Lichenometry: An Accessible Method for Dating Recent History (Geological and Manmade)

Related Links

• Science Fair Project Guide

Project Summary

Difficulty	8
Time required	Average (about one week)
Prerequisites	Suitable local lichen species in the field.
Material Availability	Availability of suitable species can depend on geographic area.
Cost	Low ($20 - $50)
Safety	No issues

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Objective

The primary objective of this project is to develop sufficient expertise through background research so that you are able to perform a local calibration of lichen growth for lichenometry dating in your area. A secondary objective is to use this calibration to date a manmade or geological feature or disturbance in your area (for example, old stone walls, or a road cut, or a rock slide).

Introduction

A trained geologist can "read" ancient history in layers of rocks. The ability to establish dates and temporal sequences of rock formations is, in fact, essential for piecing together the earth's history. Most of the methods used for dating rocks rely on specialized equipment that can measure the presence or relative proportion of specific isotopes in the rock. In this project, you'll learn a much more accessible method for dating relatively recent events (up to hundreds of years ago, or perhaps as many as ten thousand years ago).

This method of dating rocks is known as "lichenometry." The method is used with lichen species that exhibit predictable growth behavior, and is based on measuring the size of lichen colonies on exposed rock surfaces. All else being equal, those surfaces that have been undisturbed for longer times will have colonies with larger diameters. Since lichen growth rates are dependent on local conditions, growth rates must be calibrated locally. This can be done either by measuring growth rates from year to year (obviously, you won't have time for this!), or by measuring the size of colonies with known dates of origin. The method was first developed by Roland Beschel in the 1950's, by measuring lichen diameters on gravestones of different ages. The burial dates from the gravestones were taken as the starting point for lichen growth, and used to create a graph of lichen diameter vs. age. The graph could then be used to date local gravestones or rocks with unknown dates, by measuring the diameter of lichen colonies.

You can see that this method is fairly simple, at least on the surface. All you need is a ruler and some lichen-bearing rocks with a known history, right? Think for a moment about the assumptions being made. Do lichens really grow at a steady rate? Do you have to make measurements using only one type of rock? How close must the known and unknown rocks be in order for the growth rates to be comparable? Is lichen growth rate determined by mostly by local climate, or microclimate? What about changes in climate over time? It's starting to look a little more complicated, as most things do when you start looking close enough. With careful methodology, however, lichenometry can be a reliable method for dating rocks.

Terms, Concepts and Questions to Start Background Research

To do a project using lichenometry, you should do research that enables you to understand the following terms and concepts:

• biology of lichens
  • symbiosis
  • symbiont
  • mycobiont
  • photobiont
  • thallus
• characteristic lichen forms (which are suitable for lichenometry?):
  • foliose
  • fructiose
  • crustose
  • squamulose

You'll also need to answer the following question:

• Are there lichen species in your area with suitable growth patterns for dating? The species most often used in scientific studies is Rhizocarpon geographicum. Other possibilities are Aspicilia and Lecidea species (see the reference on Lichen Physiology, below).

  Rhizocarpon geographicum ("map lichen") on rock at high elevation in the Rocky Mountains.

Bibliography

• An introduction to lichens, from the New York Botanical Garden:
  http://www.nybg.org/bsci/lichens/lichen.html
• A multi-page introduction to lichenometry starts here:
  http://www.whyte.org/time/lichentrees/introduction.html
• A one-page introduction to lichenometry:
  http://www.geo.arizona.edu/palynology/geos462/13lichenometry.html
• An introduction to geological dating methods, including lichenometry:
  http://academic.emporia.edu/aberjame/ice/lec10/lec10.htm#lichen
• Detailed information on Lichen Physiology, with additional information on lichenometry (last few slides):
  http://avalon.unomaha.edu/lichens/Bio%204350%20PDF/Lichen%20Physiology.pdf
• Lichens of North America website:
  http://www.lichen.com/index.html
• Here is an interesting article on dating stone walls in New England using lichenometry. The article discusses some of the potential problems with the technique:
  http://www.primaryresearch.org/stonewalls/nylund/index.php
• William B. Bull, "Dating San Andreas Fault Earthquakes with Lichenometry":
  http://www.activetectonics.com/articles/Dating_SN_quakes_w_lichens.pdf
• A news article on William Bull's work:
  http://www.yale.edu/opa/ybc/v26.n20.news.03.html

Materials and Equipment

• field guide for identifying lichens
• ruler or calipers
• notebook

Experimental Procedure

1. First, do your background research to develop your expertise on the biology of lichens and on the methods of lichenometry. As part of your background research, you may find it helpful to seek out a mentor in your local area.
2. Identify local species of lichens with growth patterns suitable for use in dating.
3. Identify site(s) for performing your local calibration. For an accurate calibration, you'll need to know the history of disturbances at your site(s). The original calibration for the method was performed using lichens growing on gravestones.
4. Be sure to collect multiple measurements for each known time period to smooth out random variations.
5. Graph your results and use them to date other sites. If you know the history of at least some of the additional sites, you'll be able to test, and possibly refine, your calibration data.
6. Do your additional results give you more confidence in the method? Why or why not? Can you think of ways to improve your methodology? Try them out and see if your calibration is improved.

Variations

• Here's a variation that will take you into the area of Environmental Science. Some lichen species are quite sensitive to air pollution. The U.S. Forest Service has conducted a multi-year, national study to use lichen data as an indicator of air quality. While many of the methods employed in the study are beyond the scope of a science fair project, one conclusion was that lower air quality caused a decrease in diversity of lichen populations, with sensitive species dying out. It might be possible to glean information about air quality by conducting a survey of local lichen populations.
  • Data for the Pacific Northwest:
    http://www.fs.fed.us/r6/aq/lichen/
  • National data:
    http://www.nacse.org/lichenair/

• For more science project ideas in this area of science, see Geology Project Ideas

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Credits

Andrew Olson, Ph.D., Science Buddies
Idea from Olivia Kurz, Massachusetts State Science Fair

Last edit date: 2006-01-31 14:47:52

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