Investigation of Dating Techniques

Lichenometry

Lichenometry is a calibrated relative dating method that uses different species of lichen that have known growth rates in order to determine the amount of time a rock has been exposed in a specific location. The diameter of a lichen species is measured and compared to a growth curve for that known species. The lichen’s growth pattern is based in terms of its diameter, meaning that the older the lichen is the large the diameter. There can be complications with lichenometry. If you are measuring in a highly trafficked or cleaned area such as a tombstone, the results will be inaccurate.

The calibration curves are made by comparing lichen that is found on rock structures such as buildings or tombstones to their ages. This is where the calibration must be careful in the case of cleaning off tombstones. These curves are then used in order to date rocks or events that have an unknown age, such as a glacial moraine. You compare the diameter of a known lichen to the largest diameter found on the area of interest. When the age of the moraine is known then a geomorphologist can interpret when the glacier retreated leaving that area open for lichen to grow. Lichenometry is very useful in looking at the ages of events such as glacial moraines, landslides, erosion surfaces, and other processes that involve rock being disturbed or moved.

Lichenometry in the white river valley and Skolai Pass in the Wrangell and St. Elias Mountains of southern Alaska contains numerous well-dated lichen samples that are used as control dates for the surrounding area. The largest sample of Rhizocarpon geographicum lichen that was found in the area was 155 mm in diameter, which is thought to be around 3,700 years old. The issue with this study was the moraines they sampled were fairly old allowing for more vegetation to crawl over the moraines impeding the lichen measurements. There was the only success in young moraine surfaces that provided reliable lichenometric dating. Lichenometry was much more effective in the Kebnekaise and Sarek mountains of Swedish Lapland. There were 40 Holocene drifts mapped with moraines in four groups that represented the different extents of the glaciers. The data gathered proved that the lichenometric measurements were relatively uniform throughout the area. They were averaged to a growth of 3.6 mm per century. This gives evidence that the largest of the lichen they found, 480 mm in diameter, is as old as 9,000 years. (Denton & Karlén, 2018).

Lichenometry was developed by a man named Roland Beschel in 1950. "Only the lichen thallus with the maximum diameter is an indicator of surface age." This was the main assumption that Berschel based his dating technique on. He found that attempting to average large lichen is illogical because there is no way of excluding lichen that may have been transported to an area such as a moraine. Beschel was able to use his application in figuring out the absolute ages and use in tundra environments where there are not many other methods for dating in those regions. The paper explains that even though Berschels work in lichenometry was incredible, there are still drawbacks and problems with the method. There can be competition among the lichen colonies, growth rate changes, survival rates, environmental problems, and sampling problems. Failure to identify the largest lichen can provide errors in measurement and statistical interpretation and calibration for calibration curve use. (Webber & Andrews, 2018).

Citations:

Denton, G. H., & Karlén, W. (2018). Lichenometry: Its Application to Holocene Moraine Studies in Southern Alaska and Swedish Lapland. Taylor & Francis Online, 5(4), 347-372.

Heindel, R. (2013, July 09). Rhizocarpon Dreams. Retrieved October 02, 2020, from https://dartmouthigert.wordpress.com/2013/07/09/rhizocarpon-dreams/

Sedlarova, M. (2005, September). Rock slab with lichen crust Lower Tatras Mts, Slovakia [Photograph]. Department of Botany, Palack University, Czech Republic.

Webber, P., & Andrews, J. (2018). Lichenometry: A Commentary. Arctic, Antarctic and Alpine Research, 5(4), 295-302. Retrieved October 2, 2020, from https://www.tandfonline.com/doi/pdf/10.1080/00040851.1973.12003738