Vostok Ice Core Research Articles

Deep sea sedimentary analogs for the Vostok ice core

Many applications of the Vostok ice core depend critically on the ability to make stratigraphic ties to marine records in the adjacent Southern Ocean. Here we present oxygen isotopic records from high accumulation rate sites in the South Atlantic sector of the Southern Ocean, collected for the purpose of complementing the recently extended δD record from the Vostok ice core. The combination of several planktonic foraminiferal δ18O records from northern subantarctic piston cores demonstrates that all of the millennial‐scale oscillations expressed in the Vostok ice core over the last 60 ky are also present in marine records. The observations also support the assumption that the millennial‐scale oscillations common to both marine and ice archives are synchronous, thus providing a rationale for extending the marine‐ice core comparison through the last 400,000 years, making use of a marine drilled core (ODP Site 1089). By aligning the phase of these common abrupt events, we anchor the Vostok chronology to an orbitally tuned marine sediment chronology—a refinement that allows examination of a variety of paleoclimatological issues such as the relationship between deep ocean variability and Antarctic polar climate. For example, this exercise suggests that, over at least the 4 major deglaciation events, the primary (orbital scale) changes in the chemistry and, most likely, the temperature of the deep Southern ocean were synchronous with changes in atmospheric pCO2 and polar air temperatures. We also find that the deuterium excess in the ice core resembles marine (foraminiferal) δ13C records and that the deuterium excess is synchronous with an “anomalous” foraminiferal δ18O signal (the residual between normalized versions of Vostok δD and foraminiferal δ18O). These observations demand a tight link between the Vostok isotopic record and the air‐sea interaction of the subantarctic zone.

Trace elements in accreted ice from the Vostok sub-glaciallake, Antarctica: Initial results

We present in this study initial results on trace elements concentrations changes in the deepest part of the Vostok ice core. From 3310 m depth to the bottom of the ice core (3623 m), ice encountered is made of disturbed glacial ice by glacier dynamics and accreted ice from the sub-glacial lake Vostok. Since trace elements are involved in various biogeochemical processes, studying these elements in the accreted ice of the Vostok lake can lead to a better understanding of lake chemistry and life development in such extreme environment. Because concentrations to be measured in Antarctic ice are exceedingly low, such investigations on trace elements require strict control of contamination problems and the use of ultraclean protocols. Initial results obtained for Zn concentrations changes show high variability (< 0.5 to 32 pg/g), with the lowest concentrations found in the accreted ice of the Vostok lake (< 0.5 pg/g). According to crustal enrichment factors close to unity, Zn seems to be mainly associated with crustal material.

Atmospheric CO2 variations over the last three glacial-interglacial climatic cycles deduced from the Dome Fuji deep ice core, Antarctica using a wet extraction technique

A deep ice core drilled at Dome Fuji, East Antarctica was analyzed for the CO 2 concentration using a wet extraction method in order to reconstruct its atmospheric variations over the past 320 kyr, which includes three full glacial–interglacial climatic cycles, with a mean time resolution of about 1.1 kyr. The CO 2 concentration values derived for the past 65 kyr are very close to those obtained from other Antarctic ice cores using dry extraction methods, although the wet extraction method is generally thought to be inappropriate for the determination of the CO 2 concentration. The comparison between the CO 2 and Ca 2+ concentrations deduced from the Dome Fuji core suggests that calcium carbonate emitted from lands was mostly neutralized in the atmosphere before reaching the central part of Antarctica, or that only a small part of calcium carbonate was involved in CO 2 production during the wet extraction process. The CO 2 concentration for the past 320 kyr deduced from the Dome Fuji core varies between 190 and 300 ppmv, showing clear glacial–interglacial variations similar to the result of the Vostok ice core. However, for some periods, the concentration values of the Dome Fuji core are higher by up to 20 ppmv than those of the Vostok core. There is no clear indication that such differences are related to variations of chemical components of Ca 2+ , microparticle and acidity of the Dome Fuji core. DOI: 10.1034/j.1600-0889.2003.00050.x

