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The identification, examination and exploration of Antarctic subglacial lakes


Abstract and Figures

At the floor of the Antarctic ice sheet, 4 km below the Russian research base Vostok Station, lies a 2,000 km3 body of water, comparable in size to Lake Ontario. This remote water mass, named Lake Vostok, is the world's largest subglacial lake by an order of magnitude (Figure 1). Despite ice-surface temperatures regularly around -60 degrees C, the ice-sheet base is kept at the melting temperature by geothermal heating from the Earth's interior. The ice sheet above the lake has been in existence for at least several million years and possibly as long as 20 million years. The origins of Lake Vostok may therefore data back across geological time to the Miocene (7-26 Ma). The hydrology of Lake Vostok can be characterised by subglacial melting across its northern side, and refreezing over the southern section. A deep ice core, located over the southern end of the lake has sampled the refrozen ice. Geochemical analysis of this ice has found that it comprises virtually pure water. However, normal glacier ice contains impurities such as debris and gas hydrates. Subglacial melting and freezing over Lake Vostok may, therefore, leave the lake enriched in potential nutrients issued from the melted glacier ice. Many scientists expect microbial life to exist within the lake, adapted to the extreme conditions of low nutrient and energy levels. Indeed microbes have been found in the basal refrozen layers of the ice sheet. If Lake Vostok has been isolated from the atmosphere for several million years by the ice sheet that lays above it, the microbes within the lake must also date back several million years and may have undergone evolution over this time, yielding life that may be unique to Lake Vostok. Plans are currently being arranged to explore Lake Vostok and other Antarctic subglacial lakes, and identify life in these extraordinary places. Before this happens, however, much more needs to be known about the ice-sheet above subglacial lakes, and the rocks and sediment below them.
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Annu. Rev. Earth Planet. Sci. 2005. 33:215–45
doi: 10.1146/
2005 by Annual Reviews. All rights reserved
First published online as a Review in Advance on January 11, 2005
Analysis, and Future Exploration of Lake Vostok
and Other Antarctic Subglacial Lakes
Martin J. Siegert
Bristol Glaciology Center, School of Geographical Sciences, University of Bristol,
Bristol BS8 1SS, United Kingdom; email:
KeyWords Antarctica, subglacial environments, exploration
Abstract Airborne geophysics has been used to identify more than 100 lakes
beneath the ice sheets of Antarctica. The largest, Lake Vostok, is more than 250 km
in length and 1 km deep. Subglacial lakes occur because the ice base is kept warm by
geothermal heating, and generated meltwater collects in topographic hollows. For lake
water to be in equilibrium with the ice sheet, its roof must slope ten times more than the
ice sheet surface. This slope causes differential temperatures and melting/freezing rates
across the lake ceiling, which excites water circulation. The exploration of subglacial
lakes has two goals: to find and understand the life that may inhabit these unique
environments and to measure the climate records that occur in sediments on lake floors.
The technological developments required for in situ measurements mean, however, that
direct studies of subglacial lakes may take several years to happen.
1.1. Conditions Beneath the Ice Sheet
The concept of liquid water beneath the ice sheets of Antarctica is, to those unfa-
miliar with glacial processes, somewhat incongruous. The surface air temperatures
in central East Antarctica often reach below 60
C, and the coldest official tem-
perature ever recorded on Earth, 89.2
F), occurred at the Russian
Vostok Station on July 21, 1983. Yet, a little less than 4 km below the ice surface
at Vostok Station, at the ice sheet base, a huge body of water named Lake Vostok
exists. This lake is the largest (by an order of magnitude) of more than 100 known
lakes that lay under the East and West Antarctic ice sheets. Temperatures can attain
the melting value beneath an ice sheet because of three factors. First, the pressure
beneath an ice sheet (i.e., the weight of ice) causes a reduction in the temperature at
which ice melts. Beneath 4 km of ice, this value is approximately 3
C. Second,
the ice sheet insulates the base from the ultra cold temperatures at the surface.
Third, heat is generated at the ice sheet base from the Earth (geothermal heat) and
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Figure 1 Ice sheet thermal conditions and the maintenance of warm sub-
glacial conditions in East Antarctica owing to geothermal heating.
the ice sheet itself (friction heat owing to the deformation of ice and basal sliding).
Foranice sheet 4 km thick, the heat required to melt basal ice is approximately
50 mW m
, which is the background geothermal value (Figure 1). Thus, sub-
glacial water, and lakes, can occur beneath the center of a large ice sheet without
the need for unusual geothermal conditions.
Water flow beneath an ice sheet is controlled by the water pressure gradient
(a combination of gravity and ice overburden). In simple terms, water may flow
uphill if the slope of the ice surface exceeds approximately 1/10 of the ice sheet
base. In other cases, subglacial water flows downhill. The production and flow of
water at the ice sheet bed lead to its accumulation within topographic hollows and,
hence, the formation of subglacial lakes.
1.2. Identification of Subglacial Lakes
Lakes beneath the Antarctic Ice Sheet were first reported from airborne radio-echo
sounding (RES) records in the late 1960s and early 1970s (Robin et al. 1970,
Oswald & Robin 1973). The technique of RES works through the issuing of VHF
radio waves into the ice sheet, which reflect of boundaries of dielectric contrast
(Figure 2). Such boundaries occur at the ice surface, within the ice sheet (internal
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Figure 2 The technique of airborne radar sounding, and its application to identifying Lake Vostok and other subglacial lakes. In the 1970s,
airborne radar surveys were undertaken with a C130 Hercules transporter aircraft, with the wings mounted with the radar transmitter and
receiver. Aircraft navigation was accurate to approximately 5 km in the center of Antarctica. Today, most radar surveys use smaller aircraft
and GPS to navigate. Subglacial lakes are easily identified on airborne radar records owing to their uniformly strong and flat appearance.
Bedrock perturbations are recorded as hyperbolae in radar data.
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layering), and from the ice sheet base. Airborne RES at 60 MHz is often used to
penetrate to the base of ice more than 4 km thick in Antarctica (e.g., Robin et al.
1977, Drewry 1983, Morse et al. 2002, Studinger et al. 2003a). This is possible
because ice is relatively transparent to radio waves at this frequency (Johari &
Charette 1975), especially when it is several tens of degrees below freezing, as is
the case for most of the Antarctic Ice Sheet. The strength of reflection from the bed
depends to a first order on the difference between the dielectric properties of the ice
(dielectric constant ε = 3.2) and the dielectric properties of the subice material.
As the dielectric constant of water (ε = 81) is very different from typical bedrock
(ε = 4to9), a much stronger reflection is obtained from an ice-water interface
compared with an ice-rock interface. This difference is increased by the relatively
rough character of an ice-bedrock interface, which scatters energy and further
reduces echo strength. This makes RES an ideal technique for identifying water
bodies beneath ice sheets. Subglacial lakes are identified on 60 MHz RES records
by the presence of the following characteristics (Figure 3): (a) strong reflections
from the ice sheet base, which appear bright on film records and are typically
10–20 dB stronger than adjacent ice-bedrock reflections; (b) echoes of constant
strength along the track, indicative of an interface that is very smooth on the scale
of the RES wavelength; and (c)avery flat and virtually horizontal character, with
maximum slopes typically approximately 10 times the surface slope.
More than 100 lakes have now been identified beneath several regions of the
Antarctic Ice Sheet (Siegert et al. 1996, 2005; Tabacco et al. 2003; Popov &
Masolov 2004). Lakes range in size from less than a kilometer in length to Lake
Vostok (the best known of the lakes), which is the largest by an order of magnitude
(Kapitsa et al. 1996) (Figure 4, see color insert).
Figure 3 60 MHz RES data for 11 Antarctic subglacial lakes and their surrounding
ice sheet bed topography. (a)Two lakes in the Dome C area (#33 and #34). The mean ice
thickness above these lakes is 4000 m. (b) Lake in the Ridge B area (#46). The mean
ice thickness above the lake is 3700 m. (c) Lake at Titan Dome near the South Pole
(#52). The mean ice thickness above the lake is 3070 m. (d) Lake in the Whitmore
Mountains area (#68). The mean ice thickness above the lake is 2900 m. (e)Two
lakes in the Hercules Dome area (#72 and #73). The mean ice thickness above these
lakes is 3200 m (#72, right-hand side of the image) and 2800 m (#73, left-hand side
of the image). (f ) Lake Vostok, East of Ridge B. The mean thickness at this part of
the lake is 4000 m. (g) Lake at the mouth of the Astrolabe Subglacial Basin (#30).
The mean ice thickness above this lake is 4000 m. (h) Lake at the head of the Byrd
Glacier (#61). The mean ice thickness above this lake is 2580 m. (i) Lake beneath the
center of the West Antarctic Ice Sheet (#67). The mean ice thickness above this lake
is 3200 m. (a)–(f ) are adapted from Dowdeswell & Siegert (2002), (g)–(i) are from
Siegert (2002). Lakes are located by a white letter “L in each RES image. Numbers
of subglacial lakes are as defined in Siegert et al. (1996).
