East Antarctic Ice Sheet: Freezing From the Bottom Up
How radar explores the underside of Antarctica's ice sheets. Credit: Zina Deretsky/NSF.
The story of how Antarctica got its ice is about as firmly established as anything in Earth science. Although the southern continent is the world's driest, any snow that does fall there, with extraordinarily rare exceptions, stays more or less forever. Even if it only snows a fraction of an inch per year, a few million years' worth of snowfalls, piled one on top of another, eventually adds up. That's why the East Antarctic Ice Sheet is nearly two miles thick.
It's also why Antarctica and Greenland offer such valuable insight into the climate of the distant past: those fluffy snowflakes trap air, and as the snow is buried and compressed into ice, the air is preserved in tiny bubbles. Ice cores pulled out of the depths are essentialy time capusles that show what carbon dioxide (CO2) levels, air temperatures and other crucial information were hundreds of thousands of years ago. And that in turn helps climate scientists project what could happen in the future as human-generated greenhouse gases continue to build up in the atmosphere.
All of that is rock-solid — or ice-solid — textbook science. But experts on ice sheets also know that, under certain circumstances, they can also add ice from the bottom up. Thanks to heat welling up from underground, there's water — and even some lakes — where the ice meets Antarctic bedrock. This water can freeze to the underside of the ice sheet, but, says geophysicist Robin Bell, of Columbia University's Lamont-Doherty Earth Observatory, "we thought it might be five or ten meters thick." But when Bell and collaborators from a multinational project to study the continent's deeply buried Gamburtsev Mountains (which are more extensive than the Alps) took a look with powerful ice-penetrating radar, they were in for a major surprise. The bottom-forming ice wasn't ten meters thick. It was more like a thousand.
The ice laid down from snowfall, says Bell, lead author of a paper describing the new findings in the journal Science, "look like the layers of a cake."
At the bedrock level, she says, "they drape over the mountains. But we started to see these weird things that looked like giant beehives. They were huge bodies of ice pushing the overlying ice up. We thought it was a mistake at first, of course."
Elevation map of Antarctic ice sheet (in meters). Credit: NASA.
It wasn't. It turns out, in fact, that this bottom-frozen ice accounts for as much as half of the ice sheet's nearly 14,000 ft. thickness in some places round Dome A, which is the highest part of Antarctica, and lies near the Southern Pole of Inacessibility (which is even harder to reach than the South Pole, as the name suggests). The ice forms, says Bell, in two ways. First, the water that collects at the bottom of the ice simply freezes gradually onto the underside of the sheet. But in some cases, the weight of the overlying ice sheet forces the water up the sides of the buried mountains. As it gains altitude, it loses pressure, making it freeze all at once.
"You can replicate it with a bottle of seltzer," says Bell. "Put it in a bucket of ice with some salt in it, to lower the freezing point. Then unscrew the top: the pressure drops, and it will freeze instantly."
Since climate scientists depend on a steady layering of ice to tell them how ancient a particular air bubble is, you might think these intrusions from below would confuse things. There's no way to mistake bottom ice from top ice, though, says Bell. "The ice that forms from water underneath the ice sheet has no air bubbles in it. It looks completely different." In fact, she says, it could make life easier by pushing the most ancient layers of air-trapping ice closer to the surface, where drillers can get at it more easily. The bad news is that it also might have disrupted and scrambled the overlying layers, making them more challenging to interpret.
Nobody really knows, but within a couple of years, they may find out: a Chinese team wants to drill into Dome A to look for samples of million-year-old atmosphere. That would beat the existing record by about 200,000 years. If the "giant beehives" of subsurface ice have been kind to their top-forming counterpart, our ability to look into our climate future could improve significantly.