PEAK GLACIAL C-14 VENTILATION AGES SUGGEST MAJOR DRAW-DOWN OF CARBON INTO THE ABYSSAL OCEAN

Ice core records of the past 800,000 years have clearly demonstrated glacial-interglacial atmospheric CO₂ variations on the order of 100 ppm. During the last deglaciation, an estimate of about 700 GtC is required to effect the rise in atmospheric CO₂ together with land carbon uptake, when the terrestrial biosphere was recovering from glacial conditions. The origin of this carbon is usually attributed to oceanic carbon release, although the detailed mechanisms remained elusive, since an adequately old and carbon rich deep ocean reservoir seemed unlikely. Here we present evidence for changes in the deep-ocean carbon reservoir from a new ocean-wide, though still fragmentary ¹⁴C data set showing that during the Last Glacial Maximum (LGM) and parts of Heinrich stadial 1 (HS-1) the maximum ¹⁴C age difference between ocean deep waters and the atmosphere exceeded the modern values by ~1000 to 1500 ¹⁴C yr, in the extreme by up to 4500 yr. In the modern ocean the apparent ¹⁴C ventilation age of water masses below 2000 m water depth appears directly linked to the concentr­ation of dissolved inorganic carbon (DIC) in seawater, with an aging by ~400 yrs corresponding to a rise of ~60 μmol/kg DIC. This link is summarizing various independent effects of carbon supply, the ’solubility pump’ that includes the gradual aging of deep waters and the ’biological pump’ that integrates plankton productivity, the local flux of organic carbon, and calcium carbonate dissolution. Applying the modern ¹⁴C-DIC relationship to the prolonged apparent residence time of LGM deep waters suggests that the deep ocean then was indeed able to absorb and store about 750 Gt DIC in addition to the modern carbon inventory.