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The EPICA (European Project for Ice Coring in Antarctica) Dome C (EDC) ice core drilling in East Antarctica reaches a depth of 3260 m. The reference EDC chronology (AICC2012) provides an age vs depth relationship covering the last 800 kyr (thousands of years) with an absolute uncertainty rising up to 8,000 years at the bottom of the ice core. The origins of this relatively large uncertainty are threefold: (1) the δ18Oatm, δO2/N2 and total air content (TAC) records are poorly resolved and discontinuous over the last 800 kyr, (2) the three orbital tools are not used simultaneously and (3) large uncertainties are associated with their orbital targets. Here, we present new highly resolved δ18Oatm, δO2/N2 and δ15N measurements for EDC ice core covering the last five glacial – interglacial transitions as well as novel absolute 81Kr ages. We have compiled chronological and glaciological information including novel orbital age markers from new data on EDC ice core as well as accurate firn modeling estimates in a Bayesian dating tool to construct the new AICC2023 chronology. The average uncertainty of the ice chronology is reduced from 2,500 years to 1,800 years in AICC2023 over the last 800 kyr. The new timescale diverges from AICC2012 and suggests age shifts reaching 3,800 years towards older ages over Marine Isotopes Stages (MIS) 5, 11 and 19. But, the coherency between the new AICC2023 timescale and independent chronologies of other archives (Italian Lacustrine succession from Sulmona Basin, Dome Fuji ice core and northern Alpine speleothems) is improved by 1,000 to 2,000 years over these time intervals.

The region of Ridge B in central East Antarctica is one of the last unexplored parts of the continent and, at the same time, ranks among the most promising places to search for Earth's oldest ice. In January 2020, we carried out the first scientific traverse from Russia's Vostok Station to the topographical dome of Ridge B (Dome B, 3807 m above sea level, 79.02°S, 93.69°E). The glaciological programme included continuous snow-radar profiling and geodetic positioning along the traverse's route, installation of snow stakes, measurements of snow density, collection of samples for stable water isotope and chemical analyses and drilling of a 20 m firn core. The first results of the traverse show that the surface mass balance at Dome B (2.28 g cm−2 year−1) is among the lowest in Antarctica. The firn temperature below the layer of annual variations is −58.1 ± 0.2°C. A very low value of heavy water stable isotope content (-58.2‰ for oxygen-18) was discovered at a distance of 170 km from Vostok Station. This work is the first step towards a comprehensive reconnaissance study of the Ridge B area aimed at locating the best site for future deep drilling for the oldest Antarctic ice.

Recent studies have shown that stratigraphically disturbed meteoric ice bedded at Vostok Station between 3318 and 3539 m dates back to 1.2 Ma BP and possibly beyond. As part of the VOICE (Vostok Oldest Ice Challenge) initiative, a new deviation from parent hole 5G-1 was made at depths of 3270–3291 m in the 2018/19 austral season with the aim of obtaining a replicate core of the old ice. Sidetracking was initiated using the standard KEMS-132 electromechanical drill routinely employed for deep ice coring at Vostok, without significant changes to its initial design. Here we describe the method and operating procedures for replicate coring at a targeted depth in an existing slant hole, involving the use of a cable-suspended electromechanical drill. The design of the milling cutter head used for sidetracking is presented. The performance characteristics and the experience of drilling branch-hole 5G-5 at Vostok are described and discussed.

To understand the causal relationship between forcing (orbital parameters, greenhouse gas concentration…) and the climate change, dating climate archives is crucial. Ice cores are unique archives because they provide a direct record of greenhouse gas concentration. However, dating ice cores is particular since they require two chronologies: one for the ice and one for the younger air trapped in bubbles inside the core. The coherent AICC2012 chronology was established for five ice cores: EPICA Dome C (EDC), EPICA Dronning Maud Land (EDML), North Greenland Ice core Project (NGRIP), Vostok (VK) and TALos Dome Ice CorE (TALDICE). A sedimentation model was used to reconstruct past variations of three parameters: accumulation of snow at surface, ice layer thinning in depth and Lock-In-Depth (LID), the depth where air is trapped. Ice and gas ages along the core are estimated from these parameters. Then, a Bayesian tool optimised the age scale by constraining the chronology to respect chronological observations (orbital tuning, stratigraphic links between cores, tephra layers…) and by fitting the three parameters to background scenarios (accumulation deduced from ice isotopes, LID from δ15N, …). The AICC2012 chronology is associated with an uncertainty which arises up to 6 kyrs due to the discontinuity of the ice core composition records and to the poor knowledge when it comes to choose an optimised target for orbital tuning.Since AICC2012, many new data have been obtained to improve the ice core chronology and it is the right period to produce an updated coherent chronology which could also be extended to other ice cores. Here, we present a first step toward the construction of the next coherent ice core chronology by including new dating constraints from recent data on the EDC ice core: 1) air isotopes (δ18Oatm , δO2 /N2) and air content used as orbital dating constraints, 2) the δ15N signal used to estimate the background scenario for LID. In addition, we make use of the East Asian stalagmite δ18Ocalcite signal as an alternative synchronisation target for the δ18Oatm (Extier et al. 2018).This new dating experiment on EDC ice core aims to lower uncertainty of the chronology while providing a critical look on former hypotheses considered to establish AICC2012. For example, δ15N record was discontinuous at the time and it has been reconstructed based on its correlation with δD. Now that we have a continuous δ15N signal, we can evaluate the relevance of this reconstruction. Following this work, we will use new tie point constraints resulting from volcanic synchronisation which has recently been undertaken between Greenland and Antarctica (Svensson et al. 2020) and the ice cores Dome Fuji and WAIS Divide will be further studied to be included in the chronology.

In the 1990s, closed and open porosity volumes of firn samples were measured by J.-M. Barnola using the technique of gas pycnometry, on firn from three different polar sites. They are the basis of a parameterization of closed porosity in polar firn, first introduced in Goujon et al. (2003) and used in several firn physics models (e.g., Buizert et al., 2012). However, these data and their processing have not been published in their own right yet. In this short article, we detail how they were processed by J.-M. Barnola and how the closed porosity parameterization was obtained. We show that the original data processing only partially accounts for the presence of reopened bubbles in the samples. Since the proper correction to apply for this effect is hard to estimate, we also processed the data without including a correction for reopened bubbles. Finally, we made these pycnometry data available in order to be used by the glaciology community, notably for the study of polar ice formation and of the composition of gas records in ice cores. They are hosted on the PANGAEA database: https://doi.org/10.1594/PANGAEA.907678 (Fourteau et al., 2019a).

