A. Landais

Laboratoire des Sciences du Climat et l'Environnement, Gif, Île-de-France, France

Are you A. Landais?

Claim your profile

Publications (126)412.23 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Full text, figures and supplement available in open access at: http://www.clim-past.net/10/2115/2014/ Glacial climate was characterised by two types of abrupt events. Greenland ice cores record Dansgaard–Oeschger events, marked by abrupt warming in-between cold, stadial phases. Six of these stadials appear related to major Heinrich events (HEs), identified from ice-rafted debris (IRD) and large excursions in carbon- and oxygen-stable isotopic ratios in North Atlantic deep sea sediments, documenting major ice sheet collapse events. This finding has led to the paradigm that glacial cold events are induced by the response of the Atlantic Meridional Overturning Circulation to such massive freshwater inputs, supported by sensitivity studies conducted with climate models of various complexities. These models also simulate synchronous Greenland temperature and lower-latitude hydrological changes. To investigate the sequence of events between climate changes at low latitudes and in Greenland, we provide here the first 17O-excess record from a Greenland ice core during Dansgaard–Oeschger events 7 to 13, encompassing H4 and H5. Combined with other ice core proxy records, our new 17O-excess data set demonstrates that stadials are generally characterised by low 17O-excess levels compared to interstadials. This can be interpreted as synchronous change of high-latitude temperature and lower-latitude hydrological cycle (relative humidity at the oceanic source of evaporation or change in the water mass trajectory/recharge) and/or an influence of local temperature on 17O-excess through kinetic effect at snow formation. As an exception from this general pattern, stadial 9 consists of three phases, characterised first by Greenland cooling during 550 ± 60 years (as shown by markers of Greenland temperature δ18O and δ15N), followed by a specific lower-latitude fingerprint as identified from several proxy records (abrupt decrease in 17O-excess, increase in CO2 and methane mixing ratio, heavier δD-CH4 and δ18Oatm), lasting 740 ± 60 years, itself ending approximately 390 ± 50 years prior to abrupt Greenland warming. We hypothesise that this lower-latitude signal may be the fingerprint of Heinrich event 4 in Greenland ice cores. The proposed decoupling between stable cold Greenland temperature and low-latitude climate variability identified for stadial 9 provides new targets for benchmarking climate model simulations and testing mechanisms associated with millennial variability.
    Climate of the Past. 12/2014; 10:2115-2133.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Whereas millennial to sub-millennial climate variability has been identified during the current interglacial period, past interglacial variability features remain poorly explored because of lacking data at sufficient temporal resolutions. Here, we present new deuterium data from the EPICA Dome C ice core, documenting at decadal resolution temperature changes occurring over the East Antarctic plateau during the warmer-than-today last interglacial. Expanding previous evidence of instabilities during the last interglacial, multi-centennial sub-events are identified and labelled for the first time in a past interglacial context. A variance analysis further reveals two major climatic features. First, an increase in variability is detected prior to the glacial inception, as already observed at the end of Marine Isotopic Stage 11 in the same core. Second, the overall variance level is systematically higher during the last interglacial than during the current one, suggesting that a warmer East Antarctic climate may also be more variable.
    Geophysical Research Letters. 04/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Glacial climate was characterised by two types of abrupt events. Greenland ice cores record Dansgaard-Oeschger events, marked by abrupt warming in-between cold, stadial phases. Six of these stadials coincide with major Heinrich events (HE), identified from ice-rafted debris (IRD) and large excursions in carbon and oxygen stable isotopic ratios in North Atlantic deep sea sediments, documenting major ice sheet collapse events. This finding has led to the paradigm that glacial cold events are induced by the response of the Atlantic Meridional Overturning Circulation to such massive freshwater inputs, supported by sensitivity studies conducted with climate models of various complexities. This mechanism could however never be confirmed or infirmed because the exact timing of Heinrich events and associated low latitude hydrological cycle changes with respect to Greenland stadials has so far remained elusive. Here, we provide the first multi-proxy fingerprint of H4 within Stadial 9 in Greenland ice cores through ice and air proxies of low latitude climate and water cycle changes. Our new dataset demonstrates that Stadial 9 consists of three phases, characterised first by Greenland cooling during 550 ± 60 years (as shown by markers of Greenland temperature δ18O and δ15N), followed by the fingerprint of Heinrich Event 4 as identified from several proxy records (abrupt decrease in 17O excess and Greenland methane sulfonic acid (MSA), increase in CO2 and methane mixing ratio, heavier δ D-CH4 and δ18Oatm), lasting 740 ± 60 years, itself ending approximately 390 ± 50 years prior to abrupt Greenland warming. Preliminary investigations on GS-13 encompassing H5, based on the ice core proxies δ18O, MSA, δ18Oatm, CH4 and CO2 data also reveal a 3 phase structure, as well as the same sequence of events. The decoupling between stable cold Greenland temperature and low latitude HE imprints provides new targets for benchmarking climate model simulations and testing mechanisms associated with millennial variability.
