Ice core

Ice core sample taken from drill. Photo by Lonnie Thompson, Byrd Polar Research Center

An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper, and an ice core contains ice formed over a range of years. Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles, and contain ice up to 800,000 years old.

The physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core. The ratio of oxygen and hydrogen isotopes provides information about ancient temperatures, and the air trapped in tiny bubbles can be analyzed to determine the level of atmospheric gases such as carbon dioxide. Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past. These data can be combined to find the climate model that best fits all the available data.

Impurities in ice cores may depend on location. Coastal areas are more likely to include material of marine origin, such as sea salt ions. Greenland ice cores contain layers of wind-blown dust that correlate with cold, dry periods in the past, when cold deserts were scoured by wind. Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice. Some volcanic events that were sufficiently powerful to send material around the globe have left a signature in many different cores that can be used to synchronize their time scales.

Ice cores began to be studied in the early 1900s, and several cores were drilled as a result of the International Geophysical Year (1957–1958). Depths of over 400 m were reached, a record which was extended in the 1960s to 2164 m at Byrd Station in Antarctica. Soviet ice drilling projects in Antarctica include decades of work at Vostok Station, with the deepest core reaching 3769 m. Numerous other deep cores in the Antarctic have been completed over the years, including the West Antarctic Ice Sheet project, and cores managed by the British Antarctic Survey and the International Trans-Antarctic Scientific Expedition. In Greenland, a sequence of collaborative projects began in the 1970s with the Greenland Ice Sheet Project; there have been multiple follow-up projects, with the most recent, the East Greenland Ice-Core Project, expected to complete a deep core in east Greenland in 2020.

Structure of ice sheets and cores

See also: 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. As snow accumulates, each layer presses on lower layers, making them denser until they turn into firn. Firn is not dense enough to prevent air from escaping; but at a density of about 830 kg/m³ it turns to ice, and the air within is sealed into bubbles that capture the composition of the atmosphere at the time the ice formed. The depth at which this occurs varies with location, but in Greenland and the Antarctic it ranges from 64 m to 115 m. Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal. At Summit Camp in Greenland, the depth is 77 m and the age is 230 years old; at Dome C in Antarctica the depth is 95 m and the age 2500 years. As further layers build up, the pressure increases, and at about 1500 m the crystal structure of the ice changes from hexagonal to cubic, allowing air molecules to move into the cubic crystals and form a clathrate. The bubbles disappear and the ice becomes more transparent.

Two or three feet of snow may turn into less than a foot of ice. The weight above makes deeper layers of ice thin and flow outwards. 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. The outward flow can distort the layers, so it is desirable to drill deep ice cores at places where there is very little flow. These can be located using maps of the flow lines.

Impurities in the ice provide information on the environment when they were deposited. These include soot, ash, and other types of particle from forest fires and volcanoes; isotopes such as beryllium-10 created by cosmic rays; micrometeorites; and pollen. The lowest layer of a glacier, called basal ice, is frequently formed of subglacial meltwater that has refrozen. 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.

Cores are often drilled in areas such as Antarctica and central Greenland where the temperature is almost never warm enough to cause melting, but the summer sun can still alter the snow. 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.

Coring

Ice auger patented in 1932; the design is very similar to modern augers used for shallow drilling.

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 1932 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. With the aid of a tripod for lowering and raising the auger, cores up to 50 m deep can be retrieved, but the practical limit is about 30 m for engine-powered augers, and less for hand augers. 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. They can be removed by compacting them into the walls of the hole or into the core, by air circulation (dry drilling), or by the use of a drilling fluid (wet drilling). Dry drilling is limited to about 400 m depth, since below that point a hole would close up as the ice deforms from the weight of the ice above.