Antarctic paleotemperatures It has been widely thought that East Antarctica was ∼9°C cooler during the Last Glacial Maximum, close to the ∼10°C difference between then and now determined independently for West Antarctica. Buizert et al. used borehole thermometry, firn density reconstructions, and climate modeling to show that the temperature in East Antarctica was actually only ∼4° to 7°C cooler during the Last Glacial Maximum. This result has important consequences for our understanding of Antarctic climate, polar amplification, and global climate change. Science , abd2897, this issue p. 1097

The ca. 74 ka BP ‘‘super-eruption’’ of Toba volcano in Sumatra is the largest known Quaternary eruption. It expelled an estimated of 2800 km3 of dense rock equivalent, creating a caldera of 100 x 30 km. The eruption is estimated to have been 3500 greater than the Tambora eruption that created the “year without summer” in 1816 in Europe (Oppenheimer, 2002). However, the consequences of this “mega-eruption” on the climate and human evolution that could be expected for such eruption are still debated and uncertain. There is no evidence that this eruption has triggered any catastrophic climate change such as a “nuclear winter”. One of such lack of evidence lies in the ice.In the ice core community, this eruption still remains a mystery. Indeed, the estimated size of the eruption should have left a gigantic mark in the ice, at least in the form of a huge sulfuric acid layer but none of the ice records covering this period show any such singularity. The sulfate record seems so common that it is in fact difficult to allocate a specific sulfate peak to this event.In an effort to synchronize the Vostok ice core and the EPICA Dome C core, (Svensson et al., 2013) have identified three possible sulfuric acid layers for the Toba eruption in the Vostok ice core. In order to see if one of such event could have been the Toba eruption, we have performed the sulfur & oxygen isotope analysis of these three sulfuric acid layers in the hope that it could reveal some particularity. The sulfur results show that 1- all these three events have injected their products in the stratosphere and 2- the sulfur isotopic compositions of these three events share a common array, array that is in lines with other stratospheric eruptions, however one of the three acid layers shows an extremely and unusual weak oxygen anomaly, potentially indicating a major eruption. In order to remove the last doubts about the existence or not of one or a series of eruptions related to TOBA, the geochemical analysis of volcanic glasses trapped in the ice will be performed and presented.

We demonstrate that the accumulation-stake measurements in central Antarctica systematically underestimate the value of the snow build-up due to the compaction of snow layer between the stake base and the snow surface. We have developed two methods to define the corresponding correction to the snow build-up measurements at the stake farm near Vostok station. The first method is based on "Sorge's law" to calculate the rate of thinning of the snow layers using the vertical snow density profile. The second method consists of direct instrumental measurements of this thinning in the field. We have also involved the data of other two independent methods to estimate the snow accumulation rate in the vicinity of Vostok - first, geodetic data on the rate of snow layer sinking and, second, glaciological data from snow pits. The most reliable estimate of the snow accumulation rate in this region is 2.26±0.10 g cm-2 year-1, that is 8±4 % higher than initial (not corrected) value from the accumulation-stake measurements.

We measured the methane mixing ratios of enclosed air in five ice core sections drilled on the East Antarctic Plateau. Our work aims to study two effects that alter the recorded gas concentrations in ice cores: layered gas trapping artifacts and firn smoothing. Layered gas trapping artifacts are due to the heterogeneous nature of polar firn, where some strata might close early and trap abnormally old gases that appear as spurious values during measurements. The smoothing is due to the combined effects of diffusive mixing in the firn and the progressive closure of bubbles at the bottom of the firn. Consequently, the gases trapped in a given ice layer span a distribution of ages. This means that the gas concentration in an ice layer is the average value over a certain period of time, which removes the fast variability from the record. Here, we focus on the study of East Antarctic Plateau ice cores, as these low-accumulation ice cores are particularly affected by both layering and smoothing. We use high-resolution methane data to test a simple trapping model reproducing the layered gas trapping artifacts for different accumulation conditions typical of the East Antarctic Plateau. We also use the high-resolution methane measurements to estimate the gas age distributions of the enclosed air in the five newly measured ice core sections. It appears that for accumulations below 2 cm ice equivalent yr−1 the gas records experience nearly the same degree of smoothing. We therefore propose to use a single gas age distribution to represent the firn smoothing observed in the glacial ice cores of the East Antarctic Plateau. Finally, we used the layered gas trapping model and the estimation of glacial firn smoothing to quantify their potential impacts on a hypothetical 1.5-million-year-old ice core from the East Antarctic Plateau. Our results indicate that layering artifacts are no longer individually resolved in the case of very thinned ice near the bedrock. They nonetheless contribute to slight biases of the measured signal (less than 10 ppbv and 0.5 ppmv in the case of methane using our currently established continuous CH4 analysis and carbon dioxide, respectively). However, these biases are small compared to the dampening experienced by the record due to firn smoothing.

We study a firn and ice core drilled at the new "Lock-In" site in East Antarctica, located 136 km away from Concordia station towards Durmont d'Urville. High resolution chemical and physical measurements were performed on the core, with a particular focus on the trapping zone of the firn where air bubbles are formed. We measured the air content in the ice, closed and open porous volumes in the firn, firn density, firn liquid conductivity and major ion concentrations, as well as methane concentrations in the ice. The closed and open porosity volumes of firn samples were obtained by the two independent methods of pycnometry and tomography, that yield similar results. The measured increase of the closed porosity with density is used to estimate the air content trapped in the ice with the aid of a simple gas trapping model. Results show a discrepancy, with the model trapping too much air. Experimental errors have been considered but do not explain the discrepancy between the model and the observations. The model and data can be reconciled with the introduction of a reduced compression of the closed porosity compared to the open porosity. Yet, it is not clear if this limited compression of closed pores is the actual mechanism responsible for the low amount of air in the ice. High resolution density measurements reveal the presence of a strong layering, manifesting itself as centimeter scale variations. Despite this heterogeneous stratification, all layers, including the ones that are especially dense or less dense compared to their surroundings, display similar pore morphology and closed porosity as function of density. This implies that all layers close in a similar way, even though some close in advance or later compared to the bulk firn. Investigation of the chemistry data suggests that in the trapping zone, the observed stratification is partly related to the presence of chemical impurities.

