N. I. Barkov

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.

Vostok (Antarctica) ice core: Atmospheric chemistry changes over the last climatic cycle (160,000 years)

A 2083 m deep ice core from Vostok Station (East Antarctica) has been used for a comprehensive study of all major ions (i.e. Na+, NH4+, K+, Ca2+, Mg2+, H+, Cl−, NO3− and SO42− originating from aerosols deposited over the last climatic cycle (160,000 a), as depicted from the isotopic composition of the ice. For the first time in deep ice core studies, a good balance between anions and cations is obtained throughout the profile. This allows the clear identification of marine salts (i.e. sea salt and Na2SO4), terrestrial salts (calcium and magnesium associated with nitrates and sulfates) and strong mineral acids (HNO3, H2SO4 and HCl). Concentration profiles confirm that both marine and terrestrial aerosol inputs were higher during cold climatic conditions (from 110 to 15 ka B.P.) than during the Last Interglacial (centered around 130 ka B.P.) and the Holocene (the last 10,000 a) stages. High concentration peaks (up to 5 and 30 times the Holocene values of marine and terrestrial contents, respectively) are in particular observed during the very cold climate characterizing the end of the penultimate glacial age (⋍ 160 ka B.P.) and the Last Glacial Maximum which terminated around 15 ka B.P. These peaks reflect strengthened sources and transport during full glacial conditions, linked to higher wind speeds, more extensive arid areas on the continents and the greater exposure of continental shelves. More generally, marine and terrestrial aerosol concentrations measured in ice are strongly affected by climatic conditions of global (source strength and atmospheric transport efficiency) and local (rate of snow accumulation) concern. As opposed to marine and terrestrial inputs, acidic gas-derived impurity concentrations (HNO3, H2SO4) remain relatively stable over the whole climatic cycle. In particular there is no correlation between observed H2SO4 fluctuations and the isotope-temperature profile. This would indicate the absence of a long-term relationship between volcanism and climate. The mineral acid contribution represents a large part (over 50%) of ice impurities deposited during interglacial periods. For glacial ice the contribution of marine and terrestrial salts becomes preponderant (up to 75% of total soluble impurities). During interglacial stages and the relatively warm periods of the Last Glacial Age, significant quantities of either Na2SO4 or HCl are found, possibly resulting from marine aerosol alteration during atmospheric transport from sea sources towards Antarctica. On the other hand, the Cl/Nam ratio values indicate the presence of non-fractionated marine aerosols during full glacial conditions, confirming faster transport from sea sources towards Antarctica.

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.

A comparison of deep Antarctic ice cores and their implications for climate between 65,000 and 15,000 years ago

Abstract Three ice cores drilled in the central part of the Antarctic continent extend back to the last glacial period: one from West Antarctica (Byrd) and two from East Antarctica (Vostok and Dome C). This period is also partly covered by a few cores from the coastal areas. In these cores, climatic information is mostly derived from the isotopic profiles (δD or δ 18 O) from which surface temperature and, more indirectly, precipitation rate can be estimated. The main objective has been to compare thoroughly the three deep ice cores for the main part of the last glacial period (from ca. 65,000–15,000 yr B.P.). The time scales have been examined in detail and a new 40,000 yr chronology for the Dome C core adopted. Special emphasis is placed on the link between the concentration of 10 Be and past accumulation changes and on the use of peaks in the concentration of this cosmogenic isotope as stratigraphic markers. Elevation changes of the ice sheet, derived from gas content and isotopic data, bear directly on interpretations of past temperature and precipitation rate changes.

CRYSTALLINE TEXTURE OF THE 2083 m ICE CORE AT VOSTOK STATION, ANTARCTICA

Crystalline texture and c-axis orien tation of the 2083 m ice core at Vostok Station, covering more than 150 kyear, reveal the existence of strong anisotropies. Changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries. A smaller growth rate appears to be associated with cold periods. A gradual increase in the horizontal elongation of grains was observed between 350 and 680 m. But, the mean value of the coefficient of the linear dimensional orientation of grains does not cha nge below 700 m. The e-axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains, forming a vertical girdle patt ern. This charac teristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longit udi nal tension. The rotation of grains was calcu lated with respect to the total strain, simulating the form ation of the girdle fabric pattern. The fabric-enhance ment factor was calculated at various depths. It appears that Vostok ice hardens gradua lly

