Thermobaric conditions of subglacial Antarctic environment remain poorly understood, despite recent advances in radar and seismic surveying. The direct accessing to the largest subglacial lake, Lake Vostok, was carried out twice by Russian scientists in February 2012 and January 2015, opening new opportunities for assessing the thermobaric conditions at ice-water interface. According to the assumption that ice sheet is “floating” on the lake, it was predicted that the water would rise 30 - 40 m in the bottom part of the borehole, but in fact the water rose from the lake to a height of more than 500 m. To explain this phenomenon we assume that the pressure in Lake Vostok results from the external pressure of the entire mass of ice above it and the pressure of the water column that is overlaid above the point being considered. Extrapolation of temperature measurements from the deep bore-holes drilled at Vostok Station also confirmed that the bed of the ice sheet is at pressure melting point. As a result of accessing Lake Vostok, the pressure in the lake is reduced that would lead to the formation of a new additional layer of accretion ice on the lower ice sheet surface.
It is now generally recognized that a vast network of lakes, rivers, and streams exists thousands of meters beneath Antarctic Ice Sheet. As of 2010, 387 subglacial lakes have been identified; this will increase as surveys improve spatial coverage [
With dimensions of 280 km by 50 km, Lake Vostok is largest subglacial lake in Antarctica. It is located under the similarly named Russian station, Vostok Station (78˚28'S, 106˚48'E, 3488 m a.s.l.) in the central part of East Antarctica (
Drilling a deep Hole 5G started in February 1990 six years before the large subglacial lake under Vostok station was officially recognized [
The frozen subglacial water was re-drilled and the first crescent-shaped fragments of frozen water ice were got from the depth of 3367 m. At a depth of 3397 m, the first “ice core” consisting of a white substance, clathrate hydrate mixture of lake gases and densifier, HCFC-141b, was retrieved. Deeper than 3406.1 m the drill began to recover the continuous full diameter core composed from outside frozen water ice and hydrate core. However, the Hole 5G-1 is inclined from the vertical by ~6˚, and during re-drilling the drill moved away from the axis of the main hole. That is why the crescent-shaped segment of glacial ice appeared in the core shortly after, and the proportion of glacial ice increased steadily with depth. Totally 34.1 m long core with frozen lake water was recovered.
Drilling was continued to the depth of 3765 m [
Clearly, the pressure in the Lake Vostok is much higher than expected. To explain this phenomenon we assume that the pressure in Lake Vostok results from the external pressure of the entire mass of ice above it and the pressure of the water column that is overlaid above the point being considered. The following considerations attempt to give explanations for uncontrolled blowout happened at Vostok Station.
According to available data [
The Hole 5G and its upper part are practically vertical to the depth of 2400 m, below this depth the angle is 6.3˚ ± 0.9˚ [
Surface topography above Lake Vostok, determined from satellite altimetry, showed that the ice surface above the Lake Vostok is very flat, an along-lake north-south slope is prominent, tilting 60 m over a horizontal distance of ~250 km [
The small amount of superincumbent under-stress ice is located in the down warping coastal area. The evaluation of the coastal topography shows that the south and west coastal area with length of coastal line of ~200 km is inclined to the center of the lake at distance of ~5 km with the angle 3.5˚ and occupies an area of less than 6% of the entire surface of Lake Vostok. 4% of the lake surface, located along the northern part with length of coastal line of ~75 km, is inclined to the center of the lake at a distance of ~9 km with an angle of 2˚. Almost entire eastern of Lake Vostok has no up warping because the ice flow is beset by bedrock hillside. Therefore, we assume that the influence of shear stress in the coastal area of Lake Vostok on the vertical component of the gravity force of the whole ice mass is quite small and in a first approximation can be neglected.
In the assumption that the ice sheet is “floating” on the lake and is, hence, in hydrostatic equilibrium with the lake, the pressure on the ice-water interface was estimated to be equal to the local lithostatic pressure of the ice sheet at the point of drilling [
where ρi is the average ice density, kg/m3 (average ice density through the section vertically in the region of Vostok station can be taken equal to 923 kg/m3 [
According to this hypothesis the differential pressure, which is the difference between hydrostatic pressure of the drilling fluid in the borehole and the lake pressure, is approximately ?0.231 MPa (
Assuming Lake Vostok is totally isolated from the Antarctic subglacial drainage system, is sealed along its entire perimeter and forms a reservoir of water in which the pressure is hydrostatic. Then according to the Pascal’s Law, full pressure P at any point inside an isolated reservoir in homogenous field of the force of gravity consists from two components [
where Рi is the external lithostatic pressure of ice sheet, MPa; Рw is hydrostatic pressure of the water column, MPa.