A 182 m ice core has been recovered from a borehole drilled through the glacier to the bedrock at the Western Plateau of Mt Elbrus (43°20'53.9'' N, 42°25'36.0'' E; 5115 m a.s.l.), the Caucasus, Russia, in 2009. This is the first ice core in the region which represents a paleoclimate record practically undisturbed by seasonal melting. Relatively high snow accumulation rate at the drilling site enabled analysis of the intra-seasonal climate proxies' variability. Borehole temperatures ranged from −17 °C at 10 m depth and −2.4 °C at 182 m. A detailed radio-echo sounding survey showed that the glacier thickness ranged from 45 m near marginal zone of the plateau up to 255 m at the central part. The ice core has been analyzed for stable isotopes (δ18O and δ D), major ions (K+, Na+, Ca2+, Mg2+, NH4+, SO42-, NO3-, Cl-, F-), succinic acid (HOOCCH2COOH), and tritium content. The mean annual net accumulation rate was estimated from distinct annual oscillations of δ18O, δ D, succinic acid, and NH4+ and is 1455 mm w.e. for the last 140 years. Using annual layer counting also for the dating of the ice core, a good agreement with the absolute markers of the tritium 1963 bomb test time horizon located at the core depth of 50.7 m w.e. and the sulfate peak of the Katmai eruption (1912) at 87.7 m w.e. was obtained. According to mathematical modeling results, the bottom ice age at the maximal glacier depth is predicted to be about 660 years BP. As the 2009 borehole was situated downstream of this point, the estimated bottom ice age of the drilling site does not exceed 350–400 years BP. Taking into account the information that we have acquired on the Western Plateau Elbrus glacier and first results of the ice core analysis, these data can be used to reconstruct the atmospheric history of the European region.

We present the results of glaciological investigations in the megadune area located 30 km to the east of Vostok Station (central East Antarctica) implemented during the 58th, 59th and 60th Russian Antarctic Expedition (January 2013–2015). Snow accumulation rate and isotope content (δD, δ18O and δ17O) were measured along the 2 km profile across the megadune ridge accompanied by precise GPS altitude measurements and ground penetrating radar (GPR) survey. It is shown that the spatial variability of snow accumulation and isotope content covaries with the surface slope. The accumulation rate regularly changes by 1 order of magnitude within the distance δD and 17O-excess ∕ δD slopes (where dxs = δD − 8 ⋅ δ18O and 17O-excess = ln(δ17O ∕ 1000 + 1) −0.528 ⋅ ln (δ18O ∕ 1000 + 1)), we conclude that the spatial variability of the snow isotopic composition in the megadune area could be explained by post-depositional snow modifications. Using the GPR data, we estimated the apparent dune drift velocity (4.6 ± 1.1 m yr−1). The full cycle of the dune drift is thus about 410 years. Since the spatial anomalies of snow accumulation and isotopic composition are supposed to drift with the dune, a core drilled in the megadune area would exhibit the non-climatic 410-year cycle of these two parameters. We simulated a vertical profile of snow isotopic composition with such a non-climatic variability, using the data on the dune size and velocity. This artificial profile is then compared with the real vertical profile of snow isotopic composition obtained from a core drilled in the megadune area. We note that the two profiles are very similar. The obtained results are discussed in terms of interpretation of data obtained from ice cores drilled beyond the megadune areas.

We use isotopic composition (δD) data from six sites in Princess Elizabeth Land (PEL) in order to reconstruct air temperature variability in this sector of East Antarctica over the last 350 years. First, we use the present-day instrumental mean annual surface air temperature data to demonstrate that the studied region (between Russia's Progress, Vostok and Mirny research stations) is characterized by uniform temperature variability. We thus construct a stacked record of the temperature anomaly for the whole sector for the period of 1958–2015. A comparison of this series with the Southern Hemisphere climatic indices shows that the short-term inter-annual temperature variability is primarily governed by the Antarctic Oscillation (AAO) and Interdecadal Pacific Oscillation (IPO) modes of atmospheric variability. However, the low-frequency temperature variability (with period > 27 years) is mainly related to the anomalies of the Indian Ocean Dipole (IOD) mode. We then construct a stacked record of δD for the PEL for the period of 1654–2009 from individual normalized and filtered isotopic records obtained at six different sites (PEL2016 stacked record). We use a linear regression of this record and the stacked PEL temperature record (with an apparent slope of 9 ± 5.4 ‰ °C−1) to convert PEL2016 into a temperature scale. Analysis of PEL2016 shows a 1 ± 0.6 °C warming in this region over the last 3 centuries, with a particularly cold period from the mid-18th to the mid-19th century. A peak of cooling occurred in the 1840s – a feature previously observed in other Antarctic records. We reveal that PEL2016 correlates with a low-frequency component of IOD and suggest that the IOD mode influences the Antarctic climate by modulating the activity of cyclones that bring heat and moisture to Antarctica. We also compare PEL2016 with other Antarctic stacked isotopic records. This work is a contribution to the PAGES (Past Global Changes) and IPICS (International Partnerships in Ice Core Sciences) Antarctica 2k projects.

A 181.8 m ice core was recovered from a borehole drilled into bedrock on the western plateau of Mt El'brus (43°20′53.9′′ N, 42°25′36.0′′ E; 5115 m a.s.l.) in the Caucasus, Russia, in 2009 (Mikhalenko et al., 2015). Here, we report on the results of the water stable isotope composition from this ice core with additional data from the shallow cores. The distinct seasonal cycle of the isotopic composition allows dating by annual layer counting. Dating has been performed for the upper 126 m of the deep core combined with 20 m from the shallow cores. The whole record covers 100 years, from 2013 back to 1914. Due to the high accumulation rate (1380 mm w.e. year−1) and limited melting, we obtained isotopic composition and accumulation rate records with seasonal resolution. These values were compared with available meteorological data from 13 weather stations in the region and also with atmosphere circulation indices, back-trajectory calculations, and Global Network of Isotopes in Precipitation (GNIP) data in order to decipher the drivers of accumulation and ice core isotopic composition in the Caucasus region. In the warm season (May–October) the isotopic composition depends on local temperatures, but the correlation is not persistent over time, while in the cold season (November–April), atmospheric circulation is the predominant driver of the ice core's isotopic composition. The snow accumulation rate correlates well with the precipitation rate in the region all year round, which made it possible to reconstruct and expand the precipitation record at the Caucasus highlands from 1914 until 1966, when reliable meteorological observations of precipitation at high elevation began.

