Editing Ice core (section)

== Ice core data ==

=== Dating ===

Many different kinds of analysis are performed on ice cores, including visual layer counting, tests for [[Electrical resistivity and conductivity|electrical conductivity]] and physical properties, and assays for inclusion of gases, particles, [[radionuclide]]s, and various molecular [[Chemical species|species]].  For the results of these tests to be useful in the reconstruction of [[Paleoenvironment|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 parameter]]s.<ref>{{cite book|url=http://ww2.valdosta.edu/~dmthieme/Geomorph/Walker_2005_QuaternaryDatingMethods.pdf|title=Quaternary Dating Methods|last=Walker|first=Mike|publisher=John Wiley & Sons|year=2005|isbn=978-0-470-86927-7|location=Chichester|page=150|url-status=dead|archive-url=https://web.archive.org/web/20140714195126/http://ww2.valdosta.edu/~dmthieme/Geomorph/Walker_2005_QuaternaryDatingMethods.pdf|archive-date=14 July 2014}}</ref>

A difficulty in ice core dating is that gases can [[Diffusion|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.<ref>{{cite journal|last1=Bazin|first1=L.|last2=Landais|first2=A.|last3=Lemieux-Dudon|first3=B.|last4=Toyé Mahamadou Kele|first4=H.|last5=Veres|first5=D.|last6=Parrenin|first6=F.|last7=Martinerie|first7=P.|last8=Ritz|first8=C.|last9=Capron|first9=E.|last10=Lipenkov|first10=V.|last11=Loutre|first11=M.-F.|last12=Raynaud|first12=D.|last13=Vinther|first13=B.|last14=Svensson|first14=A.|last15=Rasmussen|first15=S. O.|last16=Severi|first16=M.|last17=Blunier|first17=T.|last18=Leuenberger|first18=M.|last19=Fischer|first19=H.|last20=Masson-Delmotte|first20=V.|last21=Chappellaz|first21=J.|last22=Wolff|first22=E.|title=An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology (AICC2012): 120–800 ka|journal=Climate of the Past|date=1 August 2013|volume=9|issue=4|pages=1715–1731|doi=10.5194/cp-9-1715-2013|bibcode=2013CliPa...9.1715B|doi-access=free|hdl=2158/969431|hdl-access=free}}</ref><ref>{{harvnb|Jouzel|2013}}, pp. 2530–2531.</ref> At locations with very low snowfall, such as [[Vostok Station|Vostok]], the uncertainty in the difference between ages of ice and gas can be over 1,000 years.<ref>{{harvnb|Jouzel|2013}}, p. 2535.</ref>

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.<ref name="Alley-2010">{{harvnb|Alley|2010}}, p. 1098.</ref>

[[Radiocarbon dating]] can be used on the carbon in trapped {{chem|C|O|2}}.  In the polar ice sheets there is about 15–20&nbsp;μg of carbon in the form of {{chem|C|O|2}} in each kilogram of ice, and there may also be [[carbonate]] particles from wind-blown dust ([[loess]]).  The {{chem|C|O|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 [[carbon-14|{{Chem|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 {{Chem|14|C}} produced by nuclear testing have much less impact on the results.<ref>{{Cite journal|last1=Wilson|first1=A.T.|last2=Donahue|first2=D.J.|year=1992|title=AMS radiocarbon dating of ice: validity of the technique and the problem of cosmogenic ''in-situ'' production in polar ice cores|url=https://journals.uair.arizona.edu/index.php/radiocarbon/article/viewFile/1487/1491|journal=Radiocarbon|volume=34|issue=3|pages=431–435|doi=10.1017/S0033822200063657|bibcode=1992Radcb..34..431W |doi-access=free}}</ref> Carbon in [[particulates]] can also be dated by separating and testing the water-insoluble [[Organic chemistry|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.<ref>{{cite journal|last1=Uglietti|first1=Chiara|last2=Zapf|first2=Alexander|last3=Jenk|first3=Theo Manuel|last4=Sigl|first4=Michael|last5=Szidat|first5=Sönke|last6=Salazar|first6=Gary|last7=Schwikowski|first7=Margit|title=Radiocarbon dating of glacier ice: overview, optimisation, validation and potential|journal=The Cryosphere|date=21 December 2016|volume=10|issue=6|pages=3091–3105|doi=10.5194/tc-10-3091-2016|bibcode=2016TCry...10.3091U|doi-access=free}}</ref>