    Climate of the Past Discussions 03/2014; 10:1179-1222.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Water stable isotopes in Greenland ice core data provide key paleoclimatic information, and have been compared with precipitation isotopic composition simulated by isotopically enabled atmospheric models. However, post-depositional processes linked with snow metamorphism remain poorly documented. For this purpose, monitoring of the isotopic composition (δ18O, δD) of near-surface water vapor, precipitation and samples of the top (0.5 cm) snow surface has been conducted during two summers (2011-2012) at NEEM, NW Greenland. The samples also include a subset of 17O-excess measurements over 4 days, and the measurements span the 2012 Greenland heat wave. Our observations are consistent with calculations assuming isotopic equilibrium between surface snow and water vapor. We observe a strong correlation between near-surface vapor δ18O and air temperature (0.85 ± 0.11‰ °C-1 (R = 0.76) for 2012). The correlation with air temperature is not observed in precipitation data or surface snow data. Deuterium excess (d-excess) is strongly anti-correlated with δ18O with a stronger slope for vapor than for precipitation and snow surface data. During nine 1-5-day periods between precipitation events, our data demonstrate parallel changes of δ18O and d-excess in surface snow and near-surface vapor. The changes in δ18O of the vapor are similar or larger than those of the snow δ18O. It is estimated using the CROCUS snow model that 6 to 20% of the surface snow mass is exchanged with the atmosphere. In our data, the sign of surface snow isotopic changes is not related to the sign or magnitude of sublimation or deposition. Comparisons with atmospheric models show that day-to-day variations in near-surface vapor isotopic composition are driven by synoptic variations and changes in air mass trajectories and distillation histories. We suggest that, in between precipitation events, changes in the surface snow isotopic composition are driven by these changes in near-surface vapor isotopic composition. This is consistent with an estimated 60% mass turnover of surface snow per day driven by snow recrystallization processes under NEEM summer surface snow temperature gradients. Our findings have implications for ice core data interpretation and model-data comparisons, and call for further process studies.
    Climate of the Past Discussions 01/2014; 10(1).
  • [Show abstract] [Hide abstract]
    ABSTRACT: [1] The isotopic composition of precipitation, in deuterium, oxygen 18 and oxygen 17, depends on the climatic conditions prevailing in the oceanic regions where it originates, mainly the sea surface temperature and the relative humidity of air. This dependency applies to present-day precipitation but also to past records which are extracted, for example, from polar ice cores. In turn, coisotopic measurements of deuterium and oxygen 18 offer the possibility to retrieve information about the oceanic origin of modern precipitation as well as about past changes in sea surface temperature and relative humidity of air. This interpretation of isotopic measurements has largely relied on simple Rayleigh-type isotopic models and is complemented by Lagrangian back trajectory analysis of moisture sources. It is now complemented by isotopic General Circulation Models (IGCM) in which the origin of precipitation can be tagged. We shortly review published results documenting this link between the oceanic sources of precipitation and their isotopic composition. We then present experiments performed with two different IGCMs, the GISS model II and the LMDZ model. We focus our study on marine water vapor and its contribution to precipitation over Antarctica and over the Andean region of South America. We show how IGCM experiments allow us to relate climatic conditions prevailing in the oceanic source of precipitation to its isotopic composition. Such experiments support, at least qualitatively, the current interpretation of ice core isotopic data in terms of changes in sea surface temperature. Additionally, we discuss recent studies clearly showing the added value of oxygen 17 measurements.