Planktonic foraminifera, sea surface temperatures, and mechanisms of oceanic change in the Peru and south equatorial currents, 0–150 ka BP

Planktonic foraminiferal faunas of the southeast Pacific indicate that sea surface temperatures (SST) have varied by as much as 8–10°C in the Peru Current, and by ∼5–7°C along the equator, over the past 150,000 years. Changes in SST at times such as the Last Glacial Maximum reflect incursion of high‐latitude species Globorotalia inflata and Neogloboquadrina pachyderma into the eastern boundary current and as far north as the equator. A simple heat budget model of the equatorial Pacific shows that observed changes in Peru Current advection can account for about half of the total variability in equatorial SSTs. The remaining changes in equatorial SST, which are likely related to local changes in upwelling or pycnocline depth, precede changes in polar climates as recorded by δ18O. This partitioning of processes in eastern equatorial Pacific SST reveals that net ice‐age cooling here reflects first a rapid response of equatorial upwelling to insolation, followed by a later response to changes in the eastern boundary current associated with high‐latitude climate (which closely resembles variations in atmospheric CO2 as recorded in the Vostok ice core). Although precise mechanisms responsible for the equatorial upwelling component of climate change remain uncertain, one likely candidate that may operate independently of the ice sheets is insolation‐driven changes in El Niño/Southern Oscillation (ENSO) frequency. Early responses of equatorial SST detected both here and elsewhere highlight the sensitivity of tropical systems to small changes in seasonal insolation. The scale of tropical changes we have observed are substantially greater than model predictions, suggesting a need for further quantitative assessment of processes associated with long‐term climate change.

Reconstruction of surface ocean water pCO2(aq) in Nansha area, the South China Sea during the last 30 ka

The reconstruction of pCO2 in the tropic ocean is one of the most important issues to understand global climatic changes. In this study, the high-resolution stratigraphic analysis of core 17962 was conducted, which is located in the southern South China Sea (SCS). The contents of sedimentary organic matter, the stable carbon isotopic composition of sedimentary organic matter, and the δ13C values of black carbon and terrigenous n-alkanes were determined. And the δ13Cwc value of carbon derived from aquatic was calculated. On the basis of δ13Corg-pCO2 equation proposed by Popp et al. (1989), we estimated the pCO2 in the Nansha area, SCS, since the last glaciation using δ13Cwc instead of δ13Corg. The results show that the average pCO2 was estimated at 240 ppmV during the last glaciation, and at 320 ppmV in the Holocene. A comparison of surface sea pCO2 with the atmosphere CO2 recorded in the Vostok ice core, indicates that CO2 in surface water of the southern SCS could emit into atmosphere during the last 30 ka.

The Gaia Hypothesis: Conjectures and Refutations

The uncertainties surrounding global climate change provide ample evidence, if any were necessary, of the need for a whole-system view of the Earth. Arguably the most visible - and controversial - attempt to understand Earth as a system has been Lovelock's Gaia theory. Gaia has been a fruitful hypothesis generator, and has prompted many intriguing conjectures about how biological processes might contribute to planetary-scale regulation of atmospheric chemistry and climate. In many important cases, however, these conjectures are refuted by the available data. For example, Gaia theory predicts that the composition of the atmosphere should be tightly regulated by biological processes, but rates of carbon uptake into the biosphere have accelerated by only about 2% in response to the 35% rise in atmospheric CO2 since pre-industrial times. Gaia theory would predict that atmospheric CO2 should be more sensitively regulated by terrestrial ecosystem uptake (which is biologically mediated) than by ocean uptake (which is primarily abiotic), but both processes are about equally insensitive to atmospheric CO2 levels. Gaia theory predicts that biological feedbacks should make the Earth system less sensitive to perturbation, but the best available data suggest that the net effect of biologically mediated feedbacks will be to amplify, not reduce, the Earth system's sensitivity to anthropogenic climate change. Gaia theory predicts that biological by-products in the atmosphere should act as planetary climate regulators, but the Vostok ice core indicates that CO2, CH4, and dimethyl sulfide - all biological by-products - function to make the Earth warmer when it is warm, and colder when it is cold. Gaia theory predicts that biological feedbacks should regulate Earth's climate over the long term, but peaks in paleotemperature correspond to peaks in paleo-CO2 in records stretching back to the Permian; thus if CO2 is biologically regulated as part of a global thermostat, that thermostat has been hooked up backwards for at least the past 300 million years. Gaia theory predicts that organisms alter their environment to their own benefit, but throughout most of the surface ocean (comprising more than half of the globe), nutrient depletion by plankton has almost created a biological desert, and is kept in check only by the nutrient starvation of the plankton themselves. Lastly, where organisms enhance their environment for themselves, they create positive feedback; thus Gaia theory's two central principles - first, that organisms stabilize their environment, and second, that organisms alter their environment in ways that benefit them - are mutually incon- sistent with one another. These examples suggest that the further development of Gaia theory will require more deliberate comparison of theory and data.