Drilling fluids are chosen to balance the pressure so that the hole remains stable. The fluid must have a low kinematic viscosity to reduce tripping time (the time taken to pull the drilling equipment out of the hole and return it to the bottom of the hole). 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. Historically, there have been three main types of ice drilling fluids: two-component fluids based on kerosene-like products mixed with fluorocarbons to increase density; alcohol compounds, including aqueous ethylene glycol and ethanol solutions; and esters, including n-butyl acetate. 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. This approach requires long trip times, since the entire drill string must be hoisted out of the hole, and each length of pipe must be separately disconnected, and then reconnected when the drill string is reinserted. Along with the logistical difficulties associated with bringing heavy equipment to ice sheets, this makes traditional rotary drills unattractive. In contrast, wireline drills allow the removal of the core barrel from the drill assembly while it is still at the bottom of the borehole. The core barrel is hoisted to the surface, and the core removed; the barrel is lowered again and reconnected to the drill assembly. Another alternative is flexible drill-stem rigs, in which the drill string is flexible enough to be coiled when at the surface. This eliminates the need to disconnect and reconnect the pipes during a trip.

Mechanical drill head, showing cutting teeth

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 armored 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. The drilling fluid is usually circulated down around the outside of the drill and back up between the core and core barrel; the cuttings are stored in the downhole assembly, in a chamber above 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. Hot-water drills use jets of hot water at the drill head to melt the water around the core. 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: ice melted by pressure can refreeze on EM drill bits, reducing cutting efficiency, and can clog other parts of the mechanism. 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.

Large coring projects

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 2017 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. Over the course of a drilling season, scores of people work at the camp, and logistics support includes airlift capabilities provided by the US Air National Guard, using Hercules transport planes owned by the National Science Foundation. In 2015 the EastGRIP team moved the camp facilities from NEEM, a previous Greenland ice core drilling site, to the EastGRIP site. Drilling is expected to continue until at least 2020.

Core processing

Sawing the GRIP core

With some variation from project to project, the following is the sequence of steps that must occur between the completion of a drilling run and the final storage of the ice core.

The core must be broken away from the ice below it. The drill simply removes an annulus of ice around the core; it does not cut under the core. The break may be done with a spring-loaded lever arm called a core dog, which breaks the core and holds it in place while it is brought to the surface. Once the core reaches the surface, it must be extracted from the drill barrel. The drill is usually rotated to a horizontal position so that the core can slide out sideways onto a prepared surface. The core must be cleaned of drilling fluid at this point; for the WAIS Divide coring project a vacuuming system was set up to remove as much drilling fluid as possible from the core as it was being removed from the barrel. The surface that receives the core should be aligned as accurately as possible with the position of the drill barrel to minimize mechanical stress on the core, which can easily break. The temperature in the core processing area is kept well below freezing to avoid thermal shock to the core.

Information about the core will then be logged, including the length of the core and the depth it was retrieved from, and it may be marked to show its orientation. It is usually cut into shorter sections; the standard length of ice cores in the US is one metre. The cores are then stored on site, usually in an under-snow space in order to simplify temperature maintenance, though additional refrigeration can be used. If further removal of drilling fluid is needed, air may be blown over the cores. Any samples needed for analysis performed at the drilling site 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 will be unheated to help maintain the low temperature needed; when cores are transported by ship a refrigeration unit must be provided as part of the cargo.

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 core will typically have a substantial fraction of its ice stored for archival purposes, so that it will be available for additional analyses in the future. When samples are analyzed, an outer layer may be removed to eliminate any contamination of the ice that might have occurred during drilling and handling.

Brittle ice

Bubbles in an Antarctic ice sample Illuminated with polarised light

Sliver of Antarctic ice showing trapped bubbles. Images from CSIRO.

The bubbles of air trapped in the ice are under great pressure as the depth increases. When ice cores from these depths are brought to the surface, the stress on the ice from the trapped gas can exceed the tensile strength of the ice, and the core can crack and spall. As drilling continues to greater depths, the brittle ice zone is eventually passed, as the air bubbles disappear into clathrates, and the core becomes stable again. At the WAIS Divide site, the brittle ice zone was from 520 m to 1340 m depth.