    Water Resources Research. 11/2013; 49(11).
  • [Show abstract] [Hide abstract]
    ABSTRACT: The end of the Last Glacial Maximum (Termination I), roughly 20 thousand years ago (ka), was marked by cooling in the Northern Hemisphere, a weakening of the Asian monsoon, a rise in atmospheric CO2 concentrations and warming over Antarctica. The sequence of events associated with the previous glacial–interglacial transition (Termination II), roughly 136 ka, is less well constrained. Here we present high-resolution records of atmospheric CO2 concentrations and isotopic composition of N2—an atmospheric temperature proxy—from air bubbles in the EPICA Dome C ice core that span Termination II. We find that atmospheric CO2 concentrations and Antarctic temperature started increasing in phase around 136 ka, but in a second phase of Termination II, from 130.5 to 129 ka, the rise in atmospheric CO2 concentrations lagged that of Antarctic temperature unequivocally. We suggest that during this second phase, the intensification of the low-latitude hydrological cycle resulted in the development of a CO2 sink, which counteracted the CO2 outgassing from the Southern Hemisphere oceans over this period.
    Nature Geoscience 10/2013; 6(12):1062-1065. · 11.67 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Stable isotopes of water have long been used to improve understanding of the hydrological cycle, catchment hydrology, and polar climate. Recently, there has been increasing interest in measurement and use of the less-abundant (17)O isotope in addition to (2)H and (18)O. Off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and precise measurements δ(18)O, δ(17)O, and (17)O-excess in liquid water. OA-ICOS involves no sample conversion and has a small footprint, allowing measurements to be made by researchers collecting the samples. Repeated (514) high-throughput measurements of the international isotopic reference water standard GISP demonstrate the precision and accuracy of OA ICOS: δ(18)OVSMOW-SLAP =-24.74 ± 0.07 ‰ (1σ) and δ(17)OVSMOW-SLAP = -13.12 ± 0.05 ‰ (1σ). For comparison, the IAEA value for δ(18)OVSMOW-SLAP is 24.76 ± 0.09 ‰ (1σ) and an average of previously reported values for δ(17)OVSMOW-SLAP is -13.12 ± 0.06 ‰ (1σ). Multiple (26) high-precision measurements of GISP provide a (17)O-excessVSMOW-SLAP of 23 ± 10 per meg (1σ); an average of previously reported values for (17)O-excessVSMOW-SLAP is 22 ± 11 per meg (1σ). For all these OA-ICOS measurements, precision can be further enhanced by additional averaging. OA-ICOS measurements were compared with two independent isotope ratio mass spectrometry (IRMS) laboratories and shown to have comparable accuracy and precision as the current fluorination-IRMS techniques in δ(18)O, δ(17)O, and (17)O-excess. The ability to measure accuratelyδ(18)O, δ(17)O, and (17)O-excess in liquid water inexpensively and without sample conversion is expected to increase vastly the application of δ(17)O and (17)O-excess measurements for scientific understanding of the water cycle, atmospheric convection, and climate modeling among others.
    Analytical Chemistry 09/2013; · 5.82 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A period of continental ice growth between about 80,000 and 70,000 years ago was controlled by a decrease in summer insolation, and was among the four largest ice expansions of the past 250,000 years. The moisture source for this ice sheet expansion, known as the Marine Isotope Stage (MIS) 5a/4 transition, has been proposed to be the warm subpolar and northern subtropical Atlantic Ocean. However, the mechanism by which glaciers kept growing through three suborbital cooling events within this period, which were associated with iceberg discharge in the North Atlantic and cooling over Greenland, is unclear. Here we reconstruct parallel records of sea surface and air temperatures from marine microfossil and pollen data, respectively, from two sediment cores collected within the northern subtropical gyre. The thermal gradient between the cold air and warmer sea increased throughout the MIS5a/4 transition, and was marked by three intervals of even more pronounced thermal gradients associated with the C20, C19 and C18' cold events. We argue that the warm ocean surface along the western European margin provided a source of moisture that was transported, through northward-tracking storms, to feed ice sheets in colder Greenland, northern Europe and the Arctic.
