Ice-sheet dynamics Sampling the surface of Taku Glacier in Alaska. There is increasingly dense firn between surface snow and blue glacier ice.
An ice core is a vertical column through a glacier, sampling the layers that formed through an annual cycle of snowfall and melt. At Summit Camp in Greenland, the depth is 77 m and the ice is years old; at Dome C in Antarctica the depth is 95 m and the age years. The bubbles disappear and the ice becomes more transparent. Ice is lost at the edges of the glacier to icebergs , or to summer melting, and the overall shape of the glacier does not change much with time.
These can be located using maps of the flow lines. These include soot, ash, and other types of particle from forest fires and volcanoes ; isotopes such as beryllium created by cosmic rays ; micrometeorites ; and pollen. It can be up to about 20 m thick, and though it has scientific value for example, it may contain subglacial microbial populations ,  it often does not retain stratigraphic information.
In polar areas, the sun is visible day and night during the local summer and invisible all winter. It can make some snow sublimate , leaving the top inch or so less dense. When the sun approaches its lowest point in the sky, the temperature drops and hoar frost forms on the top layer. Buried under the snow of following years, the coarse-grained hoar frost compresses into lighter layers than the winter snow.
As a result, alternating bands of lighter and darker ice can be seen in an ice core. Ice drilling Ice auger patented in ; the design is very similar to modern augers used for shallow drilling. Early cores were often collected with hand augers and they are still used for short holes. A design for ice core augers was patented in and they have changed little since. An auger is essentially a cylinder with helical metal ribs known as flights wrapped around the outside, at the lower end of which are cutting blades.
Hand augers can be rotated by a T handle or a brace handle , and some can be attached to handheld electric drills to power the rotation. Below this depth, electromechanical or thermal drills are used. The cuttings chips of ice cut away by the drill must be drawn up the hole and disposed of or they will reduce the cutting efficiency of the drill. Since retrieval of each segment of core requires tripping, a slower speed of travel through the drilling fluid could add significant time to a project—a year or more for a deep hole.
The fluid must contaminate the ice as little as possible; it must have low toxicity , for safety and to minimize the effect on the environment; it must be available at a reasonable cost; and it must be relatively easy to transport.
Newer fluids have been proposed, including new ester-based fluids, low-molecular weight dimethyl siloxane oils, fatty-acid esters , and kerosene-based fluids mixed with foam-expansion agents. It uses a string of drill pipe rotated from the top, and drilling fluid is pumped down through the pipe and back up around it. The cuttings are removed from the fluid at the top of the hole and the fluid is then pumped back down. The core barrel is hoisted to the surface, and the core removed; the barrel is lowered again and reconnected to the drill assembly.
This eliminates the need to disconnect and reconnect the pipes during a trip. These cable-suspended drills can be used for both shallow and deep holes; they require an anti-torque device, such as leaf-springs that press against the borehole, to prevent the drill assembly rotating around the drillhead as it cuts the core.
When the core is retrieved, the cuttings chamber is emptied for the next run. Some drills have been designed to retrieve a second annular core outside the central core, and in these drills the space between the two cores can be used for circulation. Cable-suspended drills have proved to be the most reliable design for deep ice drilling. Some have been designed for working in cold ice; they have high power consumption and the heat they produce can degrade the quality of the retrieved ice core.
Early thermal drills, designed for use without drilling fluid, were limited in depth as a result; later versions were modified to work in fluid-filled holes but this slowed down trip times, and these drills retained the problems of the earlier models. In addition, thermal drills are typically bulky and can be impractical to use in areas where there are logistical difficulties. More recent modifications include the use of antifreeze , which eliminates the need for heating the drill assembly and hence reduces the power needs of the drill.
The drawbacks are that it is difficult to accurately control the dimensions of the borehole, the core cannot easily be kept sterile, and the heat may cause thermal shock to the core. EM drills are also more likely to fracture ice cores where the ice is under high stress. The casing has to reach down to the impermeable ice layers.
To install casing a shallow auger can be used to create a pilot hole, which is then reamed expanded until it is wide enough to accept the casing; a large diameter auger can also be used, avoiding the need for reaming. An alternative to casing is to use water in the borehole to saturate the porous snow and firn; the water eventually turns to ice. To address this, work has been done on technology to drill replicate cores: Replicate cores were successfully retrieved at WAIS divide in the drilling season, at four different depths.
The largest projects require years of planning and years to execute, and are usually run as international consortiums. The EastGRIP project, for example, which as of is drilling in eastern Greenland, is run by the Centre for Ice and Climate , in Denmark ,  and includes representatives from 12 countries on its steering committee.
