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

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

Extended dilation of the radiocarbon time scale between 40,000 and 48,000 y BP and the overlap between Neanderthals and Homo sapiens

The new radiocarbon calibration curve (IntCal20) allows us to calculate the gradient of the relationship between 14C age and calendar age over the past 55 millennia before the present (55 ka BP). The new gradient curve exhibits a prolonged and prominent maximum between 48 and 40 ka BP during which the radiocarbon clock runs almost twice as fast as it should. This radiocarbon time dilation is due to the increase in the atmospheric 14C/12C ratio caused by the 14C production rise linked to the transition into the Laschamp geomagnetic excursion centered around 41 ka BP. The major maximum in the gradient from 48 to 40 ka BP is a new feature of the IntCal20 calibration curve, with far-reaching impacts for scientific communities, such as prehistory and paleoclimatology, relying on accurate ages in this time range. To illustrate, we consider the duration of the overlap between Neanderthals and Homo sapiens in Eurasia

Rejuvenating the ocean: mean ocean radiocarbon, CO2 release, and radiocarbon budget closure across the last deglaciation

Abstract. Radiocarbon is a tracer that provides unique insights into the ocean's ability to sequester CO2 from the atmosphere. While spatial patterns of radiocarbon in the ocean interior can indicate the vectors and timescales for carbon transport through the ocean, estimates of the global average ocean–atmosphere radiocarbon age offset (B-Atm) place constraints on the closure of the global carbon cycle. Here, we apply a Bayesian interpolation method to compiled B-Atm data to generate global interpolated fields and mean ocean B-Atm estimates for a suite of time slices across the last deglaciation. The compiled data and interpolations confirm a stepwise and spatially heterogeneous “rejuvenation” of the ocean, suggesting that carbon was released to the atmosphere through two swings of a “ventilation seesaw” operating between the North Atlantic and both the Southern Ocean and the North Pacific. Sensitivity tests using the Bern3D model of intermediate complexity demonstrate that a portion of the reconstructed deglacial B-Atm changes may reflect “phase-attenuation” biases that are unrelated to ocean ventilation and that arise from independent atmospheric radiocarbon dynamics instead. A deglacial minimum in B-Atm offsets during the Bølling–Allerød could partly reflect such a bias. However, the sensitivity tests further demonstrate that when correcting for such biases, ocean “ventilation” could still account for at least one-third of deglacial atmospheric CO2 rise. This contribution to CO2 rise appears to have continued through the Younger Dryas, though much of the impact was likely achieved by the end of the Bølling–Allerød, indicating a key role for marine carbon cycle adjustment early in the deglacial process. Our global average B-Atm estimates place further new constraints on the long-standing mystery of global radiocarbon budget closure across the last deglaciation and suggest that glacial radiocarbon production levels are likely underestimated on average by existing reconstructions. </jats:p

Paléosismicité en Auvergne à travers l'étude régionale comparée des enregistrements sédimentaires lacustres au Mont-Dore.

National audienceL’occurrence de séismes en domaine continental peut être enregistrée au sein des bassinslacustres à travers une grande variété de dépôts sédimentaires reliés à différents processus gravi- taires.A ce titre, les archives lacustres confrontées à un fort aléa sismique peuvent être assimilées à dessismomètres naturels et leur compréhension s’avère essentielle pour recenser des séismes historiques afind’évaluer le risque présent et futur.Aujourd’hui, le zonage sismique de la France continentale montre une sismicité modéréevoire forte au sein des principales chaînes de montagne. Parmi elles, la région volcanique des MontsDore permet de s’intéresser à des systèmes lacustres très contrastés, tant sur le plan limnologique,géomorphologique, que dans leur degré d’anthropisation au cours du dernier millénaire. Dans cetobjectif, une approche régionale comparée a été menée sur quatre sites entourant le massif du Sancy,incluant au total deux lacs de maar (Pavin et Chauvet) et deux lacs de barrage volcanique (Guéry etMontcineyre). Cette étude s’appuie sur une caractérisation des remplissages sédimentaires via unecartographie acoustique par sismique réflexion et des analyses multi-paramètres à haute résolution(radiographies, XRF, spectrophotométrie...) complétées par des datations au radiocarbone sur descarottes sédimentaires.La lecture des profils sismiques permet d’identifier des faciès acoustiques chaotiques assimilésà des dépôts en masse de type slumps et/ou turbidites. En complément, les analyses multiparamètrespermettent de discriminer une sédimentation évènementielle d’une sédimentation de fonddont les flux semblent reliés à l’anthropisation du milieu (Guéry, Montcineyre et Chauvet). Enfin,les modèles d’âges réalisés à partir des datations radiocarbones relient une part de ces évènements àla sismicité historique de la région, comme la récurrence d’évènements sédimentaires au sein des archivesde Guéry et Pavin durant le XIXème siècle, période durant laquelle l’activité sismique au Mont-Doreet au sud de la faille de la Limagne était assez intense. De manière similaire, des évènements synchronesdatés aux alentours de l’an 1300 ont été répertoriés au sein des quatre lacs, soulignant un facteur dedéclenchement tectonique régional jusqu’alors non recensé. Néanmoins, l’enregistrement de cettepériode de sismicité est contrasté en raison d’effet de sites importants et très différents d’un systèmeà l’autre. Les principales raisons invoquées concernent la morphologie subaquatique (degré depentes, architecture sédimentaire), la cohésion du sédiment ainsi que le degré d’anthropisation dumilieu, trois paramètres qui semblent influencer la stabilité du matériel sédimentaire et doncconditionner la génération de glissements en masse durant des épisodes sismiques

A Comparison Of New Calculations Of The Yearly 10Be Production In The Earths Polar Atmosphere By Cosmic Rays With Yearly 10Be Measurements In Multiple Greenland Ice Cores Between 1939 And 1994 - A Troubling Lack Of Concordance Paper #2

We have compared the yearly production rates of 10Be by cosmic rays in the Earths polar atmosphere over the last 50-70 years with 10Be measurements from two separate ice cores in Greenland. These ice cores provide measurements of the annual 10Be concentration and 10Be flux levels during this time. The scatter in the ice core yearly data vs. the production data is larger than the average solar 11 year production variations that are being measured. The cross correlation coefficients between the yearly 10Be production and the ice core 10Be measurements for this time period are <0.4 in all comparisons between ice core data and 10Be production, including 10Be concentrations, 10Be fluxes and in comparing the two separate ice core measurements. In fact, the cross correlation between the two ice core measurements, which should be measuring the same source, is the lowest of all, only ~0.2. These values for the correlation coefficient are all indicative of a "poor" correlation. The regression line slopes for the best fit lines between the 10Be production and the 10Be measurements used in the cross correlation analysis are all in the range 0.4-0.6. This is a particular problem for historical projections of solar activity based on ice core measurements which assume a 1:1 correspondence. We have made other tests of the correspondence between the 10Be predictions and the ice core measurements which lead to the same conclusion, namely that other influences on the ice core measurements, as large as or larger than the production changes themselves, are occurring. These influences could be climatic or instrumentally based. We suggest new ice core measurements that might help in defining more clearly what these influences are and-if possible-to correct for them.Comment: 24 pages, 6 figure