The deep holes drilled at Vostok Station by the Russian Antarctic Expedition reached the surface of Subglacial Lake Vostok twice — on February 5, 2012 and January 25, 2015. Two unsealings of the largest subglacial water body on Earth, led by Nikolay Vasiliev, have become remarkable events in the history of Antarctic science. To preserve all the twists and turns of this pioneering work for the ice-drilling community, we have compiled and carefully analyzed all the available drilling, geophysical, and glaciological observations made prior to, during, and after the lake piercings. Based on that information, in this paper we have pieced together a detailed narrative of these two unprecedented drilling operations in the hope that the lessons learned may prove useful for future environmental stewardship, scientific investigations, and technological developments related to the exploration of Lake Vostok.

In this paper, we present a new dataset on the stable water isotopic composition (δD and δ18O) in a sequence of subglacial Lake Vostok’s accreted ice (3538–3769 m) measured along three parallel ice cores. The high precision of the new data has allowed us to characterize the formation conditions of different sections of the ice. The whole lake ice interval may be divided into 3 zones: 1) “zone 0”, 3538.8–3549.8 m, is under the strong influence of the local water formed from melted meteoric ice likely entering from under the glacier on the lake’s west coast; 2) “zone 1” (accreted ice 1), 3549.8–3607.4 m, is experiencing significant variability due to the slightly different effective fractionation coefficient in the course of “water inclusions” in the ice matrix during the freezing process; 3) “zone 2” (accreted ice 2), 3607.4–3768.8 m, is under the influence of glacial melt water from the northern part of the lake and the hydrothermal flux from the lake’s bottom. We defined the exact boundary between the accreted ice 1 and ice 2, which corresponds to a sharp isotopic excursion at a depth of 3607.4 m. In this work, we present for the first time data on the “17O-excess” parameter in the lake ice and water, which allowed us to make a direct calculation of the equilibrium fractionation coefficient for oxygen 17 during water freezing.

The density of the snow and firn thickness is one of the fundamental and most important physical properties of the polar ice sheets. The data on density is used for reconstructing the past snow accumulation rate variability based on firn core studies, for correcting the results of the instrumental surface mass balance measurements and for verification of empirical and physical densification models. In this work we present a unique dataset on the snow and firn density in the upper 70 m of Antarctic glacier in the vicinity of Vostok Station based on data from 32 snow pits and firn cores. This newly obtained stacked density profile reveals different stages of the densifica-tion process. In the upper 27 cm a fast growth of the density is observed from 0.320 to 0.365 g cm-3 as a result of the initial snow grain metamorphism. Below 0.3 m much slower densification rates are observed. At the depth of 22.5 m the transition from snow to firn can be seen at the density of 0.526 g cm-3. The vertical density distri-bution can be approximated with a polynomial function with the accuracy of 0.01 g cm-3 along the whole pro-file except for the upper 0.3 m. In order to investigate the glacio-climatic conditions under which the snow-firn density has evolved, we applied a semi-empirical model by Herron and Langway (1980). In first approximation, the density distribution can be explained with the initial snow density equal to 0.35 g cm-3, surface glacier tem-perature equal to-57 C, and snow accumulation rate of 1.8-2.1 g cm-2 yr-1. The discrepancy between the data and model can be, to our opinion, explained by the recent increase of the snow accumulation rate as evidenced from the firn core studies. However, the non-stationary physical densification model failed to reproduced the density data with sufficient accuracy, which calls for revisiting the model configuration and tuning.

It is generally assumed that the gas composition and the total gas content of Lake Vostok’s water are, to a large extent, governed by the budget of atmospheric gases entering the lake together with glacier ice melt, mostly in its northern part. Since the ice accretion that prevails in the south of the lake leads to the exclusion of gases during the freezing process, these gases can build up in the lake water. Earlier theoretical works [2, 3] have demonstrated that about 30 water residence times are required to attain equilibrium between gases in solution and those in a hydrate phase, which sets the upper bounds of concentrations of nitrogen and oxygen dissolved in sub-ice water (~2.7 g N2 L–1 and ~0.8 g O2 L–1). Here we attempt to estimate the real gas content of the lake water based on the link between the pressure melting temperature of ice and the concentration of gases dissolved in the liquid phase [2]. We use the stacked borehole temperature profile extended to 3753 m depth and the measurements of temperature of sub-ice water that entered the borehole after the second unsealing of Lake Vostok to estimate the melting temperature of ice (–2.72 ± 0.1 °C) at the ice sheet-lake interface (depth 3758.6 ± 3 m, pressure 33.78 ± 0.05 MPa). The gas content of the near-surface layer of lake that corresponds to this melting temperature is calculated to be 2.23 g.L–1, meaning that the concentration of dissolved oxygen must be as high as 0.53 g.L–1, i. e. one-two orders of magnitude higher than in any other known water bodies on our planet. The inferred gas content of sub-ice water is, by a factor of 1.6, lower than the maximal solubility of air in water in equilibrium with air hydrate, though it is still higher, by a factor of 19, than the total air content of melting glacier ice. The relatively low concentration of dissolved air in the near-surface layer of the lake revealed in this study provides a new experimental constraint for understanding the gas distribution in Lake Vostok as affected by the circulation and mixing of water beneath the ice sheet.

Central Antarctica is characterized by a crucial lack of information on climate variability during the late Holocene. The study of firn cores obtained from the central part of the East Antarctic Ice Sheet is essential for the reconstruction of the paleoclimatic conditions at the continental and regional scales over the past 2,000 years (the Late Holocene). Based on glaciological and isotopic data from the shallow VK16 core drilled in the vicinity of Vostok Station over the periods of the 62th (2016-2017) and 63th (2017-2018) summer seasons of RAE, climatic characteristics have been reconstructed for the period 216 BC - 2010 AD. Studies of the VK16 core were carried out in two stages: firn density and electrical conductivity measurement (ECM), as well as sampling, was performed in the glaciological laboratory of the 5G drilling complex (Vostok Station) soon after the firn recovery, while the isotopic composition of the core samples was measured by a Picarro L-2120i laser analyzer in the Climate and Environmental Research Laboratory of AARI. In the ECM profile of the core, we discovered 14 absolute age markers (layers containing the products of known volcanic eruptions). These markers have allowed us to develop a robust chronostratigraphic scale for this core. We have shown that the main feature of the Late Holocene climate in this part of Antarctica is that the near-surface air temperature remained essentially constant throughout the whole time period under consideration. At the same time, the snow accumulation rate varied significantly around a mean value of 1.83 g cm-2 year-1, while the last 200 years were characterized by the highest snow accumulation rate, equal to 2.08 g cm-2 year-1. In this paper, we describe methods for studying firn cores, which can be useful for further research, and present first preliminary data on the climate variability in the vicinity of Vostok Station during the late Holocene.