Proglacial Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 198

to the atmosphere. Then the left-hand side of Eq. ( 6.17b ) is zero, and we can solve this
equation for the depth H:

kT f T 0

H ¼ ð 6 : 18 Þ

Q g

Here T 0 is the surface temperature of the ice sheet. The freezing point decreases with
pressure by p

C bar − 1 (see Eq. 2.7b ). Beneath 4 km of ice, the pressure is
about 350 bar and the freezing point of fresh water is

10 − 3 × 7.53 × °

50 mW m − 2 and - 2.7 ° C. If Q g *

°

T f - T 0 * 50 C, we have H * 2 km. The result is quite sensitive to the geothermal heat

fl flux. For a physically realizable subglacial lake, the solution for H must be less than the

thickness of land ice in the considered location. As discussed in Sect. 4.4 , the time-scale of
thermal diffusion is L 2 /D, where L is the length scale and D is the diffusion coefficient,
D * 0.1 m 2 day − 1 1 km the time-scale is 30,000 years.

Antarctic subglacial lakes have been categorized into three main types: (1) lakes in
subglacial basins in the ice-sheet interior; (2) lakes perched on the

for ice. Thus, for L ¼

fl flanks of subglacial

mountains; and (3) lakes close to the onset of enhanced ice ow. The bedrock topography

of the ice-sheet interior involves large subglacial basins separated by mountain ranges.
The lakes in the

fl

first category are found mostly in and on the margins of subglacial basins.
These lakes can be divided into two subgroups in regards with their location: (a) Sites
where the subglacial topography is relatively subdued, often toward the center of sub-
glacial basins; and (b) Sites where signi

cant topographic depressions exist, often closer
to subglacial basin margins, but still near the slow-

owing center of the Antarctic Ice
Sheet. Where bed topography is very subdued, deep subglacial lakes are unlikely to
develop. The subglacial lakes in Antarctica have remained unexplored except by using
remote sensing technology. There are no bottom sediment samples to tell about the history
of the ice sheet.
Theoretical models reveal that the subglacial environment may hold about 10 % of all
surface lake water on Earth, enough to cover the whole continent with a uniform water
layer with a thickness of about 1 m (see Priscu and Foreman 2009). These models further
reveal that the average water residence time in the subglacial zone is 1,000 years. The
biology of subglacial lakes is largely an open question. The evidence to date suggests that
communities of microorganisms may exist in these lakes. Recently, attention has been
focused on the possibility that subglacial environments of Antarctica may harbour
microbial ecosystems isolated from the atmosphere for as long as the continent has been
glaciated (20

fl

25 million years).

-

6.4.2 Lake Vostok

Lake Vostok (Zotikov 2006) is the largest known subglacial lake. Its surface area is about
14,000 km 2 or about the same size as Lake Ladoga, Russia. The mean depth of Lake