Carbon dioxide release from the North Pacific abyss during the last deglaciation

Out of the abyss Atmospheric carbon dioxide concentrations have varied significantly over the past two million years — they were relatively low during cold 'glacial' periods but high during the warm interglacial periods. The processes responsible for these variations remain obscure: it is thought that carbon may have been stored in the deep ocean during glacials, when deep water circulation was sluggish, and released during the transition into interglacial periods, as ventilation of the deep ocean increased, but direct evidence from this period is scarce. Galbraith et al . now use geochemical records from ocean sediment cores to shed light on the matter. They show that a poorly ventilated water mass that was rich in respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum, and that ventilation of the abyss increased during deglaciation, releasing the stored carbon dioxide. Geochemical records from ocean sediment cores are used to shed light on the composition and ventilation of water in the deep North Pacific during the Last Glacial Maximum. A poorly-ventilated water mass that was rich in respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum, and that ventilation of the abyss increased during deglaciation, releasing carbon dioxide into the atmosphere. 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 equivocal 6 . Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that—together with previously published results 7 —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