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;<ref>{{Cite news|url=https://phys.org/news/2012-10-extremely-reversal-geomagnetic-field-climate.html|title=An extremely brief reversal of the geomagnetic field, climate variability and a super volcano |work=Phys.org |date= 16 October 2012 |publisher=ScienceX network |access-date=29 May 2017}}</ref><ref>{{harvnb|Blunier et al.|2007}}, p. 325.</ref> away from that point, measurements of gases such as {{chem|C|H|4}} ([[methane]]) can be used to connect the chronology of a Greenland core (for example) with an Antarctic core.<ref>{{harvnb|Landais|Dreyfus|Capron|Pol|2012}}, pp. 191–192.</ref><ref>{{harvnb|Blunier et al.|2007}}, pp. 325–327.</ref> In cases where volcanic [[tephra]] is interspersed with ice, it can be dated using [[Argon–argon dating|argon/argon dating]] and hence provide fixed points for dating the ice.<ref name="Landais-2012">{{harvnb|Landais|Dreyfus|Capron|Pol|2012}}, p. 192.</ref><ref>{{Cite encyclopedia|encyclopedia=Encyclopedia of Quaternary Science|publisher=Elsevier|year=2013|editor-last=Elias|editor-first=Scott|location=Amsterdam|title=Volcanic Tephra Layers|editor-last2=Mock|editor-first2=Cary|isbn=9780444536426}}</ref> [[Uranium series dating|Uranium decay]] has also been used to date ice cores.<ref name="Landais-2012" /><ref>{{Cite journal|last=Aciego|first=S.|display-authors=et al.|date=15 April 2010|title=Toward a radiometric ice clock: U-series of the Dome C ice core|url=http://www.earth-prints.org/bitstream/2122/7786/1/mmc1.pdf|journal=TALDICE-EPICA Science Meeting|pages=1–2}}</ref> 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.<ref>{{harvnb|Lowe |Walker|2014}}, p. 315.</ref><ref>{{cite conference|last1=Toyé Mahamadou Kele|first1=H.|display-authors=et al.|date=22 April 2012|title=Toward unified ice core chronologies with the DatIce tool|url=http://datice.gforge.inria.fr/pdf/EGU2012_BLAYO_LEMIEUX_TOYE.pdf|conference=EGU General Assembly 2012|location=Vienna, Austria|access-date=5 September 2017|archive-date=5 September 2017|archive-url=https://web.archive.org/web/20170905233004/http://datice.gforge.inria.fr/pdf/EGU2012_BLAYO_LEMIEUX_TOYE.pdf|url-status=dead}}</ref>

The boundary between the [[Quaternary extinction event|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 [[Paleoclimatology|palaeoclimatic]] information that can be correlated with other climate proxies.<ref>{{cite journal|last1=Walker|first1=Mike|last2=Johnsen|first2=Sigfus|last3=Rasmussen|first3=Sune Olander|last4=Popp|first4=Trevor|last5=Steffensen|first5=Jørgen-Peder|last6=Gibbard|first6=Phil|last7=Hoek|first7=Wim|last8=Lowe|first8=John|last9=Andrews|first9=John|last10=Björck|first10=Svante|last11=Cwynar|first11=Les C.|last12=Hughen|first12=Konrad|last13=Kershaw|first13=Peter|last14=Kromer|first14=Bernd|last15=Litt|first15=Thomas|last16=Lowe|first16=David J.|last17=Nakagawa|first17=Takeshi|last18=Newnham|first18=Rewi|last19=Schwander|first19=Jakob|author-link10=Svante Björck|title=Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records|journal=Journal of Quaternary Science|date=January 2009|volume=24|issue=1|pages=3–17|doi=10.1002/jqs.1227|bibcode=2009JQS....24....3W|s2cid=40380068|doi-access=free}}</ref>