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1.3. Scientific Interest in Subglacial Lakes
There has been a huge degree of scientific and media interest in Lake Vostok
(and subglacial lakes in general) following the discovery that the water depth
of the lake was several hundred meters (Kapitsa et al. 1996). Discussion about
whether to make in situ measurements of the lake has been driven by two scientific
hypotheses. The first is that unique microorganisms inhabit the lake. The second
is that a complete record of ice sheet history is available from the sediments that
lie across the lake floor. Future exploration of subglacial lakes will be focused
on testing these hypotheses. If the hypotheses are correct, future investigations
of subglacial lakes could enable valuable insights into the history of Antarctica,
detailing its response to and control on climate change and our understanding of
biological functioning within extreme environments.
2.1. Locations of Subglacial Lakes
In general terms, there are two places beneath the Antarctic Ice Sheet where melting
occurs. The first is beneath the center of the ice sheet, where ice is generally thick-
est and the ice sheet is warm-based across large regions. The second is closer to
the ice sheet margin beneath warmed-based, enhanced ice flow units. Dowdeswell
& Siegert (2002) identified 10 areas of Antarctica where subglacial lake-type re-
flectors occur: Dome C (where more than 40 lakes have been identified); Ridge B
(including Lake Vostok), Dome A (where only two lakes have been found), Titan
Dome, South Pole, Hercules Dome, the Whitmore Mountains, the Transantarctic
Mountains (near Byrd Glacier), Oates Land, and George V Land (including a lake
at the mouth of the Astrolabe Subglacial Basin). More recently, Tabacco et al.
(2003) have identified a further 14 lakes across the Dome C region, and Popov &
Masolov (2004) have found evidence of 16 lakes around the Dome A and Dome
Fregions (Figure 4).
2.2. Lake Surface Areas
Approximately 75% of lakes have observed lengths of less than 5 km. Only Lake
Vostok is longer than 30 km. Lake Vostok is more than 250 km long and as much
as 80 km wide, making it unique in terms of surface area (Tabacco et al. 2002,
Studinger et al. 2003a). Around Dome C, Siegert & Ridley (1998) found that a
number of lakes thought to be relatively small lay beneath a noticeably large flat
ice surface. As is the case for Lake Vostok, Siegert & Ridley (1998) hypothesized
that the spatial extent of these flat surface features mark the actual extent of the
lakes beneath, which makes the size of the lakes much greater than had been first
thought. New RES data from Dome C confirms that relatively large lakes (three of
which are 1000 km
by area) exist between Dome C and Lake Vostok (Tabacco
et al. 2002, Tikku et al. 2002).
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2.3. Lake Depths and Water Volumes
Only Lake Vostok has been sounded by seismic methods, which reveal a depth
of between 510 m (Kapitsa et al. 1996) and 1000 m (Siegert et al. 2001). Water
depths for a number of other lakes have been inferred, however, from measure-
ments of the surrounding bedrock topography. These measurements suggest that
the depths of many lakes are between approximately 50 and 250 m (Dowdeswell
& Siegert 1999). Indeed, several lakes may be much greater than this, judging by
the side-wall slopes. According to Dowdeswell & Siegert (1999), more than 50%
of the lakes are likely to contain less than 5 km
of water, and only 10% store more
than approximately 100 km
. Lake Vostok, by contrast, is thought to hold approx-
imately 5000 km
of water (Studinger et al. 2004). Thus, Lake Vostok is unique
in terms of its volume, but this may reflect the fact that it is large by surface area,
rather than unusually deep compared with other subglacial lakes in Antarctica.
Recent inspection of aerogravity data collected over Lake Vostok reveals that its
bathymetry may involve two discrete basins: a large southern basin and a smaller
northern basin separated by a thin shallow zone (Studinger et al. 2004).
2.4. Slope of the Ice-Water Interface
RES data, combined with ERS-1 satellite altimetry, show that the ice sheet surface
above Lake Vostok has an elevation change from one end to the other of only
approximately 50 m. For the lake to be in hydrostatic equilibrium with the ice sheet
above, the slope of the ice-water interface must be approximately ten times, and
in opposite direction to, the ice surface slope (Figure 5). Thus, the ice thickness of
the northern end of Lake Vostok is approximately 500 m greater than in the south.
The relationship between ice and lake surface slopes is true for all subglacial
2.5. Types of Subglacial Lakes
Dowdeswell & Siegert (2002) characterized subglacial lakes into three main types
as follows: (a) lakes in subglacial basins in the ice-sheet interior, (b) lakes perched
on the flanks of subglacial mountains, and (c) lakes close to the onset of enhanced
ice flow.
The majority of Antarctic subglacial lakes are located within 200 km of ice
divides in the interior of the ice sheet (Figure 4). The bedrock topography of the
ice sheet interior involves large subglacial basins separated by mountain ranges
(Drewry 1983). The lakes in this category are those found in, and on the margins
of, subglacial basins. These lakes can be divided into two subgroups. First, there
are those located where subglacial topography is relatively subdued, often toward
the center of subglacial basins (e.g., Figures 3a,i). Secondly, some lakes occur in
significant topographic depressions, often closer to subglacial basin margins, but
still near the slow-flowing center of the Antarctic Ice Sheet. Where bed topography
is very subdued, deep subglacial lakes are unlikely to develop.
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Perched subglacial lakes are found mainly in the interior of the ice sheet, on the
flanks of subglacial mountain ranges (Figure 3d). In several cases, small (<10 km
long) subglacial lakes have been observed perched on the stoss face of large
(>300 m high), steep (gradient >0.1) subglacial hills.
At least 16 subglacial lakes occur at locations close to the onset of enhanced
ice flow hundreds of kilometers from the ice sheet crest (Siegert & Bamber
2000). An example is provided by three subglacial lakes near the onset of fast
flow into Byrd Glacier (e.g., Figure 3h). Byrd Glacier is fast flowing and drains
avery large interior ice sheet drainage basin into the Ross Ice Shelf (Drewry
1983). These subglacial lakes are similar in size and depth to the small and
probably shallow lakes found in parts of major subglacial basins in the ice sheet
2.6. Tectonic Setting of Lake Vostok
In recent years, a concerted effort has been made to map the size and extent of
Lake Vostok. Studinger et al. (2003a) summarized the results from an extensive
geophysical survey of the lake and its locale (Figure 6a, see color insert). The
results confirmed the aerial extent of the lake, established previously from ex-
amination of ERS-1 satellite altimetry (Ridley et al. 1993, Kapitsa et al. 1996).
A major new finding of this survey was that the western half of the lake has
distinct gravity and magnetic anomaly compared to the eastern half. The border
between the two anomalies is clear and linear along the eastern margin of the lake
(Figure 6b).
Figure 5 The dimensions and topographic setting of Lake Vostok. (a) ERS-1 altime-
try of the Antarctic Ice Sheet between Ridge B and Dome C. The location of Lake
Vostok can be identified from the anomalous flat ice surface region. SPRI (Scott Po-
lar Research Institute, University of Cambridge) radar flight lines and the location of
all known subglacial lakes around Lake Vostok (denoted as black squares) are pro-
vided. The surface ice sheet elevation, derived from the ERS-1 altimeter, is also shown.
The contour interval is 10 m. Arrows denote the direction of surface flow of ice over
Lake Vostok calculated from InSAR (Interferometric Synthetic Aperture Radar) (Kwok
et al. 2000). It must be noted that recent analysis of flow structures within the ice sheet
suggests the InSAR data may be inaccurate across the south of Lake Vostok (Tikku
et al. 2004). (b) Cross-section from north to south along the 200 km length of the lake.
(c) Cross-section from West to East along the 50 km width of the lake. The depth of
Lake Vostok can be estimated by (1) seismic information, which has revealed a water
depth of >500 m beneath Vostok Station; (2) side-wall bedrock gradient adjacent to
the lake of 0.1, which indicates several hundred meters of water depth in the center of
the lake; (3) radio-wave reflections from the lake floor, showing the water depth to be
between 10 and 20 m in the north of the lake; and (4) bedrock islands measured by
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Studinger et al. (2003b) analyzed aerogeophysical and seismological data to
establish a “conceptual tectonic model” for Lake Vostok and its locale. They con-
cluded that the tectonic framework around Lake Vostok involves a crustal boundary
(the source of the linear magnetic anomaly). The cause of this boundary is likely
to be associated with the emplacement of a thrust sheet onto a previously passive
continental margin. The age of the thrusting has been estimated as Proterozoic (i.e.,
Precambrian, in excess of 600 million years). Subsequent normal reactivation of
the thrust sheet then may have created the trough in which Lake Vostok is located
(Studinger et al. 2003b). What remains unknown about Lake Vostok’s trough is
the extent to which subglacial erosion during the onset of glaciation contributed
to its development.
