UNLOCKING SECRETS BENEATH ANTARCTIC ICE

BY BARBRA A. RODRIGUEZ

Spanning an area nearly the size of New Mexico, Antarctica’s Thwaites Glacier has the potential to increase global sea levels by a meter if it unloads its ice. What’s more, the unstable giant acts like a plug for much of West Antarctica. That means if Thwaites collapses in upcoming decades — as scientists predict it might — it could release enough ice from neighboring glaciers to increase sea levels by 3 meters.

Understanding what’s happening at Thwaites Glacier and other glaciers across the continent of Antarctica is vital and extremely challenging science. Researchers from across the globe have spent decades trying to gather data and perfect the science to determine how these glaciers work and the affect they could have on the planet as the climate changes.

Nowhere have researchers done more to overcome the scientific challenges presented by Antarctica than at the University of Texas Institute for Geophysics (UTIG). Scientists at UTIG have spent nearly 30 years looking into and under the ice, and their history of leading the way isn’t likely to stop anytime soon.

In 2016, UTIG joined an effort by the Korea Polar Research Institute to monitor Thwaites. Then in the spring of 2018, the national research foundations of the U.S. and U.K. announced $50 million in funding for an international collaboration that will also work on the Thwaites Glacier system. Both of the major research initiatives will have distinct UTIG components. The glacier models that the U.S.- and U.K.-led international collaboration will use rely on 2004 aerogeophysical data collected by the UTIG team.

Carrying out the difficult 2004 journey fell to a group led by UTIG Senior Research Scientist Don Blankenship, who teamed up with the British Antarctic Survey and glaciologist David Vaughan. Both were concerned that little was known about the West Antarctic region despite its potential for collapse.

“Up until the 2000s, there had been exactly one tractor traverse across Thwaites,” said Blankenship. “The big fear that everyone had of a runaway ice sheet collapse looked like it applied to Thwaites, and we didn’t even know the shape of this ice sheet. Can you
imagine, having a single line of data across an area the size of Texas, and trying to understand geology and hydrology from that?”

Even back then, Blankenship was no newcomer to the study of glaciers. He and other glaciologists had determined in the late 1980s how important the geology underlying a glacier was to its behavior. UTIG glaciologists have since pioneered detailed analyses of ice sheets’ subglacial geology and the forces that act upon them while revealing added sea level threats from Antarctica.

The Underpinnings of Glacial Movement

Glaciers can resemble rivers of flowing ice. Whether in the Alps or Antarctica, glaciers often rest on a bed of meltwater that hastens their movement the same way a water slide speeds swimmers into a pool.

Scientists used to think this was the most important relationship guiding a glacier’s movement. However, for many polar glaciers the scenario isn’t so simple. Blankenship helped make this clear in studies of West Antarctica in the late ’80s while at the University of Wisconsin-Madison. During those studies, he developed an array of new seismic techniques to examine glaciers feeding into West Antarctica’s Ross Ice Shelf, southeast of Thwaites and Pine Island Glacier. Unexpectedly, researchers found that there was more than water at work controlling the glacial flow.

While withstanding subzero temperatures, the Wisconsin team dug holes to drop dynamite deep within the fast-moving ice streams. Seismic reflections from the explosions revealed that about a mile beneath the glaciers that fed into the ice shelf, there was mud, not water.

The 20 feet of soupy material was glacial till, sedimentary leftovers of rocks pulverized by ancient glaciers. This mud, infused with subglacial water, would allow those ice streams to move unexpectedly dynamically, moving more slowly at first as the mud held the ice back, but changing over time.

The findings reported in 1986 Nature articles have defined much of UTIG’s glaciology studies ever since.

“We made the case that geology controls the evolution of marine ice sheets,” Blankenship said. “As the climate changes, having a structural understanding of what lies beneath these glaciers is key to predicting future glacial decay and sea level impacts.” By the time he joined UTIG in 1991, Blankenship had moved on from the ground-based seismology approach. By then, he had pioneered a multipronged aerial approach to survey glaciers and subglacial geology over thousands of miles during Antarctic field visits.

