J. R. Petit

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica

The recent completion of drilling at Vostok station in East Antarctica has allowed the extension of the ice record of atmospheric composition and climate to the past four glacial–interglacial cycles. The succession of changes through each climate cycle and termination was similar, and atmospheric and climate properties oscillated between stable bounds. Interglacial periods differed in temporal evolution and duration. Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years.

Climatic interpretation of the recently extended Vostok ice records

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.

Aeolian dust in East Antarctica (EPICA‐Dome C and Vostok): Provenance during glacial ages over the last 800 kyr

Aeolian mineral dust archived in Antarctic ice cores represents a key proxy for Quaternary climate evolution. The longest and most detailed dust and climate sequences from polar ice are provided today by the Vostok and by the EPICA-Dome C (EDC) ice cores. Here we investigate the geographic provenance of dust windborne to East Antarctica during Early and Middle Pleistocene glacial ages using strontium and neodymium isotopes as tracers. The isotopic signature of Antarctic dust points towards a dominant South American origin during Marine Isotopic Stage (MIS) 8, 10, 12, and back to MIS 16 and 20 as deduced from EDC core. Data provide evidence for a persistent overall westerly circulation pattern allowing efficient transfer of dust from South America to the interior of Antarctica over the last 800 kyr. Some small but significant dissimilarity between old and recent glacial ages suggests a slightly reduced Patagonian contribution during ancient glaciations.

Soluble and insoluble impurities along the 950 m deep Vostok ice core (Antarctica) ― Climatic implications

Simultaneous measurements of soluble and insoluble impurities were made on the 950 m deep Vostok (78°30′S, 106°54′E, 3420 m a.s.l.) ice core, spanning roughly 50000 yr, using various analytical techniques. We observed higher continental (×37) and marine (×5.1) inputs during the last glacial age than during the Holocene stage. A study of microparticle compositions and of volcanic indicators (Zn, H2SO4), shows that the high observed crustal input is not due to enhanced volcanism, but is rather of continental eolian origin. For the first time, the ionic balance along a deep ice core is established, mainly used in discussing the evolution of the Cl to Na ratio over central East Antarctica with changing climatic conditions: the presence of relatively high amounts of Na2SO4 in the marine aerosol at the Vostok site during the Holocene is demonstrated. Comparison with the Dome C (74°39′S, 124°10′E, 3040 m a.s.l.) results confirms the chronology of the major events: (i) maximum terrestrial input around the last glacial maximum (∼18 ka BP); (ii) end of the high continental flux over Antarctica near 13 ka BP; (iii) marine input varying in an opposing manner to isotopic fluctuations with rather high concentrations beginning to decrease when isotopic values increase and reaching Holocene values at the end of the transition between cold and warmer climate conditions. Detailed comparison with results provided by deep ice cores from other sites which are probably more influenced by oceanic air masses seems to indicate that most of the aerosol reaching central East Antarctica travel over large distance probably at rather high altitude through the troposphere. We can consider that central East Antarctica is well representative of the upper part of the troposphere (higher than i.e., 3000 m) and should, therefore, provide valuable data for global and Antarctic paleoclimatological models.

A model for large glacial-interglacial climate-induced changes in dust and sea salt concentrations in deep ice cores (central Antarctica): palaeoclimatic implications and prospects for refining ice core chronologies

A semi-empirical model has been developed to reproduce glacial–interglacial changes of continental dust and marine sodium concentrations (factor of ∼50 and ∼5, respectively) observed in inland Antarctic ice cores. The model uses conceptual pathways of aerosols within the high troposphere; assumes the dry deposition of impurities on the Antarctic surface; uses estimates of aerosol transit times taken independent of climate; assumes a temperature-dependent removal process during aerosol pathways from the mid-latitudes. The model is fitted to the data over the last four climate cycles from Vostok and EPICA Dome C Antarctic sites. As temperature is cooling, the aerosol response suggests different modes of climate couplings between latitudes, which can be continuous or below temperature thresholds for sodium and dust, respectively. The model estimates a southern South America dust source activity two to three times higher for glacial periods than for the Holocene and a glacial temperature over the Southern Ocean 3–5 ◦C cooler. Both estimates appear consistent with independent observations. After removal of temperature effects, dust and sodium residuals for both sites show orbital frequencies in opposite phase at the precession timescale. Such long-term insolation-related modulation of terrestrial and marine aerosol input, could provide a chemical pacemaker useful for refining ice core chronologies.