Much attention has been given to Lake Vostok as a possible habitat for life. Being
an order of magnitude larger than any other subglacial lake, Lake Vostok has been
viewed by many as the ultimate long-term target for exploratory research (Priscu
et al. 2003).
3.1. Origin and Age of the Lake
The age and origin of Lake Vostok will be critical to the biota within the lake, and
to the age and quality of the geological records on the lake floor. One published
theory concerning the origin of Lake Vostok is unlikely (Duxbury et al. 2001). In
this theory, the lake is assumed to have existed within its trough prior to glaciation,
and remain intact as the ice sheet grew across the lake to its current relatively stable
configuration. In fact, the region that is now Lake Vostok was probably occupied
by grounded ice during ice sheet buildup, even if the trough and lake were present
prior to glaciation. This is because the margin of the ice sheet would have been
far closer to the position of the lake during the early stages of ice growth (e.g.,
DeConto & Pollard 2003). The surface slopes of the ice sheet over the region of
Lake Vostok would, therefore, have been significantly greater than those at the
center of today’s ice sheet. In this situation, water would have been driven out
of the trough to the ice sheet margin. A probable analogy to Lake Vostok during
ice sheet buildup is the Astrolabe Subglacial Trough in Wilkes Land, which holds
the thickest ice in Antarctica (4776 m). This trough has a small subglacial lake at
its mouth, which indicates that the whole trough is subject to subglacial melting,
and that water is driven out of the deepest parts of the trough. As the Astrolabe
Subglacial Trough is unable to hold a large lake owing to the ice overburden, Lake
Vostok would not have been resident in its trough during the early stages of ice
sheet growth in Antarctica. Thus, the lake is most likely to postdate the formation
of the current ice sheet. The exact age of the East Antarctic Ice Sheet has been
strongly debated over the past few decades. Some believe that it has remained in
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its present form for the past 15 million years (e.g., Stroeven et al. 1998), which
could make Lake Vostok nearly as old. Others claim that the ice sheet has under-
gone substantial modification in this time (e.g., Harwood & Webb 1998), which
could make the age of the lake much younger. Although this fundamental problem
remains to be solved (see Miller & Mabin 1998), its solution may exist within the
geological record held in subglacial lake floor sediments. The extraction of lake
floor sediments is, therefore, an important driver behind subglacial lake exploratory
3.2. Origin and Age of the Lakes Water
Regardless of the origin and age of Lake Vostok, the age of the water within the lake
(and other lakes) is a function of the age of ice melting into the lake and its turnover
time. The age of the basal ice in the Vostok ice core (located at the southern end of
the lake) is an important constraint on the age of youngest water within the lake.
Preliminary examination of the isotope record (Jouzel et al. 1999), estimates of
the air-hydrate crystal growth rates (Lipenkov et al. 2000), and ice flow modeling
(Parrenin et al. 2004) provide evidence that the basal glacier ice, 230 m beneath
the 3310 m level, could be as old as one million years. This marks the maximum
possible age of the youngest lake water. This also effectively marks the date at
which Lake Vostok was last in direct contact with biotic and chemical constituents
in Earth’s atmosphere. The mean age of water within Lake Vostok is a function of
the residence time of the water and how well the meltwater mixes with the exist-
ing lake water. We can speculate that if 20% of the annual meltwater mixes with
the resident lake water before refreezing, then the residence time of Lake Vostok
would be approximately 100,000 years (Mayer & Siegert 2000, Mayer et al. 2003).
Hence the mean age of Lake Vostok’s water is most probably of the order of one
million years.
Lake Vostok is unique because of its size. To understand why this subglacial lake is
so much larger than any other, and therefore why it is unique, we must understand
the subglacial morphology of the Antarctic continent and the way in which ice
flows across this landscape.
4.1. Morphological Analysis of Subglacial Antarctica
There are several overdeepened troughs in Antarctica that, like Lake Vostok’s,
are more than 100 km in length and tens of kilometers wide (Figure 7, see color
insert). The origin of these troughs is open to debate. Some may have formed
through tectonic processes such as rifting and faulting prior to glaciation (as has
been shown for Lake Vostok; Studinger et al. 2003b). All troughs are likely to
have been affected by subglacial erosion at a time when they were occupied by
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fast-flowing ice flow units. Although several troughs are likely to be actively erod-
ing, as they are occupied by fast-flowing ice (such as in the Astrolabe Subglacial
Trench), many must be relic features from an earlier, smaller phase of Antarctic
glaciation (such as the Adventure Subglacial Trench in Dome C) (e.g., Drewry
Examination of the morphology of the seven largest troughs in Antarctica re-
veals that Lake Vostok’s trough is not unique within subglacial Antarctica in terms
of size (Figure 2). The longest is the Lambert trough; the Astrolabe Subglacial
Basin is the widest, the deepest, has the steepest sides, and houses the greatest
thickness of ice; and the largest by area is the Byrd Subglacial Basin in West
Antarctica (Figure 1). The similarity between Lake Vostok’s trough and other
glacially derived overdeepened troughs (Figure 1) suggests that it too could be a
glacially affected trench.
4.2. Ice Flow and the Existence of Lake Vostok
Ice flows onto Lake Vostok from the Ridge B ice divide, located between 200
and 250 km from the lake’s western margin (Kwok et al. 2000). ERS-1 satellite
altimetry shows the ice sheet surface above Lake Vostok to be unusually smooth
and virtually flat (Figure 5) (Kapitsa et al. 1996, Siegert & Ridley 1998, R´emy
et al. 1999). This morphology is caused by the different dynamics of ice that
is grounded compared with floating ice. The basal shear stress across the ice-
water boundary above the subglacial lake is effectively zero, so the ice sheet
should flow over the lake by vertically uniform longitudinal extension (Paterson
1994). However, numerical ice flow modeling shows that the effect of longitudinal
extension is small owing to buttressing at the downstream lake shore (Mayer &
Siegert 2000). Instead, the flow of the floating ice is controlled more by the base-
parallel shear deformation of the adjacent grounded ice. Surface ice motion across
Lake Vostok has been measured using repeat-pass InSAR from ERS-1 (Kwok
et al. 2000). The regional flow of the ice sheet upstream of the lake is from west
to east, perpendicular to the surface elevation contours. As the ice flows past the
grounding line on the lake’s western margin, a noticeable southward component
is added to the ice velocity (Bell et al. 2002, Tikku et al. 2004). At Vostok Station,
the surface ice velocity is measured at 4.2 m year
in the direction 130
N (Kwok
et al. 2000), which compares with an astronomically based measurement of 3.7 ±
0.7 m year
toward 142 ± 10
N (Kapitsa et al. 1996).
The ice above the Lake Vostok trough is distinct in that it has the lowest surface
gradient (0.0002, dipping from north to south), which is due to the ice shelf type
flow that occurs over the lake (Pattyn 2003). This situation is controlled by the flow
of grounded ice upstream of the lake. Currently, there is only 50 m worth of north-
south slope in the grounded ice that flows across the trough’s western margin,
and this translates into the elevation change over the lake itself. If the slope of
the grounded ice across the lake’s western margin were changed, so too must the
ice surface slope over the lake. For example, if there were 200 m of grounded ice
elevation change across the western margin of the lake, the ice surface change over
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the lake itself must match this value, and the elevation of the ice-water interface
would change by 2000 m. Thus, the reason that a lake exists within the Vostok
trough, and that ice shelf flow is subsequently permitted, is due primarily to the flow
direction of grounded ice, and the minimal surface elevation change that occurs
across the lake’s upstream margin. This is a fundamental concept about Lake
Vostok, and one that can be used to assess the lake’s time-dependent variability
(see Section 6). In all other cases, ice flows across large subglacial troughs at an
angle greater than 30
to their long axes (Figure 7), and this is why no large lakes
can exist within these troughs.
Lake Vostok’s physiography may be unique, therefore, as a consequence of the
flow of grounded ice around the lake. This can explain why there is only one mega
subglacial lake beneath the ice sheet, and suggests that the existence of Lake Vostok
is a function of ice sheet flow direction rather than anything unique about the lake’s
trough. Under this explanation, the formation of Lake Vostok postdates that of the
trough and the date at which the ice flow over the trough became approximately
perpendicular to the trough’s long axis.
Our knowledge of physical processes within subglacial lakes has been developed
almost exclusively from investigations of Lake Vostok for two reasons. First, it is
avery large subglacial lake. Because of this, large-scale processes within it are
more obvious and identifiable than in small subglacial lakes. For example, there
have been several models of water circulation within Lake Vostok, and these have
been developed from large-scale ocean models that have a resolution of the order
of kilometers. Such a model is applicable to Lake Vostok, which is more than
250 km in length, but not to smaller lakes that are less than 10 km because the
model simply cannot function adequately at such a small scale. Second, by chance,
the Vostok ice core is located above the southern end of Lake Vostok.