A spring-loaded lever arm called a core dog can break off the core and hold it in place while it is brought to the surface. The core is then extracted from the drill barrel, usually by laying it out flat so that the core can slide out onto a prepared surface. The surface that receives the core should be aligned as accurately as possible with the drill barrel to minimise mechanical stress on the core, which can easily break.
The ambient temperature is kept well below freezing to avoid thermal shock. It is usually cut into shorter sections, the standard length in the US being one metre.
The cores are then stored on site, usually in a space below snow level to simplify temperature maintenance, though additional refrigeration can be used. If more drilling fluid must be removed, air may be blown over the cores.
Any samples needed for preliminary analysis are taken. The core is then bagged, often in polythene , and stored for shipment. Additional packing, including padding material, is added.
When the cores are flown from the drilling site, the aircraft's flight deck is unheated to help maintain a low temperature; when they are transported by ship they must be kept in a refrigeration unit.
These locations make samples available for testing. A substantial fraction of each core is archived for future analyses. Over a depth range known as the brittle ice zone, bubbles of air are trapped in the ice under great pressure. When the core is brought to the surface, the bubbles can exert a stress that exceeds the tensile strength of the ice, resulting in cracks and spall. Some steps can be taken to alleviate the problem.
Liners can be placed inside the drill barrel to enclose the core before it is brought to the surface, but this makes it difficult to clean off the drilling fluid. In mineral drilling, special machinery can bring core samples to the surface at bottom-hole pressure, but this is too expensive for the inaccessible locations of most drilling sites. Keeping the processing facilities at very low temperatures limits thermal shocks.
Extruding the core from the drill barrel into a net helps keep it together if it shatters. Brittle cores are also often allowed to rest in storage at the drill site for some time, up to a full year between drilling seasons, to let the ice gradually relax. For the results of these tests to be useful in the reconstruction of palaeoenvironments , there has to be a way to determine the relationship between depth and age of the ice.
The simplest approach is to count layers of ice that correspond to the original annual layers of snow, but this is not always possible. An alternative is to model the ice accumulation and flow to predict how long it takes a given snowfall to reach a particular depth. Another method is to correlate radionuclides or trace atmospheric gases with other timescales such as periodicities in the earth's orbital parameters.
As a result, there are two chronologies for a given ice core: To determine the relationship between the two, models have been developed for the depth at which gases are trapped for a given location, but their predictions have not always proved reliable. The size of a crystal is related to its growth rate, which in turn depends on the temperature, so the properties of the bubbles can be combined with information on accumulation rates and firn density to calculate the temperature when the firn formed.
The CO 2 can be isolated by subliming the ice in a vacuum, keeping the temperature low enough to avoid the loess giving up any carbon. The results have to be corrected for the presence of 14 C produced directly in the ice by cosmic rays, and the amount of correction depends strongly on the location of the ice core. Corrections for 14 C produced by nuclear testing have much less impact on the results. The very small quantities typically found require at least g of ice to be used, limiting the ability of the technique to precisely assign an age to core depths.
It is more difficult to connect the timescales in different hemispheres. The Laschamp event , a geomagnetic reversal about 40, years ago, can be identified in cores;   away from that point, measurements of gases such as CH 4 methane can be used to connect the chronology of a Greenland core for example with an Antarctic core.
This approach was developed in and has since been turned into a software tool, DatIce. Formal definitions of stratigraphic boundaries allow scientists in different locations to correlate their findings.
These often involve fossil records, which are not present in ice cores, but cores have extremely precise palaeoclimatic information that can be correlated with other climate proxies. Section contains 11 annual layers with summer layers arrowed sandwiched between darker winter layers. If a pair of pits is dug in fresh snow with a thin wall between them and one of the pits is roofed over, an observer in the roofed pit will see the layers revealed by sunlight shining through.
A six-foot pit may show anything from less than a year of snow to several years of snow, depending on the location. Poles left in the snow from year to year show the amount of accumulated snow each year, and this can be used to verify that the visible layer in a snow pit corresponds to a single year's snowfall. When this turns to ice, the two layers will make up no more than a foot of ice.
The layers corresponding to the summer snow will contain bigger bubbles than the winter layers, so the alternating layers remain visible, which makes it possible to count down a core and determine the age of each layer.
Dust layers may now become visible. Ice from Greenland cores contains dust carried by wind; the dust appears most strongly in late winter, and appears as cloudy grey layers. These layers are stronger and easier to see at times in the past when the earth's climate was cold, dry, and windy. Identification of these layers, both visually and by measuring density of the core against depth, allows the calculation of a melt-feature percentage MF: MF calculations are averaged over multiple sites or long time periods in order to smooth the data.
Plots of MF data over time reveal variations in the climate, and have shown that since the late 20th century melting rates have been increasing. This requires the core to be cut lengthwise, so that a flat surface is created.
Oxygen has three stable isotopes,