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".<ref name="Alley-2010" />

=== Visual analysis ===
[[File:GISP2 1855m ice core layers.png|thumb|19 cm long section of GISP 2 ice core from 1855&nbsp;m showing annual layer structure illuminated from below by a fibre optic source. Section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers.<ref>{{Cite web|url=http://www.ncdc.noaa.gov/paleo/slides/slideset/15/15_281_slide.html|title=Summer and winter core layers|last=Gow|first=Anthony|date=12 October 2001|publisher=NOAA|archive-url=https://web.archive.org/web/20100213035305/http://www.ncdc.noaa.gov/paleo/slides/slideset/15/15_281_slide.html|archive-date=13 February 2010}}</ref>|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.<ref>{{harvnb|Alley|2000}}, pp. 44–48.</ref>

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.<ref>{{harvnb|Alley|2000}}, p. 49.</ref> 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.<ref>{{harvnb|Alley|2000}}, pp. 50–51.</ref>

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.<ref>{{harvnb|Alley|2000}}, p. 56.</ref> The problem is more acute at locations where accumulation is high; low accumulation sites, such as central Antarctica, must be dated by other methods.<ref name="Jouzel-2013-4" /> For example, at Vostok, layer counting is only possible down to an age of 55,000 years.<ref name=Ruddiman>{{Cite journal|last1=Ruddiman|first1=William F.|year=2003|title=A methane-based time scale for Vostok ice|url=http://moraymo.us/wp-content/uploads/2014/04/2003_ruddimanraymo.pdf|journal=Quaternary Science Reviews|volume=22|issue=2|pages=141–155|last2=Raymo|first2=Maureen E.|doi=10.1016/S0277-3791(02)00082-3|bibcode=2003QSRv...22..141R}}</ref>

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.<ref>{{harvnb|Jouzel|2013}}, p. 2533.</ref><ref>{{Cite journal|last=Fisher|first=David|year=2011|title=Recent melt rates of Canadian arctic ice caps are the highest in four millennia|url=http://arctic.eas.ualberta.ca/downloads/Fisheretal2011onlineGPC.pdf|journal=Global and Planetary Climate Change|volume=84–85|pages=1–4|doi=10.1016/j.gloplacha.2011.06.005}}</ref>

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.<ref>{{harvnb|Souney et al.|2014}}, p. 25.</ref>

===  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, {{chem|link=oxygen-16|16|O}}, {{chem|link=oxygen-17|17|O}} and {{chem|link=oxygen-18|18|O}}.<ref>{{Cite web|url=https://environmentalchemistry.com/yogi/periodic/O-pg2.html|last=Barbalace |first=Kenneth L.|title=Periodic Table of Elements: O – Oxygen |publisher=EnvironmentalChemistry.com|language=en-US|access-date=20 May 2017}}</ref> The ratio between {{chem|18|O}} and {{chem|16|O}} indicates the temperature when the snow fell.<ref name="Lowe-2014">{{harvnb|Lowe |Walker|2014}}, pp. 165–170.</ref> Because {{chem|16|O}} is lighter than {{chem|18|O}}, water containing {{chem|16|O}} is slightly more likely to turn into vapour, and water containing {{chem|18|O}} 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 {{chem|18|O}}/{{chem|16|O}} ratio is to subtract the ratio in a standard known as [[Standard Mean Ocean Water|standard mean ocean water]] (SMOW):<ref name="Lowe-2014" />

<math>\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},</math>

where the ‰ sign indicates [[parts per thousand]].<ref name="Lowe-2014" /> A sample with the same {{chem|18|O}}/{{chem|16|O}} ratio as SMOW has a {{delta|18|O}} of 0‰; a sample that is depleted in {{chem|18|O}} has a negative {{delta|18|O}}.<ref name="Lowe-2014" /> Combining the {{delta|18|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 the temperatures deduced from the {{delta|18|O}} data.<ref>{{harvnb|Alley|2000}}, pp. 65–70.</ref><ref name="Jouzel-2013-5" /> 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.<ref>{{harvnb|Alley|2010}}, p. 1097.</ref>