5.1. Vostok Ice Core Studies
Several deep ice cores have been extracted from the ice sheet at Vostok Station (at
the southern end of Lake Vostok) since drilling began in the mid-1960s (the first
500 m deep dry borehole was extracted in 1965), providing important information
about the climate during the last glacial cycle. The most recent and deepest (3623 m)
ice core terminated 120 m from the base of the ice sheet. The upper 3310 m of the
ice core provides a detailed palaeoclimate record spanning the past 420,000 years
(Petit et al. 1997, 1999). In addition, microbiological analysis of the ice core has
revealed a range of microbiota, some of which have been reported to be culturable
in the laboratory (Abyzov et al. 1998, Karl et al. 1999, Priscu et al. 1999).
Typical glacier ice contains a record of gases and isotopes from which palaeo-
climate information is inferred. In the Vostok ice core, this type of ice exists to a
depth of 3310 m. Lower layers of ice, between depths of 3310 and 3538 m, are
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reported to have been reworked, making the extraction of palaeoclimatic informa-
tion difficult to establish (Figure 8). The basal 84 m of the ice core, from 3539 to
3623 m (Figure 8), has a chemistry and crystallography that are distinctly different
from the normal glacier ice above. The basal ice has an extremely low conductiv-
ity, huge (up to 1 m) crystal sizes, and sediment-particle inclusions (in the upper
half) (Jouzel et al. 1999). The mineral composition of ice-bound sediments below
3539 m is dominated by micas and is clearly different than typical crustal compo-
sition and particles within the overlying glacial ice (Priscu et al. 1999). Its isotopic
composition, distinct from the meteoric ice above, suggests that it formed by the
refreezing of lake water to the underside of the ice sheet. Thus, there is 210mof
accreted Lake Vostok ice beneath Vostok Station (Jouzel et al. 1999) (Figure 8).
The accreted ice below 3608 m (and presumably extending to the ice-water inter-
face) contains no sediment-particle inclusions, implying that it formed over the
lake proper rather than along the shoreline.
Ice flows from west to east across Lake Vostok, and the accreted ice containing
sediment particles must have formed across the western side of the lake at the first
contact between ice and water. Airborne radar and seismic data suggest that the
lake may be shallow across the western side compared to the 510–1000 m water
depth recorded beneath the Station (Kapitsa et al. 1996, Lukin et al. 2000). There
are two ideas linking water depth to the entrainment of material into the accreted
ice. The first is that the basal ice scrapes against the shallow floor of the lake
across the western side, picking up debris as it does so (Jouzel et al. 1999). The
second is that the lake water is turbulent enough for fine sediment to be held in
suspension and incorporated within the formation of accreted ice (Royston-Bishop
et al. 2005).
5.2. Rates of Subglacial Melting and Freezing
Borehole temperature measurements along the full length of the Vostok ice core
have been used to establish the energy balance between the ice sheet and the lake
(Salamatin et al. 1998, Salamatin 2000). The mean basal temperature gradient is
, which relates to a heat flux through the ice from the lake ceiling
of 46 mW m
, indicating that rates of subglacial freezing above Lake Vostok
are most likely to be 4mmyear
(Salamatin et al. 1998). In the extreme case
where ice at 10
Cflows over the western lake margin, rates of melting and
freezing beneath Vostok Station will probably not be higher than approximately
11 mm year
(Salamatin 2000).
Figure 8 (a) Ice stratigraphy of the basal 550 m of ice beneath Vostok Station deter-
mined from analysis of the Vostok ice core (after Souchez et al. 2000). (bd) Chemical
records of the basal 90 m of the ice core. (b) δD with the frequency of rock particle in-
clusions, (c) δ
O, (d) deuterium excess (Souchez et al. 2000). Adapted from Souchez
et al. (2000).
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The spatial distribution of subglacial melting and freezing can be estimated
theoretically from isochronous internal radar layering by observing the loss or
gain of basal ice along a flowline. Using this technique, it has been shown that
subglacial melting occurs in the north of Lake Vostok (Siegert et al. 2000), and
freezing (accretion) takes place in the south (Bell et al. 2002, Studinger et al. 2004)
(Figure 9, see color insert). Rates of melting and freezing calculated from radar
layering have been much higher (of the order of centimeters) than those from the
ice core’s temperature record. It is possible that heat used for melting can be taken
from the lake water, but this requires a dynamic water circulation system.
5.3. Water Circulation Models
The zones of subglacial melting in the north and freezing in the south of Lake
Vostok are thought to be controlled by the slope of the ice-water interface because
the thickness of ice dictates the pressure melting temperature and the density of
meltwater. As a consequence of subglacial melting and freezing, circulation is
induced in the lake.
There are two possible ways in which water within Lake Vostok could circulate.
One is if the lake contains pure water, the other is if the lake water is saline. These
two end member possibilities are detailed below. In the first instance, circulation
of pure water is discussed.
Because the surface of Lake Vostok is inclined, the pressure melting point in
the south will be slightly (0.3
C) less than that in the north. The circulation of
pure (non-saline) water in Lake Vostok will be driven by the differences between
the density of meltwater and lake water. Geothermal heating will warm the bottom
water to a temperature higher than that of the upper layers. The water density will
decrease with increasing temperature because Lake Vostok is in a high-pressure
environment, resulting in an unstable water column (W¨uest & Carmack 2000).
This leads to convective circulation conditions in the lake in which cold meltwater
sinks down the water column and water warmed by geothermal heat ascends up the
water column (Figure 6a). However, a pool of slightly warmer and stratified water
may occur below the ice roof in the south, where the ice sheet is thinner and sub-
glacial freezing takes place (W¨uest & Carmack 2000). Here, the water would not
be involved in convective motion as heat is transferred from the ice toward the lake
(i.e., the temperature will decrease with depth). There have been three models from
which the circulation of pure water in Lake Vostok can be evaluated (Mayer et al.
2003, W¨uest & Carmack 2000, Williams 2001) (Figure 6a). The models indicate
that meltwater will be colder and denser in the northern area of Lake Vostok,
where the ice is thickest, than in both the surrounding lake water and meltwater
in areas with thinner ice cover. It appears, therefore, that this region is the main
zone of downwelling of pure water. However, the circulation is complicated by the
geometry of the lake cavity and the Coriolis Force. This means that circulation in
Lake Vostok will include horizontal transfer and, to a lesser extent, vertical over-
turning. The models agree that northern meltwater will sink and be transported
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horizontally to the south, via a clockwise circulation system, to a region where the
pressure-melting point is higher, allowing refreezing to occur (Figure 6a).
An alternate point of view is that the lake is saline to a small extent (Souchez
et al. 2000). The fresh glacier meltwater will, therefore, be buoyant compared
with the resident, more saline, lake water (Figure 6b). The northern meltwater
likely spreads southward and upward, traveling into regions of progressively lower
pressure and displacing lake water in the south if the horizontal salinity gradient
(north-south) is high enough to compensate for geothermal warming. The possi-
bility of such a regime is controlled by (a) the melting-freezing rates, (b) the rates
of mixing between the fresh ascending meltwater layer and the underlying saline
water, and (c)vertical free convection driven by the geothermal heating of water
at the lake bottom. The cold northern water will eventually enter a region where
its temperature is at the pressure melting point if the heat flux from the basal water
is not sufficiently high. The water will then refreeze back onto the ice sheet base
some distance away from where it was first melted into the lake. In this case, a con-
veyor of fresh cool meltwater is established, which migrates from north to south
immediately beneath the ice sheet, which causes displacement of warmer dense
lake water from south to north. In contrast, if the bulk salinity is not high enough,
a stable stratification will develop in the upper water layers below the tilted lake
ceiling, with more saline warmer water in the south and fresher, cooler water in the
north (W¨uest & Carmack 2000). The deep-water stratum will be subject to vertical
thermal convection because, for any reasonable level of salinity, the temperature
at the lake bottom will be high enough to start the convection.