We investigate for the first time the loss and alteration of past atmospheric information from air trapping mechanisms under low-accumulation conditions through continuous CH4 (and CO) measurements. Methane concentration changes were measured over the Dansgaard-Oeschger event 17 (DO-17, ĝ1/4 ĝ€†60ĝ€†000ĝ€†yrĝ€†BP) in the Antarctic Vostok 4G-2 ice core. Measurements were performed using continuous-flow analysis combined with laser spectroscopy. The results highlight many anomalous layers at the centimeter scale that are unevenly distributed along the ice core. The anomalous methane mixing ratios differ from those in the immediate surrounding layers by up to 50ĝ€†ppbv. This phenomenon can be theoretically reproduced by a simple layered trapping model, creating very localized gas age scale inversions. We propose a method for cleaning the record of anomalous values that aims at minimizing the bias in the overall signal. Once the layered-trapping-induced anomalies are removed from the record, DO-17 appears to be smoother than its equivalent record from the high-accumulation WAIS Divide ice core. This is expected due to the slower sinking and densification speeds of firn layers at lower accumulation. However, the degree of smoothing appears surprisingly similar between modern and DO-17 conditions at Vostok. This suggests that glacial records of trace gases from low-accumulation sites in the East Antarctic plateau can provide a better time resolution of past atmospheric composition changes than previously expected. We also developed a numerical method to extract the gas age distributions in ice layers after the removal of the anomalous layers based on comparison with a weakly smoothed record. It is particularly adapted for the conditions of the East Antarctic plateau, as it helps to characterize smoothing for a large range of very low-temperature and low-accumulation conditions. © Author(s) 2017.

Height changes of the ice surface above subglacial Lake Vostok, East Antarctica, reflect the integral effect of different processes within the subglacial environment and the ice sheet. Repeated GNSS (Global Navigation Satellite Systems) observations on 56 surface markers in the Lake Vostok region spanning 11 years and continuous GNSS observations at Vostok station over 5 years are used to determine the vertical firn particle movement. Vertical marker velocities are derived with an accuracy of 1 cm/yr or better. Repeated measurements of surface height profiles around Vostok station using kinematic GNSS observations on a snowmobile allow the quantification of surface height changes at 308 crossover points. The height change rate was determined at 1 ± 5 mm/yr, thus indicating a stable ice surface height over the last decade. It is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period. The continuous GNSS observations demonstrate that the particle heights vary linearly with time. Nonlinear height changes do not exceed ±1 cm at Vostok station and constrain the magnitude of spatiotemporal lake-level variations. ICESat laser altimetry data confirm that the amplitude of the surface deformations over the lake is restricted to a few centimeters. Assuming the ice sheet to be in steady state over the entire lake, estimates for the surface accumulation, on basal accretion/melt rates and on flux divergence, are derived.

Ion chromatography measurements of 1730 snow and firn samples obtained from three short cores and one pit in the Vostok station area, East Antarctica, allowed for the production of the combined volcanic record of the last 900 years (AD 1093–2010). The resolution of the record is 2–3 samples per accumulation year. In total, 24 volcanic events have been identified, including seven well-known low-latitude eruptions (Pinatubo 1991, Agung 1963, Krakatoa 1883, Tambora 1815, Huanaputina 1600, Kuwae 1452, El Chichon 1259) found in most of the polar ice cores. In comparison with three other East Antarctic volcanic records (South Pole, Plateau Remote and Dome C), the Vostok record contains more events within the last 900 years. The differences between the records may be explained by local glaciological conditions, volcanic detection methodology, and, probably, differences in atmospheric circulation patterns. The strongest volcanic signal (both in sulfate concentration and flux) was attributed to the AD 1452 Kuwae eruption, similar to the Plateau Remote and Talos Dome records. The average snow accumulation rate calculated between volcanic stratigraphic horizons for the period AD 1260–2010 is 20.9 mm H2O. Positive (+13%) anomalies of snow accumulation were found for AD 1661–1815 and AD 1992–2010, and negative (−12%) for AD 1260–1601. We hypothesized that the changes in snow accumulation are associated with regional peculiarities in atmospheric transport.

The recovery of a 1.5 million yr long ice core from Antarctica represents a keystone of our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we tackle the question of where such ice may still be found in the Antarctic ice sheet. We can show that such old ice is most likely to exist in the plateau area of the East Antarctic ice sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle position on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, however, we strongly suggest significantly reduced ice thickness to avoid bottom melting. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than but close to about 2500 m would be required to find 1.5 Myr old ice (i.e., more than 700 m less than at the current EPICA Dome C drill site). Within this constraint, the resolution of an Oldest-Ice record and the distance of such old ice to the bedrock should be maximized to avoid ice flow disturbances, for example, by finding locations with minimum geothermal heat flux. As the geothermal heat flux is largely unknown for the EAIS, this parameter has to be carefully determined beforehand. In addition, detailed bedrock topography and ice flow history has to be reconstructed for candidates of an Oldest-Ice ice coring site. Finally, we argue strongly for rapid access drilling before any full, deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.

We estimated the stable isotopic composition of water from the subglacial Lake Vostok using two different sets of samples: (1) water frozen on the drill bit immediately after the first lake unsealing and (2) water frozen in the borehole after the unsealing and re-drilled one year later. The most reliable values of the water isotopic composition are: -59.0 ± 0.3 ‰ for oxygen-18, -455 ± 1 ‰ for deuterium and 17 ± 1 ‰ for d-excess. This result is also confirmed by the modelling of isotopic transformations in the water which froze in the borehole, and by a laboratory experiment simulating this process. A comparison of the newly obtained water isotopic composition with that of the lake ice (-56.2 ‰ for oxygen-18, -442.4 ‰ for deuterium and 7.2 ‰ for d-excess) leads to the conclusion that the lake ice is very likely formed in isotopic equilibrium with water. In turn, this means that ice is formed by a slow freezing without formation of frazil ice crystals and/or water pockets. This conclusion agrees well with the observed physical and chemical properties of the lake's accreted ice. However, our estimate of the water's isotopic composition is only valid for the upper water layer and may not be representative for the deeper layers of the lake, so further investigations are required.

