An Archaeologist’s Guide to Dating (the Past): Part 2: Absolute Dating

This post is a continuation of the previous post (insert link) that focused on relative dating, and today’s post is going to focus on absolute dating, which provides us with a more specific, precise date of sites, objects, artifacts, and fossils.  Absolute dating techniques are usually chemically based in practice, meaning archaeologists have to conduct chemical and physical tests on the artifacts or objects in order to get that date.  So much like real life dating, you have to have the right chemistry to get that date. 

There are many absolute dating techniques because they provide us with that more precise date, which is ultimately what we want in archaeology, but I want to highlight the most common: radiocarbon dating and dendrochronology. 

Radiocarbon dating, also known as Carbon-14 dating, is a radiometric technique using the decay rate of a radioactive isotope of carbon found in organic remains (e.g. bone, hide, antler, plant remains).  This method utilizes the amount of carbon 14, an unstable isotope, left in an object as compared to carbon 12, a stable isotope, to determine the age of an artifact.  Carbon 14 is abundant in the atmosphere and readily absorbed by organic matter, particularly plants.  Plants use carbon dioxide in respiration (the process in which plants breathe and survive), and through this process, plants absorb carbon 14 as readily as stable carbon 12 elements.  Other organisms, including humans, absorb carbon 14 from eating plants or animals that ate plants.  While plants and animals, including humans, are alive, carbon is readily absorbed and lost, but after death, the carbon is no longer absorbed but it continues to be lost (Figure 1).  The carbon 14 present in organic remains continues to be lost after death through a process known as a half life, which is the amount of time it takes for half of an unstable isotope to disappear (by becoming a more stable isotope).  Scholars have a technique for measuring how much carbon 14 and carbon 12 should be in a body versus the known half life, and it is know that the half life of carbon-14 is 5,730 years (in which time it becomes a more stable nitrogen isotope).  Knowing this and measuring the amount of carbon 14 in an object allows one to date an object as long as the date is no more than 40,000 years old. 

Figure 1: The Carbon Cycle

Now this method has one serious limitation.  It is environmentally sensitive as the amount of carbon isotopes in the environment has varied over time.  The amounts of carbon in the atmosphere does vary for a variety of reasons, so the amounts of carbon that can and will be absorbed by organic materials will vary over time.  As such, dates derived over a long period of time can be skewed due to this environmental factor.  Scholars have utilized another absolute dating technique, dendrochronology, to get around this problem.

Figure 2: Tree Rings

 Dendrochronology is a dating technique that utilizes tree-ring sequences.  We know that each year a tree is alive it puts down what is known as a growth ring (Figure 2).  These tree rings provide scholars insights into the environmental conditions that the tree has been exposed to as large rings demonstrate times of plenty and narrow rings demonstrate poor conditions.  We also know that all trees in a given area will have similar tree ring sequences based on environmental conditions, although the ages will vary.  As such, scholars can get tree ring cores (Figure 3) from only a few trees in order to produce what is known as a master sequence, which is a general and relatively consistent pattern of tree-ring width variation over time within a given region.  This master sequence is created as a means of dating wood at sites. 

Figure 3: Tree Ring Cores

Once ages have been established for trees based on the master sequence and number of rings associated with the tree, the tree can be radiocarbon dated as a means of compensating for the environmental sensitivity limitations of radiocarbon dating.  This is a means of producing a calibration curve.  This calibration curve is produced by radiocarbon dating the tree with a known age and comparing the dendrochronological age and the radiocarbon date for consistencies and inconsistencies.    If the dates are consistent, then the radiocarbon date is unaffected by the environment and therefore is a reliable date, but if the dates are inconsistent, then a calibrated date that accounts for the difference between the two dates is attached to the radiocarbon dates derived from artifacts found in that region.  

Figure 4: Tree Ring Example

To put this in perspective, look at Figure 4, which depicts a tree that is approximately 550 years old based on its rings.  If we radiocarbon date this tree and that date is the same, then we know that the radiocarbon date is spot on and there is no environmental sensitivity issues.  If we, however, get a date that states that the tree is 700 years old, then we know that the date is 150 years off and any object-such as a bone tool-dated to 700 years ago with radiocarbon dating will have to have a date designation of 700 years +/-150 years because the tree date and the radiometric date are in conflict. 

This concludes the two part series focusing on archaeological dating techniques.  It is important to note that there are several different relative and absolute dating methods out there available to archaeologists for dating artifacts, objects, fossils, and the sites themselves.  These methods, however, are not discussed herein due to their uncommon use in the discipline for a variety of reasons.  You will most likely hear about these methods in your archaeological pursuits-be it in learning from professionals or hearing about them in popular culture pieces.  This information has hopefully provided you a little more detail into this matter and you have a better understanding of archaeological practice.

Reference:

Feder, Kenneth and Park M. Human Antiquity: An Introduction to Physical Anthropology and Archeology, McGraw-Hill.