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 cores are collected by cutting around a cylinder of ice in a way that enables it to be brought to the surface. 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 cutting apparatus of a drill is on the bottom end of a drill barrel, the tube that surrounds the core as the drill cuts downward. 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. Drilling fluids are chosen to balance the pressure so that the hole remains stable.
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.
Rotary drilling is the main method of drilling for minerals and it has also been used for ice drilling. 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. The need for a string of drillpipe that extends from the surface to the bottom of the borehole can be eliminated by suspending the entire downhole assembly on an armoured cable that conveys power to the downhole motor.
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. Thermal drills, which cut ice by electrically heating the drill head, can also be used, but they have some disadvantages. 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.
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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. When drilling in temperate ice, thermal drills have an advantage over electromechanical EM drills: EM drills are also more likely to fracture ice cores where the ice is under high stress. When drilling deep holes, which require drilling fluid, the hole must be cased fitted with a cylindrical lining , since otherwise the drilling fluid will be absorbed by the snow and firn.
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. Ice cores from different depths are not all equally in demand by scientific investigators, which can lead to a shortage of ice cores at certain depths.
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 logistics of any coring project are complex because the locations are usually difficult to reach, and may be at high altitude. 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.
With some variation between projects, the following steps must occur between drilling and final storage of the ice core. The drill removes an annulus of ice around the core but does not cut under it. 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. A log is kept with information about the core, including its length and the depth it was retrieved from, and the core may be marked to show its orientation. It is usually cut into shorter sections, the standard length in the US being one metre.
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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.
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- I. Methods of Dating Ice Cores.
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. There are several locations around the world that store ice cores, such as the National Ice Core Laboratory in the US. 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. The brittle ice zone typically returns poorer quality samples than for the rest of the core.
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. Many different kinds of analysis are performed on ice cores, including visual layer counting, tests for electrical conductivity and physical properties, and assays for inclusion of gases, particles, radionuclides , and various molecular species.
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.
A difficulty in ice core dating is that gases can diffuse through firn, so the ice at a given depth may be substantially older than the gases trapped in it. 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 density and size of the bubbles trapped in ice provide an indication of crystal size at the time they formed. 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. Radiocarbon dating can be used on the carbon in trapped CO 2. 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.
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Timescales for ice cores from the same hemisphere can usually be synchronised using layers that include material from volcanic events. 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. The boundary between the Pleistocene and the Holocene , about 11, years ago, is now formally defined with reference to data on Greenland ice cores.
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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.
The dating of ice sheets has proved to be a key element in providing dates for palaeoclimatic records. Cores show visible layers, which correspond to annual snowfall at the core site. 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. In central Greenland a typical year might produce two or three feet of winter snow, plus a few inches of summer snow. 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.
Any method of counting layers eventually runs into difficulties as the flow of the ice causes the layers to become thinner and harder to see with increasing depth. When there is summer melting, the melted snow refreezes lower in the snow and firn, and the resulting layer of ice has very few bubbles so is easy to recognise in a visual examination of a core. 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. In addition to manual inspection and logging of features identified in a visual inspection, cores can be optically scanned so that a digital visual record is available. This requires the core to be cut lengthwise, so that a flat surface is created.
The isotopic composition of the oxygen in a core can be used to model the temperature history of the ice sheet. Oxygen has three stable isotopes, 16 O , 17 O and 18 O. At lower temperatures, the difference is more pronounced. If the site has experienced significant melting in the past, the borehole will no longer preserve an accurate temperature record. Hydrogen ratios can also be used to calculate a temperature history.
Deuterium 2 H , or D is heavier than hydrogen 1 H and makes water more likely to condense and less likely to evaporate. It was once thought that this meant it was unnecessary to measure both ratios in a given core, but in Merlivat and Jouzel showed that the deuterium excess reflects the temperature, relative humidity, and wind speed of the ocean where the moisture originated.
Since then it has been customary to measure both. Water isotope records, analyzed in cores from Camp Century and Dye 3 in Greenland, were instrumental in the discovery of Dansgaard-Oeschger events —rapid warming at the onset of an interglacial , followed by slower cooling. Combining this information with records of carbon dioxide levels, also obtained from ice cores, provides information about the mechanisms behind changes in CO 2 over time.
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It was understood in the s that analyzing the air trapped in ice cores would provide useful information on the paleoatmosphere , but it was not until the late s that a reliable extraction method was developed. Further research has demonstrated a reliable correlation between CO 2 levels and the temperature calculated from ice isotope data.
Because CH 4 methane is produced in lakes and wetlands , the amount in the atmosphere is correlated with the strength of monsoons , which are in turn correlated with the strength of low-latitude summer insolation. Since insolation depends on orbital cycles , for which a timescale is available from other sources, CH 4 can be used to determine the relationship between core depth and age. This means that the trapped air retains, in the ratio of O 2 to N 2 , a record of the summer insolation, and hence combining this data with orbital cycle data establishes an ice core dating scheme.
Diffusion within the firn layer causes other changes that can be measured. Gravity causes heavier molecules to be enriched at the bottom of a gas column, with the amount of enrichment depending on the difference in mass between the molecules. Colder temperatures cause heavier molecules to be more enriched at the bottom of a column. Summer snow in Greenland contains some sea salt, blown from the surrounding waters; there is less of it in winter, when much of the sea surface is covered by pack ice.
Similarly, hydrogen peroxide appears only in summer snow because its production in the atmosphere requires sunlight. These seasonal changes can be detected because they lead to changes in the electrical conductivity of the ice. Placing two electrodes with a high voltage between them on the surface of the ice core gives a measurement of the conductivity at that point.
Dragging them down the length of the core, and recording the conductivity at each point, gives a graph that shows an annual periodicity. Such graphs also identify chemical changes caused by non-seasonal events such as forest fires and major volcanic eruptions. Ice cores can be dated using counting of annual layers in their uppermost layers. Dating the ice becomes harder with depth. Usually multiple methods are used to improve accuracy. Common global stratigraphic markers are palaeo-events that occur worldwide synchronously, and can allow wiggle-matching between ice cores and other palaeo archives e.
For the ice matrix, these global stratigraphic markers can include spikes in volcanic ash each volcanic eruption has a unique chemical signature , or volcanic sulfate spikes. For the gas phase, methane, and oxygen isotopic ratio of O 2 have been used Lemieux-Dudon et al. Uranium has been used to date the Dome C ice core from Antarctica. Dust is present in ice cores, and it contains Uranium. The decay of U to U from dust in the ice matrix can be used to provide an additional core chronology.
Beryillium has also been used to date ice cores. Ice cores are expensive to collect, house and keep. They must be stored continuously at a specific temperature. The American National Ice Core Laboratory provides some information on how they store and keep ice cores. When ice cores are analysed, they may be cut or sectioned, with half the sample remaining as an archive.
As the ice must be melted for analysis, the sample is usually destroyed during analysis. Lemieux-Dudon B, et al. Consistent dating for Antarctic and Greenland ice cores. Quaternary Science Reviews 29,