Editing Lichenometry

{{Short description|Geomorphic method of geochronologic aging}}
[[File:Lichen forming a Hidden Mickey.jpg|thumb|right|Lichen grown in a [[Mickey Mouse]] shape]]
[[File:Lichen Mickey 13 years later.jpg|thumb|right|13 years later]]
[[File:Lichen_17_years_later.jpg|thumb|right|17 years later]]
In [[archaeology]], [[palaeontology]], and [[geomorphology]], '''lichenometry''' is a geomorphic method of [[geochronologic]] dating that uses [[lichen]] growth to determine the [[Surface exposure dating|age of exposed rock]], based on a presumed specific rate of increase in radial size over time.<ref>Holocene climatic and glacial history of the central Sierra Nevada, California, R.R. Curry, pp. 1–47, 1969, Geological Society of America Special Paper, 123, S.A. Schumm and W.C. Bradley, eds., 1969</ref><ref name=LSN>Lichens in relation to management issues in the Sierra Nevada national parks, McCune, B., J. Grenon, and E. Martin, L. Mutch, Sierra Nevada Network, Cooperative agreement CA9088A0008. Oregon State University, Corvallis, Oregon, and Sequoia and Kings Canyon National Parks, Three Rivers, California, [http://openjournals.wsu.edu/index.php/pnwfungi/article/view/1031]</ref>{{rp|9}} Measuring the diameter of the largest lichen of a species on a rock surface can therefore be used to determine the length of time the rock has been exposed. Lichen can be preserved on old rock faces for up to 10,000 years,<ref name=Benedict>{{cite journal|journal=[[American Antiquity]]|volume=74|issue=1|date=January 2009|pages=143–172|title=A Review of Lichenometric Dating and Its Applications to Archaeology |last=Benedict |first=James B.|doi=10.1017/S0002731600047545|s2cid=83108496 }}</ref> providing the maximum age limit of the technique, but it is most accurate (within 10% error) when applied to surfaces that have been exposed for less than 1,000 years.<ref>Sowers, J.M., Noller, J.S., and Lettis, W.R., eds., 1997, Dating and Earthquakes: Review of Quaternary Geochronology and its Application to Paleoseismology. U.S. Nuclear Regulatory Commission, NUREG/CR 5562.</ref> (The practical limit of the technique might be 4,000 to 5,000 years.<ref name=Benedict/>)

Lichenometry is especially useful for dating surfaces less than 500 years old, as radiocarbon dating techniques are less accurate over this period.<ref name="innes">{{cite journal |first=John L. |last=Innes |title=Lichenometry | journal=Progress in Physical Geography | volume=9 | issue=187 | url=http://ppg.sagepub.com/cgi/reprint/9/2/187 }}</ref> The lichens most commonly used for lichenometry are those of the genera ''[[Rhizocarpon]]'' (such as the species ''[[Rhizocarpon geographicum]]'') and ''[[Xanthoria]]''. The measured growth rates of ''R. geographicum'' tends to fall within the range of 0.9–0.3&nbsp;millimeter per year, depending on several factors, including the size of the lichen patch.<ref>{{Cite journal|last=Armstrong|first=R. A.|title=Growth Curve of the Lichen ''Rhizocarpon Geographicum''|date=August 1983|journal=New Phytologist|language=en|volume=94|issue=4|pages=619–622|doi=10.1111/j.1469-8137.1983.tb04870.x|issn=0028-646X|doi-access=free}}</ref>

The technique was first employed by [[Knut Fægri]] in 1933, though the first exclusively lichenometric paper was not published until 1950, by Austrian [[Roland Beschel]],<ref>{{cite journal |last=Beschel |first=R. |author-link=Roland Beschel |year=1950 |title=Flechten als Altersmasstab rezenter Moränen |journal=Zeitschrift für Gletscherkunde und Glazialgeologie |volume=1 |pages=152–161 }}</ref> in a paper concerning the European [[Alps]].<ref>{{cite web|url=http://mc2.vicnet.net.au/home/date/web/lich.html |title=Lichenometry |access-date=2009-02-25 |url-status=dead |archive-url=https://web.archive.org/web/20080823052446/http://mc2.vicnet.net.au/home/date/web/lich.html |archive-date=2008-08-23 }}</ref>