Hydrogen ratios can also be used to calculate a temperature history. [[Deuterium]] ({{chem|2|H}}, or D) is heavier than hydrogen ({{chem|1|H}}) and makes water more likely to condense and less likely to evaporate.  A {{delta||D|Link}} ratio can be defined in the same way as {{delta|18|O}}.<ref>{{Cite web|url=http://www.iceandclimate.nbi.ku.dk/research/past_atmos/past_temperature_moisture/isotopes_delta_notation/|title=Isotopes and the delta notation|date=8 September 2009|publisher=Centre for Ice and Climate|language=en|access-date=25 May 2017|archive-date=10 July 2017|archive-url=https://web.archive.org/web/20170710163804/http://www.iceandclimate.nbi.ku.dk/research/past_atmos/past_temperature_moisture/isotopes_delta_notation|url-status=dead}}</ref><ref>{{Cite news|url=https://www.scientificamerican.com/article/how-are-past-temperatures/|first=Robert |last=Mulvaney|title=How are past temperatures determined from an ice core?|date=20 September 2004|publisher=Scientific American|access-date=25 May 2017|language=en}}</ref> There is a linear relationship between {{delta|18|O}} and {{delta|D}}:<ref name="Jouzel-2013-6">{{harvnb|Jouzel|2013}}, pp. 2533–2534.</ref>

<math>\mathrm{\delta D} = 8 \times \mathrm{\delta ^{18} O} + \mathrm{d},</math>

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 [[Jean Jouzel|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.<ref name="Jouzel-2013-6" />

Water isotope records, analyzed in cores from [[Camp Century]] and [[Dye 3]] in Greenland, were instrumental in the discovery of [[Dansgaard–Oeschger event|Dansgaard-Oeschger events]]—rapid warming at the onset of an [[interglacial]], followed by slower cooling.<ref>{{harvnb|Jouzel|2013}}, p. 2531.</ref> Other isotopic ratios have been studied, for example, the ratio between {{chem|13|C}} and {{chem|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 {{chem|C|O|2}} over time.<ref>{{cite journal|last1=Bauska|first1=Thomas K.|last2=Baggenstos|first2=Daniel|last3=Brook|first3=Edward J.|last4=Mix|first4=Alan C.|last5=Marcott|first5=Shaun A.|last6=Petrenko|first6=Vasilii V.|last7=Schaefer|first7=Hinrich|last8=Severinghaus|first8=Jeffrey P.|last9=Lee|first9=James E.|title=Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation|journal=Proceedings of the National Academy of Sciences of the United States of America|date=29 March 2016|volume=113|issue=13|pages=3465–3470|doi=10.1073/pnas.1513868113|pmid=26976561|pmc=4822573|bibcode=2016PNAS..113.3465B|doi-access=free}}</ref>

=== Palaeoatmospheric sampling ===
[[File:Vostok Petit data.svg|thumb|Graph of CO<sub>2</sub> (green), reconstructed temperature (blue) and dust (red) from the [[Vostok Station#Ice core drilling|Vostok ice core]] for the past 420,000 years|alt=Three graphs laid out one above the other; the CO<sub>2</sub> and temperature can be visually seen to be correlated; the dust graph is inversely correlated with the other two|300x300px]]
[[File:Greenland firn CFCs.png|thumb|Ozone-depleting gases in Greenland firn.<ref>{{Cite web|url=http://www.cpc.ncep.noaa.gov/products/assessments/assess_99/fig76.html|title=Climate Prediction Center – Expert Assessments|publisher=National Weather Service Climate Prediction Center|access-date=3 June 2017}}</ref>|alt=Graph showing the relationship between depth below surface, and fraction of surface concentration at the surface, for multiple gases|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 {{chem|C|O|2}} concentration was 30% less at the [[Last Glacial Maximum|last glacial maximum]] than just before the start of the industrial age.  Further research has demonstrated a reliable correlation between {{chem|C|O|2}} levels and the temperature calculated from ice isotope data.<ref name="Jouzel-2013-7">{{harvnb|Jouzel|2013}}, p. 2534.</ref>

