Rapid changes in ice core gas records Part 2: Understanding the rapid rise in atmospheric CO2 at the onset of the Bølling/Allerød

During the last glacial/interglacial transition the Earth's climate underwent rapid changes around 14.6 kyr ago. Temperature proxies from ice cores revealed the onset of the Bølling/Allerød (B/A) warm period in the north and the start of the Antarctic Cold Reversal in the south. Furthermore, the B/A is accompanied by a rapid sea level rise of about 20 m during meltwater pulse (MWP) 1A, whose exact timing is matter of current debate. In situ measured CO₂ in the EPICA Dome C (EDC) ice core also revealed a remarkable jump of 10±1 ppmv in 230 yr at the same time. Allowing for the age distribution of CO₂ in firn we here show, that atmospheric CO₂ rose by 20–35 ppmv in less than 200 yr, which is a factor of 2–3.5 larger than the CO₂ signal recorded in situ in EDC. Based on the estimated airborne fraction of 0.17 of CO₂ we infer that 125 Pg of carbon need to be released to the atmosphere to produce such a peak. Most of the carbon might have been activated as consequence of continental shelf flooding during MWP-1A. This impact of rapid sea level rise on atmospheric CO₂ distinguishes the B/A from other Dansgaard/Oeschger events of the last 60 kyr, potentially defining the point of no return during the last deglaciation

Carbon Isotope Constraints on the Deglacial CO2 Rise from Ice Cores

The stable carbon isotope ratio of atmospheric CO2 (d13Catm) is a key parameter in deciphering past carbon cycle changes. Here we present d13Catm data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in d13Catm during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the d13Catm evolution. During the Last Glacial Maximum, d13Catm and atmospheric CO2 concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then

Impact of oceanic processes on the carbon cycle during the last termination

During the last termination (from ~18 000 years ago to ~9000 years ago), the climate significantly warmed and the ice sheets melted. Simultaneously, atmospheric CO2 increased from ~190 ppm to ~260 ppm. Although this CO2 rise plays an important role in the deglacial warming, the reasons for its evolution are difficult to explain. Only box models have been used to run transient simulations of this carbon cycle transition, but by forcing the model with data constrained scenarios of the evolution of temperature, sea level, sea ice, NADW formation, Southern Ocean vertical mixing and biological carbon pump. More complex models (including GCMs) have investigated some of these mechanisms but they have only been used to try and explain LGM versus present day steady-state climates. In this study we use a coupled climate-carbon model of intermediate complexity to explore the role of three oceanic processes in transient simulations: the sinking of brines, stratification-dependent diffusion and iron fertilization. Carbonate compensation is accounted for in these simulations. We show that neither iron fertilization nor the sinking of brines alone can account for the evolution of CO2, and that only the combination of the sinking of brines and interactive diffusion can simultaneously simulate the increase in deep Southern Ocean δ13C. The scenario that agrees best with the data takes into account all mechanisms and favours a rapid cessation of the sinking of brines around 18 000 years ago, when the Antarctic ice sheet extent was at its maximum. In this scenario, we make the hypothesis that sea ice formation was then shifted to the open ocean where the salty water is quickly mixed with fresher water, which prevents deep sinking of salty water and therefore breaks down the deep stratification and releases carbon from the abyss. Based on this scenario, it is possible to simulate both the amplitude and timing of the long-term CO2 increase during the last termination in agreement with ice core data. The atmospheric δ13C appears to be highly sensitive to changes in the terrestrial biosphere, underlining the need to better constrain the vegetation evolution during the termination

Abrupt rise in atmospheric CO2 at the onset of the Bølling/Allerød: in-situ ice core data versus true atmospheric signal

During the last glacial/interglacial transition the Earth's climate underwent abrupt changes around 14.6 kyr ago. Temperature proxies from ice cores revealed the onset of the Bølling/Allerød (B/A) warm period in the north and the start of the Antarctic Cold Reversal in the south. Furthermore, the B/A was accompanied by a rapid sea level rise of about 20 m during meltwater pulse (MWP) 1A, whose exact timing is a matter of current debate. In-situ measured CO2 in the EPICA Dome C (EDC) ice core also revealed a remarkable jump of 10±1 ppmv in 230 yr at the same time. Allowing for the modelled age distribution of CO2 in firn, we show that atmospheric CO2 could have jumped by 20–35 ppmv in less than 200 yr, which is a factor of 2–3.5 greater than the CO2 signal recorded in-situ in EDC. This rate of change in atmospheric CO2 corresponds to 29–50% of the anthropogenic signal during the last 50 yr and is connected with a radiative forcing of 0.59–0.75 W m−2. Using a model-based airborne fraction of 0.17 of atmospheric CO2, we infer that 125 Pg of carbon need to be released into the atmosphere to produce such a peak. Available δ13CO2 data are neutral, whether the source of the carbon is of marine or terrestrial origin. We hypothesise that most of the carbon might have been activated as a consequence of continental shelf flooding during MWP-1A. We furthermore plan to challange our hypothesis by comparing its typical 14C signature with so far unpublished high resolution 14C data from Tahiti corals (Durant et al., 2010, Geophysical Research Abstracts, 12, EGU2010-12689-1).This potential impact of rapid sea level rise on atmospheric CO2 might define the point of no return during the last deglaciation