==Scope and limitations==
[[Image:Rhizocarpon geographicum on quartz.jpg|thumb|left|The [[map lichen]] (''Rhizocarpon geographicum''), the lichen most used in lichenometry]]
Lichenometry can provide dates for [[glacial]] deposits in [[tundra]] environments, lake level changes, glacial [[moraine]]s, [[trim lines]], palaeofloods,<ref>William Allen (2013) https://www.scribd.com/doc/156523915/A-Lichenometric-Study-of-Palaeofloods-in-the-Brecon-Beacons</ref> rockfalls, seismic events associated with the rockfalls,<ref name=LSN/> talus ([[scree]]) stabilization and former extent of [[permafrost]] or very persistent snow cover.<ref>{{cite journal |first1=Olga |last1=Solomina |first2=Mikhail |last2=Ivanov |first3=Tom |last3=Bradwell |title=Lichenometric studies on moraines in the Polar Urals |journal=Geografiska Annaler: Series A, Physical Geography |volume=92 |number=1 |pages=81–99|date=2010 |jstor=40661039}}</ref> It has also been explored as a tool in assessing the speed of [[Retreat of glaciers since 1850|glacier retreat]] due to [[climate change]].<ref name="lichens-global-warming">{{cite journal|author=Richard Armstrong |title=Lichens, Lichenometry, and Global Warming |journal=Microbiologist |date=September 2004 |pages=32–35 |url=http://www.sfam.org.uk/pdf/features/lichens.pdf |url-status=dead |archive-url=https://web.archive.org/web/20090225044120/http://www.sfam.org.uk/pdf/features/lichens.pdf |archive-date=2009-02-25 }}</ref>

Among the potential problems of the technique are the difficulty of correctly identifying the species, the delay between exposure and colonization, the varying growth rates from region to region, growth rates not always being constant over time and depend on substrate texture and composition, the climate, and determining the lichen that is the largest.<ref name="innes" />

==Methods==
[[image:Xanthoria elegans 97571.jpg|thumb|right|''[[Xanthoria elegans]]'' was one of the first lichens used for lichenometry.]]
Several methods exist for dating surfaces with help of lichenometry; the most simple relies on a single largest lichen though other methods use more. There are also differences in the way the lichen is measured; some scientists suggest that the largest [[diameter]] should be measured, but others prefer the diameter of the largest inscribed circle. A problem in dating lichens is the fact that several [[thalli]] can fuse together, making several minor lichens appears as a larger one of older age.<ref>[http://www.blackwellpublishing.com:443/Microbiology/pdfs/lichens.pdf Lichen, Lichenometry and Global Warming] {{webarchive|url=https://web.archive.org/web/20110708012620/http://www.blackwellpublishing.com:443/Microbiology/pdfs/lichens.pdf |date=2011-07-08 }}</ref> The lichenometrist Tom Bradwell has listed the following five method families as the principal ones into which most other methods can be classified:
*'''Largest lichen (LL):''' When the single largest lichen of a species is used it means that the lichen that is oldest or grows in most favorable conditions is used to date the minimum age of the exposed surface. This was the original lichenometric from which others then developed or used as reference. Despite relying upon a single lichen this technique is praised for its simplicity and allows obtaining an image of the age of rock exposure while still in the field.<ref name=GA>{{cite journal |last=Bradwell |first=Tom |year=2009 |title=Lichenometric Dating: A commentary in the light of some recent statistical studies |journal=[[Geografiska Annaler]] }}</ref>
*'''Largest five lichens (5LL):''' This method is a development of the '''LL''' and was developed in the 1970s to avoid reliance on one single potentially anomalous lichen. It has been proved that neither accuracy nor precision improves significantly by having more than five lichens.<ref name=GA/>
*'''Fixed-area largest lichen (FALL):''' This technique was initially specially designed for dating rockfalls and [[talus cone]]s with no uniform age of deposition. The largest [[thallus]] in a unit area is measured. The sample areas are usually boulders with surfaces of about 1&nbsp;m<sup>2</sup>.<ref name=GA/>
*'''Size-frequency approach (SF):''' The analysis of size and frequency of lichens was initially done in order to study lichen populations and preexisting thalli growing on surfaces, but has since been used as an effective absolute and relative dating method.<ref name=GA/>
*'''Lichen cover approach (LC):''' This method works with the premise of that the area covered by a single species will increase over time, and by measuring the total area percentage covered by a certain lichen species the age of exposure can be inferred.<ref name=GA/>

==References==
{{Reflist}}

==Further reading==
* {{cite journal |first=Vanessa |last=Winchester |title=Lichenometric dating and its limitations and problems: a guide for practitioners |journal=Land |year=2023 |volume=12 |issue=3 |page=611 |doi=10.3390/land12030611 |doi-access=free }}


{{Chronology}}
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[[Category:Incremental dating]]
[[Category:Lichenology]]