The European Project for Ice Coring in Antarctica Dome ice core from Dome C (EDC) has allowed for the reconstruction of atmospheric CO 2 concentrations for the last 800,000 years. Here we revisit the oldest part of the EDC CO 2 record using different air extraction methods and sections of the core. For our established cracker system, we found an analytical artifact, which increases over the deepest 200 m and reaches 10.1 ± 2.4 ppm in the oldest/deepest part. The governing mechanism is not yet fully understood, but it is related to insufficient gas extraction in combination with ice relaxation during storage and ice structure. The corrected record presented here resolves partly -but not completely -the issue with a different correlation between CO 2 and Antarctic temperatures found in this oldest part of the records. In addition, we provide here an update of 800,000 years atmospheric CO 2 history including recent studies covering the last glacial cycle.

24The stable carbon isotope ratio of atmospheric CO 2 (! 13 C atm ) is a key parameter to decipher 25 past carbon cycle changes. Here we present ! 13 C atm data for the last 24,000 years derived 26 from three Antarctic ice cores. We conclude that a pronounced 0.3‰ decrease in ! 13 C atm 27 during the early deglaciation can be best explained by upwelling of old, carbon-enriched 28 waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial 29 biosphere, changes in sea surface temperature, and ocean circulation governed the ! 13 C atm 30 evolution. During the Last Glacial Maximum, ! 13 C atm and CO 2 were essentially constant, 31suggesting that the carbon cycle was in dynamic equilibrium and that the net transfer of 32 carbon to the deep ocean had occurred before then. showing pronounced differences in atmospheric CO 2 rates of change in the course of the 47 last glacial/interglacial transition (3). Many processes have been involved in attempts to 48 explain these CO 2 variations, but it has become evident that none of these mechanisms 49 alone can account for the 90 ppmv increase in atmospheric CO 2 . A combination of 50 processes must have been operating (4, 5), with their exact timing being crucial. However, 51 a unique solution to the deglacial carbon cycle changes has not been yet found. 52 53

Reconstructions of atmospheric CO 2 concentrations based on Antarctic ice cores 1,2 reveal significant changes during the Holocene epoch, but the processes responsible for these changes in CO 2 concentrations have not been unambiguously identified. Distinct characteristics in the carbon isotope signatures of the major carbon reservoirs (ocean, biosphere, sediments and atmosphere) constrain variations in the CO 2 fluxes between those reservoirs. Here we present a highly resolved atmospheric d 13C record for the past 11,000 years from measurements on atmospheric CO 2 trapped in an Antarctic ice core. From mass-balance inverse model calculations 3,4 performed with a simplified carbon cycle model, we show that the decrease in atmospheric CO 2 of about 5 parts per million by volume (p.p.m.v.). The increase in d 13C of about 0.25% during the early Holocene is most probably the result of a combination of carbon uptake of about 290 gigatonnes of carbon by the land biosphere and carbon release from the ocean in response to carbonate compensation of the terrestrial uptake during the termination of the last ice age. The 20 p.p.m.v. increase of atmospheric CO 2 and the small decrease in d 13C of about 0.05% during the later Holocene can mostly be explained by contributions from carbonate compensation of earlier land-biosphere uptake and coral reef formation, with only a minor contribution from a small decrease of the land-biosphere carbon inventory.The Holocene is the current interglacial period, starting about 11,000 years before present (11 kyr BP, where present is defined as AD 1950) following the Transition (here defined as 18-11 kyr BP) from the last glacial maximum. Variations in the atmospheric concentration of CO 2 during the Holocene were significant but small compared to glacial-interglacial changes of typically 100 p.p.m.v. (refs 5, 6). Yet a decrease of about 5 p.p.m.v. from 11-7.5 kyr BP could be observed, followed by an increase of about 20 p.p.m.v. to the pre-industrial level of about 280 p.p.m.v. (refs 1, 2, 7). Different explanations for these variations were discussed 7,8 , such as changes in the carbon inventories of vegetation, soils and peatlands 9 , in anthropogenic land use 10,11 , in sea surface temperature (SST) 7,12 , coral reef growth 13,14 or carbonate compensation 15 . The latter is a multi-millennial equilibration process of the atmosphere-ocean-sediment system and the weathering cycle. Moreover, model simulations of atmospheric CO 2 and d 13C during the Holocene have not provided an unambiguous quantitative explanation 7,8,16 . The major stumbling block has been the scarcity of reconstructions of d We focus on the evolution of the carbon isotopes on a timescale of a few thousand years. Therefore, we calculated a spline and its 1s uncertainty bands with a cut-off period of 5 kyr (Fig. 2). In a Monte Carlo simulation, standard deviations smaller than 0.07% were

