|Posted on Nov 10, 2011 03:32:39 PM | Adam Voiland | 0 Comments ||
NASA recently posted a press release about an upcoming expedition to Pine Island Glacier Ice Shelf, a key piece of real estate in Antarctica that's slipping into the ocean at an increasingly worrisome pace. This month, in fact, an aircraft participating in Operation IceBridge spotted a lengthy crack cutting across the massive sheet of floating ice. There wasn't much room for many details in the release, so here's a longer description of the upcoming expedition from Goddard's cryosphere writer, María José Viñas, for polar science aficionados.
An international team of researchers will helicopter onto the Pine Island Glacier ice shelf, one of Antarctica’s most active, remote and harsh spots, in mid-December — weather permitting. Their objective: to determine how changes in the waters circulating under the fast-melting ice sheet are causing the glacier to accelerate and drain into the sea.
If all goes to plan, the multidisciplinary group of 13 scientists, led by NASA’s emeritus glaciologist Robert Bindschadler and funded by the National Science Foundation (NSF) and NASA, will depart from McMurdo station in mid-December and spend six weeks on the ice shelf. The team will use a combination of traditional tools and sophisticated new oceanographic instruments to measure the ocean cavity shape underneath the ice shelf. They aim to determine how streams of warm water enter this cavity, move toward the very bottom of the glacier and melt its underbelly, making it dump more than 19 cubic miles of ice into the ocean each year.
"The project aims to determine the underlying causes behind why Pine Island Glacier has begun to flow more rapidly and discharge more ice into the ocean," said Scott Borg, director of NSF's Division of Antarctic Sciences, the group that coordinates all U.S. research in Antarctica on the southernmost continent and surrounding oceans. "This could have a significant impact on global sea-level rise over the coming century."
“Darn hard to get to”
Pine Island Glacier has long been on the radar screen of Antarctic researchers.
“Once satellite measurements of Antarctica started to accumulate and we began to see which places were changing, this area lit up as a spot where there was a large change going on,” Bindschadler said.
Bindschadler was the first person to ever set foot on this isolated, wind-stricken corner of the world in January 2008. Previously, scientists doubted it was even possible to reach the crevasse-ridden ice shelf. But Bindschadler used satellite imagery to identify an area where planes could land safely.
“The reason we haven’t gone there before is that it’s so darn hard to get to,” Bindschadler said. “So proving that landing was doable was a relief.”
The glaciologist’s joy didn’t last: the ground proved to be too hard for the planes transporting the instruments to land repeatedly. Logistics experts determined they would have to use helicopters to transport scientists and instrumentation in and out the ice shelf, and the whole plan for field campaigns had to be redesigned around the helicopters’ availability.
Almost four years after this first landing, Bindschadler and his team will be returning to the ice shelf to study its innards.
Scientists have determined that it’s the interaction of winds, water and ice that’s driving ice loss. Gusts of increasingly stronger westerly winds push the Antarctic Circumpolar Current’s cold surface waters away from the continent: then, warmer waters that normally hover at depths below the continental shelf rise. The lifting warm waters spill over the border of the continental shelf and move along the floor, all the way back to the grounding line—the spot where the glacier comes afloat— causing it to melt. The warm salty waters and fresh glacier meltwater combine to make a lighter mixture that rises along the underside of the ice shelf and moves back to the open ocean, melting more ice on its way out. But, how much more ice melts? Bindschadler and his team need to find out to improve projections of how the glacier will melt and contribute to sea level rise.
“All existing data (satellite images, variability of winds, submarine measurements) say this a highly variable system”, said Bindschadler. “But they’re all snapshots in time. Our team will be deploying instrumentation that will get a longer record of the variability.”
Profiling ocean waters
One of the first tasks for the team will be using a hot water drill to make a 500-meter deep hole through the ice shelf. Once the drill hits the ocean, the scientists will send a camera to peer into the ocean cavity, observe the underbelly of the ice shelf and analyze the seabed lying 500 meters below the ice.