The reconstruction of the geological (tectonic) structure and environments of subglacial Lake Vostok is based on geophysical surveys and the study of mineral particles found in cores of accreted ice and frozen lake water (sampled after the lake was unsealed). Seismic reflection and refraction investigations conducted in the southern part of Lake Vostok show very thin (200–300 m) sedimentary cover overlying a crystalline basement. Most of this thin veneer is thought to have been deposited during temperate-glacial conditions in Oligocene to Middle Miocene time ( ca 34–14 Ma). The composition of the lake-bottom sediments can be deduced from mineral inclusions found in cores of accreted ice. Inclusions are represented by soft aggregates consisting mainly of clay–mica minerals and micrometre-sized quartz grains. Some of these inclusions contain subangular to semi-rounded rock clasts (siltstones and sandstones) ranging from 0.3 to 8 mm in size. In total, 31 zircon grains have been identified in two rock clasts and dated using SHRIMP-II. The ages of the studied zircons range from 0.6 to 2.0 Ga with two distinct clusters between 0.8 and 1.15 Ga and between 1.6 and 1.8 Ga. Rock clasts obviously came from the western lake shore, which is thus composed of terrigenous strata with an age of not older than 600 Ma. The sedimentary nature of the western lake shore is also confirmed by seismic refraction data showing seismic velocities there of 5.4–5.5 km s −1 at the bedrock surface. After Lake Vostok was unsealed, its water (frozen and sampled next season) was also studied with scanning electron microscopy and X-ray microprobe analysis. This study showed the existence of calcium carbonate and silica microparticles (10–20 μm across) in frozen water.

International audience; Deep drilling at the Vostok Station has reached the surface of subglacial Lake Vostok (LV) twice—in February 2012 and January 2015. As a result, three replicate cores from boreholes 5G-1, 5G-2 and 5G-3 became available for detailed and revalidation analyses of the 230 m thickness of the accreted ice, down to its contact with water at 3769 m below the surface. The study reveals that the concentration of gases in the lake water beneath Vostok is unexpectedly low. A clear signature of the melt water in the surface layer of the lake, which is subject to refreezing on the icy ceiling of LV, has been discerned in the three different properties of the accreted ice: the ice texture, the isotopic and the gas content of the ice. These sets of data indicate in concert that poor mixing of the melt (and hydrothermal) water with the resident lake water and pronounced spatial and/or temporal variability of local hydrological conditions are likely to be the characteristics of the southern end of the lake. The latter implies that the surface water may be not representative enough to study LV's behaviour, and that direct sampling of the lake at different depths is needed in order to move ahead with our understanding of the lake's hydrological regime.

To investigate factors influencing nucleation of air clathrate hydrates in polar ice sheets, we have performed high-resolution mapping of the distributions of soluble impurities, air bubbles and air- hydrate crystals versus depth in the Dome Fuji Antarctic ice. Significant correlation observed between the concentrations of air inclusions and impurities in ice along with frequent occurrence of impurities inside hydrate crystals suggest that micro-inclusions promote hydrate nucleation in the ice matrix. Our observations also show that the diffusive macroscopic-scale redistribution of air constituents in ice in the bubble-hydrate transition zone is controlled by the original sedimentary layering of soluble impurities acting as nucleation helpers. The results of this study are important for the correct interpretation of high-resolution gas analyses of ice cores and for better understanding the global bubble-to-hydrate transformation process in polar ice sheets.

A physical model for the snow/firn densification process (Salamatin and others, 2006) and Martinerie and others’ (1992, 1994) correlation for the firn density at the pore closure are employed to perform a scale analysis and computational experiments in order to deduce simplified relations for the close-off depth and ice age in quasi-stationary ice formation conditions. The critical snow density at which ice-grain rearrangement stops is used to take into account variability of snow structures subjected to densification. The results obtained are validated on a representative set of ice-core data from 22 sites which covers wide ranges of present-day temperatures and ice accumulation rates. A simple analytical approximation for the density–depth profile is proposed.

An important share of paleoclimatic information is buried within the lowermost layers of deep ice cores. Because improving our records further back in time is one of the main challenges in the near future, it is essential to judge how deep these records remain unaltered, since the proximity of the bedrock is likely to interfere both with the recorded temporal sequence and the ice properties. In this paper, we present a multiparametric study (δD-δ18Oice, δ18Oatm, total air content, CO2, CH4, N2O, dust, high-resolution chemistry, ice texture) of the bottom 60 m of the EPICA (European Project for Ice Coring in Antarctica) Dome C ice core from central Antarctica. These bottom layers were subdivided into two distinct facies: the lower 12 m showing visible solid inclusions (basal dispersed ice facies) and the upper 48 m, which we will refer to as the "basal clean ice facies". Some of the data are consistent with a pristine paleoclimatic signal, others show clear anomalies. It is demonstrated that neither large-scale bottom refreezing of subglacial water, nor mixing (be it internal or with a local basal end term from a previous/initial ice sheet configuration) can explain the observed bottom-ice properties. We focus on the high-resolution chemical profiles and on the available remote sensing data on the subglacial topography of the site to propose a mechanism by which relative stretching of the bottom-ice sheet layers is made possible, due to the progressively confining effect of subglacial valley sides. This stress field change, combined with bottom-ice temperature close to the pressure melting point, induces accelerated migration recrystallization, which results in spatial chemical sorting of the impurities, depending on their state (dissolved vs. solid) and if they are involved or not in salt formation. This chemical sorting effect is responsible for the progressive build-up of the visible solid aggregates that therefore mainly originate "from within", and not from incorporation processes of debris from the ice sheet's substrate. We further discuss how the proposed mechanism is compatible with the other ice properties described. We conclude that the paleoclimatic signal is only marginally affected in terms of global ice properties at the bottom of EPICA Dome C, but that the timescale was considerably distorted by mechanical stretching of MIS20 due to the increasing influence of the subglacial topography, a process that might have started well above the bottom ice. A clear paleoclimatic signal can therefore not be inferred from the deeper part of the EPICA Dome C ice core. Our work suggests that the existence of a flat monotonic ice-bedrock interface, extending for several times the ice thickness, would be a crucial factor in choosing a future "oldest ice" drilling location in Antarctica., SCOPUS: ar.j, info:eu-repo/semantics/published

Antarctic and Southern Ocean science is vital to understanding natural variability, the processes that govern global change and the role of humans in the Earth and climate system. The potential for new knowledge to be gained from future Antarctic science is substantial. Therefore, the international Antarctic community came together to ‘scan the horizon’ to identify the highest priority scientific questions that researchers should aspire to answer in the next two decades and beyond. Wide consultation was a fundamental principle for the development of a collective, international view of the most important future directions in Antarctic science. From the many possibilities, the horizon scan identified 80 key scientific questions through structured debate, discussion, revision and voting. Questions were clustered into seven topics: i) Antarctic atmosphere and global connections, ii) Southern Ocean and sea ice in a warming world, iii) ice sheet and sea level, iv) the dynamic Earth, v) life on the precipice, vi) near-Earth space and beyond, and vii) human presence in Antarctica. Answering the questions identified by the horizon scan will require innovative experimental designs, novel applications of technology, invention of next-generation field and laboratory approaches, and expanded observing systems and networks. Unbiased, non-contaminating procedures will be required to retrieve the requisite air, biota, sediment, rock, ice and water samples. Sustained year-round access to Antarctica and the Southern Ocean will be essential to increase winter-time measurements. Improved models are needed that represent Antarctica and the Southern Ocean in the Earth System, and provide predictions at spatial and temporal resolutions useful for decision making. A co-ordinated portfolio of cross-disciplinary science, based on new models of international collaboration, will be essential as no scientist, programme or nation can realize these aspirations alone.