5.4. Storage of Gas Hydrates (Clathrates)
Dissolved oxygen will be found in the Lake Vostok water column because gas
hydrates are released from the melting glacial ice. Gas hydrates (or clathrates)
are crystal lattices formed by water molecules around gas molecules under condi-
tions of low temperatures and high pressures. High pressures result in substantial
volumes of gas being compressed and trapped within these lattice structures. Air
hydrates are known to be present in the glacial ice above Lake Vostok (Uchida
et al. 1994). This is because gases cannot dissolve in the solid ice, and hence all
of the air is subject to the confining pressure of the ice. Some of the gases in the
air clathrates that enter the lake can dissolve in water, and hence the air clathrate
may completely or partially dissolve, dependent on the concentration of dissolved
gases already present in the lake water. Lipenkov & Istomin (2001) calculate
that the minimum oxygen concentration in Lake Vostok waters is 17 µM, just
under twice that of water saturated with oxygen at the surface, whereas the max-
imum concentration is 850 µM. The oxygenation of lake water by dissolution
of the clathrate will most likely occur near the surface of the lake, proximal to the
supply of hydrates from the melting ice sheet base. Water circulation will then
allow the transfer of oxygenated water to other parts of the lake, including the
southern side, where subglacial freezing occurs, and deeper regions. It is also
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likely that the concentration of dissolved oxygen will decrease with distance from
the source of hydrates, if there is microbial respiration in the lake water and if
there is oxidation of sulphides, ammonium, or other metabolic electron donors
in the glacial debris. This may mean that some regions, such as the floor of the
lake and the lake floor sediments, may be depleted in dissolved oxygen, potentially
making the environment there anoxic.
Clearly, the oxygen concentration in the lake water is a function of the magni-
tude of the oxygen source (from clathrate dissolution) and oxygen sinks (oxidation
of reduced compounds and incorporation in refrozen meltwater). Anoxia will oc-
cur in regions of the lake where the flux of oxygen is less than the potential oxygen
demand. A third factor that may control oxygen concentrations, if the oxygen
source exceeds the oxygen sink, is the saturation limit. The oxygen concentration
of the lake water will gradually increase over time until the maximum oxygen con-
centration is reached. Additional oxygen is then retained as clathrate, which may
effectively buffer variations in oxygen concentrations in the water column against
short-term variations in oxygen supply and sinks. Oxygen concentrations are cal-
culated to reach saturation levels in a minimum of 0.2–1.6 million years if there
are no sinks of oxygen from the lake (Lipenkov & Istomin 2001). The timescale
of nitrogen (N
) saturation is of a similar magnitude, and it is likely that nitrogen
clathrates will be found in the lake given the age of the lake and the lack of obvious
sinks. A current lack of an oxygen mass balance for the lake prevents scientists
from an unequivocal position on both the distribution of oxygen concentrations
throughout the lake and the presence or absence of oxygen clathrates.
McKay et al. (2003) cite the lack of clathrates in the Vostok ice core’s ac-
creted ice as evidence in support of clathrates sinking to the lake floor owing to
the incorporation of CO
(making the clathrates heavier). They calculate that the
concentration of N
and O
in the water is approximately 2.5 liters kg
. Such an
amount of gas is enough to cause serious problems with degassing and expansion
if the water were to be brought to the surface. Hence, plans to extract samples of
water from Lake Vostok, especially deep water, should make allowances for the
likely gas concentrations within the water.
5.5. Implications for Other Subglacial Lakes
The water circulation predictions for Lake Vostok are based on large-scale ocean
models that cannot be readily used for smaller subglacial lakes (as the single cell
width in such models is often the size of a small lake). However, the processes
identified for Lake Vostok through these modeling initiatives have important con-
sequences for other subglacial lakes. This is because the models show that the
driver of water circulation is the sloping ice roof of the lake. As all lakes have
this characteristic, it should be expected that all lakes should undergo circulation
of the lake water in response to the differential temperatures and pressures, and
rates of melting and freezing, at their surfaces. In fact, some subglacial lakes have
a noticeably higher surface slope than Lake Vostok, which may permit a dynamic
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response more vigorous than anticipated thus far. Such dynamic flow of water can
be modeled using numerical fluid dynamics tools, and these are currently employed
to study the hydrodynamics of small lakes.
A reasonable understanding of the modern glaciological setting of Lake Vostok
has been established, and some idea about the physical and chemical dynamics of
this uniquely pristine lake has been developed (Siegert et al. 2001, Studinger et al.
2003a), yet there has been little discussion as to how these processes change over
time. Today, Lake Vostok is in relative hydrostatic equilibrium with the overriding
ice sheet (i.e., the lake surface slope is about ten times, and in opposite direction to,
the ice surface slope) (Figure 10a). Under changes to the ice sheet, for hydrostatic
equilibrium to be maintained, the lake must adjust accordingly.
Processes in Lake Vostok are likely to change as a consequence of ice sheet
variations, such as those occurring over glacial-interglacial cycles (see Royston-
Bishop et al. 2004 for evidence in support of variations in the size of Lake Vostok).
The ice sheet in central Antarctica will change over this timescale in two ways:
(a)bychange to the ice thickness and ice surface elevation and (b)bymigration of
ice divides and alteration in the grounded ice flow direction. The possible effects
of these processes on the extent and volume of water in Lake Vostok are discussed
6.1. Lake Response to Variations in Ice Thickness
Today, Lake Vostok is most likely to be in a contained, pressurized environment,
which means the ice sheet response to small changes in ice surface elevation
will be distinct from those that will occur over an ice shelf. For ice shelves, a
small change in the surface elevation of the ice sheet will be associated with a
much larger (10 times) change to the draft of the ice. For Lake Vostok, how-
ever, this is very unlikely. Instead, a small increase in the ice surface elevation
will lead to either (a)anincrease in the level of the lake or (b)anincrease in
the effective depth of the lake for a steady ice sheet profile over the lake to be
If the thickness of grounded ice surrounding Lake Vostok is increased by 10 m,
the lake level, actual or effective, will also rise by 10 m. If the actual level of the
lake increases, the lake extent will also increase and no ice thickness change is
required (Figure 10b). Rates of subglacial melting and freezing are maintained in
this situation. On the other hand, if the effective lake level increases, this will have
no effect on the ice surface elevation, and so ice thickness must instead increase by
10 m. In this case, melting rates are likely to be increased as the pressure melting
point is reduced and temperatures increased beneath thicker ice. Hence, effective
lake surface change is unlikely to occur on its own; instead, both processes will
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contribute. The net effect of an increase in the grounded ice thickness is, thus, an
increase in the volume and extent of the lake.
Evidence in support of ice thickness changes over Lake Vostok comes from cal-
culations of former ice accumulation rates and numerical modeling of the ice sheet
during the last glacial cycle. During the last interglacial, ice accumulation rates at
Vostok Station (>2.5 cm year
) were greater than at present (2.3 cm year
(Siegert 2003). The grounded ice thickness in central East Antarctica is likely,
therefore, to have been greater during the Eemian than at present. Conversely,
during periods of full glaciation, the accumulation rate of ice in East Antarctica
is only approximately 1.2 cm year
, which is compatible with a reduction in ice
sheet elevation. According to the numerical modeling study of Huybrechts (2002),
the surface elevation at Vostok Station varies from the present value by between
+50 m (during interglacials) and –150 m (in periods of full glaciation) over the
last four glacial cycles.
6.2. Lake Volume and the Direction of Ice Flow
Ice currently flows onto Lake Vostok approximately orthogonal to the axis of
the lake (the lake axis is parallel to the grounded surface contours). There is
consequently very little change in ice surface elevation across the western margin
of the lake, and, hence, the ice surface over the lake is extremely flat (owing to ice
shelf type flow; Pattyn 2003). The response of Lake Vostok to changes in surface
slopes is assessed through examination of two end member situations.
First, if the grounded ice upstream of the lake flowed in a more north-south
direction, there would be an increase in the number of contours crossing the western
lake margin and, so, the ice surface over the lake would be steeper (Figure 10c).
This would cause the lake surface gradient to increase and, hence, the lake will
shrink to the south (albeit the lake could be deeper in the south under this scenario)
(Figure 10c). Second, just a subtle change to the grounded flow direction west of
the lake northward would reverse the ice profile gradient over the lake’s western
Figure 10 Ice sheet processes affecting the volume of Lake Vostok. (a) Contemporary
morphology of Lake Vostok, as estimated by Siegert et al. (2001). (b) Ice elevation
change owing to variations in the depth of the lake. Assuming a closed system, if the
water depth of Lake Vostok increased by 10 m, the ice surface elevation would also
increase by 10 m. The consequence of this process is that the lake’s volume increases
with increase in ice surface elevation. (c) Changes to the ice surface slope over lake.
Small changes to the surface elevation will result in much larger changes to the slope
of the ice-water interface. The lake responds to this change by growing when the ice
slope decreases, and shrinking under a steeper ice profile. (d) Change in subglacial
conditions that may be expected by a reversed ice surface slope. In this case, the ice-
water interface will slope in the opposite direction to at present, potentially forcing
water out of Lake Vostok to the north and into the Aurora Subglacial Basin.
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margin and, thus, over the lake itself. This would reverse the slope of the ice-water
interface and force water to the north of the lake. In this situation, water would
be evacuated from the lake if the basal slopes were <10 times the surface slope
(Figure 10d). The flowpath of such water would be toward the Aurora Subglacial
Basin to the northeast of Lake Vostok. This is an important concept to realize, as it
suggests that conditions in Lake Vostok could change with only small adjustment
to the grounded ice flow pattern upstream.
