Editing Ice core (section)

=== 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
|}