Editing Ice core (section)

=== 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" />