Scientists
working in the remotest part of Antarctica have discovered that liquid
water locked deep under the continent’s coat of ice regularly thaws and
refreezes to the bottom, creating as much as half the thickness of the
ice in places, and actively modifying its structure. The finding, which
turns common perceptions of glacial formation upside down, could reshape
scientists’ understanding of how the ice sheet expands and moves, and
how it might react to warming climate, they say. The study appears in
this week’s early online edition of the leading journal Science; it is
part of a six-nation study of the invisible Gamburtsev Mountains, which lie buried under as much as two miles of ice.
Researchers endured extreme wind and cold at a high-elevation field camp near the center of the ice sheet. (Michael Studinger/AGAP). |
Ice
sheets are well known to grow from the top as snow falls and builds up
annual layers over thousands of years, but scientists until recently
have known little about the processes going on far below. In 2006,
researchers in the current study showed that lakes of liquid water
underlie widespread parts of Antarctica. In 2008-2009, they mounted an expedition using geophysical instruments to create 3-D images of the Gamburtsevs, a range larger than the European Alps. The expedition also made detailed images of the overlying ice, and subglacial water.
“We
usually think of ice sheets like cakes—one layer at a time added from
the top. This is like someone injected a layer of frosting at the
bottom—a really thick layer,” said Robin Bell, a geophysicist at Columbia Univ.’s Lamont-Doherty Earth Observatory
and a project co-leader. “Water has always been known to be important
to ice sheet dynamics, but mostly as a lubricant. As ice sheets change,
we want to predict how they will change. Our results show that models
must include water beneath.” The Antarctic ice sheet holds enough fresh
water to raise ocean levels 200 feet; if even a small part of it were to
melt into the ocean, it could put major coastal cities under water.
The
scientists found that refrozen ice makes up 24% of the ice sheet base
around Dome A, a 13,800-foot-high plateau that forms the high point of
the East Antarctic ice sheet, at 3.8 million square miles roughly the
size of the continental U.S. In places, slightly more than
half the ice thickness appears to have originated from the bottom, not
the top. Here, rates of refreezing are greater than surface accumulation
rates. The researchers suggest that such refreezing has been going on
since East Antarctica became encased in a large ice sheet some 32
million years ago. They may never know for sure: the ice is always
moving from the deep interior toward the coast, so ice formed millions
of years ago, and the evidence it would carry, is long gone.
The radar image shows the Gamburtsev Mountains, at bottom of the image, overlain by the ice sheet, which has been deformed by a bulge of refrozen ice (center). (Courtesy Bell et al., 2011) |
Deeply
buried ice may melt because overlying layers insulate the base, hemming
in heat created there by friction, or radiating naturally from
underlying rock. When the ice melts, refreezing may take place in
multiple ways, the researchers say. If it collects along mountain ridges
and heads of valleys, where the ice is thinner, low temperatures
penetrating from the surface may refreeze it. In other cases, water gets
squeezed up valley walls, and changes pressure rapidly. In the depths,
water remains liquid even when it is below the normal freezing point,
due to pressure exerted on it. But once moved up to an area of less
pressure, such supercooled water can freeze almost instantly. Images
produced by the researchers show that the refreezing deforms the ice
sheet upward.
“When we first saw these structures in the field, we thought they
looked like beehives and were worried they were an error in the data,”
Bell said. “As they were seen on many lines, it became clear that they
were real. We did not think that water moving through ancient river
valleys beneath more than one mile of ice would change the basic
structure of the ice sheet.”
Because
the ice is in motion, understanding how it forms and deforms at the
base is critical to understanding how the sheets will move, particularly
in response to climate changes, researchers say. “It’s an extremely
important observation for us because this is potentially lifting the
very oldest ice off the bed,” said Jeff Severinghaus, a geologist at
Scripps Institution of Oceanography in San Diego who was not involved in
the study. He said it could either mean older ice is better preserved –
or, it could “make it harder to interpret the record, if it’s shuffled
like a deck of cards.”
From
November 2008 to January 2009, the researchers did fieldwork around a
California-size part of Dome A. Using aircraft equipped with ice
penetrating radars, laser ranging systems, gravity meters and
magnetometers, they flew low-altitude transects back and forth over the
ice to draw 3D images of what lay beneath. The aim was to understand
how the mountains arose, and to study the connections between the peaks,
the ice sheet, and subglacial lakes. They were also hunting for likely
spots where future coring may retrieve the oldest ice. The work took
place near the Southern Pole of Inaccessibility, the point farthest away
from any ocean, and much harder to reach than the South Pole itself.
They lived in isolated field camps, enduring high winds and temperatures
ranging down to minus 40 degrees C.
“Understanding
these interactions is critical for the search for the oldest ice and
also to better comprehend subglacial environments and ice sheet
dynamics,” said Fausto Ferraccioli, a scientist with the British
Antarctic Survey who also helped lead the project. “Incorporating these
processes into models will enable more accurate predictions of ice sheet
response to global warming and its impact on future sea-level rise.”
The
researchers now will look into how the refreezing process acts along
the margins of ice sheets, where the most visible change is occurring in
Antarctica. Based on their data, a Chinese team also hopes to drill
deep into Dome A in the next two or three years to remove cores that
would trace long-ago climate shifts. They hope to find ice more than a
million years old.
“Scientific
results from the Antarctic Gamburtsev Province Project data set are
already transforming our understanding of ice sheet behavior,” said Dr.
Alexandra Isern, program director for Antarctic earth sciences in the
National Science Foundation’s Office of Polar Programs. “This
understanding is critical for the development of climate models that can
accurately describe how our planet will react to increased global
temperatures.”
Other
co-authors of the paper include Timothy T. Creyts, Indrani Das,
Nicholas Frearson and Michael Wolovik, of Lamont-Doherty; Hugh Corr,
Thomas Jordan and Kathryn Rose of the British Antarctic Survey; David
Braaten of the Center for Remote Sensing of Ice Sheets at Kansas
Univ.; Detlef Damaske of Germany’s Federal Institute for
Geosciences and Resources; and Michael Studinger of the NASA Goddard
Space Flight Center in Maryland.
The work was funded by the U.S. National Science Foundation and launched in conjunction with the International Polar Year,
a 2007-2009 effort to study the poles by thousands of scientists from
more than 60 nations. Support also came from the Natural Environment
Research Council of Britain; the Australian Antarctic Division; and the
Polar Research Institute of China.
Project website (Lamont-Doherty Earth Observatory)
Project website (British Antarctic Survey)
Supercooling from M. T. on Vimeo.