Because {{chem|C|H|4}} (methane) is produced in lakes and [[wetland]]s, the amount in the atmosphere is correlated with the strength of [[monsoon]]s, which are in turn correlated with the strength of [[tropics|low-latitude]] summer [[Solar irradiance|insolation]].  Since insolation depends on [[Milankovitch cycles|orbital cycles]], for which a timescale is available from other sources, {{chem|C|H|4}} can be used to determine the relationship between core depth and age.<ref name="Jouzel-2013-4" /><ref name=Ruddiman/>  {{chem|N|2|O}} (nitrous oxide) levels are also correlated with glacial cycles, though at low temperatures the graph differs somewhat from the {{chem|C|O|2}} and {{chem|C|H|4}} graphs.<ref name="Jouzel-2013-7" /><ref>{{Cite journal|last1=Schilt|first1=Adrian|first2=Matthias|last2=Baumgartner|first3=Thomas|last3=Blunierc|first4=Jakob|last4=Schwander|first5=Renato|last5=Spahni|first6=Hubertus|last6=Fischer|first7=Thomas F.|last7=Stocker|year=2009|title=Glacial-interglacial and millennial-scale variations in the atmospheric nitrous oxide concentration during the last 800,000 years|url=https://ic.ucsc.edu/~acr/ocea285/articles/Schiltetal2010.pdf|journal=Quaternary Science Reviews|volume=29|issue=1–2|pages=182–192|doi=10.1016/j.quascirev.2009.03.011|access-date=2 June 2017|archive-url=https://web.archive.org/web/20170808205625/https://ic.ucsc.edu/~acr/ocea285/articles/Schiltetal2010.pdf|archive-date=8 August 2017|url-status=dead}}</ref> Similarly, the ratio between {{chem|N|2}} (nitrogen) and {{chem|O|2}} (oxygen) can be used to date ice cores: as air is gradually trapped by the snow turning to firn and then ice, {{chem|O|2}} is lost more easily than {{chem|N|2}}, and the relative amount of {{chem|O|2}} correlates with the strength of local summer insolation.  This means that the trapped air retains, in the ratio of {{chem|O|2}} to {{chem|N|2}}, a record of the summer insolation, and hence combining this data with orbital cycle data establishes an ice core dating scheme.<ref name="Jouzel-2013-4" /><ref>{{harvnb|Landais|Dreyfus|Capron|Pol|2012}}, p. 191.</ref>

[[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 {{chem|15|N}}/{{chem|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.<ref name="Jouzel-2013-5">{{harvnb|Jouzel|2013}}, p. 2532.</ref> 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.<ref name="Souney-2014-1" /> [[Chlorofluorocarbon]]s (CFCs), which contribute to the [[greenhouse effect]] and also cause [[Ozone depletion|ozone loss]] in the [[stratosphere]],<ref name="Neelin-2010">{{Cite book|title=Climate Change and Climate Modeling|author1-link=J. David Neelin|last=Neelin|first=J. David|publisher=Cambridge University Press|year=2010|isbn=978-0-521-84157-3|location=Cambridge|page=9}}</ref> can be detected in ice cores after about 1950; almost all CFCs in the atmosphere were created by human activity.<ref name="Neelin-2010" /><ref>{{cite journal|last1=Martinerie|first1=P.|last2=Nourtier-Mazauric|first2=E.|last3=Barnola|first3=J.-M.|last4=Sturges|first4=W. T.|last5=Worton|first5=D. R.|last6=Atlas|first6=E.|last7=Gohar|first7=L. K.|last8=Shine|first8=K. P.|last9=Brasseur|first9=G. P.|title=Long-lived halocarbon trends and budgets from atmospheric chemistry modelling constrained with measurements in polar firn|journal=Atmospheric Chemistry and Physics|date=17 June 2009|volume=9|issue=12|pages=3911–3934|doi=10.5194/acp-9-3911-2009|bibcode=2009ACP.....9.3911M|doi-access=free}}</ref>

