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101-580: The WAIS Divide ice core project (West Antarctic Ice Sheet Divide ice core project) investigated past climate changes, ice sheet dynamics and cryobiology. The project was completed by the United States Antarctic Program (USAP) and was funded by the National Science Foundation (NSF). The focus of the project was to develop records spanning the last ~ 80,000 years of the concentration of greenhouse gases in

202-498: A m p l e ( 18 O 16 O ) S M O W − 1 ) × 1000   o / o o , {\displaystyle \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

303-405: 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

404-417: A = 0.33‰/°C (Cuffey et al., 1995), implying that glacial-interglacial temperature changes were twice as large as previously believed. Magnesium (Mg) is incorporated into the calcite shells (tests) of planktic and benthic foraminifera as a trace element. Because the incorporation of Mg as an impurity in calcite is endothermic, more is incorporated into the growing crystal at higher temperatures. Therefore,

505-434: 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. 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

606-464: A given core, but in 1979 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

707-525: 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. Paleothermometer A paleothermometer

808-427: 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

909-439: A high Mg/Ca ratio implies a high temperature, although ecological factors may confound the signal. Mg has a long residence time in the ocean, and so it is possible to largely ignore the effect of changes in seawater Mg/Ca on the signal. Mg/Ca ratios can sometimes underestimate seawater temperatures by way of the dissolution of foraminifer shells, which lowers Mg/Ca values. Strontium (Sr) incorporates in coral aragonite, and it

1010-411: 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". Cores show visible layers, which correspond to annual snowfall at the core site. If a pair of pits is dug in fresh snow with

1111-406: 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: one for

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1212-550: A path nearly parallel to the main core. The ice core was transported to the National Ice Core Laboratory in Denver, Colorado where it was sampled and portions were distributed to 33 institutions for analysis. The analysis included the physical, chemical and isotopic properties of the gases trapped in the ice, the soluble and insoluble material in the melted ice, and the water from the melted ice. The core

1313-408: 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

1414-484: A prepared surface. The core must be cleaned of drilling fluid as it is slid out; for the WAIS Divide coring project, a vacuuming system was set up to facilitate this. 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

1515-408: 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

1616-702: 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 , originally expected to complete a deep core in east Greenland in 2020 but since postponed. 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

1717-407: A shortage of ice cores at certain depths. To address this, work has been done on technology to drill replicate cores: additional cores, retrieved by drilling into the sidewall of the borehole, at depths of particular interest. Replicate cores were successfully retrieved at WAIS divide in the 2012–2013 drilling season, at four different depths. The logistics of any coring project are complex because

1818-473: A significant component of the relationship between physiognomic states and temperature. Using CLAMP, MAT is estimated with small standard errors (e.g. CCA ± 0.7–1.0 °C). Additional temperature parameters can be estimated using CLAMP, such as the coldest month mean temperature (CMMT) and the warmest month mean temperature (WMMT) which provide estimates for winter and summer mean conditions respectively. Certain plants prefer certain temperatures; if their pollen

1919-404: A slightly different regression equation, because they have proportionally fewer smooth-margined plants. It is CLAMP is a multivariate approach largely based on a data set of primarily western hemisphere vegetation, subsequently added to with datasets from additional world regional vegetation. Canonical Correlation Analysis is used combining 31 leaf characters, but leaf margin type represented

2020-430: 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

2121-461: A special edition that includes all WAIS Divide related AGU publications as April 2016. Additional information, including a complete list of all the publications related to the project, is available at: http://waisdivide.unh.edu . Leadership for the project was provided by the following people: 79°28′03″S 112°05′11″W  /  79.467472°S 112.086389°W  / -79.467472; -112.086389 Ice core An ice core

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2222-411: 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

2323-473: 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 ones, 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 (3.2 km), and contain ice up to 800,000 years old. The physical properties of

