North Pacific deoxygenation events during the last deglaciation were sustained over millennia by high export productivity, but the triggering mechanisms and their links to deglacial warming remain uncertain 1-3 . Here we find that initial deoxygenation in the North Pacific immediately after the Cordilleran Ice Sheet (CIS) retreat 4 was associated with increased volcanic ash in seafloor sediments. Timing of volcanic inputs relative to CIS retreat suggests that regional explosive volcanism was initiated by ice unloading 5,6 . We posit that iron fertilisation by volcanic ash 7-9 during CIS retreat fuelled ocean productivity in this otherwise iron-limited region, and tipped the marine system toward sustained deoxygenation. We also identify older deoxygenation events linked to CIS retreat over the past ~50,000 years 4 . Our findings suggest that the apparent coupling between the atmosphere, ocean, cryosphere and solid earth systems occurs on relatively short timescales and can act as an important driver for ocean biogeochemical change.Reduction of dissolved oxygen (i.e., deoxygenation) in the subsurface ocean is underway and projected to worsen if modern warming trends persist 10 . This deoxygenation will have strong impacts on marine ecosystems, especially in regions that have low-oxygen backgrounds such as the Oxygen Minimum Zones (OMZs) of the North and East Pacific 11 . Identification of the mechanisms that trigger and sustain long-term deoxygenation is problematic in short modern observational records because of interannual-to-decadal variability 10 , which provides impetus for study of mechanism driving past sustained ocean deoxygenation. Among the best known of such