Greenland cores, during times of climatic transition, may show excess {{CO2}} in air bubbles when analysed, due to {{CO2}} production by acidic and alkaline impurities.<ref>{{Cite journal | doi=10.1034/j.1600-0889.1993.t01-3-00006.x| title=A natural artefact in Greenland ice-core {{CO2}} measurements| year=1993| last1=Delmas| first1=Robert J.| journal=Tellus B| volume=45| issue=4| pages=391–396}}</ref>

{{Clear}}

=== 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 [[electrode]]s 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 [[Laki#1783 eruption|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.<ref>{{harvnb|Alley|2000}}, pp. 51–55.</ref> Material from Laki can be identified in Greenland ice cores, but did not spread as far as Antarctica; the 1815 eruption of [[Mount Tambora|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.<ref name="Legrand-1997-3" /> 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.<ref>{{cite journal|last1=Sigl|first1=M.|last2=Winstrup|first2=M.|last3=McConnell|first3=J. R.|last4=Welten|first4=K. C.|last5=Plunkett|first5=G.|last6=Ludlow|first6=F.|last7=Büntgen|first7=U.|last8=Caffee|first8=M.|last9=Chellman|first9=N.|last10=Dahl-Jensen|first10=D.|last11=Fischer|first11=H.|last12=Kipfstuhl|first12=S.|last13=Kostick|first13=C.|last14=Maselli|first14=O. J.|last15=Mekhaldi|first15=F.|last16=Mulvaney|first16=R.|last17=Muscheler|first17=R.|last18=Pasteris|first18=D. R.|last19=Pilcher|first19=J. R.|last20=Salzer|first20=M.|last21=Schüpbach|first21=S.|last22=Steffensen|first22=J. P.|last23=Vinther|first23=B. M.|last24=Woodruff|first24=T. E.|title=Timing and climate forcing of volcanic eruptions for the past 2,500 years|journal=Nature|date=8 July 2015|volume=523|issue=7562|pages=543–549|doi=10.1038/nature14565|pmid=26153860|bibcode=2015Natur.523..543S|s2cid=4462058|url=https://pure.qub.ac.uk/portal/en/publications/timing-and-climate-forcing-of-volcanic-eruptions-for-the-past-2500-years(04c84f13-a3c3-48e4-81ca-1507cdd4359d).html}}</ref> There are also more ancient reference points, such as the eruption of [[Lake Toba|Toba]] about 72,000 years ago.<ref name="Legrand-1997-3">{{harvnb|Legrand|Mayewski|1997}}, pp. 222, 225.</ref>

Many other elements and molecules have been detected in ice cores.<ref name="Legrand-1997-1" /> 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.<ref>{{harvnb|Legrand|Mayewski|1997}}, pp. 231–232.</ref> 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.<ref>{{Cite journal |last=McConnell |first=Joseph R. |last2=Wilson |first2=Andrew I. |last3=Stohl |first3=Andreas |last4=Arienzo |first4=Monica M. |last5=Chellman |first5=Nathan J. |last6=Eckhardt |first6=Sabine |last7=Thompson |first7=Elisabeth M. |last8=Pollard |first8=A. Mark |last9=Steffensen |first9=Jørgen Peder |date=2018-05-29 |title=Lead pollution recorded in Greenland ice indicates European emissions tracked plagues, wars, and imperial expansion during antiquity |url=https://www.pnas.org/doi/10.1073/pnas.1721818115 |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=22 |pages=5726–5731 |doi=10.1073/pnas.1721818115 |pmc=5984509 |pmid=29760088}}</ref> The presence of nitric and sulfuric acid ({{chem|link=nitric acid|H|N|O|3}} and {{chem|link=Sulfuric acid|H|2|S|O|4}}) in precipitation can be shown to correlate with increasing fuel [[combustion]] over time. [[Mesylate|Methanesulfonate]] (MSA) ({{chem|C|H|3|S|O|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 ({{chem|link=hydrogen peroxide|H|2|O|2}}) and formaldehyde ({{chem|link=formaldehyde|H|C|H|O}}) have been studied, along with organic molecules such as [[carbon black]] that are linked to vegetation emissions and forest fires.<ref name="Legrand-1997-1">{{harvnb|Legrand|Mayewski|1997}}, p. 221.</ref> 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<sup>++</sup> 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.<ref>{{harvnb|Legrand|Mayewski|1997}}, p. 222.</ref> 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.<ref name="Legrand-1997-2">{{harvnb|Legrand|Mayewski|1997}}, p. 225.</ref>

