Editing Ice core (section)

=== Brittle ice ===
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| image1            = CSIRO ScienceImage 518 Air Bubbles Trapped in Ice.jpg
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| caption1          = Bubbles in an Antarctic ice sample. Illuminated with polarised light
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| caption2          = Sliver of Antarctic ice showing trapped bubbles. Images from [[CSIRO]].
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Over a depth range known as the [[brittle ice]] zone, bubbles of air are trapped in the ice under great pressure.  When the core is brought to the surface, the bubbles can exert a stress that exceeds the tensile strength of the ice, resulting in cracks and [[spall]].<ref name="Souney-2014-2" /> At greater depths, the air disappears into clathrates and the ice becomes stable again.<ref name="Souney-2014-2" /><ref>{{Cite journal|last1=Uchida|first1=Tsutomu|last2=Duval|first2=P.|last3=Lipenkov|first3=V.Ya.|last4=Hondoh|first4=T.|last5=Mae|first5=S.|last6=Shoji|first6=H.|year=1994|title=Brittle zone and air-hydrate formation in polar ice sheets|url=http://id.nii.ac.jp/1291/00002234/|journal=Memoirs of National Institute of Polar Research|volume=49 |issue=49|page=302}}.</ref> At the WAIS Divide site, the brittle ice zone was from 520&nbsp;m to 1340&nbsp;m depth.<ref name="Souney-2014-2">{{harvnb|Souney et al.|2014}}, pp. 20–21.</ref>

The brittle ice zone typically returns poorer quality samples than for the rest of the core.  Some steps can be taken to alleviate the problem.  Liners can be placed inside the drill barrel to enclose the core before it is brought to the surface, but this makes it difficult to clean off the drilling fluid.  In mineral drilling, special machinery can bring core samples to the surface at bottom-hole pressure, but this is too expensive for the inaccessible locations of most drilling sites.  Keeping the processing facilities at very low temperatures limits thermal shocks.  Cores are most brittle at the surface, so another approach is to break them into 1&nbsp;m lengths in the hole.  Extruding the core from the drill barrel into a net helps keep it together if it shatters. Brittle cores are also often allowed to rest in storage at the drill site for some time, up to a full year between drilling seasons, to let the ice gradually relax.<ref name="Souney-2014-2" /><ref>{{harvnb|Talalay|2016}}, pp. 265–266.</ref>