Drilling through the brittle ice zone typically results in 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 drilling fluid from the core. In mineral drilling, equipment is available to bring core samples to the surface at bottom-hole pressure, but this requires machinery that is cost-prohibitive for ice cores given the inaccessible locations of most drilling sites. Keeping the core processing facilities at very low temperatures to limit the thermal shock to the core helps reduce breakage. Cores are susceptible to breaking when being cut at the surface, so another approach is to break the cores to 1 m lengths in the hole, to avoid the need for surface cutting at the lower pressure. Extruding the core into a net as it is recovered from the drill barrel helps keep the core together if it shatters at the surface. Brittle cores are also often allowed to rest in storage at the drill site for some time, for up to a full year between drilling seasons, in order to let the ice gradually relax.

Ice core data

Dating

Graph of CO₂ (green), reconstructed temperature (blue) and dust (red) from the Vostok ice core for the past 420,000 years

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 the depth below the surface and the age of the ice at that depth: in other words, there must be a way to calibrate the depth scale so that it can be read as an age scale. The simplest approach is to count layers of ice that correspond to the original annual layers of snow, but in situations where this is impossible, other methods are used. One approach is to model the ice accumulation and flow in order to predict how long it takes a given snowfall to reach a particular depth in the ice sheet. Another method is to identify radionuclides or trace atmospheric gases in the cores, and correlate them with other timescales, such as the periodicities in the earth's orbital parameters.

A difficulty in ice core dating is that the ability of gases to diffuse through firn, before it turns to ice, means that the age of ice at a given depth may be substantially greater than the age of the trapped gases at that depth. As a result, there are two chronologies for a given ice core: one for the ice, and one for the trapped gases. To determine the relationship between the two, models have been developed that predict the depth at which gases are trapped for a given location, but the predictions these models make have not always proven to be reliable. At locations with very low snowfall, such as Vostok, the uncertainty in the difference between ages of the ice and gas at a given depth can be over 1,000 years.

The density and size of the bubbles trapped in the ice provide an indication of crystal size at the time the air was trapped. Crystal size is related to the crystal growth rate, which in turn depends on the temperature, so in combination with information about accumulation rates and firn density, the bubbles can be used to calculate the temperature at the time the firn was formed.

Radiocarbon dating can be used on the carbon in trapped CO
₂. In the polar ice sheets there is about 15–20 μg of carbon in the form of CO
₂ in each kilogram of ice, and there may also be carbon in the form of carbonate particles from loess. It is possible to separate the CO
₂ 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. Carbon in particulate form can also be dated; the water-insoluble organic components of dust can be separated and tested, though the very small quantities typically found require at least 300 g of ice to be used, limiting the ability of the technique to precisely assign an age to core depths.

It is usually possibly to synchronize timescales for ice cores from the same hemisphere by identifying layers which include material from volcanic events, which can be identified in multiple cores. It is more difficult to connect the timescales of ice cores in different hemispheres. The Laschamp event, a geomagnetic reversal about 40,000 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. In cases where volcanic tephra is interspersed with ice in a core, argon/argon dating can be used to date the tephra and hence provide fixed points for dating the ice. Uranium decay has also been used to date ice cores. Another approach to producing an integrated ice core chronology is to use Bayesian probability techniques to determine the optimal combination of multiple independent records. This approach was developed in 2010 and has since been turned into a software tool, DatIce.

The formal definition of the boundary between the Pleistocene and the Holocene, about 11,700 years ago, is now defined with reference to ice core data drawn from Greenland ice cores. Formal definitions of stratigraphic boundaries allow scientists in different locations to correlate their findings with other locations, and are often defined using fossils as markers of the change. Ice cores carry no fossil records, but the palaeoclimatic information they include is extremely precise and can be used to correlate with other climate proxies.

The dating of ice sheets, by a variety of methods, has proved to be a key element in providing dates for palaeoclimatic records. According to Richard Alley, "In many ways, ice cores are the ‘rosetta stones’ that allow development of a global network of accurately dated paleoclimatic records using the best ages determined anywhere on the planet".

Visual analysis

19 cm long section of GISP 2 ice core from 1855 m showing annual layer structure illuminated from below by a fiber optic source. Section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers.