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 {{chem|H|2|O|2}}.  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.<ref>{{harvnb|Legrand|Mayewski|1997}}, pp. 227–228.</ref>
{| class="wikitable"
|+Soluble impurities found in ice cores<ref>{{harvnb|Legrand|Mayewski|1997}}, p. 228.</ref>
!Source
!Via
!Measured in polar ice
|-
|Oceans
|Waves and wind
|Sea salt: {{chem|Na|+}}, {{chem|Cl|-}}, {{chem|Mg|2+}}, {{chem|Ca|2+}}, {{chem|S|O|4|2-}}, {{chem|K|+}}
|-
|Land
|Aridity and wind
|Terrestrial salts: {{chem|Mg|2+}}, {{chem|Ca|2+}}, {{chem|C|O|3|2-}}, {{chem|S|O|4|2-}}, [[aluminosilicate]]s
|-
|Human and biological gas emissions: {{chem|S|O|2}}, {{chem|(|C|H|3|)|2|S}}, {{chem|H|2|S}}, {{chem|C|O|S}}, {{chem|N|O|x}}, {{chem|N|H|3}}, [[hydrocarbon]]s and [[halocarbon]]s 
|Atmospheric chemistry: {{chem|O|3}}, {{chem|H|2|O|2}}, {{chem|O|H}}, {{chem|R|O|2|link=Organic peroxide}}, {{chem|N|O|3}}, 
|{{chem|H|+}}, {{chem|N|H|4|+}}, {{chem|Cl|-}}, {{chem|N|O|3|-}}, {{chem|S|O|4|2-}}, {{chem|C|H|3|S|O|3|-}}, {{chem|F|-}}, {{chem|H|C|O|O|-}}, other organic compounds
|}

=== Radionuclides ===
[[File:Upper Fremont glacier ice cl36.gif|thumb|[[chlorine|<sup>36</sup>Cl]] from 1960s nuclear testing in US glacier ice.|alt=Graph showing abundance of <sup>36</sup>Cl against snow depth, showing a spike at the time of above-ground nuclear testing|300x300px]]