Evidence in support of a change to the ice flow direction over the lake comes
from internal radar layer structures that have been mapped out across the southern
end of Lake Vostok (Bell et al. 2002). These structures are evidence of the flow
direction when the ice sheet passed across the western margin of the lake. They
are, at least in part, indicators of former ice flow paths. The orientation of these
structures is at a significant angle to the current surface velocities measured by
InSAR (Kwok et al. 2000, Bell et al. 2002). Specifically, the InSAR velocity vector
is approximately 30
south of that identified from internal ice structures. There are
two possible explanations for this mismatch. The first is that the InSAR data are
inaccurate (Bell et al. 2002, Tikku et al. 2004). The second is that at least part of
the difference between the two vectors is due to changes in the surface velocity.
Given that the surface elevation at Vostok Station changes over glacial-interglacials
cycles it is highly likely that the ice sheet surface contours (and velocity vectors)
were affected to some degree during such periods.
6.3. Consequences for Lake Circulation and
Physical Processes
As the driver for water circulation in Lake Vostok is its sloping ice roof, any change
to this slope is bound to have an impact on the water circulation. An increase in
the gradient may lead to greater variation in the basal temperature at one end
of the lake compared with the other and, hence, enhanced rates of subglacial
melting and freezing. The heat produced and released by melting (beneath thicker
ice) and freezing (beneath thinner ice), respectively, will excite circulation of
the lake water. In this way, steeper ice-water interfaces may be linked to more
dynamic subglacial lakes. Conversely, a perfectly horizontal lake surface may not
experience significant spatial change in the basal ice temperatures. Consequently,
the circulation that results will not necessarily be driven from one end to the other
and so would be organized differently. Numerical modeling of water circulation
may allow us to better understand the link between the slope of the ice water
interface and lake flow processes.
Following the realization that Antarctic subglacial lakes may house unique forms
of life and hold detailed records of past climate change, the Scientific Commit-
tee on Antarctic Research (SCAR) published recommendations for their future
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exploration (Kennicutt 2001). The report listed three specific scientific goals that
such exploration should address, and put forward advice as to how site selection
could be achieved. The first goal concerned the identification of life in the lake
waters and lake floor sediments. The second involved the extraction of palaeocli-
mate records from the ice overlying the lakes and from the sediments across their
floors. The third related to the origin and evolution of the lakes themselves, as this
knowledge would be essential in interpreting information for the first two goals.
A group of specialists was set up by SCAR to “consider and recommend mecha-
nisms for the international coordination of a subglacial lake exploration program”
(Kennicutt 2001, Priscu et al. 2003).
7.1. Which Are the Most Suitable Lakes?
No single subglacial lake is currently known to be best suited to attain the SCAR
scientific goals. Because of the surveying undertaken to date, however, only Lake
Vostok is known to be a viable location for exploration. It is, therefore, appropriate
that plans be made to study this lake further to attain SCAR’s first goal concerning
the identification of life. Further, although the goal is to find life, the ambition is
to discover endemic ancient life. To realize this ambition a lake of substantial age
and depth is required. Lake Vostok is thought to be such a candidate.
In terms of establishing records of past change in Antarctica (SCAR’s second
goal), examination of more than one lake would be preferred. All lakes may po-
tentially record glacial-interglacial changes to the ice sheet, and some located in
potentially sensitive regions such as Dome C may record more significant changes
from early in Antarctica’s glacial history. Ideally, sediment from the floors of sev-
eral lakes aligned along a transect from the ice margin across Dome C to the Vostok
Highlands and the Gamburtsev Mountains would enable an appropriate spatial ap-
preciation of glacial history. Sampling from just one lake, although useful, would
cause a restriction in the spatial interpretation of information. One key issue that
lake floor sediments could address is the whether the ice sheet has remained stable
for the past 15 Ma or whether it was more dynamic over this period (Miller &
Mabin 1998). As the most sensitive region of East Antarctica to change is likely
to be Dome C (much of the bed here is below sea level), examination of at least
one lake from this location would be needed to address this issue.
Exploration of lakes at the center of the ice sheet from Dome C, Lake Vostok, and
Ridge B would allow the first two of SCAR’s goals for subglacial lake exploration
to be addressed. Such work would also help to ascertain the origin and evolution
of these lakes. This would leave two types of lake (perched lakes and lakes found
at the onset of enhanced flow) unexplored. Hence, examination of these two types
of lakes may occur at a time after lakes at the center of the ice sheet, and after the
attainment of the SCAR-defined research goals.
A distinction should be made about the scientific merits of West Antarctic sub-
glacial lakes, compared with those in East Antarctica. Whereas the East Antarctic
Ice Sheet is expected to have been stable for several millions of years, causing the
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lakes at its base to be of a similar age, the West Antarctic Ice Sheet has most likely
fluctuated in size over this time. However, the exact nature of the fluctuation has
yet to be deciphered from the geological record. Sediments across the floors of
West Antarctic subglacial lakes may hold such a record, and it is this that makes
their exploration particularly exiting.
7.2. Specific Plans for Lake Exploration—Lake Vostok,
Russian Plan
In June 2003, the twenty-sixth annual Antarctic Treaty Consultative Meeting, held
in Madrid, discussed the Comprehensive Environmental Evaluation (CEE) of a
Russian plan to extract a sample from Lake Vostok. The plan is to use the existing
Vostok ice core (about 150 m above the lake surface), and drill down through
the lake’s ice roof. Just before penetration of the ice ceiling, drilling fluid will be
extracted from the core. Thus, as the core breaks through into the lake, it will be
underpressurized compared to the ice overburden, and so instead of drilling fluid
entering the lake, lake water will rise up the core 50 m or so. The core will be
extracted quickly, and the Lake Vostok water within the ice core will be left to
freeze. The core will then mine through the newly frozen ice and return it to the
surface for analysis. Thus, samples of Lake Vostok surface water can be extracted
from the lake without the need for in situ observation.
Critics of this plan argue that entering the lake without an ice barrier between the
lake and the ice sheet surface is potentially dangerous, given the unknown levels of
gas hydrates within the lake. One argument against the plan is that decompressing
the core could encourage the lake water to degas with potentially catastrophic
consequences. McKay et al. (2003) predict there to be large concentrations of
hydrates within the lake water, but hypothesize that these are located at depth in
the lake, rather than at the surface. Such a hypothesis could be used to argue in
favorofthe Russian plan. The only problem they would face is the decompression
and expansion of solid ice as it returns to the surface; a scientific problem certainly,
but not necessarily a catastrophic one.
Another critical issue that the planned sampling must deal with concerns the
rates of freezing that can be expected in the ice core. Maximum rates of subglacial
freezing beneath Vostok Station are of the order of several centimeters per year
(Siegert et al. 2000, Bell et al. 2002). Even at this high rate, which is much higher
than the Vostok ice core temperature profile suggests (e.g., Salamatin et al. 1998),
it would take more than a year for lake water within the lower part of the ice core
to freeze fully and allow subsequent recoring.
7.3. Other Plans for the Exploration of Lake Vostok
In 1996, when Lake Vostok was brought to the world’s attention (Kapitsa et al.
1996), scientists began questioning whether unique microorganisms existed within
the lake. Appetites were whetted by the discovery of microbes within the Vostok
ice core accreted ice (Karl et al. 1999, Priscu et al. 1999). To answer this question
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Figure 11 Experimental design to enable the in situ exploration of Antarctic subglacial
lakes. The issue of how the hydrobot moves about the lake is important to the lake environment.
It is shown here to use a water jet propulsion system that may result in mixing of the lake
water. If the lake is stratified, such mixing must be avoided. A less invasive approach is to
simply drop a string of instruments vertically down the water column and take a series of
measurements and samples without lateral navigation.
unequivocally, however, requires in situ observations (Figure 11). Several plans
were made to explore the lake, and all of them require the sampling strategy to be
as sterile as possible. As such conditions preclude the use of drilling fluids, hot
water drilling to the lake surface appears to be the only plausible way down. Of the
plans made so far, most of them revolve around using hot water drilling to a stage
above the lake ceiling. The hot water drill is removed and a tethered thermo-probe
is inserted into the hole. It then melts down into the ice, which freezes above it.
Unreeling a tether to the surface as it goes down, the sterile thermo-probe works
its way to the lake ceiling, whereupon it deploys a hydrobot to sample and measure
the environment of the lake (lake water and sediment). This plan has a far greater
potential for truly understanding the lake system than its Russian counterpart,
as it could record temperatures, flowrates, and stratification/gradients that would
not be possible from a refrozen sample. Of course it would not be possible to
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return samples to the surface under this proposal, so the thermo-probe or hydrobot
would have to include all the necessary equipment. As a consequence, the scheme
is ambitious and requires considerably more technological development than the
Russian plan.
