Fort McCoy ArtiFACT: Radiocarbon dating

Courtesy Photo | Charcoal found at an archaeological dig at Fort McCoy, Wis., is shown June 24, 2022,...... read more read more

Courtesy Photo | Charcoal found at an archaeological dig at Fort McCoy, Wis., is shown June 24, 2022, at the installation. The charcoal was radio carbon dated to be hundreds of years old. (Contributed photo)  see less | View Image Page

FORT MCCOY, WI, UNITED STATES

07.07.2022

Courtesy Story

Fort McCoy Public Affairs Office           

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One of the more significant challenges archaeologists deal with while investigating any archaeological site is determining how old it is.

At sites with historic materials, items such as bottles, jars, or plates with dates stamped on them may be present. They might have to do some research to figure out when certain things were manufactured, but that doesn’t necessarily answer the question of when that item was last used or left behind. This usually leaves the archaeologist a date range that could span decades.

At prehistoric Native American sites, the date ranges can span centuries. This is because archaeologists generally agree that changes in the appearance and shape of certain types of artifacts such as pottery and spear points change over time, but that change can be gradual, and can happen at different times in different parts of a larger area such as an individual state (Wisconsin) or region (Upper Midwest, Great Plains).

And so, archaeologists can see differences in how a prehistoric vessel was made and decorated and determine if that clay pot was produced 500 years ago, 1,200 years ago, or 2,000 years ago. They can then use the research and investigations of other archaeologists to learn whether the site where their pot was found was active 200 years ago, 2,000 years ago, or somewhere in between.

By the same token, they can compare a spear point or arrowhead from a prehistoric site with others recovered by different archaeologists in the area and assert that their projectile point was left behind 200 years ago, 10,000 years ago, or somewhere in between. This method is known as a relative dating strategy, which can only give an estimate of when a site was used by Native Americans.

The alternative to relative dating is absolute dating, but the conditions required to collect materials for absolute dating are not always available at every archaeological site. Absolute dating methods used by archaeologists include dendrochronology (dates are obtained by measuring tree-rings), optically stimulated luminescence (dates are obtained by determining the last time sediment grains, especially quartz grains, were exposed to light), and radiocarbon dating. Archaeologists working at Fort McCoy (and around the world) have used radiocarbon dating for many years.

In the 1930s and 1940s, physicist Serge Korff of New York University found that cosmic rays bombarding the Earth’s atmosphere created neutrons that joined with the naturally occurring nitrogen (isotope N-14) to produce the carbon isotope C-14, which is commonly referred to as radiocarbon. Radiocarbon dating was conceived by Willard Libby at the University of Chicago in 1946 with the assistance of his students.

Libby explained that the C-14 isotope, which is radioactive, would be absorbed by living organic matter such as plants and animals. Once that plant or animal dies, however, it stops absorbing radiocarbon (C-14), and the radiocarbon it has accumulated begins to decay and transform into a stable carbon isotope (C-12).

This decay happens at a constant rate; over 5,730 years, the total amount of radiocarbon accumulated is reduced by one half. Since the C-14 isotope decays at a constant rate as it transforms, scientists with the proper equipment can measure how long ago something stopped accumulating radiocarbon and instead began to decay. Materials that accumulate radiocarbon include, but are not limited to wood, charcoal, seeds, spores, pollen, bone, leather, hair, fur, horn, blood residue, peat, mud, soil, shells, coral, wall paintings, and paper.

During the summer of 2021, archaeologists with Colorado State University's Center for Environmental Management of Military Lands (CEMML) recovered some charcoal samples from a dark soil stain uncovered while carefully digging through an excavation unit. It was assumed that this stain might represent a fire pit, so the charcoal samples were sent to a specialized laboratory (Beta Analytic, located in Miami, Fla.) for analysis.

The laboratory sent back a date of 1,480 years before the present day with a margin of error of 30 years before or after, which was a transitional period between the end of the Middle Woodland period and the start of the Late Woodland period at Fort McCoy. This timeframe matched well with some of the archaeological materials (prehistoric pot sherds and an arrowhead) from other areas of the site that would have otherwise provided only a relative date for the site.

These results will also be useful for other archaeologists in the region when considered together with other radiocarbon dates to help refine what artifacts can be attributed to specific time periods.

Radiocarbon dating does have some limitations, one of which is how to accurately match the dates returned from analysis with actual dates in the past. The date noted above is the uncalibrated age of the charcoal sample.

Calibration is needed to account for the fact that the amount of radiocarbon in the atmosphere has varied throughout the past. Calibration curves have been developed using the trees rings associated with dendrochronology to estimate a calendar age more accurately.

The calibrated age of the charcoal samples discussed here was estimated at between AD 550-644. For comparison, the uncalibrated age of the charcoal samples represent AD 514-572 if we subtract the 60-year date range around 1,480 years before present from the current year 2022.

Another limit is how far back radiocarbon dating can provide dependable dates, but they can reach from the present day to roughly 50,000 years ago, reliably. To go further back, such as for some of the earliest human ancestors who lived millions of years ago, alternative methods need to be employed.

Potassium/Argon dating measures the decay of the potassium isotope P-40 as it becomes the stable argon isotope Ar-40 in volcanic rock. This technique can reach as far as 4.5 billion years in the past but is sometimes less effective for producing reliable dates from the past 20,000 years or so.

All archaeological work conducted at Fort McCoy was sponsored by the Directorate of Public Works Environmental Division Natural Resources Branch.

Visitors and employees are reminded they should not collect artifacts on Fort McCoy or other government lands and leave the digging to the professionals.

Any individual who excavates, removes, damages, or otherwise alters or defaces any historic or prehistoric site, artifact, or object of antiquity on Fort McCoy is in violation of federal law.

The discovery of any archaeological artifact should be reported by calling 608-388-8214.

(Article prepared by the Fort McCoy Directorate of Public Works Environmental Division Natural Resources Branch and the Colorado State University Center for Environmental Management of Military Lands under agreement with Fort McCoy.)