1. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
2. School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W 3P6, Canada
3. Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA
4. Geological Institute, Department of Earth Sciences, ETH Zürich, Zürich CH-8092, Switzerland
5. Department of Ocean Sciences, University of California, Santa Cruz, California 95064, USA
6. Department of Earth System Science, University of California, Irvine, California 92697, USA
7. Present address: Department of Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey 08544, USA.

Correspondence to: Eric D. Galbraith^1,7 Correspondence and requests for materials should be addressed to E.D.G. (Email: egalbrai@princeton.edu).

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods^{1, 2, 3, 4, 5}, but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal⁶. Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that—together with previously published results⁷—show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000–15,000 years ago)^{4, 8}, we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition 14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration⁸. We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic^{9, 10, 11}, which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir^{1, 2, 3, 4, 5, 12}, but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.

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