[[Galactic cosmic rays]] produce {{chem|10|Be}} 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 {{chem|10|Be}} in the atmosphere is a [[Proxy (climate)|proxy]] for climate.  [[Accelerator mass spectrometry]] can detect the low levels of {{chem|10|Be}} 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.<ref>{{Cite journal|last=Pedro|first=J.B.|year=2011|title=High-resolution records of the beryllium-10 solar activity proxy in ice from Law Dome, East Antarctica: measurement, reproducibility and principal trends|journal=Climate of the Past|volume=7|issue=3|pages=707–708|doi=10.5194/cp-7-707-2011|bibcode=2011CliPa...7..707P|doi-access=free}}</ref> [[Tritium radioluminescence|Tritium]] ({{chem|3|H}}), created by nuclear weapons testing in the 1950s and 1960s, has been identified in ice cores,<ref>{{cite journal|last1=Wagenhach|first1=D.|last2=Graf|first2=W.|last3=Minikin|first3=A.|last4=Trefzer|first4=U.|last5=Kipfstuhl|first5=J.|last6=Oerter|first6=H.|last7=Blindow|first7=N.|title=Reconnaissance of chemical and isotopic firn properties on top of Berkner Island, Antarctica|journal=Annals of Glaciology|date=20 January 2017|volume=20|pages=307–312|doi=10.3189/172756494794587401|doi-access=free}}</ref> and both [[Chlorine-36|<sup>36</sup>Cl]] and {{Chem|link=Plutonium-239|239|Pu}} have been found in ice cores in Antarctica and Greenland.<ref>{{cite journal|last1=Arienzo|first1=M. M.|last2=McConnell|first2=J. R.|last3=Chellman|first3=N.|last4=Criscitiello|first4=A. S.|last5=Curran|first5=M.|last6=Fritzsche|first6=D.|last7=Kipfstuhl|first7=S.|last8=Mulvaney|first8=R.|last9=Nolan|first9=M.|last10=Opel|first10=T.|last11=Sigl|first11=M.|last12=Steffensen|first12=J.P.|title=A Method for Continuous Pu Determinations in Arctic and Antarctic Ice Cores|journal=Environmental Science & Technology|date=5 July 2016|volume=50|issue=13|pages=7066–7073|doi=10.1021/acs.est.6b01108|pmid=27244483|bibcode=2016EnST...50.7066A|s2cid=206558530 |url=http://nora.nerc.ac.uk/id/eprint/513803/7/Arienzo_et_al_NWT_May2016_V1.pdf}}</ref><ref>Delmas et al. (2004), pp. 494–496.</ref><ref>{{Cite web|url=http://wwwbrr.cr.usgs.gov/projects/SW_corrosion/icecore/futurework.shtml|title=Future Work|date=14 January 2005|publisher=US Geological Survey Central Region Research|archive-url=https://web.archive.org/web/20050913192339/http://wwwbrr.cr.usgs.gov/projects/SW_corrosion/icecore/futurework.shtml|archive-date=13 September 2005}}</ref> Chlorine-36, which has a half-life of 301,000 years, has been used to date cores, as have krypton ({{Chem|link=krypton-85|85|Kr}}, with a half-life of 11 years), lead ({{Chem|link=lead-210|210|Pb}}, 22 years), and silicon ({{Chem|link=silicon-32|32|Si}}, 172 years).<ref name="Legrand-1997-2" />

===  Other inclusions ===
Meteorites and micrometeorites that land on polar ice are sometimes concentrated by local environmental processes.  For example, there are places in Antarctica where winds evaporate surface ice, concentrating the solids that are left behind, including meteorites.  Meltwater ponds can also contain meteorites.  At the [[South Pole Station]], ice in a well is melted to provide a water supply, leaving micrometeorites behind. These have been collected by a robotic "vacuum cleaner" and examined, leading to improved estimates of their flux and mass distribution.<ref>{{harvnb|Alley|2000}}, p. 73.</ref> 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.<ref>{{cite report |last1=Taylor|first1=Susan |last2=Lever |first2=James H. |last3=Harvey |first3=Ralph P. |last4=Govoni |first4=John|date=May 1997 |title=Collecting micrometeorites from the South Pole Water Well|url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a327829.pdf |archive-url=https://web.archive.org/web/20171011080819/http://www.dtic.mil/dtic/tr/fulltext/u2/a327829.pdf |url-status=live |archive-date=11 October 2017 |publisher=Cold Regions Research and Engineering Lab, Hanover, NH |pages=1–2 |docket=97–1 |access-date=14 September 2017 }}</ref> [[Pollen]], an important component of sediment cores, can also be found in ice cores.  It provides information on changes in vegetation.<ref>{{cite journal|last1=Reese|first1=C.A.|last2=Liu|first2=K.B.|last3=Thompson|first3=L.G.|title=An ice-core pollen record showing vegetation response to Late-glacial and Holocene climate changes at Nevado Sajama, Bolivia|journal=Annals of Glaciology|date=26 July 2017|volume=54|issue=63|page=183|doi=10.3189/2013AoG63A375|doi-access=free}}</ref>

===  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 [[Crystal axis|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.<ref>{{cite journal|last1=Okuyama|first1=Junichi|last2=Narita|first2=Hideki|last3=Hondoh|first3=Takeo|last4=Koerner|first4=Roy M.|title=Physical properties of the P96 ice core from Penny Ice Cap, Baffin Island, Canada, and derived climatic records|journal=Journal of Geophysical Research: Solid Earth|pages=6–1–6–2|date=February 2003|volume=108|issue=B2|doi=10.1029/2001JB001707|bibcode=2003JGRB..108.2090O|doi-access=free}}</ref>