Three different approaches to ice-core age dating are employed to develop a depth-age relationship at Dome F: (1) correlation of the ice-core isotope record to the geophysical metronome(Milankovich surface temperature cycle) inferred from the deep borehole temperature profile at Vostok,(2) importing a known chronology from another(Devils Hole) paleoclimatic signal, and(3) direct ice sheet flow modeling. Inverse Monte Carlo sampling is used to constrain the accumulation rate reconstruction and ice flow simulations in order to find the best-fit glaciological time scale matched with the two other chronologies. General uncertainty of the different age estimates varies from 2 to 6kyr on average and reaches 6-14kyr at maximum. Whatever the causes of this discrepancy might be, they are thought to be of different origins, and the age errors are assumed to be independent. Thus, the average time scale for the Dome F ice core down to a depth of 2500m(ice age of 335kyr) is deduced consistently with all three age-depth relationships within the standard deviation limits of ±3.3kyr, and its accuracy is estimated as 1.4kyr on average. The constrained ice-sheet flow model allows extrapolation of the ice age-depth curve further to the glacier bottom and predicts the ages at depths of 2800, 3000, and 3050m to be 615±70, 1560±531, and 2985±1568kyr, respectively.

We have used 16S ribosomal genes to estimate the bacterial contents of Lake Vostok accretion ice samples at 3551 m and 3607 m, both containing sediment inclusions and formed 20000–15000 yr ago. Decontamination proved to be a critical issue, and we used stringent ice chemistry-based procedures and comprehensive biological controls in order to restrain contamination. As a result, up to now we have only recognized one 16S rDNA bacterial phylotype with confident relevance to the lake environment. It was found in one sample at 3607 m depth and represents the extant thermophilic facultative chemolithoautotroph Hydrogenophilus thermoluteolus of beta-Proteobacteria, and until now had only been found in hot springs. No confident findings were detected in the sample at 3551 m, and all other phylotypes revealed (a total of 16 phylotypes, 336 clones including controls) are presumed to be contaminants. It seems that the Lake Vostok accretion ice is actually microbe-free, indicating that the water body should also be hosting a highly sparse life. The message of thermophilic bacteria suggests that a geothermal system exists beneath the cold water body of Lake Vostok, what is supported by the geological setting, the long-term seismotectonic evidence from 4He degassing and the ‘18O shift’ of the Vostok accretion ice. The seismotectonic activity that seems to operate in deep faults beneath the lake could sustain thermophilic chemolithoautotrophic microbial communities. Such a primary production scenario for Lake Vostok may have relevance for icy planets and the approaches used for estimating microbial contents in accretion ice are clearly relevant for searching for extraterrestrial life.

We performed X-ray diffraction measurements on eight ice samples taken between 3200 and 3611 m depth of the Vostok (Antarctica) ice core to observe lattice distortions of ice crystals. Selected samples represent three distinct sections of the core: (i) glacier (meteoric) ice with well-preserved climatic record (down to 3310 m), (ii) ‘shear zone’ at the base of the glacier ice (3450– 3537 m) within which the climatic record is disturbed by ice deformation, and (iii) accretion ice formed by freezing of subglacial Vostok lake waters at the base of the ice sheet (from about 3537 m depth to the bottom of the core). The dislocation density decreases from 1012 to 108m–2 with increasing depth. In the accretion ice, lattice distortion tends to decrease with depth. However, the dislocation density does not reach a level typical for laboratory-grown columnar ice even at 3610 m. This reflects plastic deformation which accretion ice has undergone after its formation.

Based on the theory of precipitation from supersaturated solutions proposed by Lifshitz and Slyozov (J. Phys. Chem. Solids 19 (1/2) (1961) 35), we develop a mathematical description of post-formation growth (ripening) of mixed air clathrate-hydrate crystalline inclusions in polar ice sheets. The growth is controlled by oxygen and nitrogen diffusion through the ice matrix. Hydrate populations in general go through three sequential stages: (1) a short transient characterized by the rapid composition relaxation and dissolution of the smallest hydrates, (2) a slow transformation of the resulting size distributions towards a steady-state pattern that is an attribute of (3) the asymptotic stage of ripening. A regularization procedure is used to numerically solve the initial value problem. Computer simulations of the hydrate size distributions are compared to the data from a 3300-m ice core from Vostok Station, East Antarctica. The asymptotic stage is likely unattainable in natural conditions. Data from the GRIP ice core (central Greenland) suggest that the activation energy of hydrate growth increases at the elevated temperature near the ice-sheet bottom. The theory predicts extinction of the climatically induced fluctuations in the hydrate number-concentration and mean-radius profiles in ice sheets with depth.

We present new ice core records showing the temporal variation in the natural Pb, Cd, Zn, and Cu concentrations in the Vostok Antarctic ice over the past 240,000 years. Our data show that concentrations of these heavy metals have varied remarkably over the last two glacial-interglacial cycles. The concentrations ranged from 0.9 to 21.3, 0.04 to 0.62, 3.12 to 126, and 2.27 to 37.4 pg/g for Pb, Cd, Zn, and Cu, respectively. These profiles provide a better understanding of climate-related variation in the occurrence of these heavy metals in ancient Antarctic ice. The concentrations were much higher during cold glacial periods than during interglacials, and poked at the coldest glacial stages. The contribution of rock and soil dust is estimated to be close to the measured concentrations of Pb, Zn, and Cu, but not Cd, in the ice during cold glacial periods.

