Average time scale for Dome Fuji ice core, East Antarctica

Three different approaches to ice-core age dating are employed to develop a depth-age relationship at Dome F: (1) correlation of the ice-core isotope record to the geophysical metronome(Milankovich surface temperature cycle) inferred from the deep borehole temperature profile at Vostok,(2) importing a known chronology from another(Devils Hole) paleoclimatic signal, and(3) direct ice sheet flow modeling. Inverse Monte Carlo sampling is used to constrain the accumulation rate reconstruction and ice flow simulations in order to find the best-fit glaciological time scale matched with the two other chronologies. General uncertainty of the different age estimates varies from 2 to 6kyr on average and reaches 6-14kyr at maximum. Whatever the causes of this discrepancy might be, they are thought to be of different origins, and the age errors are assumed to be independent. Thus, the average time scale for the Dome F ice core down to a depth of 2500m(ice age of 335kyr) is deduced consistently with all three age-depth relationships within the standard deviation limits of ±3.3kyr, and its accuracy is estimated as 1.4kyr on average. The constrained ice-sheet flow model allows extrapolation of the ice age-depth curve further to the glacier bottom and predicts the ages at depths of 2800, 3000, and 3050m to be 615±70, 1560±531, and 2985±1568kyr, respectively

Fifty years of instrumental surface mass balance observations at Vostok Station, central Antarctica

We present the surface mass balance (SMB) dataset from Vostok Station's accumulation stake farms which provide the longest instrumental record of its kind obtained with a uniform technique in central Antarctica over the last 53 years. The snow build-up values at individual stakes demonstrate a strong random scatter related to the interaction of wind-driven snow with snow micro-relief. Because of this depositional noise, the signal-to-noise ratio (SNR) in individual SMB time series derived at single points (from stakes, snow pits or firn cores) is as low as 0.045. Averaging the data over the whole stake farm increases the SNR to 2.3 and thus allows us to investigate reliably the climatic variability of the SMB. Since 1970, the average snow accumulation rate at Vostok has been 22.5 ± 1.3 kg m−2 yr−1. Our data suggest an overall increase of the SMB during the observation period accompanied by a significant decadal variability. The main driver of this variability is local air temperature with an SMB temperature sensitivity of 2.4 ± 0.2 kg m−2 yr−1 K−1 (11 ± 2% K−1). A covariation between the Vostok SMB and the Southern Oscillation Index is also observed

Air bubbles and air-hydrate crystals in the Vostok ice core

The geometrical properties of air-bubble and air-hydrate ensembles in the 3310-m deep Vostok core and in other ice cores were studied. The principle results are the following: 1) the size and abundance of air bubbles in polar ice depend on the temperature and accumulation rate prevailing over the time of the snow-ice transformation, 2) the climate signal imposed on the bubble properties at pore close-off is only slightly modified in the course of the bubble-hydrate transition (500–1250 m at present time at Vostok) and in the first, transient, phase of air-hydrate crystal growth (1150–1500 m); as a consequence, the last four glacial-interglacial cycles are resolved in variations of the number and size of air inclusions along the Vostok ice core, and 3) the air-bubble and air-hydrate records from polar ice cores can provide an independent experimental constraint on the temperature-accumulation relations in the past.International Symposium on Physics of Ice Core Records. Shikotsukohan, Hokkaido, Japan, September 14-17, 1998

キホウ ヲ フクム ヒョウショウ コオリ ノ アツミツ カンワ カテイ ノ リロン テキ ケンキュウ

この解説は, 筆者らがロシアにおいて1983年から1989年にかけて行った一連の研究のレビューである。均一な気泡を含む氷の圧密(あるいは膨張)過程の理論的検討を行う。極地氷床における密度の深度プロファイルおよび掘削後の体積膨張をシミュレーションするための数学的モデルを展開し, それを東南極ボストークにおける掘削コアに適用する。また, 気泡から空気水和物結晶への遷移に対する簡単なモデルを提案する。This paper presents a brief review of the authors\u27 earlier research on polar ice density modeling carried out and published (in the main) during 1983-1989 in Russia. A theoretical approach to macrocontinuum description of bubbly ice densification (expansion) on the basis of averaging asymptotic methods is considered. Mathematical models for the simulation of polar ice sheet density variations versus depth and for the prediction of deep ice core volume relaxation after its recovery are developed and tested on real situations at Vostok Station, East Antarctica. A simplified model of the equilibrium transformation of bubbles entrapped in ice into air hydrate crystals is proposed

Formation of the Ice Core Isotopic Composition

Main processes of the ice core isotopic composition formation are overviewed. Theory of isotope-temperature relationship is discussed and confirmed by a number of experimental data. The factors related to wind-driven spatial snow redistribution and post-depositional isotopic changes that may alter or weaken this relationship, are also considered. For high-resolution isotopic time-series obtained at sites with low accumulation of snow, the signal-to-noise ratio is shown to be as low as 0.25, which means that noise accounts for about 80 % of the total variance. It is demonstrated that "classical isotopic method" (based on the present-day geographical isotope-temperature slope) underestimates the amplitude of past temperature changes in Antarctica. The most likely reason for the discrepancy is the change in the moisture source conditions. After correction for the latter, the paleo-temperature reconstructions produced by the isotopic method become consistent with those obtained from borehole temperature measurements. We show that in the case of the Vostok ice core, both approaches lead to the same temperature shift of 10℃ between LGM and the present time. The isotopic composition of the basal part of the Vostok ice core, comprising frozen subglacial Lake Vostok water, is also discussed.IV. Chemical properties and isotope

Formation of air clathrate hydrates in polar ice sheets : heterogeneous nucleation induced by micro-inclusions

To investigate factors influencing nucleation of air clathrate hydrates in polar ice sheets, we have performed high-resolution mapping of the distributions of soluble impurities, air bubbles and air-hydrate crystals versus depth in the Dome Fuji Antarctic ice. Significant correlation observed between the concentrations of air inclusions and impurities in ice along with frequent occurrence of impurities inside hydrate crystals suggest that micro-inclusions promote hydrate nucleation in the ice matrix. Our observations also show that the diffusive macroscopic-scale redistribution of air constituents in ice in the bubble-hydrate transition zone is controlled by the original sedimentary layering of soluble impurities acting as nucleation helpers. The results of this study are important for the correct interpretation of high-resolution gas analyses of ice cores and for better understanding the global bubble-to-hydrate transformation process in polar ice sheets