    Nature Geoscience 09/2013; 6(10):837-841. · 11.67 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In order to reconstruct Greenland NGRIP temperature, measurements of δ15N from the beginning of the Holocene to Dansgaard-Oeschger (DO) event 8 have been performed. Together with previously measured and mostly published δ15N data, we are now able to present for the first time a NGRIP temperature reconstruction for the whole last glacial period (beginning of the Holocene back to 120 kyr) including every DO event based on δ15N isotope measurements using a firn densification and heat diffusion model. The detected temperature rises at DO events range from 5 °C (DO 25) up to 16.5 °C (DO 11), ± 3 °C. To bring measured and modelled data into agreement, we had to reduce the accumulation rate given by the ss09sea06bm time scale in some periods significantly, especially during the last glacial maximum (LGM). A comparison between reconstructed temperature and δ18Oice data confirms that the isotopic composition of the stadial was strongly influenced by seasonality. We continuously calculated α (δ18Oice to temperature sensitivity) on a 10 kyr running time window. α variations show an anticorrelation with obliquity, in agreement with a simple Rayleigh distillation model, and moreover seem to be influenced by Northern Hemisphere ice sheet volume.
    Climate of the Past Discussions 07/2013; 9(4):4099-4143.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Combined measurements of water isotopologues of a snow pit at Vostok over the past 60 y reveal a unique signature that cannot be explained only by climatic features as usually done. Comparisons of the data using a general circulation model and a simpler isotopic distillation model reveal a stratospheric signature in the (17)O-excess record at Vostok. Our data and theoretical considerations indicate that mass-independent fractionation imprints the isotopic signature of stratospheric water vapor, which may allow for a distinction between stratospheric and tropospheric influences at remote East Antarctic sites.
    Proceedings of the National Academy of Sciences 06/2013; · 9.81 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: During the last glacial cycle, Greenland temperature showed many rapid temperature variations, the so called Dansgaard-Oeschger (DO) events. The past atmospheric methane concentration closely follows these temperature variations, which implies that the warmings recorded in Greenland were probably hemispheric in extent. Here, we present 931 new methane measurements along the North Greenland Ice Core Project (NGRIP) ice core. We therefore substantially extend and complete the NGRIP methane record from Termination 1 back to the end of the last interglacial period in high resolution. We relate the amplitudes of the methane increases associated with DO events to the amplitudes of the temperature increases derived from stable nitrogen isotope (δ15N) measurements, which have been performed along the same ice core (see poster by P. Kindler). We find the sensitivity to oscillate between 5 and 20 ppbv/°C with the approximate frequency of the precessional cycle. A remarkable high sensitivity of 26 ppbv/°C is reached during Termination 1. Conservative analysis of the timing of the fast methane and temperature increases reveals significant lags of the methane increases for the DO events 5, 9, 10, 11, 13, and 15.
    04/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: During the last ice age dramatic temperature variations of up to 16 °C took place in Greenland which are now known as Dansgaard-Oeschger-events (DO-events). They most probably originate from the North Atlantic oceanic and atmospheric circulation system and are characterised by an abrupt warming within decades followed by a gradual cooling over hundreds to thousands of years. We have determined local temperature variations for DO-event 1 to 25 in Greenland based on δ15N measurements from the NorthGRIP ice core, corresponding to the period from 10 to 110 kyr b2k. The record is a composite of measurements from two laboratories, Laboratoire des Sciences du Climat et de l'Environnement, Paris (DO 18 to 25) and the Climate and Environmental Physics Division of the Physics Institute of the University of Bern (DO 1 to 17) with new measurements from the beginning of the Holocene to DO 8. Temperature variations were reconstructed by reproducing the measured 15N/14N ratio of air enclosed in ice bubbles by the firn densification and heat diffusion model from Schwander. The reconstruction show temperature amplitudes for the DO-events ranging from 5 to 16 °C, thereby the corresponding rates of change can exceed 0.5 °C/decade. In order get an agreement between measured δ15N, Δdepth and Δage values with their modelled analogues, a lower accumulation rate than the one associated with the used ss09sea06bm1 time scale had to be assumed. We had to reduce the accumulation rate time dependently by 0 to nearly 40% with a mean reduction over the whole time period of 16%. With these adjustments both the Δdepth and the Δage values agree between model and measurements.