Constraints on the atmospheric CO2 deglacial rise based on its δ13CO2 evolution

The analysis of air bubbles trapped in polar ice permits the reconstruction of atmospheric evolution of greenhouse gases, such as carbon dioxide (CO2 ), on various timescales. Within this study, the simultaneous analysis of the CO2 mixing ratio and its stable carbon isotope composition (δ 13 CO2 ) over the last two deglaciations allows us to better constrain the global carbon cycle. Based on the different isotopic signatures of the ocean and the terrestrial biosphere (major reservoirs responsible for the CO2 oscillations on a glacial interglacial scale), δ 13 CO2 contributes in distinguishing the major sources of CO2 for the studied periods. The new LGGE analytical method applied to samples from the EPICA / Dome C ice core provides a 1-sigma uncertainty over 3 measurements on the same extracted gas of 0.98 and 1.87 ppmv for CO2 , for the last and penultimate deglaciation respectively, accompanied by an averaged 0.1 1-sigma for δ 13 CO2 for both periods. This allows us to reveal significant changes in the signal through time. The time resolution of our results (∼250 and ∼730 years, for last and penultimate deglaciation) allows us to divide Terminations (T) into sub-periods, based on the different slope of CO2 rate of changes. The ∼80 ppmv CO2 increase throughout TI, coherent with previously published studies, is accompanied by a ∼0.6 decrease of δ 13 CO2 with additional clear trends during the different sub-periods. TII shows similar trends as for TI but of a larger magnitude: we therefore observe a ∼110 ppmv rise associated with an overall ∼0.9 decrease. In addition, δ 13 CO2 appears overall lighter during TII than TI. The two datasets are jointly evaluated using two C cycle box models. We conclude that oceanic processes involving stratification breakdown of the austral ocean, combined with reduction of sea ice cover and biological pump, can explain a large part of the signal. In addition, continental biosphere buildup during the Bolling/Allerod and thermohaline circulation fluctuations could have imprinted our signals in the second half of TI

A detailed atmospheric carbon isotopic constraint on the causes of the deglacial CO2 increase

Paleo-environmental records and extensive modeling studies have demonstrated thatthe Sahara was largely covered by grass and steppe vegetation in the early to midHolocene. The orbitally controlled incoming summer insolation is the primary forcingfactor during the Holocene. It is well-documented that internal feedback-mechanismsbetween the vegetation and the atmosphere-ocean system caused a sudden shift fromthe vegetated humid Sahara state to a arid desert climate about 50004000 years ago.Proxy evidence suggests also an abrupt onset of the African Humid Period between14,000 and 11,000 yr BP. However, the attribution of the rapid onset to orbitally driveninsolation anomalies or to the Bølling-Allerød, Younger- Dryas transitions is non-trivial. Here we show in transient simulations with climate and vegetation modelsof different complexity that the abrupt change of the African Monsoon/vegetationsystem from dry/deserted glacial state to wet/green conditions is accelerated by thevegetation-albedo feedback. The non-linear response of the climate-vegetation sys-tem to precessional forcing leads to a rapid onset of the African Humid Period at∼11,000 yr BP

Abrupt rise in atmospheric CO2 at the onset of the Boølling/ Alleroød: In-situ ice core data versus true atmospheric signals

During the last glacial/interglacial transition the Earth's climate underwent abrupt changes around 14.6 kyr ago. Temperature proxies from ice cores revealed the onset of the Boølling/Alleroød (B/A) warm period in the north and the start of the Antarctic Cold Reversal in the south. Furthermore, the B/A was accompanied by a rapid sea level rise of about 20 m during meltwater pulse (MWP) 1A, whose exact timing is a matter of current debate. In-situ measured CO2 in the EPICA Dome C (EDC) ice core also revealed a remarkable jump of 10 ± 1 ppmv in 230 yr at the same time. Allowing for the modelled age distribution of CO2 in firn, we show that atmospheric CO2 could have jumped by 20-35 ppmv in less than 200 yr, which is a factor of 2-3.5 greater than the CO2 signal recorded in-situ in EDC. This rate of change in atmospheric CO2 corresponds to 29-50% of the anthropogenic signal during the last 50 yr and is connected with a radiative forcing of 0.59-0.75 W m−2. Using a model-based airborne fraction of 0.17 of atmospheric CO2, we infer that 125 Pg of carbon need to be released into the atmosphere to produce such a peak. If the abrupt rise in CO2 at the onset of the B/A is unique with respect to other Dansgaard/Oeschger (D/O) events of the last 60 kyr (which seems plausible if not unequivocal based on current observations), then the mechanism responsible for it may also have been unique. Available δ13CO2 data are neutral, whether the source of the carbon is of marine or terrestrial origin. We therefore hypothesise that most of the carbon might have been activated as a consequence of continental shelf flooding during MWP-1A. This potential impact of rapid sea level rise on atmospheric CO2 might define the point of no return during the last deglaciation. © 2011 Author(s).SENS

