Lichenometry dating curve
Some studies have used these dated moraine records to make climatic inferences spanning the last three centuries e. The measured growth rates of R. Table des illustrations Titre Tab. Gravestones have been regarded to be the most reliable dated surfaces, if it is assumed that their surfaces have never been cleaned, because they are dated.
Time scale is therefore crucial. The fallacy of the assumption that the largest lichens on a surface have been there since the surface was exposed was pointed out by Joachimsen and Osborn et al. Through examples, the validity of lichenometry as an absolute dating method is questioned. And that does not agree with the acquisition of quick results during short and scarce, far and costly field seasons. This minimizes the risk of under- or overestimating the age of analysed surfaces due to the misinterpretation of a lichen-dating curve constructed in a different environment.
This point is also made by Osborn et al. Atuagkat, Rhodos and Danish Polar Center.
This was the original lichenometric from which others then developed or used as reference. Both approaches were retaining only a limited amount of lichens measured on each surface, proposing an average result that limits the weight of the potential anomalous largest lichen. As a result, some of the lichen-bearing material has been much reworked, and several lichen populations may have been mixed. In Iceland, most lichenometric studies have been performed in proglacial environments to date moraine sequences e.
This method also relies on a large number of measurements, and integrates confidence intervals for the computed ages. That is the case for debris flow no. The reason for the disparity is probably that lichens measured on boulders constituting the moraine are not the first generation ones. Furthermore, gravestones are small and do not compare well in size to the geomorphic features being dated.
More isolated investigations regarding debris-flow and snow-avalanche frequency have been carried out in Northwestern and North Iceland Decaulne et al. Very few authors used keys to help lichen identification e. Lichenometric dating of debris-flow occurrence in the given path.
Lichenometry in Iceland, results and application
However the data are processed, if variations in size-age relationship over an area are not accounted for, the results will not be robust. Most papers dealing with lichenometry in Iceland do not describe the lichen s that had been used e. Rhizocarpon geographicum, whilst others identify only to section level viz.
Relying on few data sets only, the method is considered weak from a statistical point of view McKinsey et al. Most authors having worked in Iceland were aware of the rough Icelandic nature, and aimed at finding a proxy that could date landforms they observed, lacking all source of accurate dating.
Both are crustose thalli, yellow-green in color, and can hardly be distinguished in the field. However, recent history is rich in eruptions that severely affected the Icelandic environment e. This point is made especially important when the different developmental stages of lichen growth and the different growth rates are properly considered i. Columbia University Press, New York, p.
Due to the overlap with more accurate historical data during these years, lichenometry loses interest for the most recent years. The largest thallus in a unit area is measured. 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.
Progress in Physical Geography. The University of Wisconsin Press, Wisconsin, p.
For instance in Tröllaskagi, North Iceland, lichenometric studies date frontal moraines deposits e. Kirkbride and Dugmore, and in the north e. However, the lichenometrical analysis proposes that the moraine dates from Evans et al. Gletscher- und landschaftsgeschichtliche Untersuchungen in Nordisland. It is worth considering whether lichenometry is best considered a relative or absolute dating tool.
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