Evidence for a large surface ablation zone in central East Antarctica during the last Ice Age

Internal isochronous ice sheet layers, recorded by airborne ice-penetrating radar, were measured along an ice flowline across a large (>1 km high) subglacial hill in the foreground of the Transantarctic Mountains. The layers, dated through an existing stratigraphic link with the Vostok ice core, converge with the ice surface as ice flows over the hill without noticeable change to their separation with each other or the ice base. A two-dimensional ice flow model that calculates isochrons and particle flowpaths and accounts for ice flow over the hill under steady-state conditions requires net ablation (via sublimation) over the stoss face for the predicted isochrons to match the measured internal layers. Satellite remote sensing data show no sign of exposed ancient ice at this site, however. Given the lack of exposed glacial ice, surface balance conditions must have changed recently from the net ablation that is predicted at this site for the last 85,000 years to accumulation.

Atmospheric CO2 variations over the last three glacial’interglacial climatic cycles deduced from the Dome Fuji deep ice core, Antarctica using a wet extraction technique

A deep ice core drilled at Dome Fuji, East Antarctica was analyzed for the CO2 concentration using a wet extraction method in order to reconstruct its atmospheric variations over the past 320 kyr, which includes three full glacial—interglacial climatic cycles, with a mean time resolution of about 1.1 kyr. The CO2 concentration values derived for the past 65 kyr are very close to those obtained from other Antarctic ice cores using dry extraction methods, although the wet extraction method is generally thought to be inappropriate for the determination of the CO2 concentration. The comparison between the CO2 and Ca2× concentrations deduced from the Dome Fuji core suggests that calcium carbonate emitted from lands was mostly neutralized in the atmosphere before reaching the central part of Antarctica, or that only a small part of calcium carbonate was involved in CO2 production during the wet extraction process. The CO2 concentration for the past 320 kyr deduced from the Dome Fuji core varies between 190 and 300 ppmv, showing clear glacial—interglacial variations similar to the result of the Vostok ice core. However, for some periods, the concentration values of the Dome Fuji core are higher by up to 20 ppmv than those of the Vostok core. There is no clear indication that such differences are related to variations of chemical components of Ca2×, microparticle and acidity of the Dome Fuji core.

Glacial–interglacial variations in central East Antarctic ice accumulation rates

Past rates of ice accumulation in central East Antarctica are defined poorly for the Last Glacial period. Ice cores, from which current estimates are based, are limited in number (there is one deep ice core at Vostok Station, one at Dome F and two others currently being extracted at Dome C and Dronning Maud Land), and are restricted spatially (they are one-dimensional). Here, spatial variations in ice accumulation rates during the Last Glacial are calculated from isochronous internal ice sheet layering, measured by airborne ice penetrating radar data. The layers are used to identify the stratigraphy around five East Antarctic ice domes. Isochrons are dated by linking them to the Vostok ice core and, in one case, the EPICA Dome C ice core. This chronostratigraphy allows the depth–age function to be identified across the ice sheet. A simple ice flow model, using the depth–age measurements as input, is then used to determine the spatial and temporal variation in East Antarctic ice accumulation over the Last Glacial cycle. The lowest rates of ice accumulation were predicted in the full glacial period to the south of Dome A and Ridge B. There are two possible explanations for this: (1) the ice divide topographies may have caused a ‘precipitation shadow’ in the ice age; (2) with the expansion of the Ross ice shelf the area to the south of Dome A and Ridge B would be at the very centre of the ice sheet and, hence, at a maximum distance from the ocean moisture source.

