No single mechanism can account for the full amplitude of past atmospheric carbon dioxide (C[O.sub.2]) concentration variability over glacial-interglacial cycles (1). A build-up of carbon in the deep ocean has been shown to have occurred during the Last Glacial Maximum (2,3). However, the mechanisms responsible for the release of the deeply sequestered carbon to the atmosphere at deglaciation, and the relative importance of deep ocean sequestration in regulating millennial-timescale variations in atmospheric C[O.sub.2] concentration before the Last Glacial Maximum, have remained unclear. Here we present sedimentary redox-sensitive tracemetal records from the Antarctic Zone of the Southern Ocean that provide a reconstruction of transient changes in deep ocean oxygenation and, by inference, respired carbon storage throughout the last glacial cycle. Our data suggest that respired carbon was removed from the abyssal Southern Ocean during the Northern Hemisphere cold phases of the deglaciation, when atmospheric C[O.sub.2] concentration increased rapidly, reflecting--at least in part--a combination of dwindling iron fertilization by dust and enhanced deep ocean ventilation. Furthermore, our records show that the observed covariation between atmospheric C[O.sub.2] concentration and abyssal Southern Ocean oxygenation was maintained throughout most of the past 80,000 years. This suggests that on millennial timescales deep ocean circulation and iron fertilization in the Southern Ocean played a consistent role in modifying atmospheric C[O.sub.2] concentration.

Ice-core and surface-ocean proxy records provide a wealth of evidence supporting a central role of the Southern Ocean in modulating the air-sea partitioning of carbon during ice ages (4,5). The Southern Ocean could exert a substantial control on the partial pressure of C[O.sub.2] ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]), owing to its leverage on the efficiency of the global soft- tissue pump (2) (STP) by which the photosynthetic production, sinking and remineralization of organic matter store dissolved inorganic carbon (DIC) in the ocean interior. At present, vertical exchange of water causes deeply sequestered C[O.sub.2] and nutrients to be brought rapidly to the Southern Ocean surface, while iron (Fe) limitation of phytoplankton prevents the exposed nutrients from being entirely fixed back to organic matter before the water sinks again, allowing a net release of C[O.sub.2] to the atmosphere. Either decreasing the rate of vertical exchange or enhancing export production would raise DIC concentrations in deep waters, lowering atmospheric C[O.sub.2]. Higher DIC, in turn, enhances deep water corrosivity, dissolving carbonate (CaC[O.sub.3]) minerals and thereby increasing ocean alkalinity, which produces a further C[O.sub.2] drawdown (2).

Phytoplankton productivity in the Subantarctic Zone of the Southern Ocean surface was stimulated during glacial periods by an enhanced supply of Fe-bearing dust, which would have reduced the leakage of C[O.sub.2] to the atmosphere, strengthening the overall STP (6,7). Meanwhile, radiocarbon measurements on deep-sea corals and benthic foraminifera have been interpreted as showing weakened exchange between the surface and the deep Southern Ocean during the Last Glacial Maximum (LGM) (3,8); if this is correct, such a weakened exchange would have complemented the effect of dust (9). From the LGM to the Holocene, the combination of dwindling dust inputs and accelerated vertical exchange in the...