Contributions of an ancient evaporitic-type reservoir to subglacial Lake Vostok chemistry

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Abstract

We present here the first comprehensive study of the chemical composition of accretion ice from Lake Vostok. Ion chromatographic analyses were performed on samples obtained along the deeper part of the Vostok ice core. Samples were taken from 3350 down to 3611 m depth, both in glacier ice and subglacial lake ice. The total ionic contents of two accretion ice layers—a few meters thick and centered around 3540 and 3590 m depth—are several times lower than those of glacier ice. Very low concentrations were also observed in the deeper part of accretion ice, below 3609 m depth. Elsewhere, the total ionic content is variable but remains 5 to 50 times higher than in glacier ice. Whatever its total ionic content, the ionic composition of accretion ice is significantly different from what is observed in glacier ice. It is dominated by sodium chloride, homogeneously distributed throughout the ice lattice, as well as calcium and magnesium sulfate, likely located in solid inclusions, or to a lesser extent at grain boundaries. Chemical considerations combined with additional studies of sulfur and oxygen isotopes in sulfate, and iron measurements strongly suggest that glacier water recycling and bedrock hydrolysis do not play a prominent role in providing impurities to accretion ice. It is more likely that NaCl rich water carrying fine sulfate salt particles is sporadically incorporated in the ice accreting in a shallow bay upstream from Vostok. The origin of such salty water, which should also contribute to Lake salinity, is discussed.

Introduction

The largest subglacial lake identified under the Antarctic ice cap is Lake Vostok [1], [2]. The lake is 230 km long and 50 km wide. The water depth varies with ice thickness and reaches 600 m in its southern part below the drilling site where the ice is 3750 m thick. The configuration of the ice, the underlying lake and the bedrock, along with main glacier flow pattern, is shown schematically in Fig. 1.

Evidence from water isotope studies and ice properties such as crystal sizes, electrical conductivity and total gas content show that the deeper part of the Vostok ice core is made of two distinct types of ice separated by a sharp transition. Above 3538 m depth, the ice is meteoric, originating from the ice sheet, while below 3539 m it originates from accretion, i.e. refreezing of the lake water [3]. The ice thickness above the lake varies from 3750 m in its southern part to 4150 m in its northern part so that the ice ceiling is tilted. Based on radio-echo soundings, subglacial melting occurs at the ice–lake interface in the northern part of the lake while accretion takes place below Vostok station, in the southern part of the lake [4], [5]. The isotopic composition of the accretion ice supports the hypothesis that lake ice forms by freezing and consolidation of a slush made of frazil ice and host water, and clearly indicates that the lake does not contain sea water [6]. It also suggests that the lake is not a closed water reservoir. However, very few data are available regarding its chemistry, which would provide valuable information concerning the formation and dynamics of the lake.

Two main types of accreted ice may be distinguished (Fig. 1): in the upper part, the ice (accretion ice-1) contains many visible inclusions, mainly located between 3540 and 3608 m depth [3]. These inclusions vanish when samples are melted, and their composition remains unknown. However, preliminary studies of their insoluble part have been carried out by optical microscopy and show that some contain fine particles (clays or quartz, and dust of unidentified origin). Below 3608 m, the ice is clear, without visible inclusions (accretion ice-2).

Although not represented in Fig. 1, two other thin layers of ice containing no or very few visible inclusions (ice-2 type) are observed: the first is located just below the glacier ice (3538–3553) and the second from 3588 to 3594 m depth. Based on topography data from radio-echo-sounding studies [7] and flow trajectory calculation, the first 60 m of accretion ice (accretion ice-1) were likely formed in a shallow bay located northeast of the drilling site and partly separated from the deeper main lake by a submerged reef (see Fig. 1), while deeper on (below 3608 m), the ice (accretion ice-2) originates from the main lake. Taking into account the ice flow velocity (3 m year−1) and trajectory, apparent accretion rates varying from 3.8 cm year−1 over the shallow bay to 1.4–2.5 cm year−1 over the main lake may be calculated.

This paper presents the chemical composition of glacier and accretion ice and discusses the possible composition of lake water and origin of sub-glacial deposits. The interpretation of this first set of data is supported by additional measurements of sulfate isotopic composition and total iron in a few selected samples.

Section snippets

Methods and sampling

Ice cross-sections, a few centimeters thick and roughly 10 cm long, were cut from the inner part of available core pieces with a band saw. They were cleaned in three successive baths of ultra-pure water until roughly half the initial ice volume was removed. Despite the small initial size of these ice cross-sections and strong core contamination by drilling fluid, the concentrations of trace species that are very sensitive to contamination, such as K+, NH4+ and carboxylic ions (HCOO− or CH3COO−

Concentration trends

From 3350 m down to 3535 m, samples were taken every 3 m and none of the ionic species studied displays an observable trend. Mean values calculated on this core section are presented in Table 1 along with concentration ranges corresponding to typical interglacial and full glacial climate conditions. Although stratigraphic perturbations have been observed in this part of the core [9], [13], concentrations show very low variations (less than 50% of mean values). Minima are a few times higher than

Discussion

It can be conclude from Section 3 that glacier and accreted ice have different chemical fingerprints. This is puzzling and leads to the question of where the varying and sometimes very large amounts of NaCl and sulfate salts observed in accreted ice come from and how they are incorporated in the ice.

Conclusion

A detailed study of the ionic content of the deepest part of the Vostok core, combined with specific investigations of selected samples, shows that accretion ice is significantly different in composition from glacier ice. Large and varying concentrations of salts, dominated by halite and calcium or magnesium sulfate, are observed in ice accreted ice above the subglacial lake. Some of these salts are homogeneously distributed throughout the ice lattice (halite, fluoride and nitrate), while the

Acknowledgements

Vostok ice cores were recovered by a joint Russia, France and USA program. We thank the Russian Antarctic Expeditions, the Institut Polaire Paul Emile Victor and the Division of Polar Programs (NSF) for logistic support. We are grateful to the drilling team (St. Petersburg Mining Institute) for field work, and R. Souchez acknowledges the support of the Belgian Antarctic Program (Science Policy Office). We wish to thank Paul Duval for helpful discussion as well as Becky Alexander and Justin

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