On the ice edge
ANDRILL gathers data to drill through moving ice shelf into ancient seafloor sediments
Posted July 29, 2011
For some scientists, Antarctica’s ice is the main focus of their research. For others, the continent’s vast cover of frozen water is an obstacle to be overcome, an impediment to the secrets buried below.
In the case of a team of researchers hoping to recover sediments tens of millions of years old in the Ross Sea region, the Ross Ice Shelf is not an insignificant barrier. In fact, a proposal to drill into the seafloor in the next few years meant traveling to the edge of Antarctica’s largest ice shelf this past summer season to determine if such a feat is even possible.
The answer appears to be “yes” based on the results of a three-month operation that required driving tractors and sleds through crevasse fields and melting multiple holes through ice more than 250 meters thick.
“It was a good test of the full capability of the ANDRILL hotwater drill system,” said Frank Rack at McMurdo Station as the field season drew to a close. “We ended up making quite a few holes.”
ANDRILL, for ANtarctic geological DRILLing, is a multinational program with its own tagline — drilling back to the future. The line refers to the project’s main goal of reconstructing Antarctica’s past environment by coring and recovering sediments from the continent’s margin to help predict how the continent’s ice sheets might respond to future environmental changes.
Sediment cores — composed of things like glacial rocks, sand, volcanic ash, siltstone and mudstone — represent layers of time millions of years in the past. Physical and chemical analyses of the sediment can tell scientists what environmental conditions dominated and how the ice sheets responded at a given interval in the past.
For example, glacial rock types represent periods when the ice sheets were large, while the presence of fossil diatoms (planktonic algae) indicates marine conditions when ice sheets had retreated, and the oceans were too warm to freeze.
ANDRILL scientists drilled two sediment cores near McMurdo Station in 2006 and 2007, each more than a kilometer long, covering about 20 million years. One significant find: Between 2 million and 5 million years ago, when Antarctica was warmer than today and atmospheric carbon dioxide levels higher, variations in temperature caused numerous advances and retreats of the West Antarctic Ice Sheet.
Now the target is much deeper in time, more than 34 million years ago when Antarctica went from the so-called greenhouse world to the current icehouse where it has remained relatively frozen through the millennia.
“Now we’re truly trying to step back to the period before Antarctica got glaciated,” said Rack, principal investigator for the site survey project on the Ross Ice Shelf at a location called Coulman High.
“If we can get back in the Eocene, we can look at what ice-free Antarctica looked like,” added Rack, who serves as executive director of the ANDRILL Science Management Office at the University of Nebraska-Lincoln.
The problem is that those sediments sit underneath the 250-meter-thick Ross Ice Shelf, which spreads northward toward the ocean at a rate of about 2 meters per day. ANDRILL’s drilling platform, which sits on the ice, sends a riser through the ice after first melting a hole with a hotwater drill. The riser drops down through the ice and the ocean below before hitting the seafloor. The drill string itself then snakes down this protective tube and begins its work collecting cores.
The shifting shelf means that the riser will bend over time, eventually stopping the drilling. Rack and his team need to understand the fine details about how quickly the shelf truly moves and in which direction. The oceanographic conditions below the ice shelf are also important variables as they develop a drilling strategy.
That required hauling people and equipment nearly 150 kilometers from the U.S. Antarctic Program’s largest research base at McMurdo Station to the edge of the Ross Ice Shelf using tracked vehicles, as well as helicopters. A third of the journey closest to the station crossed the so-called shear zone, an area of crevasses in the ice.
Using ANDRILL’s hotwater drill system, the scientists and support personnel punched several holes in the ice shelf. Oceanographers from Woods Hole Oceanographic Institution and the National Institute of Water and Atmospheric Research in New Zealand deployed instruments through two holes to measure tidal and ocean currents, as well as the temperature and salinity of the water.
“Both of those deployments went really well,” said Rack, adding that the current meter moorings had brought back the first real-time velocity and water property data throughout the water column under the Ross Ice Shelf.
“We really tried to think about what was the ancillary science that could be done as part of this survey activity,” he added.
Through another hole, the scientists dropped a video camera that allowed them to examine the ice shelf cavity and seafloor. Starfish, sponges and other benthic, or bottom-dwelling marine critters, were spotted.
“That turned out to be real interesting,” Rack said.
The Coulman High site visit also offered an opportunity to test the capabilities of a underwater robot called SCINI (Submersible Capable of under Ice Navigation and Imaging), developed by engineer Bob Zook at Moss Landing Marine Laboratories.
Shaped like a torpedo and designed to fit through holes made in sea ice that is only a few meters thick, SCINI sports a manipulator arm and camera. The remotely operated vehicle (ROV) made a series of dives over 10 days, reaching a maximum depth of about 300 meters, and made a significant biological discovery that is currently being documented.
“It roamed all underneath the ice shelf,” Rack said. “It bodes well for trying to increase its depth capability to the seafloor and also give it more missions than just seeing and looking.”
SCINI could play a pivotal role in the final drilling operation, which will likely involve coring into one site on the seafloor for as long as technically feasible. SCINI could observe the bending of the riser and help determine when to terminate drilling. The drillers would then start a second hole, drilling as fast as possible to reach a depth that matches up well with where the first hole left off before returning to coring.
Rack said the ANDRILL team would submit a proposal to drill two sites at Coulman High within a couple of years. The New Zealand Antarctic science program is already on board, and he expects other nations will also join the effort. South Korea, Brazil, the United Kingdom and other European countries have expressed interest the Coulman High Project, according to Rack.
The two previous field campaigns involved Germany, Italy, New Zealand and the United States, with about two-thirds of the $30 million coming from the National Science Foundation.
Eventually, Rack said, he would like to see a sustained international ANDRILL effort, akin to the International Trans-Antarctic Scientific Expedition (ITASE), a program involving about 20 nations studying recent climate change through the collection of shallow ice cores and other data from field observations.
“The preparation for this future effort would first involve conducting surveys to identify prospective sites where scientific drilling could take place,” Rack said. “What we’re trying to do is coordinate site-specific activities, while also expressing a circum-Antarctic, long-term sciences strategy.”
NSF-funded research in this story: Frank Rack, University of Nebraska-Lincoln, Award No. 0839108.
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