Lichenometry

The map lichen (Rhizocarpon geographicum), the lichen most used in lichenometry.

In archaeology, palaeontology, and geomorphology, lichenometry is a geomorphic method of geochronologic dating that uses lichen growth to determine the age of exposed rock, based on a presumed specific rate of increase in radial size over time.[1][2]: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[citation needed] 10,000 years,[3] providing the maximum age limit of the technique, though it is most accurate (within 10% error) when applied to surfaces that have been exposed for less than 1,000 years.[4] (The practical limit of the technique might be 4,000 to 5,000 years.[3]) Lichenometry is especially useful for dating surfaces less than 500 years old, as radiocarbon dating techniques are less accurate over this period.[5] The lichens most commonly used for lichenometry are those of the genera Rhizocarpon (e.g. the species Rhizocarpon geographicum) and Xanthoria. The measured growth rates of R. geographicum tends to fall within the range of 0.9 - 0.3 millimeter per year, depending on several factors, including the size of the lichen patch. [6]

It was first employed by Knut Fægri in 1933, though the first exclusively lichenometric paper was not published until 1950, by Austrian Roland Beschel[7] (1928-1971),[8] in a paper concerning the European Alps.[9]

Lichenometry can provide dates for glacial deposits in tundra environments, lake level changes, glacial moraines, trim lines, palaeofloods,[10] rockfalls, seismic events associated with the rockfalls,[2] talus (scree) stabilization and former extent of permafrost or very persistent snow cover.[11] It has also been explored as a tool in assessing the speed of glacier retreat due to climate change.[12]

Among the potential problems of the technique are the difficulty of correctly identifying the species, delay between exposure and colonization, varying growth rates from region to region as well as the fact that growth rates are not always constant over time, dependence of the rate of growth upon substrate texture and composition, climate, and determining which lichen is the largest.[5]

Methods[]

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 while other methods use more. There are also differences in the way the lichen is measured; while some suggest that the largest diameter should be measured, other scientists 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.[13] Lichenometrist Tom Bradwell has listed the following five method families as the principal ones into which most other methods can be classified:

References[]

  1. ^ 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
  2. ^ a b 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, [1]
  3. ^ a b Benedict, James B. (January 2009). "A Review of Lichenometric Dating and Its Applications to Archaeology". American Antiquity. 74 (1): 143–172. doi:10.1017/S0002731600047545.
  4. ^ 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.
  5. ^ a b John L. Innes. "Lichenometry". Progress in Physical Geography. 9 (187).
  6. ^ ARMSTRONG, R. A. (August 1983). "GROWTH CURVE OF THE LICHEN RHIZOCARPON GEOGRAPHICUM". New Phytologist. 94 (4): 619–622. doi:10.1111/j.1469-8137.1983.tb04870.x. ISSN 0028-646X.
  7. ^ Beschel R. (1950). "Flechten als Altersmasstab rezenter Moränen". Zeitschrift für Gletscherkunde und Glazialgeologie 1: 152–161.
  8. ^ http://pubs.aina.ucalgary.ca/arctic/Arctic24-4-316.pdf
  9. ^ "Archived copy". Archived from the original on 2008-08-23. Retrieved 2009-02-25.
  10. ^ William Allen (2013) https://www.scribd.com/doc/156523915/A-Lichenometric-Study-of-Palaeofloods-in-the-Brecon-Beacons
  11. ^ Olga Solomina, Mikhail Ivanov, Tom Bradwell. Lichenometric studies on moraines in the Polar Urals.
  12. ^ Richard Armstrong (September 2004). "Lichens, Lichenometry, and Global Warming" (PDF). Microbiologist: 32–35. Archived from the original (PDF) on 2009-02-25.
  13. ^ Lichen, Lichenometry and Global Warming Archived 2011-07-08 at the Wayback Machine.
  14. ^ a b c d e Bradwell, Tom 2009. Lichenometric Dating: A commentary in the light of some recent statistical studies. Geografiska Annaler.