2424-593: Is a methodology that provides an estimate of the ambient temperature at the time of formation of a natural material. Most paleothermometers are based on empirically-calibrated proxy relationships, such as the tree ring or TEX 86 methods. Isotope methods, such as the δ O method or the clumped-isotope method , are able to provide, at least in theory, direct measurements of temperature. The isotopic ratio of O to O, usually in foram tests or ice cores. High values mean low temperatures. Confounded by ice volume - more ice means higher δ O values. Ocean water

2525-540: 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

2626-466: Is available from other sources, CH 4 can be used to determine the relationship between core depth and age. N 2 O (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

2727-447: Is based on the observation that the proportion of woody dicot species with smooth (i.e. non-toothed) leaf margins (0 ≤ P margin ≤ 1) in vegetation varies proportionately with mean annual temperature (MAT ). Requires the fossil flora to be segregated into morphotypes (i.e. ‘species’), but does not require their identification. The original LMA regression equation was derived for East Asian forests, and is: The error of

2828-427: 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 from 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

2929-400: 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

3030-436: Is expressed as and a similar formula for δD . δ O values for precipitation are always negative. The major influence on δ O is the difference between ocean temperatures where the moisture evaporated and the place where the final precipitation occurred; since ocean temperatures are relatively stable the δ O value mostly reflects the temperature where precipitation occurs. Taking into account that

3131-427: Is found one can work out the approximate temperature. There is a slight thermodynamic tendency for heavy isotopes to form bonds with each other, in excess of what would be expected from a stochastic or random distribution of the same concentration of isotopes. The excess is greatest at low temperature (see Van 't Hoff equation ), with the isotopic distribution becoming more randomized at higher temperature. Along with

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3232-450: Is gradually trapped by the snow turning to firn and then ice, O 2 is lost more easily than N 2 , and the relative amount of O 2 correlates 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

3333-505: 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. 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

3434-464: Is mostly H 2 O, with small amounts of HD O and H 2 O. In Standard Mean Ocean Water (SMOW) the ratio of D to H is 155.8 × 10 and O/ O is 2005 × 10 . Fractionation occurs during changes between condensed and vapour phases: the vapour pressure of heavier isotopes is lower, so vapour contains relatively more of the lighter isotopes and when the vapour condenses the precipitation preferentially contains heavier isotopes. The difference from SMOW

3535-466: Is not dense enough to prevent air from escaping; but at a density of about 830 kg/m 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,

3636-849: Is often continuously variable between sites along climatic gradients, such as from hot to cold climates, or high to low precipitation. This variation between sites along environmental gradients reflects adaptive compromises by the species present to balance the need to capture light energy, manage heat gain and loss, while maximising the efficiency of gas exchange, transpiration and photosynthesis . Quantitative analyses of modern vegetation leaf physiognomy and climate responses along environmental gradients have been largely univariate , but multivariate approaches integrate multiple leaf characters and climatic parameters. Temperature has been estimated (to varying degrees of fidelity) using leaf physiognomy for Late Cretaceous and Cenozoic leaf floras, principally using two main approaches: A univariate approach that

3737-580: 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 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): 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

3838-497: 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 2 O 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

3939-452: Is slightly more likely to condense from vapour into rain or snow crystals. At lower temperatures, the difference is more pronounced. The standard method of recording the O / O ratio is to subtract the ratio in a standard known as standard mean ocean water (SMOW): δ 18 O = ( ( 18 O 16 O ) s

4040-414: 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

4141-482: Is the measured difference in concentration between isotopologues with a mass of 47 u (as compared to 44) in a sample and a hypothetical sample with the same bulk isotopic composition, but a stochastic distribution of heavy isotopes. Lab experiments, quantum mechanical calculations, and natural samples (with known crystallization temperatures) all indicate that Δ 47 is correlated to the inverse square of temperature . Thus Δ 47 measurements provide an estimation of

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4242-555: Is well established that the precise Sr/Ca ratio in the coral skeleton shows an inverse correlation with the seawater temperature during its biomineralization. Distributions of organic molecules in marine sediments reflect temperature. The characteristic leaf sizes, shapes and prevalence of features such as drip tips (‘leaf or foliar physiognomy’) differs between tropical rainforests (many species with large leaves with smooth edges and drip tips) and temperate deciduous forests (smaller leaf size classes common, toothed edges common), and

