Radiocarbon (14C) is a naturally occurring radioactive isotope of carbon that is produced in the upper atmosphere through the impact of cosmic radiation on nitrogen atoms, 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 exchange of carbon between the various carbon-bearing Earth system reservoirs. The figure above (from Skinner & Bard, 2022) illustrates the cycling of radiocarbon in the surface environment (inset A), whereby the timescale associated with the exchange of carbon (e.g. between the ocean and atmosphere, inset B) and its transport within reservoirs, leaves its imprint on radiocarbon activities. Of course, as with all proxy measurements, sedimentary impacts on the fossil material that is used to reconstruct past radiocarbon activities, are a further concern that must always be borne in mind (inset C).

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 about changes in the transport of carbon between and within key Earth surface reservoirs such as the ocean and the atmosphere (e.g. 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 atmosphere or terrestrial biosphere for example) it is virtually certain that insights into the role of the ocean circulation in glacial-interglacial CO2 change are to be found in the record of past marine radiocarbon 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 (e.g. Skinner et al., 2023).

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 inferred ~700 14Cyr-equivalent ‘aging’ of the ocean’s carbon inventory, perhaps as much as 60 ppm of atmospheric CO2 could have been drawn out of the atmosphere. These estimates have since been backed up my model sensitivity tests with the Bern3D EMIC (e.g. Skinner et al., 2023).  Ongoing work is seeking to better quantify this impact using auxiliary trace-element proxy analyses.

In the figure above: Upper panels show zonal average interpolated radiocarbon fields for the Atlantic and Pacific at the LGM; lower panels show offsets at the LGM versus the pre-industrial ocean (from Skinner & Bard, 2022).

More recently, the 3D Bayesian time-slice interpolation method used to reconstruct the zonal average sections shown above has been extended to six time-slices spanning the last deglaciation (Skinner et al., 2023). This has allowed global average radiocarbon ‘ventilation age’ estimates to be derived across the last deglaciation, demonstrating a step-wise ‘rejuvenation’ of the global ocean, and a consistent scaling with atmospheric CO2 anomalies.

An animation of the results can be seen by clicking on the image below:

RELEVANT PUBLICATIONS:

Skinner, L., F. Primeau, A. Jeltsch-Thommes, F. Joos, P. Koehler and E. Bard (2023). “Rejuvenating the ocean: mean ocean radiocarbon, CO2 release, and radiocarbon budget closure across the last deglaciation.” Climate of the Past.

Skinner, L. C. and E. Bard (2022). “Radiocarbon as a Dating Tool and Tracer in Paleoceanography.” Reviews of Geophysics 60(1): e2020RG000720.

Skinner, L., E. Freeman, D. Hodell, C. Waelbroeck, N. Vasquez Riveiros and A. Scrivner (2021). “Atlantic Ocean ventilation changes across the last deglaciation and their carbon cycle implications.” Paleoceanography and Paleoclimatology.

Skinner, L., F. Muschitiello and A. E. Scrivner (2019). “Marine Reservoir Age Variability Over the Last Deglaciation: Implications for Marine Carbon Cycling and Prospects for Regional Radiocarbon Calibrations.” Paleoceanography and Paleoclimatology 34(11): 1807-1815.

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.