Reassessing Lake Vostok’s behaviour from existing and new ice core data

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Abstract

Interpretation of new ice core data and reappraisal of existing data, both from the basal part of the Vostok ice core, give strong support to a kind of thermohaline circulation in Lake Vostok. Although the salinity of the lake is considered as weak (less than 1‰), the prominent influence of salinity at high pressure and low temperature on water density makes such a circulation possible. As a consequence, subglacial melting along the northern shores of the lake is balanced, further south, by frazil ice production in the upper water column, its accretion and consolidation at the ice–water interface followed by accreted ice export out of the system together with the southeasterly glacier flow. The dynamics of the system is documented by a stable water isotope budget estimate, by inferences concerning accreted ice formation and by an investigation of ice properties at the transition between meteoric ice and accreted ice. This complex behaviour is the controlling factor on water, biota and sediment fluxes in the lake environment.

Introduction

Lake Vostok is a huge subglacial lake, one of the 70 lakes identified beneath the Antarctic Ice Sheet [1]. 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.

Interpretation of radio-echo-soundings [1], [2] has promoted a lake system with melting occurring in the northern part of the lake while ice accretion characterises the area around Vostok station in the southern part of the lake. Later radio-echo-sounding studies [3] have indicated that accreted ice is exported out of the lake system along with glacier flow at the south-eastern margin of the lake. Ice flowing above the subglacial lake is coming from the Ridge B area (Fig. 1) and from Dome B in the northern part of the lake where subglacial melting is taking place.

The completion of ice drilling at the Russian Vostok station down to 3623 m, about 130 m above subglacial Lake Vostok, 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 timescales [4]. Below 3310 m depth, however, the climatic record cannot be directly deciphered because of complex ice deformation [5], [6]. Below 3539 m depth, the ice is no more of meteoric origin (ice formed at the surface by snowfall) but is accreted ice produced by freezing of subglacial Lake Vostok waters [7], [8].

The aim of this paper is mainly to reassess the isotopic properties of this accreted ice and to develop the consequences for Lake Vostok’s behaviour.

Section snippets

Stable isotopes in accreted ice: previous interpretation

Major shifts in deuterium and deuterium excess are observed at the 3539 m transition. Within less than 50 cm, deuterium values increase by about 10 per mil (‰) and deuterium excess d (d=δD−8δ18O) values shift from around 14‰, a value typical for meteoric ice in the Vostok area, to 7 or 8‰ [6], [7]. In a δD–δ18O diagram, data points above 3539 m lie on the Vostok precipitation line with a slope of 7.93 calculated over the past 420 000 years. In contrast, data from the zone below 3539 m depth

A new isotopic model

Accreted ice sampled in the Vostok ice core was formed in the postglacial period. Because of ice flow (3 m per year) older accreted ice, if present, was exported out of the lake system. Within this timescale with no significant climatic change, the lake can be considered as having a constant volume (dV=0). This assumption is perhaps questionable at glacial–interglacial timescale where climate and ice sheet configuration have changed. Even at this timescale, it still represents a reasonable

Reassessment

What are the consequences of this model on the interpretation of the isotopic data?

A first problem concerns the deuterium values. In Fig. 4, the deuterium concentration in surface snow samples is given along traverses Vostok–Komsomolskaya (a station north of the lake) and Dome B–Komsomolskaya as a function of the distance from Vostok station (upper straight line). Assuming that the mean deuterium concentration for meteoric ice of the four climatic cycles shows the same geographic variation than

The transition zone

A study of ice properties in the transition zone between meteoric ice and accreted ice in the Vostok core supplies additional information in the context of this paper. Fig. 2 gives the electrical conductivity measurements (ECM), mineral particle aggregates (inclusions) number, gas content and crystal size at a 1 m sampling length across the meteoric ice–accreted ice contact. Also shown is the δD profile with a 0.1 m sampling length between 3535 and 3541 m depth. A striking feature of this

Conclusion

The physical environment of Lake Vostok is primarily the consequence of the kind of thermohaline circulation implying ice formation in the upper water column of the lake. Strong arguments for such a dynamics are developed thanks to a detailed investigation of isotopic properties in the accreted ice present in the basal part of the Vostok ice core. In a lake system having been renewed several times, owing to meltwater input and accreted ice export, the isotopic concentration of the accreted ice

Acknowledgements

R.S. and J.-L.T. are grateful for the support of the Belgian Antarctic programme (Science Policy Office). Dr R. Koerner is gratefully acknowledged for the very careful review which led to a significant improvement of the manuscript.[BARD]

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