The geophysical metronome (Milankovitch components of the past surface temperature variations) and the isotope–temperature transfer function deduced from the borehole temperature profile at Vostok station, Antarctica, are applied to date the 2500 mdeep ice core from Dome Fuji station, Antarctica, and to reconstruct paleoclimatic conditions at the drilling site on the basis of the local δ18O isotope record. Special attention is paid to consistency of this depth–age relation with the mass-balance reconstruction and predictions of ice-flow modeling. the present-day ice mass-balance rate at Dome Fuji is estimated as 3.2 cm a–1. the ice age at the borehole bottom (590m above the bedrock) is around 335 ± 4.5 kyr and may reach 2000 kyr at about 3000 mdepth.The difference in the ice-sheet surface temperatures between Holocene optimum and Last Glacial Maximum is found to be 17.8˚C at the temporal isotope/temperature slope, about 30% lower than the modern geographical estimates. A good agreement between modeled and measured (preliminary data) borehole temperatures is obtained at the geothermal flux 0.059 Wm–2 and ice-fusion temperature (–2˚C) at the ice–rock interface with minimum (zero) melt rates.

X-ray diffraction experiments were carried out on large ice crystals from accreted ice above Lake Vostok, a subglacial lake lying beneath the East Antarctic ice sheet. Results indicate a surprisingly very low lattice distortion. This crystalline quality does not seem to be affected by impurities. Abnormal grain growth should occur and could explain both the large grain size and the low lattice distortion. Accreted ice is therefore supposed to be non-plastically deforming. These results should be taken into account for further studies of the permeability of accreted ice to drilling fluid present in the borehole.

Nucleation of air clathrate hydrates in air bubbles and diffusive air-mass exchange between coexisting ensembles of bubbles and hydrate crystals are the major interrelated processes that determine the phase change in the air–ice system in polar ice. In continuation of Salamatin et al. (J. Crystal Growth 193 (1998) 197; Ann. Glaciol. 29 (1999) 191) where the post-nucleation conversion of single air bubbles to hydrates was considered, we present here a statistical description for transformation of air bubbles to air clathrate hydrates based on the general theory of evolution of these two ensembles, including the gas fractionation effects. The model is fit to data on ice cores from central Antarctica, and then compared to other ice-core data. The focus is on the rate of clathrate–hydrate nucleation, which is determined to be the product of the inverse relative bubble size raised to the power λ≈5.8 with the relative supersaturation to the power β≈2. The clathration-rate constant is k0≈3.2–4.5×10−6 yr−1 at 220 K. The N2- and O2-permeation coefficients in ice, at 220 K, are inferred to be DN20≈1.8–2.5×10−8 mm2 yr−1 and DO20≈5.4–7.5×10−8 mm2 yr−1, respectively. Comparison of observations to simulations of bubble-to-hydrate transformation in Greenland ice sheet gave estimates for activation energies of hydrate formation and air diffusion of QJ≈70 kJ mol−1 and Q d ≈50 kJ mol−1, respectively.

We measured the incoherent inelastic neutron scattering (IINS) of Vostok Antarctic ice in order to investigate the arrangement of protons in the ice and to verify the proton ordering in Antarctic ice previously observed by IINS measurements on Dome Fuji Antarctic ice (FUKAZAWA et al., Chem. Phys. Lett., 294,554,1998a). The IINS spectrum of Vostok ice at 500m in depth has a peak at 19 meV in the region of translational lattice vibrations, while the peak in Vostok ice at 2452m in depth is much lower. Ice XI with a proton-ordered arrangement has a sharp peak at 19meV, while ordinary ice (ice Ih) with a proton-disordered arrangement does not have such a peak. These results of analysis of the IINS spectra indicate that Vostok ice at 500m in depth has a proton-ordered arrangement.

A recently developed theory of post-nucleation conversion of an air bubble to air-hydrate crystal in ice is applied to simulate two different types of air-hydrate formation in polar ice sheets. The work is focused on interpretation of the Vostok (Antarctica) ice-core data. The hydrostatic compression of bubbles is the rate-limiting step of the phase transformation which is additionally influenced by selective diffusion of the gas components from neighboring air bubbles. The latter process leads to the gas fractionation resulting in lower (higher) N2/O2 ratios in air hydrates (coexisting bubbles) with respect to atmospheric air. The typical time of the post-nucleation conversion decreases at Vostok from 1300-200 a at the beginning to 50-3 a at the end of the transition zone. The model of the diffusive transport of the air constituents from air bubbles to hydrate crystals is constrained by the data of Raman spectra measurements. The oxygen and nitrogen self-diffusion (permeation) coefficients in ice are determined at 220 K as 4.5 × 10−8 and 9.5 × 10−8 mm2 a−1, respectively while the activation energy is estimated to be about 50 kJ mol−1. The gas-fractionation time-scale at Vostok, τF ∼300 a, appears to be two orders of magnitude less than the typical time of the air-hydrate nucleation, τz ∼30-35 ka, and thus the condition for the extreme gas fractionation, τF ≪ τz is satisfied. Application of the theory to the GRIP and GISP2 ice cores shows that on average, a significant gas fractionation cannot be expected for air hydrates in central Greenland. However, a noticeable (statistically valid) nitrogen enrichment might be observed in the last air bubbles at the end of the transition.

We present an attempt to model the process of conversion of an air bubble, trapped in ice, to clathrate air–hydrate crystal after its nucleation on the air–ice interface. Both counterparts of the transformation are considered: diffusion of interstitial water and air molecules through the growing hydrate layer that coats the bubble surface, and compressive deformation of the three-phase (air–hydrate–ice) system at a given temperature and load pressure. The mathematical model is constrained by laboratory experiments covering a wide range of thermodynamic conditions. Computational tests show that either diffusion or bubble compression can be the rate-limiting step in the post-nucleation growth of air–hydrate crystal. As a plastic material, air–hydrate appears to be, at least, one order harder than ice. The mass transfer coefficient for the diffusion of air and water molecules in air–hydrate is estimated to be 0.6–1.3 mm 2 /yr at 263 K with the activation energy not higher, than 30–50 kJ/mol. The mass flux of air, although small in comparison with that of water, plays an important role in the conversion. Special attention is paid to the case of air–hydrate growth in air bubbles in polar ice sheets.

