In The Field

Antarctica 2010: Ice core drilling

After four nights at the West Antarctic Ice Sheet Divide field camp, an LC-130 plane finally landed and brought us back to McMurdo Station, the final stop on the trip before leaving the continent. WAIS Divide gets some of the worst weather in Antarctica — but that’s why it’s the home of one of the most highly anticipated ice-coring projects in the world.

Ice cores trap samples of the atmosphere in small bubbles, offering snapshots of ancient atmospheric conditions to the scientists who dig the ice up and extract the gas. If the ice is thick enough, paleoclimatologists can count the layers back tens of thousands of years to catalog what the climate was like when that ice fell from the sky as snow. Other methods such as comparison with methane records can be used to date the ice, but thicker layers allow for greater temporal resolution and smaller error bars on the final analysis years down the road. And to have thick layers, you need lots of annual snowfall, so that’s why glaciologists drill at WAIS: It gets about half a metre of snow per year (not too shabby for Earth’s driest continent), creating beautiful annual ice layers 22 centimeters thick. freshcoresm.JPG

A look at the science of the WAIS Divide ice coring project will be forthcoming in the print edition of Nature, but in the meantime I thought it would be neat to see how drillers pull up 3.5 kilometers of ice, the last few metres of which are over 100,000 years old. I’ve spent lots of time in the drilling arch over the last few days, and I was fortunate enough to see the ice coring operation in action.

Ice cores like to tease the drillers who pull them up: The Deep Ice Sheet Coring Drill at WAIS grabs ice in 3.3-meter segments (each grabbing is called a “run”), so the closer you get to the bedrock and the further back you go in time, the longer it takes the drill to swim down the hole and reach the next piece. “Swim” really is the operative term: The hole is filled with a drilling fluid to maintain its structural integrity, so the drill is outfitted with a pump that allows it to move quickly down the hole. When the drill comes up, drilling fluid spills out, giving the core a nice fresh sheen.

An automated crane-like system lifts the barrel of the drill to a transfer station, where the core is pushed out of the barrel through a hole to the processing side of the drilling arch. The drilling side remains at whatever temperature it is outside (about -10C to -14C), but the processing side, or “science side,” is cooled to about -27C, and is never allowed to get warmer than -20C, the temperature at which certain gases will begin to leak out of the core. To keep the room cool, four refrigerator units make the science side quite chilly. Suffice it to say that air conditioning in Antarctica has been one of the major surprises of the trip.

Once the core is pushed through to the science side, the core handlers take over. Core handling is, by admission of the people who do it, a rather thankless trade, but all the core handlers are highly qualified scientists doing a very important job. They take a slew of measurements, mark up the core with lines to indicate its proper orientation, and carefully document the condition of the core, including any fractures or breaks the core may have suffered. One of the most important measurements is the precise length of the segment: There is no depth-o-meter to know how far down a core came from, so the handlers measure depth by adding the length of each piece to that of the previous piece and making a cumulative measurement. Certain landmark events like volcanic eruptions (which show up as ash layers in the core) can be used to roughly check the measurements, but scientists are interested in millimetre-scale features of the core, so precise measurements are crucial.

One factor that can make this measurement tricky is the break made by the drill at the end of each run to separate the core from what will become the next segment. Usually these breaks are clean, but sometimes they can be diagonal, making it difficult to say exactly how long a segment of core is. Should you end your measurement when the core ceases to be cylindrical? Or should you extend it to very tip of the ice, even if it’s a thin point? To solve this problem, the core handlers keep the previous segment of core on hand to be butted up against each new segment, and they take the measurement of the two-core combo, which fits together even with diagonal breaks.

Many ice coring projects make a number of scientific measurements on-site, but the remoteness of the WAIS Divide camp and the costs of getting material out there mean handlers can only take the basic measurements mentioned earlier. After that, the ice sits for a few days until it becomes less cloudy, and can be inspected again if anything special comes up. On my visit, we were shown a piece of core from a depth of 1,586m (approximate age: 8,200 years ago) with an ash layer believed to have originated from the eruption of Mt. Takahe, a volcano a few hundred miles from WAIS Divide.lightedcoresm.JPG

After the initial processing stages, the ice can remain on site for quite a while. Last year, drillers were pulling up ice from the “brittle zone,” an area from about 500m in depth to 1,200m, where the ice is known to crack and break into pieces even if handled delicately. Last year’s brittle ice was still on site, “resting” until it could be moved, so this year’s ice will stay on site another year as well (it would be shipped off, but there are only resources to ship one season of ice each year), which gives perspective to the speed of ice-core research. Ken Taylor, the chief scientist at WAIS, told me he doesn’t expect to see publications from the ice pulled up today for another 4-5 years! First, there’s the year of sitting, then another half-year to get to the U.S. National Ice Core Laboratory in Denver, Colorado, and another few months to make it to labs across the world. That’s nearly two years just to get into the hands of scientists, who are suddenly bombarded with a whole drilling season’s worth of ice. If it takes them two years to measure the ice, analyze the data and write up and publish the results (a breakneck pace!), that would be about 4 years from drilling time to publication, and it could certainly take longer.

But it’s worth it to be patient. The work going on here is some of the most important in all of climate science.

Editor’s note: See Chaz’s full story about the WAIS Divide drilling project here.


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