About Ice Cores
Drilling Ice Cores
Ice cores are drilled in glaciers and on ice sheets on all of Earth's continents. Most ice cores, however, come from Antarctica and Greenland, where the longest ice cores extend to 3 km—over 2 miles—or more in depth. Ice cores from the cold interior regions of polar ice sheets provide exceptionally well-preserved and detailed climate records. This is because the lack of melt at these locations does not corrupt the record of trapped gases or blur the record of other impurities. The oldest continuous ice core records extend to 130,000 years in Greenland, and 800,000 years in Antarctica.
Ice cores are typically drilled by means of either a mechanical or thermal drill. Both types of drills incise an annulus, or circle, around a central, vertical core.
A mechanical drill is simply a rotating pipe, or drill barrel, with cutters at the head. When the drill barrel is rotated, the cutters incise a circle around the ice to be cored until the barrel is filled with ice. The cuttings—also referred to as chips—are transported to a chip chamber in the drill. The drill barrel is rotated by either physical force, as in the case with simple hand augers, or with an electromechanical motor drive, as in the case with sophisticated electromechanical drills. Thermal drills, in contrast, use a ring-shaped heating element to melt an annulus around the ice to be cored and the melt water is stored in a tank in the drill.
At sites where the ice is well below freezing, such as the interiors of the polar ice sheets, mechanical drills must be used. In contrast, thermal drills are particularly effective at coring through warmer ice (e.g., ice approximately above -10°C) and are frequently used on mid- or low-latitude glaciers. On many non-polar glaciers, however, ice conditions can vary from "cold" ice to "warm" ice, requiring both mechanical and thermal drilling to retrieve the best possible core quality over the entire depth interval that is drilled.
The length of the drill barrel determines the maximum length of a core section that can be retrieved in a single drill run. Ice cores are typically retrieved in sections that are 1 meter to as much as 6 meters in length, and typically 50-132 millimeters in diameter. The collection of a long ice core therefore requires many repeated cycles—or drill runs—of lowering the drill, cutting a core section, raising the drill back to the surface, removing the drilled ice core section from the barrel, and readying the drill to go back down the hole to retrieve more ice.
When scientists are interested in collecting ice cores from the top 20- to 30-meters of a glacier or ice sheet a hand auger is commonly used. In the United States, there are four types of hand augers commonly used: the SIPRE auger, the PICO auger, the Kovacs auger, and the IDDO auger.
The hand auger is the most basic of the mechanical drills and consists of a barrel and cutting head. The barrel is typically 1 m long and with an internal diameter of either 3 or 4 inches. A protruding thread spiraling up and around the barrel is used to remove the ice chips from the cutting face. The cutter head includes 2 or 3 hardened steel or carbide teeth, and the protrusion of the teeth can be adjusted to control the depth of cut. If the depth of cut is set too small, the cutter head will skid over the ice surface rather than cut down into it. If the depth of cut is set too large, the teeth will become lodged in the ice halting the drilling altogether.
The hand auger is driven from the surface by a series of extensions that are added as drilling proceeds into the ice. The hand auger, like all other coring drills, has to be retrieved each time a core section is recovered. Hand augers are either driven by hand, using an attached T-bar, or adapted to use a powered motor drive or drill. The maximum depth to which a hand auger and its extensions can be raised and lowered in and out of a borehole by two people is ~20-30 m. A mechanical winch/lifting system is generally required at greater depths. The depth limit for hand augers is limited by the strength and flexibility of the extension rods to ~40 m.
Shallow Ice Coring
When depths greater than ~40 meters need to be reached, ice coring projects typically use specialized electromechanical or electrothermal drills that hang on a cable. The cable runs from a winch over a top wheel—called a sheave—on a vertical tower. The cable has electrical wires inside that power the drill and allow for operation of the drill from the surface. This type of drilling is referred to as cable-suspended drilling, and is the preferred drilling method when drilling intermediate and deep ice cores. While several early ice drilling projects used conventional surface-driven rotary and wireline drill rigs to collect ice cores, the use of specialized cable-suspended drills to collect ice cores is now the common practice. Cable-suspended drill systems are preferred because they significantly decrease the weight of the drill and its power consumption, shorten the time of travel in and out of the borehole, and simplify the process of removing the cuttings from the borehole.