    04/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: This presentation will be dedicated to the comparison of sets of information on polar climate extracted from an array of deep ice cores from Greenland (GISP2, GRIP, NGRIP, NEEM) and from East Antarctica (EDML, EDC, Vostok and TALDICE). This comparison will benefit from the synchronised AICC2012/GICC05 chronologies which have recently been established (Bazin et al, Clim. Past, submitted). It will rely on water stable isotope records, estimates of past accumulation rates derived from ice core chronologies, and estimates of past Greenland Summit temperature change derived from firn gas fractionation. The sequences of events between Greenland and Antarctic temperature, d18O, deuterium excess and accumulation will be investigated and discussed. The inter-ice core spread will be assessed throughout the time period from 30 to 11 ka, in order to extract the common climatic signals and the local deposition noise. The timing of the coldest period in polar temperature, water stable isotopes, and deuterium excess will be compared to the timing of the driest period as indicated by estimates of past accumulation rates. The bipolar sequence of events including changes in moisture sources and changes in polar climate will be identified and discussed in relationship with information on past changes in sea level, marine circulation and atmospheric composition.
    04/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have for the last four summer seasons since 2009 measured the isotopic composition of the water vapor in continuous mode on top of the Greenland Ice Sheet as part of the NEEM deep ice core-drilling project (77.45 N 51.06 W, 2484 m a.s.l). The purpose of this campaign has been to improve our understanding of the climatic factors controlling the ice core isotope signal, which can then be used to reconstruct past climate. To achieve such an understanding general circulation models provide a valuable tool. It is therefore crucial to test the ability of the models to simulate the present day hydrological cycle and its isotopic counterparts. We therefore compare the observed water vapor isotopic composition with model outputs from two isotope-enabled general circulation models (LMDZiso and isoGCM). We are thereby able to both validate, but also point to weaknesses in the modeled isotopic values. This gives us information about which parameterizations in the atmospheric hydrological cycle may need improvement. Together with the atmospheric water vapor observations on the Greenland Ice Sheet we also collected snow surface samples from the top one cm of the snow pack. The isotope values of these snow surface samples constitute the climate signal, which are stored in the ice core. We find that between precipitation events large variations are observed in the snow surface isotopic composition - potentially lagging the atmospheric water vapor isotopic composition. This finding has great importance for understanding the ice core isotope signal.
    04/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The last glacial period was affected by the occurrence of rapid climatic events at the millennial time scale known as Dansgaard-Oeschger (DO) events. In Greenland, these events are composed of a rapid temperature increase of 5-16° in less than a century, a warm phase lasting several centuries (InterStadial, GI) followed by a more gradual temperature decrease, and finally a cold phase (Stadial, GS). An Antarctic counterpart to each GI of the Last Glacial Period has been identified in Antarctic ice cores. In the North Atlantic Ocean, marine cores also record changes in surface temperature as well as the occurrence during cold phases of ice rafted debris horizons, corresponding to massive icebergs discharges, known as Heinrich (H) events. It has never been possible to identify the presence of H events from temperature proxies in Greenland ice cores. It thus remains difficult to compare the durations of H events and GS. Here, we focus on the time period covering DO 9 to 7 (41 to 34 ka b2k according to the GICC05/AICC2012 time scales), with H event 4 occurring during GS 9. We present a compilation of high resolution measurements (about 60 years) of this period based on Greenland and Antarctic ice cores data (ice and gas) synchronized on the new time scale AICC2012. Proxies for local Greenland temperature (δ15N-N2, δ18O-H2O) record GS9 as a uniform period lasting ~1850 years, followed by a sharp transition to GI8. This pattern is also seen in continuous methane concentration data (NEEM ice core, Greenland) showing a large increase by ~100 ppbv at the GS9 - GI8 transition. However, using additional proxies and a detailed inspection of the methane profile, GS9 can be divided into 3 phases. The first 600 years of GS9 (phase 1) are characterized by low CO2 and methane concentration, intermediate δD of CH4 (tracer of methane sources), high NEEM 17O-excess (proxy for vapor source relative humidity) and a progressive increase in EDML δ18O. The transition between phase 1 and phase 2 is marked by an abrupt increase of CO2 and CH4 concentration and δD-CH4 as well as a decrease of NEEM 17O-excess. All proxies stay constant during the 850 years of phase 2. At the transition between phase 2 and phase 3, 400 years before the onset of GI8, we observe an abrupt increase in NEEM 17O-excess and an abrupt decrease in δD-CH4 while methane concentration remains constant. In Antarctica, EDML δ18O progressively decreases. We speculate that these different phases may be linked to different atmospheric and/or oceanic conditions during GS9 probably related to the occurrence of H event 4. To explore this hypothesis, we propose a synchronization of North Atlantic marine records to ice cores records.