UTIG’s polar aerogeophysical survey methodology has since evolved to include improved instruments,
acquisition and flight systems, and it has been adapted for many different aircraft.

“UTIG has provided important boundary information from our airplanes and helicopters that researchers need to answer questions about past glacial behavior and to improve predictions of future change,” said Jamin Greenbaum, a UTIG postdoctoral fellow who has participated in every Antarctic summer season with Blankenship since UTIG began studying East Antarctica in 2008.

Widening the Antarctic Lens

By the 2000s, the UTIG team had broadened its focus beyond West Antarctica. Why only study glaciers in the west, the thinking went, when their ice payload could be dwarfed by largely unstudied glacial systems elsewhere that might have similar instabilities? To learn more, they ventured to the vast southern reaches of East Antarctica, developing the ICECAP (International Collaborative Exploration of the Cryosphere through Aerogeophysical Profiling) program, which relies on the research stations of multiple international collaborators to conduct broad airborne surveillance.

Nicknamed the “Sleeping Giant” at the time for its assumed stability, East Antarctica covers about two-thirds of the continent. However, satellite monitoring by UT Austin’s Center for Space Research in the 2000s revealed that areas in East Antarctica such as Totten Glacier were losing ice and mass (based on GRACE satellite imaging).

Moreover, field research led by Blankenship, Professor Martin Siegert of Imperial College of London, and Tas van Ommen of the Australian Antarctic Division had shown that parts of East Antarctica included unstable reservoirs of ice held within rock basins below sea level. This was concerning because the lower a basin’s rim is, the more likely the ocean can erode and destabilize the ice sheet.

From outlining the structure of the subglacial basin that fed Thwaites, Blankenship knew these basins resemble a soup bowl in profile where water couldn’t pour in until it tops the rim. East Antarctic basins had been assumed to be too high up along the coast for this to occur, but the UTIG team had shown otherwise.

“Everyone thought that little of East Antarctica was below sea level,” Blankenship said, “but Martin and I knew that everything that was going on at Thwaites and Pine Island in West Antarctica could go on there as well.”

Unraveling Glacial Collapse

To help clarify the potential of East Antarctica to contribute to future sea level rise, Blankenship and Research Scientist Duncan Young developed a map of the major basin-feeding glaciers such as Totten’s Aurora Subglacial Basin.

“We chose to focus on the Aurora Subglacial Basin because it may represent the weak ‘underbelly’ of the East Antarctic Ice Sheet,” Blankenship said of the ice sheet.

Using ice-penetrating radar, magnetometers and other instruments attached to an airplane, they developed the first high-resolution map of the basin’s structure. In a 2011 Nature paper, they revealed that about one-fifth of the California-size basin extended more than 1 kilometer below sea level, making it susceptible to being destabilized by the Southern Ocean. The ice contained in that basin could raise sea level by at least 3.5 meters, more than the total amount believed to be vulnerable to being lost in West Antarctica.

UTIG researchers also found that the ice sheet filling the basin had a history of instability when they discovered signs that the ice had advanced and
retreated across the basin enough times in the past to erode a large system of fjords into the subglacial topography. The carvings in the Earth’s crust had a different orientation than the way the East Antarctic Ice Sheet is flowing today and came from faster moving ice sheets during warmer millennia, whencoastlines were farther inland.

“Duncan made the convincing argument that just looking at the shape of the basin’s bedrock provided compelling evidence that this ancient ice sheet advanced and retreated multiple times, that it was a dynamic ice sheet,” Greenbaum said.

This demonstration that glacial carnage had occurred in East Antarctica before, back when atmospheric CO2 levels resembled those of today, added weight to the notion that the basin could substantially retreat again. Furthermore, the satellite data sets showed that Totten Glacier (the basin’s main ice outlet) had become the most rapidly thinning glacier in East Antarctica. Independent studies confirmed that surface melting could not explain the thinning, indicating other processes at work.

The prevailing wisdom was that deep warm water couldn’t reach the base of Totten Glacier to thaw it. But Greenbaum and Blankenship thought it was worth investigating whether there were troughs in the sea floor deep enough to allow warm water observed on the continental shelf to reach Totten Glacier’s grounding line, the area where the ice transitions from resting on the sea floor to floating.