Then, they will lower a set of instruments that Tim Stanton, an oceanographer with the Naval Postgraduate School, has built. The primary instrument in the package is an ocean profiler, which will move up and down a vertical cable that connects it to a communication instrument package on the surface of the ice shelf. As it moves, the profiler will measure temperature, salinity and currents from 3 meters below the ice to just above the seabed. It can also be instructed to park at specific depths and gauge water turbulence and vertical transport of heat and salt along the water column. The device will send all data to the surface tower that will then transmit it to Stanton’s laboratory via a satellite phone.
The profiler is controlled remotely, and Stanton can vary its sampling rate. I t will initially do fast sampling, to observe daily changes in water properties and circulation within the ocean cavity.
“After about a month of fast sampling, we’ll make it reduce the number of profiles it takes each day, to capture seasonal changes in water properties and circulation,” Stanton said. “If it survives its first year, we’ll switch to super slow sampling, to measure how much heat is coming into the cavity every year.”
A second hole will support another instrument array similar to the profiler but fixed to a pole stuck to the underside of the ice shelf. The fixed-depth flux package will make measurements very close to the interface where ice and water exchange heat.
Another gadget connected to the fixed-depth package will be a string of 16 small temperature sensors deployed within the lowermost ice to freeze in and become part of the ice shelf. Their mission: to measure the vertical temperature profile, data that can tell scientists how fast heat is transmitted upwards through the ice whenever hot flushes of water enter the ocean cavity.
“Since the temperature of the ice shelf determines its strength, we hypothesize that strength may decrease as warm melting events occur within the ocean cavity,” Stanton said.
Stanton plans on deploying up to two sets of instruments during this field season, and a third one next year. “If we get one in, I’ll be happy. If I get two, I’ll be extraordinarily happy,” he said. One of the biggest challenges in building his pack of instruments, Stanton said, was designing it to fit the hole in the ice shelf, only 20 centimeters wide and 500 meters long. A tight, long hole also means that the team will only get one shot at deploying the instruments: once the package is lowered into the ocean cavity, it cannot be pulled out.
“I have been deploying instruments into ice floes in the Arctic for the last 10 years, so I got quite used to just putting them in and turning on my heels and walking away. But it’s still quite hard to do,” Stanton said.
Explosions and sledgehammers
A geophysicist with Penn State University, Sridhar Anandakrishnan, will create tiny earthquakes to study the shape of the ocean cavity and the properties of the bedrock under the PIG ice shelf. He will be doing measurements in about three-dozen spots in the glacier, using helicopters to hop from one place to another.
Anandakrishnan’s technique, formally called reflection seismology, involves generating waves of energy by setting up small explosions or by using instruments similar to sledgehammers to bang the ice. He’ll record how long it takes for the waves to travel through ice and water, bounce off the seabed and return, and he’ll analyze the strength of the echo. Both factors will tell him about the thickness of the ice and water.
“[The technique] is identical to the way bats and dolphins do echolocation: they send out a sound and listen to the echo – both the time and direction of the echo tell them about the distance to their prey,” he said.
Anandakrishnan also wants to study the properties of the bedrock beneath the ice.
“When glaciers are sliding over the bedrock, they do it very differently depending on whether it is rough or smooth,” he said.
Finally, the geophysicist will inspect a mysterious ridge that runs across the ocean cavity under the ice sheet. This ridge was unknown to researchers when they designed their project in the early 2000s; it wasn’t until 2009 that an unmanned submarine operated by the British Antarctic Survey detected it. Its existence has made the scientists rethink where they will place their oceanographic instruments under the ice shelf, so that they don’t hit the ridge while the glacier advances toward the sea.
"The Pine Island Glacier ice shelf continues to be the place where the action is taking place in Antarctica," Bindschadler said. "It only can be understood by making direct measurements, which is hard to do. We're doing this hard science because it has to be done. The question of how and why it is melting is even more urgent than it was when we first proposed the project over five years ago."
Text by Maria-José Viñas. Pine Island Glacier ice tongue image originally published on the Pine Ice Glacier Ice Shelf page. Image of Bob Bindschadler on the ice shelf originally published here. Ocean profiler image originally published on the Pine Island Oceanography Program website. Image of Sridhar Anandakrishnan originally posted by the National Science Foundation.
Tags : Antarctica, General, Pine Island Glacier, polar