7.4. Exploration of West Antarctic Subglacial Lakes
Lake Vostok is not the only subglacial lake worthy of exploration. In East Antarctica
there are over 90 known subglacial lakes that could contain unique life and hold
records of past climate change. As these lakes could be several millions of years old,
the requirement for their preservation as unique ecosystems makes their exploration
challenging. The same is not necessarily true in West Antarctica, where there are
several known subglacial lakes (Figure 4). The exploration of West Antarctic
subglacial lakes has three advantages over the exploration of their East Antarctic
The first advantage concerns the age of West Antarctic subglacial lakes. As
the West Antarctic Ice Sheet probably decayed a number of times during the
Quaternary, West Antarctic subglacial lakes must be considerably younger than
those in East Antarctica. The lakes will not, therefore, be ancient systems as is an-
ticipated for Lake Vostok. Nevertheless, the lakes comprise the same environment
as in any subglacial lake (they are under the same boundary conditions), which
means that biological selection processes are as likely to occur in West Antarctic
subglacial lakes as they are anywhere else.
The second relates to environmental considerations and the preservation of
ancient environments. The base of the East Antarctic ice sheet has never been
reached by drilling (although the EPICA ice core in Dome C plans to), and this
makes the planning of East Antarctic lake exploration particularly difficult in terms
of environmental conservation. However, there have been numerous occasions
when the base of the WAIS has been reached, sampled, and measured. In partic-
ular, wet ice-bed contacts have been observed several times on the Siple Coast
(e.g., Gow et al. 1968, Kamb 2001). Hence, although the environmental issues
relating to West Antarctic subglacial lake exploration are important, they may
not be as insurmountable as the issues relating to East Antarctic subglacial lake
The third advantage relates to the elevation of the ice surface above subglacial
lakes. The ice sheet surface in West Antarctica is no higher than 2400 m above sea
level, which is over a kilometer lower than the ice surface over most East Antarctic
subglacial lakes. Altitude-related problems encountered by scientists at the center
of the East Antarctic Ice Sheet will not, therefore, be as much of an issue during
the study of West Antarctic subglacial lakes.
These advantages have led a U.K.-U.S. consortium to propose the exploration
of a 10-km-long West Antarctic subglacial lake, named Subglacial Lake Ellsworth
S, 90
W) (Siegert et al. 2004). Geophysical surveying of Lake Ellsworth
is planned for 2006-7, with direct measurement and sampling to take place dur-
ing the period of the forthcoming International Polar Year (IPY, 2007–2009). In
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fact, the IPY may witness a considerable advance in our understanding of sub-
glacial lake environments in both East and West Antarctica, as several international
teams have proposed scientific programs on a variety of subglacial lakes for this
7.5. Extraction of Sediment from Subglacial Lakes
The acquisition of climate records from subglacial lake floor sediments represents
a significant challenge. No one has yet designed a drilling system capable of
coring and retrieving material beneath several kilometers of ice and a water column
potentially hundreds of meters deep. Knowledge of how to do this is being gained,
however, by the ANDRILL (ANtarctic DRILLing) program, which over the next
few years will drill through the Ross Ice Shelf in a number of locations and extract
the sea floor sediment beneath to get a record of Cenozoic climate and ice sheet
variability. Use of a traditional drilling rig, as in ANDRILL, is likely to be disruptive
to the lake environment to some degree, as sediment will be disturbed and forced
into the water column. If the sediment is fine, as is expected, it may take some
time to settle out completely. Consequently, investigations focused solely on lake
floor sediments (other than the thin layer of material that could be analyzed by
ahydrobot) may have to take place after biological investigations or, at the very
least, a sufficient distance away.
Approximately ten years ago, an inventory of Antarctic subglacial lakes was pub-
lished, detailing the size of more than 70 lake-type features (Siegert et al. 1996).
The water depth of the largest of these lakes, Lake Vostok, was discovered to be
more than 500 m deep (Kapitsa et al. 1996). This information led biologists to
regard the lake as a potential habitat for unique microorganisms, and led geol-
ogists to suggest that sediments across the floor of the lake will hold important
environmental records. In the past decade there has been a concerted international
scientific effort to understand Antarctic subglacial lakes and to plan the exploration
of these unique environments.
More than 100 subglacial lakes have now been identified from data collected
from radar sounding of the ice base and satellite altimetry of the ice surface.
They exist at the ice sheet base owing to geothermal heating, which at a
background level is enough to maintain melting beneath several kilometers
of ice. Meltwater produced at the ice base collects in topographic hollows to
form pools of water.
Lake Vostok, the largest subglacial lake, is more than 250 km long, more
than 50 km wide, and, in at least one place, is more than 1 km deep. It resides
beneath 3.7 and 4.2 km of ice in central East Antarctica. The lake occupies a
huge subglacial trough, which, interpretation of geophysical data suggests,
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may have been developed by faulting in preglacial times and subsequent
glacial erosion.
There are several large troughs beneath the Antarctic ice sheet, but only
one is occupied by a large subglacial lake (Lake Vostok). This is because
grounded ice flows approximately perpendicular to Lake Vostok trough’s
long axis, which permits an extremely low ice-surface gradient to exist and
subglacial water to pond. In all other cases, ice flows at an angle (>30
the trough axis, which causes enhanced water pressure gradients at the ice
base and the evacuation of water in even the deepest trough (the Astrolabe
Subglacial Basin). Thus, Lake Vostok may be unique as a consequence of
the ice flow around it, rather than its surrounding topography.
The ceiling of Lake Vostok slopes by ten times the ice surface above it (to
be in hydrostatic equilibrium). This causes the pressure and temperature at
one end of the lake to be different to the other, which results in differential
rates of melting and freezing and, in turn, water circulation.
Even small changes to the ice surface that occur over glacial-interglacial
timescales may have potentially significant consequences for the ceiling gra-
dient over Lake Vostok. Thus, water circulation may be affected by changes
to the ice sheet that occur over glacial cycles.
All subglacial lakes have a sloping ice roof. Thus, the processes identified
thus far for Lake Vostok that are driven by the differential conditions across
the lake roof are likely to be applicable to other, smaller subglacial lakes.
The exploration of subglacial lakes has two science goals. The first is to
identify and understand the microorganisms that live in these extreme envi-
ronments. The second is to extract and measure the climate record that will
be held in the sediments across the floors of subglacial lakes.
In terms of the first goal, plans to explore Lake Vostok range from a relatively
simple experiment involving the trapping, freezing, and mining of lake water
in the Vostok ice core’s borehole, to highly sophisticated in situ measurement
of the lake system using apparatus aboard remote vehicles. Plans are also
being developed to explore other subglacial lakes, including Lake Ellsworth
in West Antarctica, for the period of the IPY.
The extraction of sediments from a subglacial lake is also some way off.
However, relevant experience is being gained by geologists who are coring
through sea floor sediments beneath sea ice and ice shelves in the Ross Sea (in
the ANDRILL program). The technology developed in these investigations
could one day be used to retrieve sediments from a subglacial lake.
I thank members of the SCAR SALE group of specialists for discussions held in
Chamonix Mont Blanc and the University of Bristol during 2003, during which
many of the ideas covered in this chapter were developed. Funding in support
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of this work was provided by NERC Grant NER/A/S/2000/01144 and a Philip
Leverhulme Prize.
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Figure 4 Map of the Antarctic Ice Sheet showing the locations of Antarctic subglacial lakes (triangles).
The ice sheet surface is contoured at 500 m intervals (after Drewry 1983).
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Figure 6 The tectonic setting of Lake Vostok. (a) The subglacial elevation of Lake Vostok.
(b) The magnetic field anomaly across the Lake Vostok region. Reprinted from Studinger
et al. 2003a, with permission from Elsevier.
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Figure 7 Physiography of Antarctic subglacial trenches as denoted in the bed map of
Antarctica (Drewry 1983). The lengths, mean widths, shapes, and areas of troughs were
calculated from a contour that depicts the shape of the trough, namely the –1000 m con-
tour (for Astrolabe, Adventure, and Lambert), –750 m (for Peacock), and –1500 m (for
Bentley, Byrd, and north of the Horlick Mountains). Side slopes were calculated from the
lowest point in the trough (assumed to be –1000 m beneath Vostok Station), and the near-
est topographic high point (i.e., the top of the head wall).
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See legend on next page
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Figure 9 Water circulation patterns within Lake Vostok under fresh and saline conditions.
(a) Circulation calculated by numerical modeling, assuming that the water is pure
(Williams 2001, Mayer et al. 2003). The white arrows show the bottom water circulation
and the black arrows denote the higher-level circulation close to the ice base. Dots refer to
upwelling of lake water, crosses denote downwelling. There are two clockwise circulation
paths in the upper and lower regions of the lake. Most of the vertical mixing takes place in
the southern two thirds of the cavity, but this exchange is rather limited. Blue shading
refers to predicted zones of subglacial freezing, red shading indicates subglacial melting.