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Past atmospheric methane concentrations show strong fluctuations in parallel to rapid glacial climate changes in the Northern Hemisphere 1,2 superimposed on a glacial-interglacial doubling of methane concentrations [3][4][5] . The processes driving the observed fluctuations remain uncertain but can be constrained using methane isotopic information from ice cores 6,7 . Here we present an ice core record of carbon isotopic ratios in methane over the entire last glacial-interglacial transition. Our data show that the carbon in atmospheric methane was isotopically much heavier in cold climate periods. With the help of a box model constrained by the present data and previously published results 6,8 , we are able to estimate the magnitude of past individual methane emission sources and the atmospheric lifetime of methane. We find that methane emissions due to biomass burning were about 45 Tg methane per year, and that these remained roughly constant throughout the glacial termination. The atmospheric lifetime of methane is reduced during cold climate periods. We also show that boreal wetlands are an important source of methane during warm events, but their methane emissions are essentially shut down during cold climate conditions.The atmospheric concentration of CH 4 , the second most important anthropogenic greenhouse gas, is determined by a balance between natural and anthropogenic CH 4 sources and sinks that is still debated. Photochemically induced oxidation in the troposphere and stratosphere, and uptake by methanotrophic bacteria in aerated soils, represent the most important sinks 9,10 . The dominating natural CH 4 sources comprise tropical and boreal wetlands, ruminants, and biomass burning 10,11 . These sources all differ in their carbon and hydrogen isotopic signature. In addition, a release of CH 4 from marine gas hydrates 12,13 and emissions from plants under aerobic conditions are currently debated 14,15 . Most probably all those sources and sinks were subject to palaeoclimatic changes, as reflected by CH 4 being as low as 360 parts per billion (10 9 ) by volume (p.p.b.v.) during the Last Glacial Maximum (LGM), compared with up to 725 p.p.b.v. in the preindustrial Holocene epoch 3,16,17 . Throughout the glacial period and during the last transition, CH 4 changed by up to 200 p.p.b.v. (refs 3, 18) in parallel with rapid climate changes. Using the interhemispheric CH 4 gradient in ice cores, an increase of high-latitude CH 4 sources in the Northern Hemisphere was derived for warm periods 3,17 . However, a more detailed quantitative source attribution is still missing.Such quantitative constraint on the sources can be derived from methane isotopic measurements on ice cores 6,7 , making use of the different isotopic signatures of the CH 4 sources and the different isotopic fractionation factors for the individual removal processes (Supplementary Table 1). In Fig. 1

Continuous flow analysis (CFA) is a well-established method to obtain information about impurity contents in ice cores as indicators of past changes in the climate system. A section of an ice core is continuously melted on a melter head supplying a sample water flow which is analyzed online. This provides high depth and time resolution of the ice core records and very efficient sample decontamination as only the inner part of the ice sample is analyzed. Here we present an improved CFA system which has been totally redesigned in view of a significantly enhanced overall efficiency and flexibility, signal quality, compactness, and ease of use. These are critical requirements especially for operations of CFA during field campaigns, e.g., in Antarctica or Greenland. Furthermore, a novel deviceto measure the total air content in the ice was developed. Subsequently, the air bubbles are now extracted continuously from the sample water flow for subsequent gas measurements.

Atmospheric methane (CH 4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era 1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH 4 emissions, but the precise magnitude of these contributions is a subject of debate 2,3. Carbon-14 in CH 4 (14 CH 4) can be used to distinguish between fossil (14 C-free) CH 4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14 CH 4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century 4,5. Moreover, the partitioning of total fossil CH 4 emissions (presently 172 to 195 teragrams CH 4 per year) 2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH 4 per year 6,7. Geological emissions were less than 15.4 teragrams CH 4 per year at the end of the Pleistocene, about 11,600 years ago 8 , but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14 CH 4 measurements to show that natural geological CH 4 emissions to the atmosphere were about 1.6 teragrams CH 4 per year, with a maximum of 5.4 teragrams CH 4 per year (95 per cent confidence limit)-an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH 4 emissions are underestimated by about 38 to 58 teragrams CH 4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH 4 budget, and will help to inform strategies for targeted emission reductions 9,10. 14 CH 4 emissions from nuclear power plants 4,5. By contrast, palaeoatmospheric 14 CH 4 measurements from ice cores offer a direct constraint on natural geological CH 4 emissions without these complications. Whereas geological CH 4 emissions have the potential to change on tectonic-and glacial-cycle timescales 14 , they have very probably been constant over the past few centuries. The preindustrial-era emission estimates can therefore be applied to the modern CH 4 budget with confidence. Ice core 14 CH 4 analysis is challenging owing to both the very large sample requirement (~1,

Abstract. Continuous records of the atmospheric greenhouse gases (GHGs) CO 2 , CH 4 , and N 2 O are necessary input data for transient climate simulations, and their associated radiative forcing represents important components in analyses of climate sensitivity and feedbacks. Since the available data from ice cores are discontinuous and partly ambiguous, a well-documented decision process during data compilation followed by some interpolating post-processing is necessary to obtain those desired time series. Here, we document our best possible data compilation of published ice core records and recent measurements on firn air and atmospheric samples spanning the interval from the penultimate glacial maximum (∼ 156 kyr BP) to the beginning of the year 2016 CE. We use the most recent age scales for the ice core data and apply a smoothing spline method to translate the discrete and irregularly spaced data points into continuous time series. These splines are then used to compute the radiative forcing for each GHG using well-established, simple formulations. We compile only a Southern Hemisphere record of CH 4 and discuss how much larger a Northern Hemisphere or global CH 4 record might have been due to its interpolar difference. The uncertainties of the individual data points are considered in the spline procedure. Based on the given data resolution, time-dependent cutoff periods of the spline, defining the degree of smoothing, are prescribed, ranging from 5000 years for the less resolved older parts of the records to 4 years for the densely sampled recent years. The computed splines seamlessly describe the GHG evolution on orbital and millennial timescales for glacial and glacial-interglacial variations and on centennial and decadal timescales for anthropogenic times. Data connected with this paper, including raw data and final splines, are available at https://doi