If so, warm water hundreds of meters below the Southern Ocean’s surface could be the impetus of Totten’s melting. This deep water came from other oceans over the centuries and is relatively warm compared with water near the surface. Most of it circulates around East Antarctica without reaching the coast. However, if deep water were to extend inland, its warmth could enhance ice melt along Totten Glacier’s grounding line.

To map the shape of the sea floor beneath Totten, Greenbaum and Blankenship used airborne gravity measurements to infer where the seafloor rock begins. They used this method because although aerial radar signals can penetrate the ice, they bounce off the ice-water interface. In a March 2015 Nature Geoscience article, the team reported finding two unexpected seafloor troughs along the glacier’s southwestern flank that could allow seawater beneath Totten.

“We showed that the troughs not only existed,” Greenbaum said of the multinational work, “but were deep enough that they could let warm water reach Totten’s cavity in two places, including one that wasn’t thought to be an entry to the ice shelf cavity at all.”

However, before this could occur, the warm, deep water would have to breach the ridge that juts up at the edge of Antarctica’s continental shelf. Chad Greene, a doctoral candidate advised by Blankenship, collaborated with Australian colleagues to look at one possibility: The winds over the Sabrina Coast, affected by global changes in atmospheric conditions, were stirring up enough deep water to breach the ridge. Evaluations of wind stress data confirmed that the waters close to Totten flowed in the reverse direction of the waters farther offshore — a phenomena that allowed the divergent waters to slide past each other along the line of the continental ridge. These conditions allowed upwelling to occur, in which wind displaces the surface water, and deeper and warmer water rises to replace it.

“The warm water that’s deep starts rising over the continental shelf,” said Greene. “From there it can start attacking Totten, just the way it had been shown to be attacking Thwaites and Pine Island.”

Greene had also developed a detailed assessment of Totten Glacier’s elevation loss by combining hundreds of aerial laser altimetry profiles UTIG gathered while monitoring the region. As described in a November 2017 Science Advances study, ice loss at Totten Glacier peaked during periods when the wind conditions offshore allowed the upwelling of warmer seawaters.

“Jamin and Chad’s remarkable research showed how and where the ocean was delivering heat, as well as how Totten Glacier is responding,”Blankenship said of the seafloor trough and wind-driven melting discoveries, which helped spur acceptance that East Antarctica was awakening.“Now we’re focusing on what that glacial undercutting is doing to the interior of ice sheets.”

The answer has come in part from work that Greenbaum, Greene, Young and Blankenship have done with a Canadian researcher and others that appeared in Science Advances in June 2018. The work focused on signs of decay in four Antarctic ice shelves and one in Greenland. It showed that wherever an ice shelf was eroded from below by warm ocean water, matching grooves had formed on the surface. Warmer Antarctic air temperatures in recent years have added meltwater into those grooves, causing surface-driven melting.

The researchers showed that in one of the glacier systems, the separation of a very large iceberg from the main ice sheet was associated with this process.

They also found that many other glacier systems have all the pieces in place for it to occur as air temperatures continue to rise and cause additional surface meltwater.

“What’s going on beneath the ice, unseen, is modulating what’s happening on the surface that satellites have captured,” Blankenship said. “Those two things are conspiring to disintegrate ice sheets faster, which is not good.”

Charting New Territory

Never content to relax, the UTIG researchers are planning to study the forces at play along the coast of the Wilkes Subglacial Basin in East Antarctica, as well as revisiting Thwaites in the west. In the coming years, the international Thwaites Glacier collaboration will use research icebreakers to investigate the glacier’s coastline and conduct seismic and radar investigations over the ice sheet and floating ice tongue. As part of this effort, UTIG has proposed a five-year international project called the LIONESS (Land-Ice/Ocean Network Exploration with Semi-autonomous Systems) to work with helicopters from the Korea Polar Research Institute to monitor the evolution of the Thwaites Glacier system.