Cores show visible layers, which correspond to annual snowfall at the core site. Digging a pair of pits in fresh snow with a thin wall between them, and roofing over one of the pits, reveals obvious layers in the snow wall to an observer in the roofed pit, who will see sunlight shining through the layers. 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. As the depth increases to the point where the ice structure changes to a clathrate, the bubbles are no longer visible, and the layers can no longer be seen. However, 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. The problem is more acute at locations where accumulation is high; low accumulation sites, such as central Antarctica, must be dated by other methods. For example, at Vostok, layer counting is only possible down to an age of 55,000 years.

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 recognize 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): an MF of 100% would mean that every year's deposit of snow showed evidence of melting. 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.

Isotopic analysis

The isotopic composition of the oxygen in the core can be used to model the temperature history of the ice sheet. There are three stable isotopes of oxygen: ¹⁶
O, ¹⁷
O and ¹⁸
O; measuring the ratio between two of these, ¹⁸
O and ¹⁶
O, provides an indication of the temperature at the time the snow (that formed the ice) fell. Because ¹⁶
O is lighter than ¹⁸
O, water containing ¹⁶
O is slightly more likely to turn into vapor, and water containing ¹⁸
O is slightly more like to condense from vapor into rain or snow crystals. This difference between ¹⁸
O and ¹⁶
O is dependent on temperature: at lower temperatures, the difference is more pronounced. The standard method of recording the ratio between ¹⁸
O and ¹⁶
O in a given sample is to calculate the difference between the ¹⁸
O/¹⁶
O ratio in the sample, and the ratio in a standard known as standard mean ocean water (SMOW):

$\mathrm {\delta ^{18}O} ={\Biggl (}\mathrm {\frac {{\bigl (}{\frac {^{18}O}{^{16}O}}{\bigr )}_{sample}}{{\bigl (}{\frac {^{18}O}{^{16}O}}{\bigr )}_{SMOW}}} -1{\Biggr )}\times 1000\ ^{o}\!/\!_{oo}$

where the ‰ sign indicates parts per thousand. A sample with the same ¹⁸
O/¹⁶
O ratio as SMOW has a δ¹⁸O of 0‰; a sample which is depleted in ¹⁸
O has a negative δ¹⁸O. This provides an estimate of historical temperatures. Combining the δ¹⁸O measurements of ice core samples with the borehole temperature at the depth at which the sample was taken provides additional information, in some cases leading to significant corrections to the temperatures deduced from the δ¹⁸O data. Not all boreholes can be used in these analyses; if the site has experienced significant melting in the past, the borehole will no longer preserve an accurate history of the temperature record. Similarly, deuterium (²
H, or D) is heavier than hydrogen (¹
H), and water containing deuterium is more likely to condense and less likely to evaporate than water containing no deuterium. A δD ratio can be defined in the same way as δ¹⁸O, and measuring δD in ice cores provides another method of calculating historical temperatures. There is a linear relationship between δ¹⁸O and δD:

$\mathrm {\delta D} =8\times \mathrm {\delta ^{18}O} +\mathrm {d}$

where d is the deuterium excess. It was once thought that this meant it was unnecessary to measure both δ¹⁸O and δD in a given core, as they would both provide the same information about past climates, but in 1979 it was realized that measuring the deuterium excess provided additional information about temperature, relative humidity, and wind speed, and 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. Other isotopic ratios have been studied: for example, the ratio between ¹³
C and ¹²
C can provide information about past changes in the carbon cycle. 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.

Palaeoatmospheric sampling

Ozone-depleting gases in Greenland firn.