    04/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Past interglacial periods are characterized by global warm climatic conditions comparable to the ones of the current interglacial period, the Holocene. As perfect targets to better understand natural climate variability and benchmarks for climate models, they have been widely documented in recent years, at multi-millennial and millennial scale in particular. However, due to a lack of records at sufficient temporal resolutions, past interglacial climate variability has been barely explored at sub-millennial scale. Using a new water stable isotope record at decadal resolution from the EPICA Dome C ice core, we here characterize patterns and changes in Antarctic millennial to centennial climate variability occurring over the Last Interglacial period (LIG). Multi-centennial climatic sub-events, bearing comparison in terms of intensity with glacial Antarctic Isotopic Maxima, are for the first time identified in a past interglacial context. From observed changes in variance further arise two major points. First, the end of the LIG is marked by an increase in high frequency variability, preceding the cooling trend into glacial inception (as already observed for Marine Isotopic Stage 11). No such increase in variance can be detected so far over the last millennia. Second, the LIG variance level is significantly higher than the Holocene one. It questions the role of ocean-atmosphere-sea ice interactions in such enhanced variability, during a warmer climatic state likely associated with reduced Antarctic ice sheet.
    04/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Understanding the role of atmospheric CO during past climate changes requires clear knowledge of how it varies in time relative to temperature. Antarctic ice cores preserve highly resolved records of atmospheric CO and Antarctic temperature for the past 800,000 years. Here we propose a revised relative age scale for the concentration of atmospheric CO and Antarctic temperature for the last deglacial warming, using data from five Antarctic ice cores. We infer the phasing between CO concentration and Antarctic temperature at four times when their trends change abruptly. We find no significant asynchrony between them, indicating that Antarctic temperature did not begin to rise hundreds of years before the concentration of atmospheric CO, as has been suggested by earlier studies.
    Science 03/2013; 339(6123):1060-3. · 31.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling (‘NEEM’) ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 ± 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 ± 250 metres, reaching surface elevations 122,000 years ago of 130 ± 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
    Nature 01/2013; 493:489-494. · 38.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An accurate and coherent chronological frame-work is essential for the interpretation of climatic and envi-ronmental records obtained from deep polar ice cores. Un-til now, one common ice core age scale had been devel-oped based on an inverse dating method (Datice), combin-ing glaciological modelling with absolute and stratigraphic markers between 4 ice cores covering the last 50 ka (thou-sands 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 δ 18 O atm over Marine Isotope Stage (MIS) 11–12 on EDC and a complete δ 18 O atm record of the TALDICE ice cores permit us to derive additional or-bital gas age constraints. The coherency of the different orbitally deduced ages (from δ 18 O atm , δO 2 /N 2 and air con-tent) 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 be-tween speleothems and methane. For instance, the largest de-viation 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.
