What is the deepest part of the Vostok ice core telling us?
Introduction
The completion of ice drilling at the Russian Vostok station in Central Antarctica down to 3623 m, about 130 m above subglacial Lake Vostok (Masolov et al., 2001), has allowed the extension of the record of atmospheric composition and climate to the past four glacial–interglacial cycles. The Vostok core has thus given one of the most interesting palaeoenvironmental record at interglacial–glacial time scales (Petit et al., 1999). Below 3310 m depth, however, the climatic record cannot be directly deciphered because of complex ice deformation. This deepest part of the Vostok ice record, between 3310 m depth and the bottom of the drilling hole at 3623 m depth, is the subject of this review paper. As will be developed hereafter, this deepest part consists of two major sections. Between 3310 and 3539 m depth (section length 229 m), the ice is still of meteoric origin but has been subjected to interactions with the bedrock and contains glacial flour in the lowest 89 m. By contrast, ice between 3539 m and the bottom of the core at 3623 m depth (section length 84 m) is lake ice produced by freezing of subglacial Lake Vostok waters.
Lake Vostok is a huge subglacial lake, one of the 70 lakes identified beneath the Antarctic Ice Sheet Siegert et al., 1996, Dowdeswell and Siegert, 1999. It is about 240 km long and 50 km wide and lies under 3750 m of ice in its southern part and under 4150 m of ice in its northern part. The ice ceiling is tilted, being at 750 m below sea level in the North and at 250 m below sea level in the South. The water depth near Vostok station in the southern part of the lake is about 680 m, reaching 1200 m 60 km NW from Vostok (Masolov et al., 2001). The lake itself will not be considered here in detail since a review paper on Antarctic subglacial lakes (Siegert, 2000) was recently published and another one on Lake Vostok itself (Siegert et al., 2001). Reference to the situation in the lake will however sometimes be indicated.
The aim of this review paper is to understand the ice properties of the Vostok ice core below the depth where the palaeoclimatic signal is lost and to develop inferences which can be deduced from the ice core data. The approach takes into account all available information on the deepest part of the Vostok ice core with a special emphasis on isotope geochemistry including D/H, 18O/16O, 3He/4He.
Section snippets
Two major ice sections
Between a depth of 3310 m where interglacial ice from the marine isotope stage 11.3 (Petit et al., 1999) 420,000 years old is displayed, and the bottom of the core at a depth of 3623 m, the hydrogen isotope record shows remarkable features (Fig. 1). First, the δD profile oscillates between interglacial-type and glacial-type values. Then, the amplitude of the oscillations diminishes downward until nearly constant δ-values are displayed. This decrease by a factor of 3 or more cannot be of
Ice properties in the upper section
At about 3311 m depth, three volcanic ash layers are observed dipping in opposite directions. Such a feature is indicative of complex ice deformation; it is most probably due to folding of the ice. In the section of the core considered in this paper, the variations of total dust mass with depth and also the mass contribution of particles smaller than 2.5 μm (Simões et al., in press) show an inverse relationship between microparticles mass and δD variations down to 3450 m. Firstly, down to 3346
Stable isotope evidence for a lake ice origin for the lower section
As the diffusion coefficients of HDO and H218O molecules in ice are very low—in the order of 10−11 cm2 s−1—melting of compact ice is likely to occur without isotopic change. Since diffusion coefficients in liquid water which can be considered as isotopically homogeneous are relatively high—in the order of 10−5 cm2 s−1—freezing involves isotopic fractionation as a consequence of different water molecules freezing at slightly different temperatures. Once formed, the ice has an isotopic
Inferences on ice formation in subglacial Lake Vostok
The deduced isotopic composition of Lake Vostok waters (δD=−449.3‰, δ18O=−57.9‰) implies that most of the water constituting Lake Vostok comes from glacial melt. This lake water differs significantly from that of the overlying ice. This can either be the result of a warmer Antarctic climate before 420,000 years (Jouzel et al., 1999) but it must also be said that glacier ice is richer in heavy isotopes 200 km north of Vostok Station, above the northern part of the lake. Melting at the ice–lake
The helium profile
Unlike most gases, helium can be incorporated into the crystal structure of ice during freezing. Helium also diffuses quite rapidly into ice: its diffusion coefficient is in the order of 5×10−6 cm2 s−1 with lower values if diffusion occurs parallel to the optic axis and higher values if diffusion is normal to the optic axis (Jean-Baptiste, personal communication).
Helium in ice or in Lake Vostok may have three possible origins. First, atmospheric helium with an isotopic ratio 3He/4He close to
Two scenarios
A foremost way to understand such helium profiles is to consider the progressive build up of the lake ice. In Fig. 8, three simulations are given, differing from each other in the time required for the formation of the lake ice. Progressive growth occurs respectively in 103, 104 and 105 years. It is clear that the curve for 103 years match very well the data implying a rapid growth of the lake ice. This is the consequence of the high diffusion coefficient of helium in ice, which rapidly levels
Conclusion
Ice core data are generally used for reconstructing the environment of the past. The Vostok ice core has given a lot of information along these lines. Because of low snow accumulation rates in this area of central Antarctica, most of the data give records at interglacial–glacial time scales or at best at pluri-millenial time scales. The time resolution in this ice core does not allow to study climatic variations of high frequency.
Below 3310 m depth, the picture changes radically. The
Acknowledgements
We are indebted to the Russian drill engineers from the St. Petersburg Mining Institute who developed the drilling equipment and conducted the field operation at Vostok and we thank all participants for field work and ice sampling. We acknowledge the Russian Antarctic Expeditions (RAE), the Institut Français de Recherches et Technologies Polaires (IFRTP), and the Division of Polar Programs (NSF) for logistic support. R.S. is grateful for the support of the Belgian Antarctic programme (Science
R. Souchez is a Professor in the Department of Earth Sciences at the University of Brussels, Belgium. His major research interests include glaciology, mainly the study of basal ice composition (water isotopes, gases and particles embedded in the ice).
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R. Souchez is a Professor in the Department of Earth Sciences at the University of Brussels, Belgium. His major research interests include glaciology, mainly the study of basal ice composition (water isotopes, gases and particles embedded in the ice).
P. Jean-Baptiste is an Isotope geochemist at LSCE, Saclay, France. His main research interests include application of isotope tracer techniques to the study of natural systems and of exchange of elements between the main terrestrial reservoirs.
J.-R. Petit is an Ice geochemist at LGGE, Grenoble, France. His main research interests include reconstruction of climate and environment from ice cores using electrical methods, dust and chemical analyses together with stable isotopes. He is also involved in 11 Antarctic campaigns at Vostok station.
V. Lipenkov is the leading scientist at the Arctic and Antarctic Research Institute in St. Petersburg, Russia. His main research interests include Antarctic glaciology, ice core physical properties and palaeoclimate research. He is also one of the P.I.'s for the deep drilling at Vostok station.
J. Jouzel is currently the Director of Institut Laplace, Paris, France. His main research interests include global change studies, use of water isotopes for reconstructing past climate stages from ice cores at various timescales. He is involved in major European research programs in the polar regions.