It was understood in the 1960s that analyzing the air trapped in ice cores would provide useful information, but it was not until the late 1970s that a reliable extraction method was developed. Early results included a demonstration that the CO
₂ concentration at the last glacial maximum was 30% less than the CO
₂ level just before the start of the industrial age. 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 of CH
₄ 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 measured in cores can be used to determine the relationship between core depth and age. N
₂O (nitrous oxide) levels are also correlated with glacial cycles, though at low temperatures the graph differs somewhat from the CO
₂ and CH
₄ graphs. Similarly, the ratio between N
₂ (nitrogen) and O
₂ (oxygen) can be used to date ice cores: as air is gradually trapped by the snow turning to firn and then ice, O
₂ is lost more easily than N
₂ from the trapped gas, and the relative amount of O
₂ loss has been shown to correlate with the strength of local summer insolation. 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. Temperature changes the amount of enrichment: colder temperatures cause heavier molecules to be more enriched at the bottom of a column. These fractionation processes in trapped air, determined by the measurement of the ¹⁵
N/¹⁴
N ratio and of neon, krypton and xenon, have been used to infer the thickness of the firn layer, and determine other palaeoclimatic information such as past mean ocean temperatures. Some gases, such as helium, can rapidly diffuse through ice, so it may be necessary to test samples of ice for these "fugitive gases" within minutes of the core being retrieved in order to obtain accurate data. Chlorofluorocarbons (CFCs), which contribute to the greenhouse effect and also cause ozone loss in the stratosphere, can be detected in ice cores after about 1950; almost all CFCs in the atmosphere were created by human activity.

Glaciochemistry

Summer snow in Greenland contains some sea salt, blown from the surrounding waters; in winter, with much of the sea surface covered by pack ice, there is less sea salt. Similarly, hydrogen peroxide appears only in summer snow; in this case it is because production of hydrogen peroxide 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 of the core at that point. Dragging the electrodes down the length of the core, and recording the conductivity at each point, gives a graph that shows an annual periodicity. These graphs also identify chemical changes caused by non-seasonal events, such as forest fires and major volcanic eruptions. When a known volcanic event, such as the eruption of Laki in Iceland in 1783, can be identified in the ice core record, it provides a cross-check on the age determined by layer counting. Material from Laki can be identified in Greenland ice cores, but did not spread as far as Antarctica; the 1815 eruption of Tambora in Indonesia injected material into the stratosphere, and can be identified in both Greenland and Antarctic ice cores. If the date of the eruption is not known, but it can be identified in multiple cores, then dating the ice can in turn give a date for the eruption, which can then be used as a reference layer. This was done, for example, in an analysis of the climate for the period from 535 to 550 AD, which was thought to be influenced by an otherwise unknown tropical eruption in about 533 AD; but which turned out to be caused by two eruptions, one in 535 or early 536 AD, and a second one in 539 or 540 AD. There are also more ancient reference points, such as the eruption of Toba about 72,000 years ago.

Many other elements and molecules have been detected in ice cores. In 1969, it was discovered that lead levels in Greenland ice had increased by a factor of over 200 since pre-industrial times, and increases in other elements produced by industrial processes, such as copper, cadmium, and zinc, have also been recorded. The presence of nitric and sulphuric acid (HNO
₃ and H
₂SO
₄) in precipitation can be shown to correlate with increasing fuel combustion over time. Methanesulphonate (MSA) (CH
₃SO⁻
₃) is produced in the atmosphere by marine organisms, so ice core records of MSA provide information on the history of the oceanic environment. Both hydrogen peroxide (H
₂O
₂) and formaldehyde (HCHO) have been studied, along with organic molecules such as carbon black that are linked to vegetation emissions and forest fires. Some species, such as calcium and ammonium, show strong seasonal variation. In some cases there are contributions from more than one source to a given species: for example, Ca⁺⁺ comes from dust as well as from marine sources; the marine input is much greater than the dust input and so although the two sources peak at different times of the year, the overall signal shows a peak in the winter, when the marine input is at a maximum. Seasonal signals can be erased at sites where the accumulation is low, by surface winds; in these cases it is not possible to date individual layers of ice between two reference layers.

Some of the deposited chemical species may interact with the ice, and as a result, what is detected in an ice core is not necessarily what was originally deposited. Examples include HCHO and H
₂O
₂. Calculating the original atmospheric concentration of a species is also complicated by the possibility that in areas with low accumulation rates, deposition from fog can increase the concentration in the snow, sometimes to the point where the atmospheric concentration could be overestimated by a factor of two.

