Dating Many different kinds of analysis are performed on ice cores, including visual layer counting, tests for
electrical conductivity and physical properties, and assays for inclusion of gases, particles,
radionuclides, and various molecular
species. For the results of these tests to be useful in the reconstruction of
palaeoenvironments, there has to be a way to determine the relationship between depth and age of the ice. The simplest approach is to count layers of ice that correspond to the original annual layers of snow, but this is not always possible. An alternative is to model the ice accumulation and flow to predict how long it takes a given snowfall to reach a particular depth. Another method is to correlate radionuclides or trace atmospheric gases with other timescales such as periodicities in the earth's
orbital parameters. A difficulty in ice core dating is that gases can
diffuse through firn, so the ice at a given depth may be substantially older than the gases trapped in it. As a result, there are two chronologies for a given ice core: one for 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 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 . In the polar ice sheets there is about 15–20 μg of carbon in the form of in each kilogram of ice, and there may also be
carbonate particles from wind-blown dust (
loess). The 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 carbon-14| produced directly in the ice by cosmic rays, and the amount of correction depends strongly on the location of the ice core. Corrections for 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 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 (
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, 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 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 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".|left|alt=A series of dark and light bands, with arrows identifying the lighter bands Cores show visible layers, which correspond to annual snowfall at the core site. If a pair of pits is dug in fresh snow with a thin wall between them and one of the pits is roofed over, an observer in the roofed pit will see the layers revealed by sunlight shining through. A six-foot pit may show anything from less than a year of snow to several years of snow, depending on the location. Poles left in the snow from year to year show the amount of accumulated snow each year, and this can be used to verify that the visible layer in a snow pit corresponds to a single year's snowfall. In central Greenland a typical year might produce two or three feet of winter snow, plus a few inches of summer snow. When this turns to ice, the two layers will make up no more than a foot of ice. The layers corresponding to the summer snow will contain bigger bubbles than the winter layers, so the alternating layers remain visible, which makes it possible to count down a core and determine the age of each layer. 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 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. 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 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 a core can be used to model the temperature history of the ice sheet. Oxygen has three stable isotopes, , and . The ratio between and indicates the temperature when the snow fell. Because is lighter than , water containing is slightly more likely to turn into vapour, and water containing 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 / ratio is to subtract the ratio in a standard known as
standard mean ocean water (SMOW): Hydrogen ratios can also be used to calculate a temperature history.
Deuterium (, or D) is heavier than hydrogen () and makes water more likely to condense and less likely to evaporate. A ratio can be defined in the same way as . There is a linear relationship between and : \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 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. Other isotopic ratios have been studied, for example, the ratio between and 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 over time.
Palaeoatmospheric sampling for the past 420,000 years|alt=Three graphs laid out one above the other; the CO2 and temperature can be visually seen to be correlated; the dust graph is inversely correlated with the other two|300x300px It 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 concentration was 30% less at the
last glacial maximum than just before the start of the industrial age. Further research has demonstrated a reliable correlation between levels and the temperature calculated from ice isotope data. Because (methane) is produced in lakes and
wetlands, the amount in the atmosphere is correlated with the strength of
monsoons, which are in turn correlated with the strength of
low-latitude summer
insolation. Since insolation depends on
orbital cycles, for which a timescale is available from other sources, can be used to determine the relationship between core depth and age. Similarly, the ratio between (nitrogen) and (oxygen) can be used to date ice cores: as air is gradually trapped by the snow turning to firn and then ice, is lost more easily than , and the relative amount of correlates with the strength of local summer insolation. This means that the trapped air retains, in the ratio of to , a record of the summer insolation, and hence combining this data with orbital cycle data establishes an ice core dating scheme.
Diffusion within the firn layer causes other changes that can be measured. Gravity causes heavier molecules to be enriched at the bottom of a gas column, with the amount of enrichment depending on the difference in mass between the molecules. Colder temperatures cause heavier molecules to be more enriched at the bottom of a column. These
fractionation processes in trapped air, determined by the measurement of the / 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. 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 in air bubbles when analysed, due to production by acidic and alkaline impurities.
Glaciochemistry Summer snow in Greenland contains some sea salt, blown from the surrounding waters; there is less of it in winter, when much of the sea surface is covered by pack ice. Similarly,
hydrogen peroxide appears only in summer snow because its production in the atmosphere requires sunlight. These seasonal changes can be detected because they lead to changes in the
electrical conductivity of the ice. Placing two
electrodes with a high voltage between them on the surface of the ice core gives a measurement of the conductivity at that point. Dragging them down the length of the core, and recording the conductivity at each point, gives a graph that shows an annual periodicity. Such graphs also identify chemical changes caused by non-seasonal events such as forest fires and major volcanic eruptions. 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. 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. Analysis of the elemental composition of ice cores has even been used to determine the activities of ancient societies: the presence of lead in Greenland ice cores, for instance, corresponds to periods of war and resource extraction during the Roman empire. The presence of nitric and sulfuric acid ( and ) in precipitation can be shown to correlate with increasing fuel
combustion over time.
Methanesulfonate (MSA) () 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 () and
formaldehyde () 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, so what is detected in an ice core is not necessarily what was originally deposited. Examples include HCHO and . Another complication is 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.
Radionuclides from 1960s nuclear testing in US glacier ice.|alt=Graph showing abundance of 36Cl against snow depth, showing a spike at the time of above-ground nuclear testing|300x300px
Galactic cosmic rays produce in the atmosphere at a rate that depends on the solar magnetic field. The strength of the field is related to the intensity of
solar radiation, so the level of in the atmosphere is a
proxy for climate.
Accelerator mass spectrometry can detect the low levels of in ice cores, about 10,000 atoms in a gram of ice, and these can be used to provide long-term records of solar activity.
Tritium (), created by nuclear weapons testing in the 1950s and 1960s, has been identified in ice cores, and both
36Cl and have been found in ice cores in Antarctica and Greenland. Chlorine-36, which has a half-life of 301,000 years, has been used to date cores, as have krypton (, with a half-life of 11 years), lead (, 22 years), and silicon (, 172 years). The well is not an ice core, but the age of the ice that was melted is known, so the age of the recovered particles can be determined. The well becomes about 10 m deeper each year, so micrometeorites collected in a given year are about 100 years older than those from the previous year.
Pollen, an important component of sediment cores, can also be found in ice cores. It provides information on changes in vegetation.
Physical properties In addition to the impurities in a core and the isotopic composition of the water, the physical properties of the ice are examined. Features such as crystal size and
axis orientation can reveal the history of ice flow patterns in the ice sheet. The crystal size can also be used to determine dates, though only in shallow cores. == History ==