Late Glacial Input of Eolian Continental Dust in the Dome C Ice Core: Additional Evidence from Individual Microparticle Analysis

399 individual microparticles in nine samples from the Dome C ice core were studied under ~ scanning electron microscope and analysed by an energy dispersive X-ray system. The studied particles were either continental quartz or various silico-aluminates of continental or volcanic origin. Observations lead to the conclusion that the increase in microparticle concentration by a factor of 10 to 20 during the last glacial stage is explained by a large input of continental dust, as already indicated by trace element analysis (Petit and others 1981) and previously suggested by chemical analysis of other polar ice cores (Cragin and others 1977). This increase is considered to be a consequence of the ice-age climate and earth surface conditions which were characterized by the increase of arid regions and more vigorous atmospheric circulation. Both these conclusions are further supported by the existence of a higher quartz content in the Antarctic ice core as was already found in tropical deep-sea core studies. INTRODUCTION Assuming that chemical concentrations in Antarctic air and snow are correlated (Pourchet and others in press), the atmospheric composition in the past can probably be deduced from the chemical composition of ice samples taken from pit walls (hundreds of years old) or deep ice cores (thousands of years). In order to study the possible 1 ink between atmospheric aerosol content and climate we measured the trace element composition and microparticle concentration of ice samples from the Dome C (74°S,124°E) core, each one covering a 3 to 10 a period. The 905 m deep ice core spans approximately 30 ka. The depths corresponding to the last glacial maximum ( ~ 18 000 BP), to the climatic transition, and to the Holocene period were taken from the isotopic profile by Lorius and others (1979) (see periods 3, 2, and 1 on Figure 1). The first step in our experiment involved removing the outer part of the samples, contaminated by field procedures, and measuring components already known to be of crustal origin (Al, V, Mn, Zn, insoluble microparticles) and marine origin (Cl and Na). The detailed analytical procedure along with complete results and discussion have been published elsewhere (Petit and others 1981). The main results are that the conti nenta 1 (see Fi g.l) and ma ri ne inputs were respectively about 20 and 5 times higher during the late Fig.l. Stable isotope content (o~18%o), microparti cle and alul1inium (continental indicator) concentrations for the Dome C ice core: (a) microparticle data from light-scattering technique (LS) in rel ative units, (b) Coulter counter concentrations (CC), units in 10 3 particles (r>0.4~m) g-1 of snow. Data from Thompson and others (1981) were also used. The chronology and isotope profile with distinct isotopic periods were presented by Lorius

Interannual variation of water isotopologues at Vostok indicates a contribution from stratospheric water vapor

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 17O-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.

CLIMATIC INTERPRETATION OF A CONTINUOUS DEUTERIUM PROFILE OBTAINED FROM THE VOSTOK ICE CORE, ANTARCTICA (160 000 YEARS) (Abstract)

Oceanic studies have convincingly demonstrated that there is a link between the Pleistocene ice ages and the variations in the elements of the Earths orbit (Imbrie and others 1984). In contrast, the climatic conditions which prevailed over continental areas have been far less well documented and then rarely on a quantitative basis. In this context, the 2083 m ice core recovered by the Soviet Antarctic Expeditions at Vostok (East Antarctica) is of fundamental importance because it covers fully the last glacial-interglacial cycle, back to the ice age which preceded the last interglacial (-160 ka B.P .). Potentially it allows access to many climatic and climate-related parameters as illustrated by the oxygen-18 data we have recently published (Lorius and others 1985), from lOBe measurements (Yiou and others 1985, Raisbeck and others 1987), from aerosol concentration (De Angelis and others 1987) and from CO 2 measurement (Barnola and others 1988, this volume). Our first isotopic data set was largely discontinuous over the last 100 ka (only about 7% of the core was analyzed), but continuous beyond that time. Sampling of the ice was completed later, in the field, and we now have continuous deuterium data for the whole core (total ice recovery is about 85%), combining the data of the 2083 m core below 138 m and a complementary data set above. The core chronology was established using a two-dimensional ice-flow model and, for snow accumulation, taking into account change with time (Lorius and others 1985). There is a general correspondence between this curve and the previously published S 18 0 record (Lorius and others 1985). However, there is obviously far more information in this continuous SD record , which we will examine from the deduced temperature record.