Soluble impurities found in ice cores

Source	Via	Measured in polar ice
Oceans	Waves and wind	Sea salt: Na+ , Cl− , Mg2+ , Ca2+ , SO2− 4, K+
Land	Aridity and wind	Terrestrial salts: Mg2+ , Ca2+ , CO2− 3, SO2− 4, aluminosilicates
Human and biological gas emissions: SO 2, (CH 3) 2S, H 2S, COS, NO x, NH 3, hydrocarbons and halocarbons	Atmospheric chemistry: O 3, H 2O 2, OH, RO 2, NO 3,	H+ , NH+ 4, Cl− , NO− 3, SO2− 4, CH 3SO− 3, F− , HCOO− , other organic compounds

Radionuclides

³⁶Cl from 1960s nuclear bombs in US glacier ice.

Galactic cosmic rays produce ¹⁰
Be in the atmosphere at a rate that is dependent on the solar magnetic field. The strength of the field is related to the intensity of solar radiation,so the level of ¹⁰
Be in the atmosphere at a given time in the past is a proxy for historical climate information. Accelerator mass spectrometry can detect the extremely low levels (about 10,000 atoms in a gram of ice) of ¹⁰
Be in ice cores, and these can be used to provide long-term records of solar activity. Tritium (³
H), created by nuclear weapons testing in the 1950s and 1960s, has been identified in ice cores, as have both ³⁶Cl and ²³⁹
Pu, which have been found in ice cores in Antarctica and Greenland. ³⁶
Cl, which has a half-life T = 301,000 years, has been used to date cores, as have krypton (⁸⁵
Kr, with half-life T = 11 years), lead (²¹⁰
Pb, T = 22 years), and silicon (³²
Si, T = 172 years).

Other inclusions

Meteorites and micrometeorites that land on polar ice are sometimes concentrated in particular locations by local environmental processes. For example, there are places in Antarctica where winds evaporate surface ice, which concentrates the solids that are left behind, including meteorites. Meltwater ponds can also contain meteorites. At the South Pole Station, the cavity created by melting ice to provide a water supply retains the micrometeorites that were in the ice in the cavity, and these have been collected by a robotic "vacuum cleaner" and examined, leading to improved estimates of the flux and mass distribution of micrometeorites. The well is not an ice core, but it shares some characteristics with cores: the age of the ice at the bottom of the well is known, so the age of the recovered particles can be determined—the well becomes about 10 m deeper each year, which means micrometeorites collected in a given year are about 100 years older than those from the previous year.

Pollen, which is an important element of sediment cores, can also be found in ice cores. Pollen provides information about the environment of the past beyond the reconstructed temperature, since changes in vegetation are reflected in the types of pollen found.

Physical properties

In addition to studying the impurities in cores and the isotopic composition of the water, the physical properties of the ice are also examined. Features such as crystal size and axis orientation can reveal the history of ice flow patterns in the ice sheet from which the core is taken. The crystal size can also be used in some cases to determine dates, though this is only applicable in shallow cores.

History

A store of core samples

Early years

In 1841 and 1842, Louis Agassiz drilled holes in the Unteraargletscher in the Alps; these were drilled with iron rods and did not produce cores. The deepest hole achieved was 60 m. Another early scientific use for ice drilling was on Erich von Drygalski's Antarctic expedition; 30 m holes were drilled in an iceberg south of the Kerguelen Islands in 1902 and 1903, and temperature readings were taken. The first scientist to create a snow sampling tool was J.E. Church, described by Pavel Talalay as "the father of modern snow surveying". In the winter of 1908–1909, Church constructed steel tubes with slots and cutting heads to retrieve cores of snow up to 3 m long. Similar devices are in use today, with modifications that allow samples to a depth of about 9 m to be obtained. These tools are used by simply pushing them into the snow and rotating them by hand.

