No single mechanism can account for the full amplitude of past atmospheric carbon dioxide (CO2) concentration variability over glacial–interglacial cycles1. A build-up of carbon in the deep ocean has been shown to have occurred during the Last Glacial Maximum2,3. 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 CO2 concentration before the Last Glacial Maximum, have remained unclear. Here we present sedimentary redox-sensitive trace-metal 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 CO2 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 CO2 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 CO2 concentration.
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S.L.J. and A.M.-G. were funded by the Swiss National Science Foundation (grants PP00P2-144811 and PZ00P2_142424, respectively), E.D.G. by NSERC, and R.F.A. by the US NSF. Sediment samples were provided by the core repository at the Lamont-Doherty Earth Observatory. Computational resources were provided to E.D.G. by Compute Canada and the Canadian Foundation for Innovation. We thank C. Buizert, H. Fischer, F. Herman and T. Pedersen for discussions.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Idealized model experiments illustrating the impact of AABW production on dissolved oxygen relative to the core locations.
Shaded contours show the difference in dissolved oxygen (∆O2) averaged between 25° W and 10° E, for a coupled model simulation with strong Weddell convection compared to a simulation with moderate Weddell convection (Methods). Squares indicate the location of sediment cores TN057-13PC and TN057-14PC.
Extended Data Figure 2 Biogenic particle flux reconstructed by 230Th normalization for four independent proxies covering the last glacial termination at site TN057-13PC.
a, 230Th-normalized total organic carbon flux43. b, 230Th-normalized CaCO3 flux. c, 230Th-normalized biogenic barium (bioBa) flux. d, 230Th-normalized biogenic opal flux29. CaCO3 and bioBa data are from this study. The accumulation of biogenic CaCO3 above glacial background values during HS1 and the YD is consistent with enhanced ventilation of bottom waters during these intervals. Enhanced ventilation of bottom waters would have lowered the regenerated DIC concentration of the bottom water by releasing excess CO2 to the atmosphere, raising the [CO32−] (ref. 44) and calcite saturation state of the bottom water9 and thus reducing CaCO3 dissolution.
Red/black dots indicate the location of sedimentary records for which authigenic U concentrations/mass accumulation rates were higher/lower during the LGM when compared to the Holocene, respectively. White dots highlight cores where authigenic U concentrations did not change much between these two intervals (see Extended Data Table 1 for details). Shadings show the modern bottom water dissolved oxygen concentrations26.
Extended Data Figure 5 Comparison of bulk sediment accumulation rates and authigenic U concentrations in sediment core TN057-14PC for the interval 20–80 kyr ago.
a, Bulk sediment accumulation rates (SedRate). b, Authigenic U concentrations.
a, δ18Opachy (where ‘pachy’ refers to the planktonic foraminifera Neogloboquadrina pachyderma) (ref. 46) and 230Th-normalized biogenic opal flux28 in core TN057-14PC. b, δ18Opachy (ref. 47) and 230Th-normalized biogenic opal flux28 in core TN057-13PC. The grey shading highlights the disturbed portion of core TN057-14PC. The black triangle highlights the presence of planktonic foraminifera deposited during the Younger Dryas (that is, 12.18 kyr ago; ref. 46), which have been mixed down into late LGM sediments after the hiatus occurred.
Extended Data Figure 7 Cross-plot of Mn/Al and authigenic U across the last glacial termination in sediment core TN057-13PC.
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Jaccard, S., Galbraith, E., Martínez-García, A. et al. Covariation of deep Southern Ocean oxygenation and atmospheric CO2 through the last ice age. Nature 530, 207–210 (2016). https://doi.org/10.1038/nature16514
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