Unusual iceberg
© Andrew Shepherd, University
Unusual iceberg at Rothera Research Station, Antarctic Peninsula.
A region of West Antarctica is behaving differently from most of the rest of the continent: A large patch of ice there is thickening, unlike other parts of West Antarctica that are losing ice. Whether this thickening trend will continue affects the overall amount that melting or collapsing glaciers could raise the level of the world's oceans.

The track hidden in the middle of the ice sheet suggests that the current thickening is just a short-term feature that may not affect the glacier over the long term, the new study indicates. It also suggests that similar clues to the past may be hiding deep inside the ice sheet itself.

SHUTTING IT DOWN

"What's exciting about this study is that we show how the structure of the ice sheet acts as a powerful record of what has happened in the past," says first author Nicholas Holschuh, a postdoctoral researcher in Earth and space sciences at the University of Washington.

The data come from the ice above Mount Resnik, a 1.6-kilometer (mile-high) inactive volcano that currently sits under 300 meters (0.19 miles) of ice. The volcano lies just upstream of the thickening Kamb Ice Stream, part of a dynamic coastal region of ice that drains into Antarctica's Ross Sea.

antarctic ice
Studies show Kamb Ice Stream has flowed quickly in the past but stalled more than a century ago, leaving the region's ice to drain via the four other major ice streams, a switch that glaciologists think happens every few hundred years. Meanwhile, the ice inland of Kamb Ice Stream is beginning to bulge, and it is unclear what will happen next.

"The shutdown of Kamb Ice Stream started long before the satellite era," Holschuh says. "We need some longer-term indicators for its behavior to understand how important this shutdown is for the future of the region's ice."

BENEATH THE ICE

The paper analyzes two radar surveys of the area's ice. Coauthors Robert Jacobel and Brian Welch collected one using the ice-penetrating radar system at St. Olaf College in Minnesota in 2002. Coauthor Howard Conway, a research professor of Earth and space sciences, collected the other in 2004. Conway noticed the missing layers and asked his colleagues to investigate.

"It wasn't until we had spent probably six months with this data set that we started to piece together the fact that this thing that we could see within the ice sheet was forming in response to the subglacial volcano," Holschuh says.

The study shows that the mysterious feature originates at the ice covering Mount Resnik. The authors believe that the volcano's height pushes the relatively thin ice sheet up so much that it changes the local wind fields, and affects depositing of snow. So as the ice sheet passes over the volcano a section missed out on a few annual layers of snow.

"These missing layers are common in East Antarctica, where there is less precipitation and strong winds can strip away the surface snow," Holschuh says. "But this is really one of the first times we've seen these missing layers in West Antarctica. It's also the first time an unconformity has been used to reconstruct ice sheet motion of the past."

CHECKING THE RECORD

Over time, the glacial record shows that this feature followed a straight path toward the sea. During the 5,700-year record, the five major coastal ice streams are thought to have sped up and slowed down several times, as water on the base lubricates the glacier's flow and then periodically gets diverted, stalling one of the ice streams.

"Despite the fact that there are all these dramatic changes at the coast, the ice flowing in the interior was not really affected," Holschuh says.

What the feature does show is that a change occurred a few thousand years ago. Previous UW research shows rapid retreat at the edge of the ice sheet until about 3,400 years ago, part of the recovery from the most recent ice age. The volcano track also shows a thinning of the ice at about this time.

"It means that the interior of the ice sheet is responding to the large-scale climate forcing from the last glacial maximum to today," Holschuh says. "So the long-timescale climatic forcing is very consistent between the interior and the coast, but the shorter-timescale processes are really apparent in the coastal record but aren't visible in the interior."

Holschuh cautions that this is only a single data point and needs confirmation from other observations. He is part of an international team of Antarctic scientists looking at combining the hundreds of radar scans of Antarctic and Greenland glaciers that researchers originally did to measure ice thickness. Those data may also contain unique details of the glacier's internal structure that researchers can use to recreate the history of the ice sheet's motion.

"These persistent tracers of historic ice flow are probably all over the place," Holschuh says. "The more we can tease apart the stories of past motion told by the structure of the ice sheet, the more realistic we can be in our predictions of how it will respond to future climate change."