Specifically, they will use the instruments Blankenship’s team has developed over 27 years to look at where Thwaites extends from its grounding line near the Amundsen Sea to where the lip of its basin begins farther inland.

“No one understands what’s occurring about 100 kilometers upstream of any Antarctic glacier’s grounding line,” Blankenship said. “In the case of Thwaites, we’re within a few decades of the ocean disrupting its subglacial boundary conditions to the point where there’s potentially a big collapse because we’ve got ocean water melting coastal ice about 40 to 50 kilometers from the lip of the Thwaites’ basin.”

Blankenship and the UTIG team are determined to lead the way and keep up the important work.

“We’ve always been where everyone else is not to get at these problems,” he said. “We now need to understand how soon Thwaites will become a full blown problem that society cannot ignore.”

ANTARCTIC BEDROCK UPLIFT COULD DELAY ICE SHEET COLLAPSE

By Monica Kortsha

The weight of the West Antarctic Ice Sheet is a  considerable load for the planet to carry. Research published in June 2018 in the journal Science has found that as the ice melts the underlying bedrock is showing signs of creating at least temporary relief, with the rock rising up to 4.1 centimeters per year.

The international team who conducted the research — which includes the Jackson School’s Ian Dalziel — said that the uplift could stave off ice sheet collapse by raising parts of it out of reach from encroaching sea water. However, Dalziel said that in the long run, even rapidly rising bedrock won’t be able to outpace the effects of climate change.

“That pinning bedrock isn’t going to hold it back forever,” said Dalziel, a research professor at the Institute for Geophysics (UTIG) and a professor in the Department of Geological Sciences.

“Eventually the ice will retreat beyond that and nasty things are going to happen to our sea level globally.”

As the climate has been warming, the ice sheet has been losing mass near its seaward edge, notably south of the Pacific Ocean. The mass loss is allowing sea water to get closer and closer to vulnerable parts of the ice sheet grounded below sea level on the Antarctic continent. In the study, an international team of scientists found that the rising bedrock is creating a barrier that could block water from reaching these critical parts of the ice sheet.

In addition, the uplifted bedrock slows down the flow of ice and the reduction in local gravitational attraction caused by the ice loss lowers sea level along the Antarctic coast, which could help keep the water at bay.

The team discovered the uplift using data collected by the United States Antarctic component of POLENET (Polar Observing Network), an international network of six continuous GPS monitors and seismometers installed on areas of the ice sheet where bedrock is exposed over an area roughly equivalent to the United States east of the Mississippi River.

This part of the POLENET system stems from a monitoring campaign started in the early 2000s and led by Dalziel called WAGN (the West Antarctic GPS Network) where monitors were run for a few weeks instead of year-round. UTIG has been highly involved with both projects, Dalziel said, with a team of UTIG scientists leading the initial installation of four of the six POLENET monitors used in the study.

The uplift was not unexpected. Rock rebounding as an overlying load lightens is a phenomenon that has been documented around the world. But no one expected such rapid uplift.

The rate is so rapid, geologically speaking, that the researchers discovered that satellite data used to measure ice loss in the West Antarctic region needed to be readjusted to account for the uplift. Their work revealed that up to 10 percent more ice has been lost from the West Antarctic ice sheet than previously thought.

“That’s a critical correction for being able to use the satellites to measure what’s actually happening and hence to contribute to modelling of future ice sheet behavior,” Dalziel said.

And as the ice sheet loses more ice over time, the rate of uplift is only expected to speed up. Within 100 years the bedrock could be moving upward at a rate of more than 14 centimeters (5.5 inches) per year.

The rapid rebound indicates that the mantle — the semimolten layer of the Earth that separates the crust from the core — is in a low-viscosity state  beneath the ice sheet. Like a water bed mattress filling a vacated space faster than a foam mattress, the mantle rock is quick to flow into the lower pressure areas created by a lightening ice load. Dalziel said that this behavior probably relates to the long history of the Pacific margin of Antarctica as part of the volcanically and seismically active ‘Ring of Fire’.

The research findings exemplify the planet’s interconnected nature, and show that ice loss in Antarctica results in uplift of its own bedrock even as it contributes to global sea level rise.