Editing Ice core (section)

===  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>