The first systematic study of snow and firn layers was by Ernst Sorge, who was part of the Alfred Wegener Expedition to central Greenland in 1930–1931. Sorge dug a 15 m deep pit to examine the snow layers, and his results were later formalized into Sorge's Law of Densification by Henri Bader, who went on to do additional coring work in northwest Greenland in 1933. In the early 1950s, a SIPRE expedition obtained pit samples over much of the Greenland ice sheet, obtaining early oxygen isotope ratio data. Three other expeditions in the 1950s began ice coring work: a joint Norwegian-British-Swedish Antarctic Expedition (NBSAE), in Queen Maud Land in Antarctica; the Juneau Ice Field Research Project (JIRP), in Alaska; and Expéditions Polaires Françaises, in central Greenland. Core quality was poor, but some scientific work was done on the retrieved ice.

The International Geophysical Year (1957–1958) saw increased glaciology research around the world, with deep cores in polar regions one of the high priority research targets. SIPRE conducted pilot drilling trials in 1956 (to 305 m) and 1957 (to 411 m) at Site 2 in Greenland; the second core, with the benefit of the previous year's drilling experience, was retrieved in much better condition, with fewer gaps. In Antarctica, a 307 m core was drilled at Byrd Station in 1957–1958, and a 264 m core at Little America V, on the Ross Ice Shelf, the following year. The success of the IGY core drilling led to increased interest in improving ice coring capabilities, and was followed by a CRREL project at Camp Century, where in the early 1960s three holes were drilled, the deepest reaching the base of the ice sheet at 1387 m in July 1966. The drill used at Camp Century then went to Byrd Station, where a 2164 m hole was drilled to bedrock before the drill was frozen into the borehole by sub-ice meltwater, and had to be abandoned.

French, Australian and Canadian projects from the 1960s and 1970s include a 905 m core at Dome C in Antarctica, drilled by CNRS; cores at Law Dome drilled by ANARE, starting in 1969 with a 382 m core; and Devon Ice Cap cores recovered by a Canadian team in the 1970s.

Antarctica deep cores

Composite data for Dome C, CO₂ levels (ppm) going back nearly 800,000 years, and related glacial cycles.

Soviet ice drilling projects began in the 1950s, in Franz Josef Land, the Urals, Novaya Zemlya, and at Mirny and Vostok in the Antarctic; not all these early holes retrieved cores. Over the following decades work continued at multiple locations in Asia. Drilling in the Antarctic focused mostly on Mirny and Vostok, with a series of deep holes at Vostok begun in 1970. The first deep hole at Vostok reached 506.9 m in April 1970; by 1973 a depth of 952 m had been reached. A subsequent hole, Vostok 2, drilled from 1971 to 1976, reached 450 m, and Vostok 3 reached 2202 m in 1985 after six drilling seasons. Vostok 3 was the first core to retrieve ice from the previous glacial period, 150,000 years ago. Drilling was interrupted by a fire at the camp in 1982, but further drilling began in 1984, eventually reaching 2546 m in 1989. A fifth Vostok core was begun in 1990, reached 3661 m in 2007, and was later extended to 3769 m. The estimated age of the ice is 420,000 years at 3310 m depth; below that point it is difficult to interpret the data reliably because of mixing of the ice.

The EPICA Dome C and Vostok ice cores compared

EPICA, a European ice coring collaboration, was formed in the 1990s, and two holes were drilled in East Antarctica: one at Dome C, which reached 2871 m in only two seasons of drilling, but which took another four years to reach bedrock at 3260 m; and one at Kohnen Station, which reached bedrock at 2760 m in 2006. The Dome C core had very low accumulation rates, which mean that the climate record extended a long way; by the end of the project the usable data extended to 800,000 years ago.

