International audienceA high-resolution deuterium profile is now available along the entire European Project for Ice Coring in Antarctica Dome C ice core, extending this climate record back to marine isotope stage 20.2, 800,000 years ago. Experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation. We assessed the general correspondence between Dansgaard-Oeschger events and their smoothed Antarctic counterparts for this Dome C record, which reveals the presence of such features with similar amplitudes during previous glacial periods. We suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is Ͻ1.0 m thick and rests with sharp planar contacts on stagnant glacier ice reportedly of Miocene age, older than 8.1 Ma. The age of the ice is based on 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice.Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patternedground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feed-*back associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondaryice lenses contrast markedly with glacial ice by lying on a ␦D versus ␦ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs.The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains Ͼ25% striated clasts in the Ͼ16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarsegrained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ؎ 0.08 Ma and 0.18 ؎ 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimat...

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ABSTRACT. A model for the isotopic composition in .5 0 and .5 18 0 of ice for med by refreezing at the glacier sole is developed. This model predicts relatively well the distribution of points representing samples from basal layers of an Arctic a nd an Alpine glac ie r on a .50-.5 18 0 diagram . The frozen fraction which is the part of the liquid that refreezes can be determined for each basal ice layer. This may have implications on the stud y of the ice-wa ter system a t th e ice-rock interface.R ESUME. Fusion et regel it la base d'lIll glacier et composition isotopique de la g lace. Un modele pour I'etude de la composition isotopique en .50 et en .5 18 0 de la glace formee pa r regel a la base du glacier est develop pe dans cet article. Ce modele predit relativemen t bien la distribution des points represen ta nt des echantillons de glace basale d 'un glacier pol a ire et d'un glacier a lpin sur un diagramme .5 0 _.5 18 0. La fraction gelee qui est la pa rt d 'eau qui regeie pour co ns tituer une couche de glace basale peut etre determinee dans c haque cas. Ceci est s usceptible d'avoir des implications sur les et udes du comportement de la phase liquide a I'inte rface glacier-rocher.

Isotope studies show that the Vostok ice core consists of ice refrozen from Lake Vostok water, from 3539 meters below the surface of the Antarctic ice sheet to its bottom at about 3750 meters. Additional evidence comes from the total gas content, crystal size, and electrical conductivity of the ice. The Vostok site is a likely place for water freezing at the lake-ice interface, because this interface occurs at a higher level here than anywhere else above the lake. Isotopic data suggest that subglacial Lake Vostok is an open system with an efficient circulation of water that was formed during periods that were slightly warmer than those of the past 420,000 years. Lake ice recovered by deep drilling is of interest for preliminary investigations of lake chemistry and bedrock properties and for the search for indigenous lake microorganisms. This latter aspect is of potential importance for the exploration of icy planets and moons.

ABSTRACT. Expe rim ents on progressive unidirectional freezing are ducted to determine rhe evol ution in 00 and (51 BO of successive water samples and ice laye rs ta ken during the course of freezing. R es ults indicate thal lhis evoluti on takes pl ace, in a bD-bIBO diagram, a long a strai ght line wi th a characte risti c slope. This slope, different from that due to the precipitation effec t, gives a finger-print of the occurrence of a freezing or of a melt ing-refreez ing process in the stud ied reservoir. RESUME. Composition isotopique efl DD el bill 0 de ['eau et de la glace au cours de la ronge/ation. D es experiences de gel progre?sif unidirectionnel ont ete realisees cn vue de detenniner I'evolution de la composition isotopique en c5D el en c5 1 "0 d ' echantilJons d 'eau pris slI ccessivemenl a u co urs du processus de congclali on el d ' cc hanlillons de couches de glace successivemen 1 formces. Les rcsultats indiquclll quc cctte evolution se produit, dans un diagramme

A model for the isotopic composition in δD and δ18O of ice formed by refreezing at the glacier sole is developed. This model predicts relatively well the distribution of points representing samples from basal layers of an Arctic and an Alpine glacier on a δD–δ18O diagram. The frozen fraction which is the part of the liquid that refreezes can be determined for each basal ice layer. This may have implications on the study of the ice–water system at the ice–rock interface.

Abstract. The ice core recently drilled at the Dome Concordia site on the East Antarctic plateau provides a new high resolution isotope record covering part of the last glacial, the last transition and the Holocene. The two step shape of the deglaciation is remarkably similar for all the ice cores now available on the East Antarctic plateau. The first warming trend ends about 14000 years ago and is followed by the well marked Antarctic Cold Reversal (ACR) with a secondary peak common to all records. During the deglaciation, there are more similarities between the near coastal site of Taylor Dome and inland East Antarctica than between Taylor Dome and central Greenland. However, the results for EPICA do appear to confirm the Taylor Dome timescale after about 14 ka, showing cooling into the ACR roughly in phase between Greenland and Antarctica. While the overall deglacial pattem is asynchronous, this suggests that the now classical picture of a temperature seesaw between Antarctica and Greenland may be too simplistic.