Lichen grown in aMickey Mouse shape13 years later17 years later
Inarchaeology,palaeontology, andgeomorphology,lichenometry is a geomorphic method ofgeochronologic dating that useslichen growth to determine theage 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 10,000 years,[3] 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.[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 generaRhizocarpon (such as the speciesRhizocarpon geographicum) andXanthoria. The measured growth rates ofR. 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]
The technique was first employed byKnut Fægri in 1933, though the first exclusively lichenometric paper was not published until 1950, by AustrianRoland Beschel,[7] in a paper concerning the EuropeanAlps.[8]
Themap lichen (Rhizocarpon geographicum), the lichen most used in lichenometry
Lichenometry can provide dates forglacial deposits intundra environments, lake level changes, glacialmoraines,trim lines, palaeofloods,[9] rockfalls, seismic events associated with the rockfalls,[2] talus (scree) stabilization and former extent ofpermafrost or very persistent snow cover.[10] It has also been explored as a tool in assessing the speed ofglacier retreat due toclimate change.[11]
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 uuon substrate texture and composition, the climate, and determining the lichen that is the largest.[5]
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 largestdiameter should be measured, but others prefer the diameter of the largest inscribed circle. A problem in dating lichens is the fact that severalthalli can fuse together, making several minor lichens appears as a larger one of older age.[12] 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.[13]
Largest five lichens (5LL): This method is a development of theLL 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.[13]
Fixed-area largest lichen (FALL): This technique was initially specially designed for dating rockfalls andtalus cones with no uniform age of deposition. The largestthallus in a unit area is measured. The sample areas are usually boulders with surfaces of about 1 m2.[13]
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.[13]
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.[13]
^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
^abLichens 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]
^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.
^abInnes, John L."Lichenometry".Progress in Physical Geography.9 (187).
^Solomina, Olga; Ivanov, Mikhail; Bradwell, Tom (2010). "Lichenometric studies on moraines in the Polar Urals".Geografiska Annaler: Series A, Physical Geography.92 (1):81–99.JSTOR40661039.