Extending the ice core record beyond half a million years

Ice cores have been a crucial source of information about past changes in the climate and atmosphere. The Vostok ice core from Antarctica has provided key global change data sets extending 400,000 years in the past [Petit et al., 1999], while Japanese scientists drilling at Dome Fuji have obtained records extending to 330,000 years. Now, a new core being drilled by a consortium of European laboratories has surpassed these ages, and looks like extending the ice core record several hundred thousand years into the past.Ice cores are unique: of all the paleo‐records, they have the most direct linkage with the atmosphere. At some sites, the time resolution is sufficient to study extremely fast climate changes; and they have information about many forcing factors for climate (including greenhouse gas concentrations) displayed in the same cores as the resulting climate changes.

Orbital tuning chronology for the Vostok climate record supported by trapped gas composition

We present data on the O 2/N 2 ratios of trapped gas samples for the entire length of the Vostok climate record. As in other cores, O 2/N 2 ratios in these samples are less than the atmospheric ratio, by a small and variable amount, because O 2 is selectively excluded during the gas trapping process, and because O 2 is also preferentially lost in poorly preserved core samples. Samples younger than 150 ka have large and variable O 2 depletions. Samples older than 200 ka have O 2/N 2 ratios that replicate well and vary smoothly with depth. We plot O 2/N 2 ratios of well replicated samples older than 160 ka, using a chronology derived by matching the δ 18O of paleoatmospheric O 2 (δ 18O atm) to northern hemisphere June insolation. On this timescale, O 2/N 2 varies coherently with local (78°S) summertime insolation. Based on time series analysis of the O 2/N 2 record and the dynamics of snow metamorphism at the surface, we conclude that summertime insolation influences physical properties of ice grains that control the degree of O 2 exclusion during bubble closeoff. O 2/N 2 in Vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core. We show that the δ 18O atm chronology, supported by the coincidence of O 2/N 2 ratios with insolation, is also compatible with recent radiometric dating of corals from high sea stands. We further successfully test the δ 18O atm tuning chronology by showing that it predicts a chronology for the GISP2 core which is essentially indistinguishable from the standard GISP2 chronology and, therefore, in excellent agreement with the radiometric chronology of Hulu Cave, China. An accurate chronology for the Vostok ice core is now in place.

Feedback between aeolian dust, climate, and atmospheric CO2 in glacial time

Enhanced aeolian supply of iron to the biota of the Southern Ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric CO2. Declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in CO2, but the reasons behind the dust decline itself are as yet unknown. Here we show that the dust record from the Vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of Patagonian sources of aeolian material and the efficiency with which it is transported through the atmosphere. We suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that, in turn, climate is responsive to the dust due to iron fertilization. This positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states, characterized by distinct CO2 concentrations. Recognition of this “glacial subcycle” may help to account for the timing and sequence of events at the terminations.

Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores

The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice‐rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO2 variation depends on the age model as 14C determinations cannot be obtained for the time interval of 29.5–14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO2 by approximately 2000 years. Our diatom‐based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO2 change must be sought elsewhere, rather than in the Southern Ocean.