External lithostatic pressure Pi is determined by the integral of the total weight of superincumbent ice over the entire area of the lake (on
Parameters | Notation | Version I | Version II |
---|---|---|---|
Lake pressure under Vostok Station, MPa | P | 33.747 | 38.090 |
Differential pressure at the bottom of Hole 5G-2, MPa | ΔP | ?0.231 | ?4.574 |
Estimated rise of upwelling water in the borehole, m | - | 23.6 | 3 883 |
Temperature at ice-water interface (pressure melting point), ˚C | tm | ?2.68 | ?3.06 |
Depth of pressure melting point due to extrapolation of temperature profile, m | - | 3776.7 | 3758.3 |
point at the ice-water interface and is determined with the equation:
where V0 is the entire ice volume above the lake’s surface, m3; S0 is the horizontal area of the surface of ice-wa- ter interface, m2;
Hydrostatic pressure Pw is created inside lake’s volume by the action of gravity force of water column, the height of which is determined by the difference of height from the selected point to the point with maximum absolute altitude at the entire water-ice boundary:
where ρw is the lake’s water density, kg/m3, Zw is the height of water column, m.
The height of water-column Zw can be estimated according to ice thickness as following (
where Z is the ice thickness at selected point, m; Zhw is the thickness of ice at the high-water point with maximum absolute altitude at the entire water-ice boundary, m; DH is the elevation difference between these points, m.
The height of water-column between bottom of the hole 5G-2 and highest point of ice-water interface (height between points B and A on
where
So, assuming that the pressure in the Lake Vostok is composed of the external pressure of the entire mass of ice above the lake and the pressure of the water column that is overlaid above the point being considered, we obtain:
In this case, the rise of water level in the borehole is greater than the 3761 m vertical height of the Hole 5G-2. This means that even if the entire borehole is filled with subglacial water, the excessive pressure in the Lake Vostok would not be compensated, and the outflow of the water through the borehole mouth would continue like artesian well until the water freezes in the borehole. Such artesian supraglacial fountains are well known phenomenon and were observed in valley glaciers, e.g. at Trapridge Glacier, Yukon, Canada [
There is good reason to believe that the real interface between the ice sheet and the subglacial water lies a few meters below the bottom of the hole, and that lake water rose into the hole through intergranular cracks formed secondary to the large pressure difference between the lake and the fluid in the borehole. This phenomenon is widely known in geology and mining as fracturing, and it occurred in Greenland in 2004, when subglacial water rose into the hole although the ice sheet bed was about 6 m beyond the bottom of the borehole [
Radar profiling has recorded a chain of subglacial water layers at 30 km west of the Vostok Station with elevations that are up to 850 m higher than the edge of the Lake Vostok (point C on
When a subglacial lake is accessed by a borehole with negative differential pressure, the pressure in the lake is reduced to some extent, and as a result the accretion ice would be formed at least in the area around the bottom of the drill-hole. In this way, the accessing of subglacial lake would lead to significant change of the system’s condition, which has existed for hundreds of thousands of years without external influences.
Temperature measurements in deep boreholes at Vostok Station were regularly carried out since beginning of 1970-s [
This equation gives the opportunity to predict the temperature at the bed of ice sheet taking into account the temperature of the ice-water interface which is equal to the pressure melting point. As the pressure P [in MPa] increases, the melting temperature tm [in K] of ice decreases as follows (modified from [
Using the pressure from Equation (8), ice temperature at the bed of the sheet would be equal to -3.06˚C. Extrapolation and solving of Equation (9), gives the depth of 3758.3 m at pressure melting that is in close proximity to the true vertical depth of Lake Vostok accessing (~3761 m).
The temperature gradient up to the depth of 2500 m is close to linear, and in lower part of the ice sheet it decreases to 0.02206˚C/m as the ice-water interface is approached (
The estimation of the pressure at ice-water interface in isolated subglacial reservoirs being equal to the local lithostatic pressure of the ice at the drilling point does not account the hydrostatic features of subglacial environment. Repercussion of Lake Vostok showed that the pressure difference between hydrostatic pressure of drilling fluid and lake pressure was incorrectly calculated, and the pressure in the Lake Vostok is much higher than expected. Perhaps, the assumption that the ice sheet is simply “floating” on the lake, is only appropriate for active subglacial lakes which are draining into other lakes or into isolated pathways that connect to the ocean [
Our most probable scenario assumes that the pressure in this totally isolated reservoir which has a complex ice-water interface, results from the external pressure of the entire mass of ice above it and the pressure of the water column that is overlaid above the point being considered. It is possible that rising subglacial water rising into the borehole could, and, outflows through the mouth similar to artesian aquifer flow if the borehole does not freeze closed.
When Lake Vostok was accessed by a borehole with negative differential pressure, subglacial water inevitably upwells into the borehole and the condition of thermobaric equilibrium is disturbed to some extent. At this point thermobaric conditions in the lake changed, leading to the freezing-on of additional accreted ice to the ice sheet base. These assumptions should be considered for future subglacial lakes exploration.
This paper describes the research done under support of the National Science Foundation of China (Projects No. 41276189 and No. 41327804), Geological Survey of China (Project No. 3R212W324424) and Ministry of Land and Resources of China (Project No. 201311041). The authors thank R.N. Vostretsov (St.-Petersburg Sated Mining University, passed away in October 2002) and S.V. Popov (Polar Marine Geological Expedition, St.- Petersburg) for provided data and V. N. Golubev (Research Laboratory of Avalanches and Landslides, Geographical Faculty, Moscow State University) for very constructive and pertinent remarks. The author also thanks K. Makinson (British Antarctic Survey) for healthy criticism and editing of this paper.