Constraints on the causes of CO2 rise during deglaciations: atmospheric stable carbon isotope ratio of CO2 from Antarctic ice cores

The analysis of air bubbles trapped in polar ice permits the reconstruction of the evo-lution of major greenhouse gases over various timescales. This study leans on thepast behaviour of the most important human-induced greenhouse gas, carbon dioxide(CO2). The past origin of CO2 is better comprehended when studying concomitantlythe evolution of its stable carbon isotope composition, as it is affected by various frac-tionation processes in and between carbon reservoirs.The LGGE dry extraction method of gases occluded in ice was used in combinationwith a new instrumental setup to investigate the CO2 mixing ratio and its stable car-bon isotope composition (delta13CO2) in air from the last deglaciation at the EPICADome Concordia site (Antarctica). The resolution of our results (250 years in average)allows us to divide Termination I (TI) into four sub-periods, each representing differ-ent climatic features at the Earth surface (Heinrich I, Bølling/Ållerød, Antarctic ColdReversal, Younger Dryas). We observe that CO2 and delta13CO2 are not correlated.Delta13CO2 shows positive and negative excursions associated with changes in thegrowth rate of atmospheric CO2. This illustrates the dynamic character of the carboncycle and its coupling to climate change during the deglaciation. The use of two car-bon cycle box models highlight oceanic mechanisms as the major contributors to theCO2 evolution during these periods of TI, and the terrestrial biosphere for the warmBølling/Ållerød event.We will also present pioneering delta13CO2 data obtained in the course of the penul-timate deglaciation (TII); this is expected to bring some more light in the carbon cyclequestion during glacial-interglacial transitions although the existing challenge on icephysics (clathrate ice for TII vs bubbly ice for TI) should not be neglected

Constraints on the atmospheric carbon dioxide (CO2) deglacial rise based on its stable carbon isotopic ratio increase (δ13CO2)

The analysis of air bubbles trapped in polar ice permits the reconstruction of atmospheric components over various timescales. Past evolution of greenhouse gases, such as carbon dioxide (CO2), lies on the frontline of paleorecords understanding. Within this study, the glacial interglacial oscillations of CO2 will be examined for the last 160,000 years. This period encompasses two deglaciations.The simultaneous analysis of the stable carbon isotope composition (δ13CO2) allows to better constrain the global carbon cycle. Based on the different isotopic signatures of the ocean and the terrestrial biosphere (major reservoirs responsible for the CO2 oscillations on a glacial interglacial scale), δ13CO2 contributes in distinguishing the major sources of CO2 for the studied periods.The LGGE method of gas extraction from ice was used in combination with a new instrumental setup to investigate the CO2 mixing ratio and its stable carbon isotope composition in air from the two last deglaciations at the EPICA Dome Concordia site in Antarctica. Being challenged from the different ice properties corresponding to the two major periods (being in bubble form for the last and in clathrate form for the penultimate deglaciation), the resulting averaged 3-expansion 1-sigma uncertainty (0.98 and 1.87 ppmv for CO2, respectively), accompanied by an averaged 0.1 1-sigma for δ13CO2 for both periods were satisfying enough to exclude any artefact scenario in the experimental protocol. The resolution of our results (~250 and ~730 years, for last and penultimate deglaciation) allows us to divide Terminations (T) into sub-periods, based on the different slope CO2 experiences. For TI, the four sub-periods revealed climatic events for both hemispheres (e.g.: Heinrich I, Bölling/Alleröd, Antarctic Cold Reversal, Younger Dryas), as also shown from polar and oceanic proxies. For the case of TII, a similar dynamic pattern between CO2 and δ13CO2 is seen as for TI, but the synchronization of oceanic events in our atmospheric record is more delicate due to higher data uncertainties one encounters for such a time scale.Our results show a ~80 ppmv CO2 increase throughout TI, which is coherent with previously published studies. The δ13CO2 shows a deglacial ~0.6 decrease accompanying the CO2 rise, showing clear trends during the different sub-periods. TII shows similar trends as for TI but of a larger magnitude: we therefore observe a ~110 ppmv rise associated with a ~0.9 decrease. Several scenarii can explain the abrupt deglacial CO2 increase, but there is presently no consensus on the exact causes and their respective role. Still, it is presumed that the ocean reservoir contributes the most. As a first interpretation of the obtained TI coupled CO2 and δ13CO2 dataset, the use of two C cycle box models is applied, validating the initial dominant oceanic role. The use of polar and oceanic proxies for the atmosphere and the ocean, superposed with our atmospheric signal should provide some responses on the similarities and differences of both deglaciations. Similarities potentially concern forcing factors and the amplifying role of the climatic system towards the external forcing, while differences mainly concern the different relative timing and magnitudes