This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by Nature Publishing Group. ; Changes in deep ocean ventilation are commonly invoked as the primary cause of lower glacial atmospheric CO_2. The water mass structure of the glacial deep Atlantic Ocean and the mechanism by which it may have sequestered carbon remain elusive. Here we present neodymium isotope measurements from cores throughout the Atlantic that reveal glacial-interglacial changes in water mass distributions. These results demonstrate the sustained production of North Atlantic Deep Water under glacial conditions, indicating that southern-sourced waters were not as spatially extensive during the Last Glacial Maximum as previously believed. We demonstrate that the depleted glacial ?^13C values in the deep Atlantic Ocean cannot be explained solely by water mass source changes. A greater amount of respired carbon therefore must have been stored in the abyssal Atlantic during the Last Glacial Maximum. We infer that this was achieved by a sluggish deep overturning cell, comprised of well-mixed northern- and southern-sourced waters. ; Sample material was provided by the Godwin Laboratory for Paleoclimate Research at the University of Cambridge, the International Ocean Discovery Program, the GeoB Core Repository at the MARUM ? Center for Marine Environmental Sciences, University of Bremen and Petrobras. Jo Kerr and Aurora Elmore are thanked for providing additional samples. The data reported in this paper are listed in supplementary information and archived in Pangaea (www.pangaea.de). Thiago Pereira dos Santos is thanked for providing the unpublished age model data for GL1090; Jo Clegg and Vicky Rennie are thanked for technical support and Natalie Roberts for helpful discussions. Radiocarbon analyses were supported by NERC radiocarbon grant 1752.1013 and Nd isotope analyses by NERC grant NERC NE/K005235/1 and NERC NE/F006047/1 to AMP. JNWH was supported by a Rutherford Memorial Scholarship. SM was funded through the DFG Research Center/Cluster of Excellence ?The Ocean in the Earth System?. CMC acknowledges financial support from FAPESP (Grant 2012/17517-3).