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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 3. 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 proportions of different oxygen and hydrogen isotopes provide information about ancient temperaturesand the air trapped in tiny bubbles can be analysed 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 Ice guard 30 plus dating 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 testingcan be used to date the layers of ice. Some volcanic events Ice guard 30 plus dating were sufficiently powerful to send material around the globe have left a signature in many different cores that can be used to synchronise their time scales.

Ice cores have been studied since the early 20th century, and several cores were drilled as a result of the International Geophysical Year — Depths of over m were reached, a record which was extended in the s to m at Byrd Station in Antarctica.

Soviet ice drilling projects in Antarctica include decades of work at Vostok Stationwith the deepest core reaching m. Numerous other deep cores in the Antarctic have been completed over the Ice guard 30 plus dating, 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 s with the Greenland Ice Sheet Project ; there have been multiple follow-up projects, with the most recent, the East Ice guard 30 plus dating Ice-Core Projectexpected to complete a deep core in east Greenland in 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.

Two or three feet of snow may Ice guard 30 plus dating into less than a foot of ice. Ice is lost at the edges of the glacier to icebergsor 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.

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Impurities in the ice provide information on the environment from when they were deposited. 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[7] it often does not retain stratigraphic information. Cores are often drilled in areas such as Antarctica Ice guard 30 plus dating 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 "Ice guard 30 plus dating"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 handleand some can be attached to handheld electric drills to Ice guard 30 plus dating 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 Ice guard 30 plus datingfor 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 estersand kerosene-based fluids mixed with foam-expansion agents. Rotary drilling is the main method Ice guard 30 plus dating 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 Ice guard 30 plus dating through the pipe Ice guard 30 plus dating 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 Ice guard 30 plus dating is hoisted to the surface, and the core removed; the barrel is lowered again and reconnected to the drill assembly. This eliminates "Ice guard 30 plus dating" need to disconnect and reconnect the pipes during a trip.

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The need for a string of "Ice guard 30 plus dating" 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 Ice guard 30 plus dating 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.

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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 antifreezewhich eliminates the need for heating Ice guard 30 plus dating 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.

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 liningsince otherwise the drilling fluid will be absorbed by the snow and firn. The casing has to reach down to the Ice guard 30 plus dating 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 Ice guard 30 plus dating 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 "Ice guard 30 plus dating" drilling in eastern Greenland, is run by the Centre for Ice and Climatein DenmarkIce guard 30 plus dating 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 Ice guard 30 plus dating 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 "Ice guard 30 plus dating" 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.

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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 polytheneand 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. There are several locations around the world that store ice cores, such as the National Ice Core Laboratory in the US. Ice guard 30 plus dating 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 Ice guard 30 plus dating. When the core is brought to the surface, the bubbles can exert a stress that exceeds Ice guard 30 plus dating 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 Ice guard 30 plus dating 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, radionuclidesand various molecular species.

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