New insights into Southern Hemisphere temperature changes from Vostok ice cores using deuterium excess correction

The combination of both Vostok ice core deuterium and deuterium excess histories over the last four climate cycles back to 420 ka BP offers a unique opportunity for reconstructions of vapor source region and local temperature variations (ΔTsource and ΔTsite respectively) relative to their modern values. Our study is based on an inversion of ice isotopic composition using a Rayleigh-based isotopic model which estimates the dependence of Vostok precipitation isotopic composition on different climate controls (moisture source temperature, local deposition temperature and ocean isotopic composition). Both reconstructed local and source temperatures show no substantial effects from this correction, in the sense that none of the rapid or large variations in raw deuterium and deuterium excess profiles can be entirely attributed to a second order climate control. Our discussion is focused on moisture source temperature and meridional temperature gradient history (ΔTsource−ΔTsite). First, using sea surface temperature time series, we confirm that moisture source temperature signal recorded in Vostok deuterium excess over the last 150 ka fully reflects the obliquity time-varying relative contribution of low and high ocean latitudes to Vostok precipitation. Second, comparisons of source-to-site meridional temperature gradient with the Vostok sodium record over 420 ka suggest that moisture transport and sea salt transport to the ice sheet are controlled by processes in different latitudes (the sea salt being more of a high-latitude signal).

Precessionally forced productivity variations across the equatorial Pacific

Measurements of combustion oxygen demand (COD) in two sediment cores provide a record of paleoproductivity driven by surface‐ocean dynamics in the equatorial eastern and western Pacific for the past 400,000 years. The COD time series are well correlated with each other over this time span and show pronounced precessionally forced peaks of higher productivity during globally colder periods. The phase of this signal in the two cores is identical, to within chronological uncertainties, suggesting a common insolation forcing mechanism for the upper ocean across the equatorial Pacific. COD is also in phase with the precessionally forced component of global ice volume, as indicated by oxygen isotopes, and with atmospheric methane in the Vostok ice core. These relationships imply that the COD relative paleoproductivity index provides an important diagnostic measure of the mechanisms of tropical ocean dynamics and climate change.

What is the deepest part of the Vostok ice core telling us?

This review paper is mainly concerned with a geochemical investigation of the deepest part of the Vostok ice core between 3310 m, the depth at which the palaeoenvironmental record present in the ice above is lost, and the bottom of the core about 130 m above subglacial Lake Vostok. Two sections constitute this part of the core. The upper section (3310–3539 m depth) still consists of ice of meteoric origin but subjected to widespread complex deformation. This deformation is analysed in light of a δD–deuterium excess diagram and information on microparticles, crystal sizes and chemical elements distributions in that part of the core. Such ice deformation occurred when the ice was still grounded upstream from Vostok station in a region with subfreezing temperatures. The lower section from 3539 m to the bottom of the core at 3623 m depth is lake ice formed by freezing of subglacial Lake Vostok waters. This is indicated by the isotopic properties ( δD, δ 18O and deuterium excess), by electrical conductivity measurements (ECM), crystallography and gas content of the ice. These ice core data together with data on ionic chemistry favour an origin of the lake ice by frazil ice generation in a supercooled (below pressure melting point) water plume existing in the lake followed by accretion and consolidation by subsequent freezing of the host water. The helium profile of this deepest part of the Vostok core is quite unusual and surprising. It has important implications for the interactions between the ice sheet and the lake. Two constrasting scenarios can be satisfactorily constructed so that the lake residence time is not well constrained.

Comparison of ice-rafted debris and physical properties in ODP Site 1094 (South Atlantic) with the Vostok ice core over the last four climatic cycles

Visual counts of ice-rafted debris (IRD), foraminifera, and radiolaria were made for ∼1500 samples in Site 1094 spanning the last four climatic cycles (marine isotope stages 1–11). Most, but not all, of the IRD variability is captured by whole-core physical properties including magnetic susceptibility and γ-ray attenuation bulk density. Glacial periods are marked by high IRD abundance and millennial-scale variability, which may reflect instability of ice shelves in the Weddell Sea region. Each interglacial period exhibits low IRD and high foraminiferal abundance during the early part of the interglacial, indicating relatively warm sea-surface temperatures and reduced influence of sea ice. IRD increases and foraminiferal abundances decrease during the latter part of each interglacial, indicating a return to more glacial-like conditions. Glacial terminations I and V are each characterized by a step-wise reduction in ice-rafting punctuated by a brief pulse in IRD delivery and reversal in δ 18O. The coarse fraction of the sediment is dominated by ash and radiolaria, and the relative abundance of these components is remarkably similar to the concentration of Na + in Vostok. Each of these variables is believed to be controlled mainly by sea-ice cover, thereby providing a means for sediment–ice core correlation.

