ALBERT

Description: 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.

Description: 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.

Description: 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.

Description: 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.