|
By
Steven Profaizer
On the frozen Antarctic continent, subglacial lakes are
a hot spot of scientific interest, but the information
they contain remains untapped.
“The ice sheet in Antarctica can be as much as 5
kilometers thick, and at the bottom point, it can be quite
warm, as warm as the melting point of ice,” said
Sridhar Anandakrishnan, who led a science team to the
South Pole this season to learn more about one lake that
rests about 16 kilometers away from the U.S. Antarctic
Program station.
Geothermal activity and the 3 kilometers of insulating
ice at Pole can trap heat and melt sections of the ice
sheet’s base. If some of that water begins to collect
in basins, it can form a lake as it would anywhere else
on Earth.
“Subglacial lakes are potentially very valuable treasure
troves for interesting biota and paleoclimate information,”
Anandakrishnan said. He added that sediments left in the
lakes may give clues as to the history of the ice sheet
itself.
Aerial radar surveys have flagged hundreds of lakes with
large surface areas dwelling under the ice. The subglacial
lake at the South Pole was first identified using this
method in the late 1970s.
The largest is Lake Vostok, which sits 4 kilometers beneath
Russia’s Vostok Station. At 10,000 square kilometers,
it is a little bigger than Lake Ontario.
“Lake Vostok is sort of the crown jewel of subglacial
lakes, so we have to be very careful before we try to
sample there,” Anandakrishnan said of the lake that
reaches more than 500 meters deep. “However, there
are hundreds of these other lakes. And while we need to
be very careful with them as well, we can experiment a
little bit with sampling technologies.”
But before any sampling efforts can even be proposed near
the South Pole, scientists need to get a better idea of
the subglacial lake’s properties.
“The problem is that Vostok is the only one that
has been definitively defined as a lake with significant
volumes of water,” Anandakrishnan said.
Radar was used to scout out the 15-by-15-kilometer lake
near the South Pole as well as the other lakes spread
across the continent, but it can only provide a two-dimensional
picture because water reflects radar signals. Therefore,
radar can provide information on a lake’s area, but
it can’t penetrate the surface to explore what lies
beneath.
Depth is crucial in determining the scientific value of
a subglacial lake. Deep lakes mean longevity, giving the
organisms there time to take hold. Shallow lakes are much
harder on any life that does exist there because the water
is more susceptible to drying up or refreezing.
Anandakrishnan’s team turned to a method called seismic
reflection profiling to obtain the vital third dimension.
“Radar kind of gives you indication that there is
a lake and seismic really nails it down,” Anandakrishnan
said. “We don’t really know what is there at
the South Pole yet. As far as the radar is concerned,
water that is a meter or a few meters thick is the same
as water that is tens of meters thick.”
The team also went back over sections of the lake with
ground-based radar, which gives a much finer picture of
the lake’s shape than its airborne cousin.
“In a way, I think that will be the most valuable
thing we have. There’s lots and lots of radar data
on the rest of the continent, but there’s very little
seismic data,” Anandakrishnan said. “We’ll
have a seismic record, and we’ll have a radar record
in exactly the same spot. … If we could do a really
good job of identifying what the characteristics of this
normal lake are, then we hope all the other normal lakes
around the continent would pop into focus.”
The principle behind seismic reflection profiling is much
more straightforward than its name suggests – make
a sound at the surface and listen for the echo.
Anandakrishnan and his team needed an extremely loud sound
to make it down several kilometers to the lake and then
echo back to the surface again, and so they turned to
Pentaerythritol Tetranitrate (PETN) – one of the
strongest known high explosives.
The team made 5-centimeter-wide holes 18 to 30 meters
into the ice with a hot water drill that spits out 93-degree-Celsius
water and lowered an explosive into each one.
Each of the approximately 100 charges was detonated one
at a time and ranged in weight from a fifth of a kilogram
to almost 5 kilograms.
“At the surface we don’t feel very much, but
the sound travels down into the ice,” Anandakrishnan
said.
When the sounds hit the lake’s surface, part of the
signals were reflected, but part of them continued to
the bottom of the lake, where they then bounced back to
the scientists on the surface.
The reflected signals were recorded on the surface of
the ice by 150 geophones, which operate much like microphones
but record vibrations in the ice instead of in the air.
Anandakrishnan and his team are now back in the United
States with the seismic records of their month in the
field. It takes a lot of data processing to turn the EKG-like
seismic graphs into a profile of the lake.
To determine the lake’s depth, the team looks at
the difference between arrival times of the signal reflected
off the surface of the lake and the signal reflected off
the bottom.
“In the field, we have limited capability for processing
the data,” Anandakrishnan said. “We can look
at them, see that they are of high quality, and say, ‘Yep,
that looks good. We got data.’ But we can’t
really interpret them for the properties of the subglacial
material until we get home and can do a more thorough
job.
“I’m hopeful that we will have an answer soon,
but I’m not sure what it is yet.”
-
Antarctic
Sun
|
