This Month in Archeology

2008 February 13
Dating advice from an archeologist

Thermoluminescence, radiocarbon, stratigraphic superposition, typological sequence. . . Not exactly sweet nothings the object of your affection would appreciate this Valentine’s Day, but certainly exciting words to an archeologist. In archeological terms, “dating” means determining the age or time period of an artifact. Few tasks are more important in archeology – we need good dates to situate sites within an understandable timeline and to help us make sense of what people did in the past. And, I think, there are lessons from archeological dating that apply to the more passionate kind.

Lesson #1: Never trust a single date. Romantics may believe in love at first site, but archeologists demand multiple lines of evidence before we accept anything as a “fact”. A single date may not tell the whole tale at an archeological site. Perhaps it was occupied for a long period of time, or re-occupied several times over the centuries. The only way to tell is to get multiple dates, from different types of artifacts found at different locations within the site.

Lesson #2: Diamonds may be a girl’s best friend, but charcoal’s the stuff that makes an archeologist’s heart flutter. All living organisms (plants and animals) contain carbon. Most carbon is naturally stable, while a small percent is radioactive: with time, it decays and becomes stable carbon. Because organisms incorporate carbon from the atmosphere, the proportion of radioactive to stable carbon within them remains the same as long as they are alive. As soon as a plant or animal dies, this proportion begins to change as the amount of radioactive carbon decreases. Radiocarbon dating measures the carbon ratio within a piece of organic material to determine how long ago the radioactive decay process has been operating – in other words, how long ago the organism died. Charcoal was once alive (as a bit of tree or other plant), and is often preserved in ancient hearths or fire pits, making it a prime candidate for this dating method.

Lesson #3: Always look more than skin-deep. The simplest and most reliable dating method is stratigraphic superposition, the basic geologic principle that sediments are deposited from the bottom up. The deeper we dig, the farther back into the past we see. This serves as a check on all other dating methods: if other dates don’t agree with stratigraphy (for example, if a radiocarbon date from the top of a site is older than one at the bottom), we know that the stratigraphy is disturbed or that we’ve misinterpreted one of the dates.

Lesson #4: Know your type. The most common dating method used in archeology is typological dating, based on the principal that the types or styles of artifacts change through time. Consider automobiles, a common artifact of the 20th and 21st centuries. Most everyone would be able to date a car with tailfins and big chrome bumpers to the 1950s or 60s, because we know these traits were popular then. When we’ve dated hundreds or thousands of prehistoric artifacts of the same type, such as Clovis spear points (dating to the Paleoindian time period, about 12,000 to 10,000 years ago), we can be pretty sure that other Clovis points we find were also made about the same time. Dating several styles of stone points or other artifacts creates a typological sequence: a timeline of artifact types.

Lesson #5: Don’t be afraid to heat it up. Thermoluminescence is used to date clay that was exposed to high temperatures. The mineral structure of clay traps radioactive ions that occur naturally in the atmosphere. When clay is exposed to high temperatures, the ions are released and it slowly begins accumulating them again. We know that prehistoric clay pots were exposed to high temperatures when they were fired in kilns, and sometimes when they were used for cooking. These actions effectively “reset the clock” of ion accumulation. By heating bits of prehistoric clay pots today, and measuring the amount of ions released, thermoluminescence dating can reveal how long ago the ion clock was reset, and therefore how long ago the pot was made or last used.

Lesson #6: You’re never too old to date. Mystery writer Agatha Christie, married to an archeologist, once quipped that the older she grew the more interested in her he became. Some dating methods, like radiocarbon, have age limitations. After about 50,000 years the radioactive carbon in any organic material is too decayed to be measured. Luckily there are many different dating methods, each with their own strengths and weaknesses, some of them stretching back millions of years in time.

All this may not sound as romantic as candy or flowers, but it’s exciting to those of us in the business anyway. And if you ever ask an archeologist for a date, be sure they know which kind you mean.

Dr. Gregory Vogel is Director of Research at the Center for American Archeology in Kampsville, Illinois. He may be contacted at: gvogel@caa-archeology.org, or P.O. Box 366, Kampsville, Illinois, 62053.