventory of ‘excess’ carbon retained by the glacial ocean was remobilised very rapidly indeed, within just a few millennia. The rapidity of this remobilisation, and in particular the ?¹⁴C_atm evidence for the release of radiocarbon depleted CO₂ into the atmosphere (which cannot easily be explained by a marked reduction in biological carbon export from the sea surface for example), strongly suggests that a physical or dynamical ‘barrier’ between the deep ocean and the atmosphere was rapidly removed during deglaciation. Here we present new radiocarbon and neodymium isotope evidence from different sectors of the Southern Ocean that sheds light on the possible existence of such a ‘dynamical barrier’ and describes its evolution across the last deglaciation.