With fall coming to a close, there is no better time to talk about tree rings and their use in archaeology.
You probably know that trees have rings, which you can see and count when you look at a stump after a tree has been cut, but did you know that the rings of a tree let you know how old it is?
Dendrochronology or tree-ring dating is the scientific method of dating using the annual nature of tree growth in suitable tree species.
Together older trees and younger trees are used to create long, chronological, growth sequences that can help us date artifacts and archaeological sites that are hundreds, even, thousands of years old.
The sequence is created by overlaping the tree rings so a long series of rings can be seen.
The size of the rings can also depend on the age of the tree, because as a tree gets older it produces narrower rings as well. Dendrochronology has two uses in archaeology: it can be used to calibrate (correct) radiocardon dates, and it can be used to date things all on its own.
Archaeologists look at other trees of the same species in the area because they have the same ring patterns.
It is based on the fact that amino acids (the building blocks of all proteins) exist in two mirror image forms, both of which otherwise have the same chemical structures.
The L-amino acid molecule form has an extension to the left, while the D-amino acid form has an extension to the right.
Since weather patterns tend to run in cycles of a number of years, the sequence of tree-rings in a region will also reflect the same cycling, as illustrated by the graph below.
By cross-linking core samples from living and dead trees, a master sequence of annual tree-ring widths can be compiled.
Maybe you’ve heard of carbon dating, and are wondering “Why do archaeologists use tree-ring dating at all? ” Yes, you could, but carbon dating (which our final blog post in the series will be about next week) always has an error range of as many as 50-100 years, meaning that we can only have a general idea of how old something is.
Tree-ring dating lets us find out the exact year that a tree was cut down! It is too bad that we do not find wood more often in Ontario! Be sure to check out the series introduction or follow the links below!
Fortunately, there are other methods available to researchers.