ROGUE-14: Revisiting the ventilation paradigm

Revisiting the paradigm of ocean ventilation and glacial CO2 sequestration using radiocarbon

ROGUE-14 is a NERC-funded project (2021-2024) that aims to extend our understanding of global marine radiocarbon ‘ventilation’ back beyond the Last Glacial Maximum (LGM), to at least 35 ka BP. The goal is to test the hypothesis that the ocean was poorly ventilation well before the LGM, as required by the prevailing hypothesis regarding the role of ocean ‘ventilation’ in reducing atmospheric CO2 during past glacials.

a. Modelled atmospheric CO2 anomalies and associated changes in the ocean’s average radiocarbon ‘age’ (vs. the atmosphere), driven by changes in air-sea gas-exchange or transport/mixing in the ocean interior using a range of models (see Skinner et al., Climate of the Past, 2023). b. Observed atmospheric CO2 anomalies and associated changes in the mean ocean radiocarbon age (vs. the atmosphere), based on 3D interpolations of compiled data that have been averaged at a series of deglacial time-slices (see Skinner et al., 2023). The horizontal bar in plot b shows the atmospheric CO2 level observed during the last glacial period prior to the LGM, suggesting that a similar radiocarbon ‘ventilation age’ to the LGM would be expected.

The prevailing account of past glacial-interglacial atmospheric CO2 change attributes a major role to ‘ocean ventilation’ (encompassing both overturning rates and air-sea gas exchange efficiency). Marine radiocarbon data spanning the last ~20,000 years, from the LGM to the current interglacial period, support this paradigm: they demonstrate an increase in ocean ventilation across the last deglaciation that has been widely interpreted to reflect enhanced exchange of the marine- and atmospheric carbon pools, and a release of CO2 to the atmosphere (e.g. see Skinner & Bard, 2022). However, it has not been possible to reconcile this theory with a complete closure of the global radiocarbon- and carbon cycles since the last glacial period (Skinner et al., 2023), and it is not even known if reduced ocean ventilation persisted prior to the LGM, when atmospheric CO2 was already near minimum glacial levels, at ~205 ppm: the data simply do not yet exist. 

This project seeks to confront the prevailing paradigm for CO2 rise after the LGM with new data from the onset of the LGM. More specifically, we will test the hypothesis that ocean ventilation was not significantly different from modern during the 20,000 years that preceded the LGM, when atmospheric CO2 was already close to minimum levels. We will generate the first marine radiocarbon data that are capable of testing this hypothesis.  If the hypothesis is rejected, our new data will provide the missing key to closing the global carbon budget since the last glacial period, based on a necessary balance of radiocarbon production rates, radiocarbon decay, and the ratio of the atmospheric and marine radiocarbon inventories. If the hypothesis cannot be rejected, then our new data will also require the prevailing paradigm for glacial-interglacial CO2 change to be completely revisited, to account for ~80 ppm of atmospheric CO2 draw-down prior to the LGM in the absence of an ocean ventilation contribution. This project therefore stands to revitalize, and potentially to reorient, research into the ocean’s role in late Quaternary climate and carbon cycle change.

Participants: L Skinner (PI), A. Scrivner (RA), S. Radionovskaya (PDRA)

Relevant publications:

Watch this space! 🙂