Editing Ice core (section)

=== 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>

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