Highly deformed basal ice in the Vostok core, Antarctica

Our paper documents the build‐up of a highly deformed basal ice layer in the basal part of the Vostok ice core. This is done mainly on the basis of an isotopic composition investigation of the ice. Complex deformation in the lower 228 m of the ice sheet has resulted in folding and intermixing of ice at a submetric scale and, for the upper part of this basal sequence, in interbedding of ice layers from distinct origins at a larger scale. This complex deformation occurred at a temperature largely below the pressure‐melting point. The basal ice layer has built upwards and size‐selective incorporation of bed material into the ice has taken place. The documentation of this complex basal deformation has implications for the maximum age of ice that will be useful in paleoclimate studies and for ice sheet dynamics.

A vertical girdle fabric in the NorthGRIP deep ice core, North Greenland

AbstractA high-resolution study of c-axis fabrics has been performed on the NorthGRIP deep ice core from North Greenland. A newly developed automatic ice-fabric analyzer has been used to measure c-axis orientations on vertical thin sections at 142 different depths between 100 and 2920 m in the core. Detailed comparison studies show that this new method produces results that are in full accordance with those obtained from conventional manual measurements. Fabric development in ice of the NorthGRIP core can be explained by rotation of c axes, controlled by the prevailing stress systems. In the upper 800 mthe c axes appear to distribute uniformly, but in the depth interval 900–2500m they tend to cluster around a vertical plane. This vertical girdle pattern is strengthened with depth, forming the first clear evidence of this fabric type in a Greenland deep ice core. Such fabric development has previously been observed in the Vostok ice core from East Antarctica, where converging flow is believed to occur. the most likely interpretation of the NorthGRIP girdle fabric is that the c axes are rotating away from a horizontal-tension axis across the main ice divide, which runs north-northwest–south-southeast through the NorthGRIP drilling site. This is supported by information available from surface velocity measurements. Within the girdle fabric, increased concentration of c axes parallel to the core axis is observed with increasing depth, indicating combined effects of vertical compression and horizontal tension on the fabric development. from about 2500 m depth, the girdle-type fabric starts to give way to a strong vertical single-maximum fabric, which persists to 2920m depth, where drilling was terminated 150 mabovebedrock in the year 2000. the single maximum seems to suggest that bed-parallel simple shear is exerting a strong influence on the fabric in the lowest part of the ice sheet down to 2920 m.

Evidence of glacial flour in the deepest 89 m of the Vostok ice core

The 3623 m long Vostok ice core is divided into two main parts: (1) from the surface to a depth of 3310 m, the ice layers are undisturbed and may be used to reconstruct the climatic record over the past 420 kyr; (2) from 3538 m to the bottom, the core is made up of accreted ice formed by freezing of the lake. In between, the glacier ice is disturbed by ice dynamics, as shown, for example, by inclined volcanic-ash layers. Microparticle concentrations and distributions as well as ionic and isotopic content were used to subdivide this third section. The identification of layers with bedrock material provides clues as to the entrainment processes at sub-freezing temperatures. Dust concentration and size distribution as well as ionic content are comparable with values found in ice of glacial and interglacial periods. Below 3450 m depth, however, the mode of the volume size distribution clearly shifts from 2.1 μm to 3.4 μm in diameter. Particles as large as 30 μm in diameter are observed and cannot have an aeolian origin. From microscopic observations, we suggest that they originate from the bedrock and represent glacial flour entrained in a shear layer up to 89 m from the glacier sole. This process most likely occurred when the ice sheet was grounded before flowing over the lake.