Other deep Antarctic cores included a Japanese project at Dome F, which reached 2503 m in 1996, with an estimated age of 330,000 years for the bottom of the core; and a subsequent hole at the same site which reached 3035 m in 2006, estimated to reach ice 720,000 years old. US teams drilled at McMurdo Station in the 1990s, and at Taylor Dome (554 m in 1994) and Siple Dome (1004 m in 1999), with both cores reaching ice from the last glacial period. The West Antarctic Ice Sheet (WAIS) project, completed in 2011, reached 3405 m; the site has high snow accumulation so the ice only extends back 62,000 years, but as a consequence, the core provides high resolution data for the period it covers. A 948 m core was drilled at Berkner Island by a project managed by the British Antarctic Survey from 2002 to 2005, extending into the last glacial period; and an Italian-managed ITASE project completed a 1620 m core at Talos Dome in 2007.

In 2016, cores were retrieved from the Allan Hills in Antarctica in an area where old ice lay near the surface. The cores were dated by potassium-argon dating; traditional ice core dating is not possible as not all layers were present. The oldest core was found to include ice from 2.7 million years ago—by far the oldest ice yet dated from a core.

Greenland deep cores

In 1970, scientific discussions began which resulted in the Greenland Ice Sheet Project (GISP), a multinational investigation into the Greenland ice sheet that lasted until 1981. Years of field work were required to determine the ideal location for a deep core; the field work included several intermediate-depth cores, at Dye 3 (372 m in 1971), Milcent (398 m in 1973) and Crete (405 m in 1974), among others. A location in north-central Greenland was selected as ideal, but financial constraints forced the group to drill at Dye 3 instead, beginning in 1979. The hole reached bedrock at 2037 m, in 1981. Two holes, 30 km apart, were eventually drilled at the north-central location in the early 1990s by two groups: GRIP, a European consortium, and GISP-2, a group of US universities. GRIP reached bedrock at 3029 m in 1992, and GISP-2 reached bedrock at 3053 m the following year. Both cores were limited to about 100,000 years of climatic information, and since this was thought to be connected to the topography of the rock underlying the ice sheet at the drill sites, a new site was selected 200 km north of GRIP, and a new project, NorthGRIP, was launched as an international consortium led by Denmark. Drilling began in 1996; the first hole had to be abandoned at 1400 m in 1997, and a new hole was begun in 1999, reaching 3085 m in 2003. The hole did not reach bedrock, but terminated at a subglacial river. The core provided climatic data back to 123,000 years ago, which covered part of the last interglacial period. The subsequent North Greenland Eemian (NEEM) project retrieved a 2537 m core in 2010 from a site further north, extending the climatic record to 128,500 years ago; NEEM was followed by EastGRIP, which began in 2015 in east Greenland and is expected to be complete in 2020.

Non-polar cores

Ice cores have been drilled at locations away from the poles, notably in the Himalayas and the Andes. Some of these cores reach back to the last glacial period, but they are more important as records of El Niño events and of monsoon seasons in south Asia. Cores have also been drilled on Mount Kilimanjaro, in the Alps, and in Indonesia, New Zealand, Iceland, Scandinavia, Canada, and the US.

Future plans

IPICS (International Partnerships in Ice Core Sciences) has produced a series of white papers outlining future challenges and scientific goals for the ice core science community. These include plans to:

• Retrieve ice cores that reach back over 1.2 million years, in order to obtain multiple iterations of ice core record for the 40,000-year long climate cycles known to have operated at that time. Current cores reach back over 800,000 years, and show 100,000-year cycles.
• Improve ice core chronologies, including connecting chronologies of multiple cores.
• Identify additional proxies from ice cores, for example for sea ice, marine biological productivity, or forest fires.
• Drill additional cores to provide high-resolution data for the last 2,000 years, to use as input for detailed climate modeling.
• Identify an improved drilling fluid
• Improve the ability to handle brittle ice, both while drilling and in transport and storage
• Find a way to handle cores which have pressurized water at bedrock
• Come up with a standardized lightweight drill capable of drilling both wet and dry holes, and able to reach depths of up to 1000 m.
• Improve core handling to maximize the information that can be obtained from each core.

See also

• List of ice cores

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External links

• Ice Core Gateway
• Byrd Polar Research Center – Ice Core Paleoclimatology Research Group