Typically, a single core will be cut lengthwise into several narrower pieces for multiple chemical analyses. A larger-diameter ice core gives you more ice to analyze for each year of accumulation, but increases the weight and number of boxes needed to haul and ship back. On our traverse, we brought three different ice drilling rig models, but because of problems with two of them, we ended up drilling all the needed cores with the large Trapanelle model, which can drill down to 200 m and makes cores with a 4-inch (10-cm) diameter. Although the PALEO location was specifically chosen for a core for the reasons listed above, the exact location of the core at the general PALEO site wasn’t important. When coring a lake or even a mountain glacier, more focus must be made to choose a specific coring spot with the best likelihood for high quality and long record.
Here on the ice sheet, the snow and ice are pretty much all the same anywhere for miles in any direction, so we simply set up the drilling rig at a spot a dozen yards upwind of where we parked our caravan and got to work. We hoisted the Trapanelle off the side of the caravan where it had been stored with a crane attached to one of our driving tractors and placed it at our chosen drilling spot. Power is supplied from the main caravan generator, and we erected three large tarps upwind to give us some blocking of the chilly winds. Temperatures were relatively mild most days—between 15 and -22°F (-25 and -30°C)—with two windier and cold afternoons and one extraordinarily calm and sunny afternoon that approached 0°F (-17°C). Work generally started after breakfast around 8 am and continued until dinner after 6 pm, with some breaks for coffee and lunch. In our full polar gear and wind blocking, we mostly stayed plenty warm (and even got a little too hot on the extra-warm day!). And at these temperatures, there is no danger of the ice melting, even in direct sunlight, so all work is simply done outside. To start an ice core, the drilling piece of the Trapanelle is raised to vertical and a winch lowers it until the drilling bit makes contact with the snow surface. The drilling operator then activates the drill motor through the control panel. This drill motor is located directly above the drill bit, and it spins the bit to carve into the snow.
The bit will carve down until it reaches its maximum depth of 3 ft (1 m), at which point the drill motor is turned off and the winch reversed to retrieve the drilling piece. Spring-loaded backward facing prongs at the bottom of the drill bit serve as a “core-catcher” and keep the newly drilled core from falling out the bottom during retrieval. After the first core is taken, subsequent rounds of drilling are guided into the same borehole and the drilling piece is lowered down to the last cored depth to drill another 3 ft (1 m). Three metal pieces above the drill motor press against the walls of the borehole and keep the drill bit from wildly spinning or bouncing as it descends and ascends. The “snow” created as the bit cuts down into the ice is gathered and held under the protective bit cover to keep it from piling up in the borehole. Once the drill is back at the surface, the drill motor is reversed to remove this “snow” trapped between the protective casing and the drill bit. The drilling piece is then moved to horizontal and the drill bit detached from the larger piece. The 3-ft (1-m) drill bit is removed, with the core still inside, and moved over to a wooden support structure. The top of the bit (and core) are pointed down, and a plastic plug that kept the drilling-produced “snow” away from the core is removed. With one team member standing at the drill bit top, another member raises the bottom of the drill bit so that the ice core inside slides out the top. The ice core is then moved over to a nearby table and placed in a V-shaped aluminum trough. It is very important that the top of the ice core is placed the left in the trough, as there is no way to tell the top of a core from the bottom once it is out of the bit. Instead, we make sure that the top of the core is always facing to the left through the entire processing period. As two team members place the drill bit back in the rig and start the process of drilling the next core, another member begins to measure and process the ice core just drilled. If we are lucky, the core will be in one piece and close to the desired 3-ft (1-m) length. However, the actual core length varies based on where exactly the bottom of the core broke from the main ice mass when pulling the drill bit back to the surface. The Styrofoam boxes we use to store and transport the ice cores are only 3 ft (1 m) long inside, so cores are packaged at this length for maximum efficiency. If a core is too long, we saw the excess off and simply add it to the top of the next core. If a core is shorter than 3 ft (1 m), we will wait and take a section of the next drilled core to get it up to the desired length. After the length of the core is measured and logged, we bag the core in clean plastic with the top and bottom of the core clearly labeled along with the core ID. After bagging, the core is placed in the Styrofoam storage box ready for transport. At the beginning of drilling a core, everyone on the team (usually three people) is in constant motion, with cores rapidly being drilled and delivered to the processing table. I usually worked as the table processor, measuring and cutting cores to length and also preparing the bags and Styrofoam boxes. The actual drilling of the core and extracting it from the bit only takes 5-10 minutes, and processing takes a little longer than that, so for the first 20 cores, I was mainly trying to avoid getting too backlogged. However, the longest part of the drilling process soon became apparent. After 60 ft (20 m) or so, it takes 5-10 minutes simply to raise and lower the drilling bit, and the pace of work becomes more relaxed.
While the bit is being lowered or raised, there’s not much to do other than keep an eye on the bit depth. After 100 m of drilling, the wait between cores grows to over 20 minutes, with a quick rush of activity upon core retrieval followed by another stretch of waiting. Toward the end of our coring, we could get only two or three cores per hour, and cores would increasingly come up shorter than a meter and broken as the ice was more brittle at depth. Sometimes the core came up in several fractured pieces, and it became a bit of a puzzle to figure out their orientation and order. By the time we got to a depth of 590 ft (180 m), the cores were breaking too much for us to make sense of the pieces and the length of time to core was impractical.
As we shut down the operation after five straight days of coring, we celebrated the end of our full core, which covers over 5000 years of snowfall and climate history. Above: A timelapse taken over a few hours at PALEO when our team was drilling at depths below 400 ft (120 m).