Air content paleo record in the Vostok ice core (Antarctica) : a mixed record of climatic and glaciological parameters

Under present-day climatic conditions the air content of ice shows a high sensitivity to the atmospheric pressure and hence to the elevation at the surface of the ice sheet. This observation has been used to infer past ice sheet thickness variations of Antarctica and Greenland. A high-resolution air content profile (more than 1000 measurements) covering approximately the last 200,000 years was obtained along the 2546-m long Vostok ice core. Three analytical techniques were used, leading to consistent results which show large amplitude and rapid air content variations. The Vostok results support thicker/thinner ice in the central part of East Antarctica during warm/cold periods

A comparison of the Vostok ice deuterium record and series from Southern Ocean core MD 88-770 over the last two glacial-interglacial cycles

Taking advantage of the fact that the Vostok deuterium (δD) record now covers almost two entire climatic cycles, we have applied the orbital tuning approach to derive an age-depth relation for the Vostok ice core, which is consistent with the SPECMAP marine time scale. A second age-depth relation for Vostok was obtained by correlating the ice isotope content with estimates of sea surface temperature from Southern Ocean core MD 88-770. Both methods lead to a close correspondence between Vostok and MD 88-770 time series. However, the coherence between the correlated δD and insolation is much lower than between the orbitally tuned 8D and insolation. This reflects the lower accuracy of the correlation method with respect to direct orbital tuning. We compared the ice and marine records, set in a common temporal framework, in the time and frequency domains. Our results indicate that changes in the Antarctic air temperature quite clearly lead variations in global ice volume in the obliquity and precession frequency bands. Moreover, the average phase we estimated between the filtered δD and insolation signals at precessional frequencies indicates that variations in the southern high latitude surface temperature could be induced by changes in insolation taking place during a large period of the summer in northern low latitudes or winter in southern low latitudes. The relatively large lag found between Vostok δD variations and obliquity-driven changes in insolation suggests that variations in the local radiative balance are not the only mechanism responsible for the variability in surface temperature at those frequencies. Finally, in contrast to the cross-spectral analysis method used in previous studies, the method we use here to estimate the phases can reveal errors in cross-correlations with orbitally tuned chronologies.

Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record

In the interpretation of the Antarctic deep ice-core data, little attention has been given to the Holocene part of the records. As far as translation of the stable isotope content in terms of temperature is concerned, this can be understood because expected temperature changes may be obscured by isotopic noise of various origins and because no 14C dating has yet been available for this type of sequence. In this article, we focus on the Dome C and Vostok cores and on a new 850-m long ice core drilled out at Komsomolskaïa by the Soviet Antarctic Expeditions. These three sites are located in East Antarctica, on the Antarctic plateau, in a region essentially undisturbed by ice-flow conditions, so that their detailed intercomparison may allow us to identify the climatically significant isotopic signal. Our results compare well with the proximal records of Southern Hemisphere high latitudes and support the existence of a warmer “climatic optimum” between 10 and 6 ka y BP. Maximum temperatures are reached just at the end of the last deglaciation, which confirms previous observations at high latitudes, in contrast with later dates for the Atlantic and hypsithermal optima in Europe and North America.

Volcanic layers in Antarctic (Vostok) ice cores: Source identification and atmospheric implications

Fifteen visible volcanic ash layers (tephra) from Vostok ice cores have been analyzed for major elements, trace elements, and Sr and Nd isotope composition. Comparison of their geochemical signatures to lava composition from the inventory of Antarctic and subantarctic volcanoes, which have been active over the last 0.5 million years, indicates that nine layers originate from activity of the South Sandwich volcanic arc, three from southern South America, one from the Antarctic Peninsula (Bransfield Strait), and one from West Antarctica (Marie Byrd Land province). The large size of the tephra (up to 50 μm) requires rapid atmospheric transfer from the volcanic centers to East Antarctica. Rapid tropospheric transport from the southwestern Atlantic, penetrating East Antarctica, therefore predominates during the period studied, whether in glacial or interglacial climatic mode. In spite of the low frequency of occurrence of visible tephra layers in Vostok core (one event every 20 kyr), the overall atmospheric pathway of these ash events appears consistent with the almost continuous advection of continental dust from South America.

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.