While most cable-suspended drills are similar in design, the details do vary from drill to drill, often with specific projects and field sites in mind. For instance, due to the remoteness and high-altitude of most non-polar glaciers, the drills used at these locations must be lightweight and of modular design to allow for transportation by helicopter, or for glaciers located above the range of helicopter operation (e.g., above 5500 m elevation), transportation by either porters or pack animals. In the polar regions, extremely low temperatures and very remote and logistically complicated field sites influence the design of the drills used.
Below are general descriptions for two types of cable-suspended ice coring drills: electromechanical and electrothermal. More detailed information can be found by reading the literature noted in the References and Links.
Cable-Suspended Electromechanical Drills
There are many versions of the electromechanical (EM) drill used in ice coring. The portion of the drill that goes down the borehole is called the sonde, and its only physical connection to the surface is through its suspending cable. The components of the sonde typically include a cutting head that is attached to an inner barrel, an outer barrel, a motor to rotate the inner barrel, and an anti-torque system that counteracts the rotational cutting action.
The cutting head typically has 3 to 4 hardened steel or carbide teeth that shave ice in an annulus around the central, vertical core. The protrusion of the teeth and their subsequent depth of cut into the ice are controlled with small, adjustable buttons called shoes that are located on the bottom face of the cutter head. The vertical distance between the bottom of the shoes and the cutters determines the depth of cut into the ice.
A protruding thread, also referred to as flights, spiraling up and around the inner barrel is used to remove the ice chips from the cutting face. As the cutting head spins around and the cutters shave the ice, the chips are transported up the flights between the two barrel sections. The inner barrel spins while the outer barrel stays stationary, and it is that difference that actually drives the chips upward along the flights to the top of the sonde thus removing them from the cutting face.
Most cable-suspended EM drills are designed as double-barreled (e.g., an inner barrel and an outer barrel) for several reasons. Double-barreled drills collect ice chips more efficiently than single-barreled drills, and the added stiffness of the double-barreled design typically results in fewer breaks in the cores that are drilled, as well as straighter boreholes.
Because of the great depths that can be reached, cable-suspended EM drills are often used with a drilling fluid to prevent closure of the borehole at depth (see Deep Ice Coring).
The anti-torque system on EM drills often consists of three or four leaf springs that anchor the sonde to the wall of the borehole. If there is no anti-torque section, the motor will cause the entire EM drill and cable to rotate inside the borehole, preventing the ice from being cut and causing the cable to become wrapped.
When a drill run is finished and the barrel is full with ice, the core is typically held in the inner barrel by spring-loaded lever arms called core dogs that grip, break, and retain the ice core when the cable and sonde is pulled upward. The sonde is then hoisted to the surface with the winch and the core removed. In many modern cable-suspended EM drill systems, the sonde and tower both tilt from the vertical position to the horizontal position to aid in the removal of the ice core and the chips.
Scientists at Allan Hills, Antarctica use the Badger-Eclipse electromechanical drill to look for ice as old as 2.5 million years. Note the inner barrel on the left and the tilting outer barrel/tower design. [See article - The Ultimate Classroom: Fieldwork at Allan Hills, Antarctica provides lifetime of learning]
—Credit: Melissa Rohde, Univ. Maine
Cable-Suspended Electrothermal Drills
The electrothermal (ET) drill uses a ring-shaped heating element in the coring head to melt an annulus around the ice to be cored, rather than shaving it away with cutters like the EM drill. The electrical power to heat the coring head is fed through the drill's suspending cable.
The meltwater that is produced by ET drills may either be left down the hole or stored in a tank within the sonde. When englacial temperatures are low and there is concern that the meltwater may re-freeze causing the borehole to close-in on itself, the meltwater is either stored in a tank in the sonde and emptied once on the surface, or the meltwater is mixed with an antifreeze solution and left down the hole. For depths below which borehole closure in an open hole is a concern, the meltwater is left down the hole to help prevent hole closure. If re-freezing is also a concern, the meltwater that is left down the hole is mixed with an antifreeze solution.
