Permafrost thawing as a possible source of abrupt carbon release at the onset of the Bølling/Allerød

Peter.Koehler [ at ]


One of the most abrupt and yet unexplained past rises in atmospheric CO2 (10 ppmv in two centuries in the EPICA Dome C [EDC] ice core) occurred in quasi-synchrony with abrupt northern hemispheric warming into the Bølling/Allerød, about 14.6 ka ago. In Köhler et al. (2014) we used a U/Th-dated record of atmospheric Δ14C from Tahiti corals to provide an independent and precise age control for this CO2 rise. We also used model simulations to show that the release of old (nearly 14C-free) carbon can explain these changes in CO2 and Δ14C. The Δ14C record provides an independent constraint on the amount of carbon released (125 PgC). We suggest, in line with observations of atmospheric CH4 and terrigenous biomarkers, that thawing permafrost in high northern latitudes could have been the source of carbon, possibly with contribution from flooding of the Siberian continental shelf during meltwater pulse 1A. Our findings highlight the potential of the permafrost carbon reservoir to modulate abrupt climate changes via greenhouse-gas feedbacks. These calculations and conclusions were challenged by the new CO2 data (Marcott et al. 2014) from the West Antarctic Ice Sheet Divide Ice Core (WDC), which have a higher temporal resolution. We therefore revised our carbon release experiments in order to meet these new WDC CO2 data. We furthermore used a new age distribution during gas enclosure in ice which includes the most recent understanding of firn densification. We then can align EDC and WDC CO2 data and propose a peak amplitude in atmospheric CO2 of about 15 ppmv around 14.6 ka BP corresponding to a C pulse of 85 PgC released in 200 years (0.425 PgC per year). This is 68% of the initial suggested strength of the C pulse of 125 PgC, that then led to a peak amplitude in true atmospheric CO2 of 22 ppmv. CO2 data from other ice cores suggest that the amplitude in atmospheric CO2 was in-between both these scenarios. The revised scenario proposes a carbon release that is still large enough to explain the atmospheric Δ14C anomaly of – (50 – 60) ‰ in 200 –250 years derived from Tahiti corals. However, in the revised scenario the released carbon needs to be essentially free of 14C, while in the previously suggested scenario there was still the possibility that the released carbon still contained some 14C and had a difference in the Δ14C signature to the atmosphere Δ(Δ14C) of –700 ‰. The previous scenario, therefore, contained a larger possibility that the released carbon might eventually been released from the deep ocean. The revised interpretation proposed here strengthens the idea that the carbon was released from permafrost thawing, since this had more likely a nearly 14C-free signature than any other known source. We therefore conclude, that the new WDC CO2 data are not in conflict with our permafrost thawing hypothesis, but indicate only that the magnitude of the released carbon might have been smaller than initially suggested. References: Köhler, P., Knorr, G., and Bard, E. 2014. Permafrost thawing as a possible source of abrupt carbon release at the onset of the Bølling/Allerød. Nature Communications 5, 5520. DOI: 10.1038/ncomms6520. Marcott, S. A., Bauska, T. K., Buizert, C., Steig, E. J., Rosen, J. L., Cuffey, K. M., Fudge, T. J., Severing­ haus, J. P., Ahn, J., Kalk, M. L., McConnell, J. R., Sowers, T., Taylor, K. C., White, J. W. C., and Brook, E. J. 2014. Centennial scale changes in the global carbon cycle during the last deglaciation. Nature 514: 616–619. DOI: 10.1038/nature13799.

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XI. International Conference on Permafrost (ICOP2016), 20 Jun 2016 - 25 Jun 2016, Potsdam, Germany.
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Köhler, P. , Knorr, G. and Bard, E. (2016): Permafrost thawing as a possible source of abrupt carbon release at the onset of the Bølling/Allerød , XI. International Conference on Permafrost (ICOP2016), Potsdam, Germany, 20 June 2016 - 25 June 2016 .

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