(b) Circulation of Lake Vostok thought to occur as a result of saline conditions (Mayer
et al. 2003) (i.e., 1.2–0.4‰). It should be noted that understanding Lake Vostok’s water
circulation may be complicated by the discovery of a distinct bathymetric basin beneath
the “freeze zone” (Studinger et al. 2004). Numerical modeling that may uncover the bathy-
metric influence on water flow has yet to be undertaken, however. Adapted from Siegert
et al. (2001).
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March 23, 2005 16:53 Annual Reviews AR233-FM
Annual Review of Earth and Planetary Sciences
Volume 33, 2005
M. GARRELS, D.E. Canfield 1
Okan T
uz, Caner
Imren, Mehmet Sakınc¸, Haluk Eyido
gan, Naci G
Xavier Le Pichon, and Claude Rangin 37
ARE THE ALPS COLLAPSING?, Jane Selverstone 113
EARLY CRUSTAL EVOLUTION OF MARS, Francis Nimmo and Ken Tanaka 133
PREDICTION, T.N. Palmer, G.J. Shutts, R. Hagedorn, F.J. Doblas-Reyes,
T. Jung, and M. Leutbecher 163
Hiroo Kanamori 195
SUBGLACIAL LAKES, Martin J. Siegert 215
FEATHERED DINOSAURS, Mark A. Norell and Xing Xu 277
STRESS TRANSFER, Andrew M. Freed 335
Ryosuke Motani 395
THEIR ECOSYSTEMS, Guy M. Narbonne 421
Garry Willgoose 443
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QUESTIONS, Tristan Guillot 493
MOON, Stein B. Jacobsen 531
e 571
MOIST CONVECTION, Bjorn Stevens 605
Subject Index 673
Cumulative Index of Contributing Authors, Volumes 23–33 693
Cumulative Index of Chapter Titles, Volumes 22–33 696
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... Varečka). in the ice layer buried more than 3 km under the earth's surface [2,12,[16][17][18]21]. The geological age of the ice layer was estimated at up to 500 thousand years. ...
Microorganisms were isolated from lignite freshly excavated in the Záhorie coal mine (southwestern Slovakia) under conditions excluding contamination with either soil or air-borne microorganisms. The isolates represented both Prokarya and Eukarya (fungi). All were able to grow on standard media, although some microorganisms were unstable and became extinct during storage of coal samples. Bacteria belonged to the genera Bacillus, Staphylococcus, and Rhodococcus, according to both morphological criteria and ITS sequences. Several bacterial isolates were resistant to antibiotics. The presence of anaerobic bacteria was also documented, although they have not yet been identified. Fungal isolates were typified by using their ITS sequences. They belonged to the genera Trichoderma (Hypocrea), Penicillium, Epicoccum, Metarhizium (Cordyceps), and Cladosporium. Several fungi produced compounds with antibiotic action against standard bacterial strains. The evidence for the presence of microorganisms in native lignite was obtained by means of fluorescence microscopy, scanning electron microscopy, and electron microprobe analysis. Results demonstrated that microorganisms were able to survive in the low-rank coal over a long time period.
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Large subglacial lakes manifest themselves as flat regions on the ice surface. ERS-1 satellite radar altimetry of the Dome C region of East Antarctica was analyzed to correlate unusually flat areas on the ice surface with known locations of subglacial lakes identified from airborne radio-echo sounding (RES) data. The mean length of subglacial lakes which have an expression in the ice-sheet surface was ~8.3 km, whilst those that did not exhibit a surface morphological manifestation had a mean length of ~3.3 km. Thus, lakes up to about 4 km in length arc unlikely to be detected from satellite radar altimetry of the ice surface. Given that the spacing of radio-echo flight tracks within the SPRI-NSF-TUD Antarctic database is 50-100 km in many areas, a number of subglacial lakes probably lie undetected beneath the ice sheet. RES information from two large, flat surface regions within Dome C, and a further flat area located at 80° S, 127° E, indicates the absence of subglacial lakes beneath the ice-surface features. However, these areas are characterised by relatively strong radio-echo returns which may indicate the presence of water-saturated basal sediments. We suggest that (1) blankets of water-saturated basal sediments may cause similar surface morphological features to those produced by subglacial lakes; and (2) misidentification of subglacial lakes from satellite altimeter observations of the ice-sheet surface is possible without the support of RES information relating to the ice-sheet base. Furthermore, our study indicates a lack of subglacial lake signals from RES data over relatively thick regions of East Antarctica such as the Adventure Subglacial Trough. We conclude that subglacial water produced in such regions may be transported by a basal hydrological system, driven by overburden pressure, to less thick regions of the ice sheet where subglacial lakes have been identified.
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The conditions of air hydrate stability in Lake Vostok are quantified. The upper limit of concentration of O 2 dissolved in the subglacial water is predicted to be 50 times as high as the O 2 concentration in the air-supersaturated perennially ice-covered surface lakes in Antarctica.
Radar-altimeter data from ERS-1 allow detailed topographic mapping of Antarctica to 82° S to be carried out, revealing several notable surface features. Among these is the large subglacial lake near Vostok which is mapped here in detail. The central part of the lake is found to have an exceptionally smooth surface with r.m.s. elevation variations of less than 0.2 m. A search for other large Antarctic lakes in the data is made based on the smoothness and low gradient of the surface. A number of other flat areas are identified with lake locations previously determined from radio-echo sounding observations. However, radar-altimeter observations show that a minimum lake size of 20 km is required for a surface above a lake to become flat. Numerous bowl-like features can be seen in the surface topography, and these may be associated with intermediate-sized subglacial lakes. It is determined that high spatial-resolution radar altimetery could be used to identify subglacial lakes greater than 10 km in lateral extent. Flat regions of the ice sheet are particularly useful as they may be used as height-reference surfaces to help fix the orbits of radar-altimeter satellites.
Microscopic observation of air-hydrate crystals was carried out using 34 deep ice-core samples retrieved at Vostok Station, Antarctica. Samples were obtained from depths between 1050 and 2542 m, which correspond to Wisconsin/Sangamon/Illinoian ice. It was found that the volume and number of air-hydrate varied with the climatic changes. The volume concentration of air-hydrate in the interglacial ice was about 30% larger than that in the glacial ice. In the interglacial ice, the number concentration of air-hydrate was about a half and the mean volume of air-hydrate was nearly three times larger than that in the glacial-age ice. The air-hydrate crystals were found to grow in the ice sheet, about 6.7 × 10−12 cm3 year-1, in compensation for the disappearance of smaller ones. The volume concentration of air-hydrate was related to the total gas content by a geometrical equation with a proportional parameter α. The mean value of α below 1250 m, where no air bubbles were found, was about 0.79. This coincided with an experimentally determined value of the crystalline site occupancy of the air-hydrate in a 1500 m core obtained at Dye 3, Greenland (Hondoh and others, 1990). In the depth profile of calculated α for many samples, α in the interglacial ice was about 30% smaller than that in the glacial-age ice.
The relative permittivity ∊’ and attenuation α in laboratory-grown, polycrystalline and single-crystal ice Ih are reported at 35 and 60 MHz in the temperature range —25°C to — 0.2 ° C. The ∊’ and α at 35 MHz and — 1°C are 3.208±0.010 and 6.2±0.1 dB/100 m, respectively. From a comparison between the respective ∊’ and α of the polycrystalline and single-crystal ice measured perpendicular to the c-axis, it is concluded that any anisotropy of polarization at these frequencies is so small as to be undetectable. Amongst several factors that may contribute to anisotropy in ice, electronic polarization contributes 0.0037 to the difference between the relative permittivity measured parallel and perpendicular to the c-axis at — 1° C and at frequencies less than 500 THz. Experiments have shown that the plastic deformation resulting from a uniaxial compressive stress of up to 100 bar does not influence the ∊’ and α of ice at 35 and 60 MHz.
Microorganisms were detected in samples taken from different horizons of the glacier of Central Antarctica (at depths of 1500-2750 m), whose age is over 240 000 years. Microscopic studies of microorganisms from thawed water samples taken from the center of the glacier core (precipitated on membrane filters and stained with fluorescent dyes) revealed that they contained a wide variety of microorganisms. Bacteria prevailed, although yeasts, fungi, and microalgae also occurred. Some horizons also contained the pollen of higher plants and dust particles of various origins. Consumption of C-14-labeled organic compounds by most thawed samples testified to the presence of viable cells in them. The total microorganism number at depths of 1500-2750 m was 0.8 x 10(3) to 10.8 x 10(3) cells/ml. Fluctuations in the number of microorganisms throughout the glacier correlated with changes in the number of mineral particles detected by glaciologists in the respective glacier horizons, which, according to their calculations, depended on alternations of warm and cold periods an Earth.