International audience; An interpretation of the deuterium profile measured along the Vostok (East Antarctica) ice core down to 2755 m has been attempted on the basis of the borehole temperature analysis. An inverse problem is solved to infer a local “geophysical metronome,” the orbital signal in the surface temperature oscillations expressed as a sum of harmonics of Milankovich periods. By correlating the smoothed isotopic temperature record to the metronome, a chronostratigraphy of the Vostok ice core is derived with an accuracy of ±3.0–4.5 kyr. The developed timescale predicts an age of 241 kyr at a depth of 2760 m. The ratio δD/δTi between deuterium content and cloud temperature fluctuations (at the top of the inversion layer) is examined by fitting simulated and measured borehole temperature profiles. The conventional estimate of the deuterium‐temperature slope corresponding to the present‐day spatial ratio (9 per mil/°C) is confirmed in general. However, the mismatch between modeled and measured borehole temperatures decreases noticeably if we allow surface temperature, responsible for the thermal state of the ice sheet, to undergo more intensive precession oscillations than those of the inversion temperature traced by isotope record. With this assumption, we obtain the long‐term temporal deuterium‐temperature slope to be 5.8–6.5 per mil/°C which implies that the glacial‐interglacial temperature increase over central Antarctica was about 15°C in the surface temperature and 10°C in the inversion temperature. Past variations of the accumulation rate and the corresponding changes in the ice‐sheet surface elevation are simultaneously simulated.

A mathematical model for simulating the densification of bubbly glacier ice is used to interpret the following experimental data from the Vostok (central Antarctica) ice core: two ice-porosity profiles obtained by independent methods and a bubble-pressure profile obtained by direct measurements of air pressure within individual bubbles. The rheological properties of pure polycrystalline ice are deduced from the solution of the inverse problem. The model and the inferred ice-flow law are then validated, using porosity profiles from seven other ice cores drilled in Antarctica and Greenland, in the temperature range from -55° to -20° C. The following expression is adopted for the constitutive law:where ė and τ are the effective strain rate and stress, respectively, α is the creep exponent taken as 3.5, Rs is the gas constant and T(Ts) is the temperature (standard temperature). The numerical values obtained for the “linear” and “non-linear” viscosities are: μ1 = 2.9 ± 1.3 M Pa year and μ2 = 0.051 ± 0.019 M Paα year, and the apparent activation energy Q is confirmed to be 60 k J mole−1. The corresponding flow law is in good agreement with results of both mechanical tests and independent estimations based on the analysis of different natural phenomena associated with glacier-ice deformation. When the model is constrained by the porosity and bubble-pressure profiles from Vostok, the mean air content in Holocene ice is inferred to be about 0.088 cm3g−1. The corresponding mean air pressure in bubbles at the end of pore closure is about 0.083 M Pa, whereas the atmospheric pressure at this depth level would be 0.063 M Pa. The influence of the climatic change on the ice-porosity profile is discussed. It resulted in an increased air content in ice at Vostok during the Last Glacial Maximum: 0.096 cm3 g−1.

Dry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn. Bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure. This stage extends to the transition zone where, due to high pressures and low temperatures. air trapped in bubbles and ice begins to form the mixed air clathrate hydrates, while the gas phase progressively disappears. Here we develop a model of bubbly-ice rheology and ice-sheet dynamics taking into account glacier-ice compressibility. The interaction between hydrostatic compression of air bubbles, deviatoric (uniaxial) compressive deformation of the ice matrix and global deformations of the glacier body is considered. The ice-matrix pressure and the absolute-load pressure are distinguished. Similarity theory and scale analysis are used in examine the resultant mathematical model of bubbly-ice densification. The initial rate of bubble compression in ice sheets appears to be relatively high, so that the pressure (density) relaxation process takes place only 150-200 m in depth (below pore close-off) to reach its asymptotic phase, wherein the minimal drop between bubble and ice pressures is governed by the rate of loading (ice accumulation). This makes it possible to consider densification under stationary (present-day) conditions of ice formation as a special case of primary interest. The computational tests performed with the model indicate that both ice-porosity and bubble-pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice. However, only the bubble-pressure distinguishes between the rheological properties at low and high stresses. The porosity profile at the asymptotic phase is mostly determined by the air content in the ice. In the companion paper (Lipenkov and others, 1997), we apply the model to experimental data from polar ice cores and deduce, through an inverse procedure, the rhelogical properties of pure ice as well as the mean air content in Holocene and glacial ice sediments at vostok Station (Antarctica).

An accurate and coherent chronological framework is essential for the interpretation of climatic and environmental records obtained from deep polar ice cores. Until now, one common ice core age scale had been developed based on an inverse dating method (Datice), combining glaciological modelling with absolute and stratigraphic markers between 4 ice cores covering the last 50 ka (thousands of years before present) (Lemieux-Dudon et al., 2010). In this paper, together with the companion paper of Veres et al. (2013), we present an extension of this work back to 800 ka for the NGRIP, TALDICE, EDML, Vostok and EDC ice cores using an improved version of the Datice tool. The AICC2012 (Antarctic Ice Core Chronology 2012) chronology includes numerous new gas and ice stratigraphic links as well as improved evaluation of background and associated variance scenarios. This paper concentrates on the long timescales between 120–800 ka. In this framework, new measurements of δ18Oatm over Marine Isotope Stage (MIS) 11–12 on EDC and a complete δ18Oatm record of the TALDICE ice cores permit us to derive additional orbital gas age constraints. The coherency of the different orbitally deduced ages (from δ18Oatm, δO2/N2 and air content) has been verified before implementation in AICC2012. The new chronology is now independent of other archives and shows only small differences, most of the time within the original uncertainty range calculated by Datice, when compared with the previous ice core reference age scale EDC3, the Dome F chronology, or using a comparison between speleothems and methane. For instance, the largest deviation between AICC2012 and EDC3 (5.4 ka) is obtained around MIS 12. Despite significant modifications of the chronological constraints around MIS 5, now independent of speleothem records in AICC2012, the date of Termination II is very close to the EDC3 one.

A new ice core drilled at the Russian station of Vostok in Antarctica reached 2755 m depth in September 1993. At this depth, the glaciological time scale provides an age of 260 ky BP (±25). We refine this estimate using records of dust and deuterium in the ice and of δ18O of O2 in the entrapped air. δ18O of O2 is highly correlated with insolation over the last two climatic cycles if one assumes that the EGT chronology overestimates the increase of age with depth by 12% for ages older than 112 ky BP. This modified age-depth scale gives an age of 244 ky BP at 2755 m depth and agrees well with the age-depth scale of Walbroeck et al. (in press) derived by orbital tuning of the Vostok δD record. We discuss the temperature interpretation of this latter record accounting for the influence of the origin of the ice and using information derived from deuterium-excess data. We conclude that the warmest period of stage 7 was likely as warm as today in Antarctica. A remarkable feature of the Vostok record is the high level of similarity of proxy temperature records for the last two climatic cycles (stages 6 and 7 versus stages 1–5). This similarity has no equivalent in other paleorecords.