Radiocarbon and CO2

Radiocarbon cycle_2

Radiocarbon (14C) is a naturally occurring radioactive isotope of carbon that is constantly produced in the upper atmosphere through the impact of cosmic radiation on nitrogen molecules, and which naturally ‘decays’ away at a constant rate.

If 14C was produced and removed from the atmosphere at a constant rate through all time, it could be used as a perfect geological ‘clock’ reaching back some 50,000 years. However, this is not the case; the 14C-budget of the atmosphere is in fact modulated by fluctuating levels of cosmic radiation and by changes in the distribution of carbon (a nearly constant proportion of which is 14C) within the natural environment.

For geochronologists changes in the atmospheric 14C budget are a problem, as they distort the ‘radiocarbon-clock’. However, for climate scientists these changes are very useful, as they can tell us when carbon has been transferred between the deep sea and the atmosphere (as the greenhouse gas carbon dioxide, CO2). We already know pretty well how the atmospheric CO2 and 14C budgets have changed in the recent geological past, though we are still unable to explain exactly why they have changed as observed. This represents an important test, and training ground, for our knowledge of the climate system and its carbon cycle feedbacks.

Given that the deep-ocean is by far the largest ‘dynamic carbon reservoir’ on earth (far larger than the terrestrial biosphere for example) it is virtually certain that insights into the role of the ocean circulation in glacial-interglacial CO2 change will be found in the record of marine 14C variability. Our research seeks to provide a more complete picture of how the marine radiocarbon budget changed through time and why, thus shedding light on the ocean’s role in modulating the atmospheric CO2 budget.  For example, as illustrated below, evidence for a stronger radiocarbon offset between the deep ocean and the atmosphere during the last glacial period (e.g. Skinner et al., 2010; Skinner et al., 2017) would suggest less effective mixing of carbon-rich deep waters with the surface ocean, and therefore a greater capacity for biological carbon export to sequester carbon in the deep ocean.

The next figure below illustrates how marine and atmospheric radiocarbon evolved across the last deglaciation, in parallel with atmospheric CO2 change.  Atmospheric CO2 increased (bottom plot) while atmospheric radiocarbon decreased (top plot).  However, only a portion of the atmospheric radiocarbon record can be explained by cosmogenic radiocarbon production changes (dashed lines in top plot). This means that the remainder was likely due to carbon cycle changes affecting the radiocarbon budget of the atmosphere. If these changes occurred because of the release of previously ‘sequestered’ respired CO2 from the deep ocean to the atmosphere, then we would expect the deep ocean radiocarbon activity to increase (i.e. deep ocean radiocarbon ages decreasing; dashed lines middle plot). This is indeed what we find: the deep ocean got ‘younger’ across the last deglaciation (e.g. Skinner et al., 2010, 2014, 2015, 2017; de la Fuente et al., 2015; Freeman et al., 2016)

Recent results from the project OCEAN-14 (funded by the Natural Environment Research Council, NERC) have served to demonstrate that the glacial ocean was indeed much more poorly ventilated than the modern ocean (Skinner et al., 2017). Tentatively, these results suggest a 2 to 8-fold decrease in the rate of turn-over of the deep ocean’s carbon pool, which would almost certainly have contributed to a greater accumulation of respired carbon in the deep ocean.  ‘Back of the envelope’ calculations suggest that for the ~690 yr equivalent ‘aging’ of the ocean’s radiocarbon inventory that has been identified, perhaps as much as 60 ppm of atmospheric CO2 could have been drawn out of the atmosphere.  Ongoing work is seeking to better quantify this impact using auxiliary trace-element proxy analyses.

In the figure above: a. and b. show meridional average interpolated fields for radiocarbon ventilation ‘age’ in the Atlantic and Pacific at the LGM, whereas c. and d. show offsets at the LGM versus the Holocene/pre-industrial.

RELEVANT PUBLICATIONS:

Skinner, L.C., Primeau, F., Freeman, E., de la Fuente, M., Goodwin, P., Gottschalk, J., Huang, E., McCave, I.N., Noble, T. and Scrivner, A.E. (2017) Radiocarbon constraints on the ‘glacial’ ocean circulation and its impact on atmospheric CO2. Nature Communications.

Gottschalk, J., Skinner, L.C., Lippold, J., Vogel, H., Frank, N., Jaccard, S.L. and Waelbroeck, C. (2016) Biological and physical controls in the Southern Ocean on past millennial-scale atmospheric CO2 changes. Nature Communications 7, 11539.

Freeman, E., Skinner, L., Waelbroeck, C. and Hodell, D. (2016) Radiocarbon evidence for enhanced respired carbon storage in the deep Atlantic at the Last Glacial Maximum. Nature Communications 7.

de la Fuente, M., Skinner, L., Calvo, E., Pelejero, C. and Cacho, I. (2015) Increased reservoir ages and poorly ventilated deep waters inferred in the glacial Eastern Equatorial Pacific. Nat Commun 6.

Skinner, L., McCave, I.N., Carter, L., Fallon, S., Scrivner, A. and Primeau, F. (2015) Reduced ventilation and enhanced magnitude of the deep Pacific carbon pool during the last glacial period. Earth Planet. Sci. Lett. 411, 45-52.

Skinner, L., C. Waelbroeck, A. Scrivner, S. Fallon, Radiocarbon evidence for alternating northern and southern sources of ventilation of the deep Atlantic carbon pool during the last deglaciation, Proceedings of the National Academy of Sciences 111 (2014), 5480-5484

Skinner, L., A. Scrivner, D. Vance, S. Barker, S. Fallon, C. Waelbroeck, North Atlantic versus Southern Ocean contributions to a deglacial surge in deep ocean ventilation, Geology 41 (2013) 667-670

Skinner, L.C., S. Fallon, C. Waelbroeck, E. Michel, S. Barker, Ventilation of the deep Southern Ocean and deglacial CO2 rise, Science 328(2010) 1147-1151.

Skinner, L.C., Revisiting the absolute calibration of the Greenland ice-core age-scales, Clim. Past 4(2008) 295-302.

Skinner, L.C., N.J. Shackleton, Rapid transient changes in Northeast Atlantic deep-water ventilation-age across Termination I, Paleoceanography 19(2004) 1-11.