4343-409: Is ~ 50 m above the bottom of the ice sheet. The last 50 m of ice was left in place to provide a barrier between the borehole and the pristine aqueous basal environment. The ice at the bottom of the hole fell as snow 67,748 years ago. In the 2012-2013 season additional core was collected in zones of high scientific interest. This was done by drilling through the side of the main bore hole and coring along

4444-450: The N / 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 for these "fugitive gases" within minutes of the core being retrieved to obtain accurate data. Chlorofluorocarbons (CFCs), which contribute to

4545-424: 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. Greenland cores, during times of climatic transition, may show excess CO 2 in air bubbles when analysed, due to CO 2 production by acidic and alkaline impurities. Summer snow in Greenland contains some sea salt, blown from

4646-486: The ability of the technique to precisely assign an age to core depths. 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 40,000 years ago, can be identified in cores; away from that point, measurements of gases such as CH 4 ( methane ) can be used to connect

4747-477: 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 ice 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

4848-471: The atmosphere and Antarctic climate , and to do this with the highest possible time resolution. The project is best known for producing records of atmospheric carbon dioxide and methane with high time resolution and dating accuracy. This was accomplished by collecting and analyzing an ice core from a site named WAIS Divide (79.468° S 112.086° W) that is on the WAIS ice flow divide. This can cause confusion because

4949-412: 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. The need for a string of drillpipe that extends from

5050-421: 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 . In

5151-426: The chronology of a Greenland core (for example) with an Antarctic core. In cases where volcanic tephra is interspersed with ice, it can be dated using argon/argon dating and hence provide fixed points for dating the ice. Uranium decay has also been used to date ice cores. Another approach is to use Bayesian probability techniques to find the optimal combination of multiple independent records. This approach

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5252-461: The closely related phenomenon of equilibrium isotope fractionation , this effect arises from differences in zero point energy among isotopologues . Carbonate minerals like calcite contain CO 3 groups that can be converted to CO 2 gas by reaction with concentrated phosphoric acid. The CO 2 gas is analyzed with a mass spectrometer, to determine the abundances of isotopologues. The parameter Δ 47

5353-404: 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

5454-438: 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. There are several locations around

5555-596: 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. With some variation between projects,

5656-401: 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

5757-440: 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

5858-428: 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. Cores are most brittle at

5959-610: 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 sulfuric acid ( HNO 3 and H 2 SO 4 ) in precipitation can be shown to correlate with increasing fuel combustion over time. Methanesulfonate (MSA) ( CH 3 SO 3 )

6060-472: The estimate for LMA is expressed as the binomial sampling error: where c is the slope from the LMA regression equation, P margin as used in ( 1 ), and r is the number of species scored for leaf margin type for the individual fossil leaf flora. LMA calibrations have been derived for major world regions, including North America, Europe, South America, and Australia. Riparian and wetland environments have

6161-400: 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. These fractionation processes in trapped air, determined by the measurement of

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6262-405: 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

6363-400: 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

6464-422: 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 temperatures , and 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

6565-719: 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

6666-409: The ice by cosmic rays, and the amount of correction depends strongly on the location of the ice core. Corrections for C produced by nuclear testing have much less impact on the results. Carbon in particulates can also be dated by separating and testing the water-insoluble organic components of dust. The very small quantities typically found require at least 300 g of ice to be used, limiting

6767-452: 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

6868-496: 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

6969-403: The ice, and one for the trapped gases. 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. At locations with very low snowfall, such as Vostok , the uncertainty in the difference between ages of ice and gas can be over 1,000 years. The density and size of

7070-494: 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 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 (1957–1958). Depths of over 400 m were reached, a record which

7171-580: 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 ( Niels Bohr Institute , University of Copenhagen ) in Denmark , and includes representatives from 12 countries on its steering committee. Over

7272-416: 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

7373-478: The name "WAIS Divide" can refer to the WAIS Divide ice flow divide or the WAIS Divide drill site that is at a single spot on the ice flow divide. Site selection started in 2000 with the goal of finding the best place meeting the following requirements. Site preparation started in the 2005/2006 season with the construction of the skiway, and a steel arch shelter for drilling and core processing. Camp logistics