ET drills have only one barrel, so the size of the borehole is generally smaller than a borehole from an EM drill for a similar diameter ice core. ET drills are mechanically much simpler than EM drills because they contain fewer moving parts. They also do not require an anti-torque system since there is no rotational cutting action. ET drills are also generally more compact in size and weight than EM drills, in part because no chip storage is required.
While ET drills are particularly effective at coring through warmer ice (e.g., ice approximately above -10°C) they have also been used to drill in cold ice in Antarctica. The main disadvantage of using ET drills to drill cold ice (e.g., ice approximately below -15°C) is the large thermal shock that occurs to the core, which is bad for many types of measurements that are typically made on ice cores.
A 2 meter section of ice core from Combatant Col, British Columbia, is removed from an electrothermal drill. Note the absence of cutters on the drill head, as well as the absence of an anti-torque system on the sonde. [See article - Ice Core Paleoclimate Records from Combatant Col, British Columbia]
—Credit: Doug Clark, Univ. Washington
Deep Ice Coring
For depths below which borehole closure in an open hole is a concern, a drilling fluid is used to make sure that the pressure in the borehole is approximately the same as the surrounding ice to prevent closure of the borehole. The plastic nature of ice, which is what produces the hole closure, is strongly temperature dependent and therefore the depth at which a drilling fluid is required to prevent borehole closure is also strongly temperature dependent. A drilling fluid may be required as early as 100 m depth on glaciers that are at their melting point (e.g., temperate glaciers), or as deep as 1000 m on the high, extremely-cold interior plateau of East Antarctica. In general, drilling fluids are used whenever depths greater than ~300 m are required.
Deep ice core drilling, that is, to depths where the use of a drilling fluid is required, is typically carried-out using cable-suspended EM drills that use a pump to circulate both the drilling fluid and the chips through screens that separate the chips from the fluid.
The deepest ice core records come from Antarctica and Greenland, where the very deepest ice cores extend to 3 km (over two miles) in depth. The oldest continuous ice core records extend to 130,000 years in Greenland, and 800,000 years in Antarctica. The United States (U.S.), Denmark, Russia, France, Germany and Japan have all developed highly specialized cable-suspended EM drills for the purpose of recovering ice cores to depths of 2 km or more.
The most recent U.S. deep ice coring project is the West Antarctic Ice Sheet Divide (WAIS Divide) project in West Antarctica, funded by the National Science Foundation. After seven field seasons of preparation and drilling, the WAIS Divide project reached its final depth goal of 3,405 meters on December 31, 2011, completing the longest U.S. ice core ever recovered from the polar regions.
The WAIS Divide ice core was drilled using the U.S. Deep Ice Sheet Coring Drill (DISC Drill). The DISC Drill is a cable-suspended EM drill system capable of cutting and retrieving cores of ice—in 3 meter long sections—to depths of ~4,000 meters. The sonde consists of a cutter head, a core barrel in which the core is collected, screens to remove chips from the drill fluid, a motor and transmission to drive the cutter head, a pump to circulate ice cuttings in the drill fluid through the sonde, an instrumentation/control section, an anti-torque section to stabilize the drill in the borehole, and the mechanical, electrical power, and fiber optic terminations of the cable. The 15 mm diameter cable suspends the drill sonde in the borehole and also provides electrical power and fiber optic communication to the sonde. The drill's cutter head contains four razor-sharp cutters that shave out an annulus of ice, which the 14 m long sonde slides down into. As the sonde slides down into the annulus it slides over the core, which is 12.2 cm in diameter. When a cable pulls up the sonde, four cams grab the core and fracture it. After the sonde is pulled back to the surface, the sonde and tower are tilted as one unit from a vertical to a horizontal orientation and the core is pushed out.
The 3,405 meter long core was drilled over the course of six field seasons. Typically, ~40 days were available each season for drilling, and drilling operations were conducted on a 24-hours per day, 6-days per week schedule by a crew of ~9 drillers (3 shifts of 3 drillers/shift) each season.