Title: Carbon storage and its release from the glacial ocean: leaky pumps and seesaws
Prof. Luke Skinner, University of Cambridge
It has long been postulated that the key contributor to lower atmospheric CO2 during past glacial periods was a more efficient marine biological pump, which sequestered more of the ocean’s carbon pool as respired carbon deep in the ocean’s interior. A further long-standing question is whether the putative increase in pump efficiency was achieved primarily by an increase in the ‘strength’ of the soft tissue pump (overall, or relative to the carbonate pump), or a reduction in its ‘leakiness’. The former would implicate a major role for changing nutrient supply to the ocean, in particular from atmospheric sources (e.g. dust-driven ‘iron fertilization’). The latter would implicate the ocean circulation and ocean-atmosphere gas exchange processes instead. However, these two regulators of biological pump efficiency are not mutually exclusive: both can operate together, and to greater effect.
This seminar consists of two parts: the first focuses on the causes of respired carbon sequestration in the ocean interior at the Last Glacial Maximum (LGM), while the second focuses on the release of this carbon to the atmosphere during deglaciation. In both instances, the role of the ocean circulation is emphasised, though not to the exclusion of other contributing mechanisms. Emerging radiocarbon data thus demonstrate quite clearly that the ocean was more poorly ‘ventilated’ at the Last Glacial Maximum, perhaps by the equivalent of ~700 14Cyrs on average. A significant impact on atmospheric CO2 is implied, and supported by new oxygenation and carbonate ion reconstructions, as well as simple scaling arguments and box-model experiments. During deglaciation, globally distributed records of radiocarbon, stable isotopes and oxygenation track the release of previously sequestered respired carbon back into the atmosphere. These records paint a picture of two ‘ventilation seesaws’ operating in the North Pacific and Southern Ocean, and apparently triggered via the North Atlantic. Together, and via their connection to opposing ventilation changes in the North Atlantic, these seesaws might help to explain both a ‘fast but limted’ and a ‘slow but strong’ component to deglacial atmospheric CO2 rise.