    Climate of the Past 01/2013; 9:1715-1731. · 3.56 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The deep polar ice cores provide reference records commonly employed in global correlation of past climate events. However, temporal divergences reaching up to sev-eral thousand years (ka) exist between ice cores over the last climatic cycle. In this context, we are hereby introduc-ing the Antarctic Ice Core Chronology 2012 (AICC2012), a new and coherent timescale developed for four Antarctic ice cores, namely Vostok, EPICA Dome C (EDC), EPICA Dronning Maud Land (EDML) and Talos Dome (TALDICE), alongside the Greenlandic NGRIP record. The AICC2012 timescale has been constructed using the Bayesian tool Dat-ice (Lemieux-Dudon et al., 2010) that combines glaciolog-ical inputs and data constraints, including a wide range of relative and absolute gas and ice stratigraphic markers. We focus here on the last 120 ka, whereas the companion paper by Bazin et al. (2013) focuses on the interval 120–800 ka. Compared to previous timescales, AICC2012 presents an improved timing for the last glacial inception, respecting the glaciological constraints of all analyzed records. Moreover, with the addition of numerous new stratigraphic markers and improved calculation of the lock-in depth (LID) based on δ 15 N data employed as the Datice background scenario, the AICC2012 presents a slightly improved timing for the bipo-lar sequence of events over Marine Isotope Stage 3 associ-ated with the seesaw mechanism, with maximum differences of about 600 yr with respect to the previous Datice-derived chronology of Lemieux-Dudon et al. (2010), hereafter de-noted LD2010. Our improved scenario confirms the regional differences for the millennial scale variability over the last glacial period: while the EDC isotopic record (events of tri-angular shape) displays peaks roughly at the same time as the NGRIP abrupt isotopic increases, the EDML isotopic record (events characterized by broader peaks or even extended pe-riods of high isotope values) reached the isotopic maximum several centuries before. It is expected that the future contribution of both other long ice core records and other types of chronological constraints to the Datice tool will lead to further refinements in the ice core chronologies beyond the AICC2012 chronology. For the time being however, we recommend that AICC2012 be used as the preferred chronology for the Vostok, EDC, EDML and TALDICE ice core records, both over the last glacial cycle Published by Copernicus Publications on behalf of the European Geosciences Union. 1734 D. Veres et al.: The Antarctic ice core chronology (AICC2012) (this study), and beyond (following Bazin et al., 2013). The ages for NGRIP in AICC2012 are virtually identical to those of GICC05 for the last 60.2 ka, whereas the ages beyond are independent of those in GICC05modelext (as in the construc-tion of AICC2012, the GICC05modelext was included only via the background scenarios and not as age markers). As such, where issues of phasing between Antarctic records in-cluded in AICC2012 and NGRIP are involved, the NGRIP ages in AICC2012 should therefore be taken to avoid in-troducing false offsets. However for issues involving only Greenland ice cores, there is not yet a strong basis to rec-ommend superseding GICC05modelext as the recommended age scale for Greenland ice cores.
    Climate of the Past 01/2013; 9:1733-1748. · 3.56 Impact Factor

Publication Stats

2k Citations
412.23 Total Impact Points

Institutions

  • 2003–2014
    • Laboratoire des Sciences du Climat et l'Environnement
      Gif, Île-de-France, France
  • 2013
    • University Joseph Fourier - Grenoble 1
      • Laboratoire de Glaciologie et Géophysique de l'Environnement
      Grenoble, Rhone-Alpes, France
  • 2004–2012
    • Institut Pierre Simon Laplace
      Lutetia Parisorum, Île-de-France, France
    • University of Copenhagen
      • Centre for Ice and Climate
      København, Capital Region, Denmark
  • 2011
    • Cea Leti
      Grenoble, Rhône-Alpes, France
  • 2010
    • Atomic Energy and Alternative Energies Commission
      • LSCE - Laboratoire des Sciences du Climat et de l'Environnement
      Gif-sur-Yvette, Ile-de-France, France
    • Université de Versailles Saint-Quentin
      • Laboratoire des Sciences du Climat et de l'Environnement (LSCE)
      Versailles, Île-de-France, France
  • 2004–2010
    • French National Centre for Scientific Research
      • Laboratoire de glaciologie et géophysique de l'environnement (LGGE)
      Lutetia Parisorum, Île-de-France, France
  • 2006–2008
    • Hebrew University of Jerusalem
      Yerushalayim, Jerusalem District, Israel
    • Weizmann Institute of Science
      • Department of Environmental Sciences and Energy Research
      Jerusalem, Jerusalem District, Israel