7474-412: The onset of an interglacial , followed by slower cooling. Other isotopic ratios have been studied, for example, the ratio between C and 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. It

7575-409: The oxygen in a core can be used to model the temperature history of the ice sheet. Oxygen has three stable isotopes, O , O and O . The ratio between O and O indicates the temperature when the snow fell. Because O is lighter than O , water containing O is slightly more likely to turn into vapour, and water containing O

7676-470: 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

7777-422: 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 carbonate particles from wind-blown dust ( loess ). 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 C produced directly in

7878-408: 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

7979-520: The precipitation forms above the inversion layer , we are left with a linear relation: which is empirically calibrated from measurements of temperature and δ O as a = 0.67‰/°C for Greenland and 0.76‰/°C for East Antarctica . The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations (Jouzel and Merlivat, 1984). More recently, borehole thermometry has shown that for glacial-interglacial variations,

8080-437: 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

8181-422: 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

8282-440: 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

8383-427: The same way as δ O . There is a linear relationship between δ O and δ D: δ D = 8 × δ 18 O + d , {\displaystyle \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 ratios in

8484-412: 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

8585-491: The summer sunlight 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

8686-501: 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. When a known volcanic event, such as the eruption of Laki in Iceland in 1783, can be identified in

8787-403: 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

8888-601: The surface, so another approach is to break them into 1 m lengths in the hole. 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

8989-403: 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

9090-543: The temperature at which a carbonate formed. C- O paleothermometry does not require prior knowledge of the concentration of O in the water (which the δ O method does). This allows the C- O paleothermometer to be applied to some samples, including freshwater carbonates and very old rocks, with less ambiguity than other isotope-based methods. The method is presently limited by the very low concentration of isotopologues of mass 47 or higher in CO 2 produced from natural carbonates, and by

9191-486: The temperatures deduced from the δ O 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 temperature record. Hydrogen ratios can also be used to calculate a temperature history. Deuterium ( H , or D) is heavier than hydrogen ( H ) and makes water more likely to condense and less likely to evaporate. A δ D ratio can be defined in

9292-420: The tensile strength of the ice, resulting in cracks and spall . At greater depths, the air disappears into clathrates and the ice becomes stable again. At the WAIS Divide site, the brittle ice zone was from 520 m to 1340 m depth. 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

9393-622: 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

9494-409: 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 1932 and they have changed little since. An auger

9595-409: 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

9696-407: The ‰ sign indicates parts per thousand . A sample with the same O / O ratio as SMOW has a δ O of 0‰; a sample that is depleted in O has a negative δ O . Combining the δ O measurements of an ice core sample with the borehole temperature at the depth it came from provides additional information, in some cases leading to significant corrections to

9797-486: Was dated using two methods. The top was dated by counting annual layers to an age of 31,200 years ago. The bottom was dated using stratigraphic methods to an age of 67,748, years at the bottom of the core. The dating was more accurate than other Antarctic ice cores and enables a better understanding of the causes of previous climate changes. This information helps improve predictions of future climate changes. The project developed information on: Paleoceanography will publish

9898-696: Was developed in 2010 and has since been turned into a software tool, DatIce. The boundary between the Pleistocene and the Holocene , about 11,700 years ago, is now formally defined with reference to data on Greenland ice cores. 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

9999-722: 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,

10100-746: Was provided by Raytheon Polar Services Company and the air transport by the New York Air National Guard using LC-130 aircraft. The deep coring started in the 2006/2007 season using the Deep Ice Sheet Coring (DISC) drill developed and operated by the Ice Drilling Design and Operations group at the University of Wisconsin, Madison. The coring was stopped in December 2011 at a depth of 3,405 m, which

10201-420: Was understood in the 1960s that analyzing the air trapped in ice cores would provide useful information on the paleoatmosphere , 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 was 30% less at the last glacial maximum than just before the start of the industrial age. Further research has demonstrated

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