Radiocarbon dating can unravel historical mysteries

Radiocarbon dating measures the amount of a radioactive type of carbon, called carbon-14, in a sample. The process begins in the Earth’s air, about 78% of which is nitrogen. Cosmic rays (highly energetic atomic particles) strike nitrogen molecules, changing some of them into carbon-14.

Plants continuously absorb carbon dioxide and are eaten by animals, so carbon-14 enters living organisms. Although this carbon-14 decays at an approximately known rate, turning back into nitrogen, it is always replaced through uptake. Anything alive maintains around the same ratio as the atmosphere of radioactive carbon to nonradioactive carbon.

When a plant or animal dies, it stops taking up new carbon-14 and its carbon-14 decays without replacement. The ratio of radioactive carbon to nonradioactive carbon in the dead organism changes. Measuring this ratio indicates for how long carbon-14 has not been replaced and so how long ago the organism died.

Consequently, only samples that were part of a living organism can be used. Organic material includes wood, parchment, grain, and bone. Ceramic, metal, and stone cannot be radiocarbon dated. Radiocarbon dating can determine age from a few centuries ago up to 50,000 to 55,000 years ago.

From graves or garbage dumps, the bones of people or animals, as well as charcoal from wood fires and remains of plants used for food, materials, or buildings, can be radiocarbon dated.

Photo: Hans Verburg / Shutterstock
A cross section of large cut pine tree shows the growth rings and bark. Wood is another type of sample that can be used for radiocarbon dating.

The earliest radiocarbon date from a site could suggest when humans first arrived there. Certainly, older materials, graves, or dumps could have been destroyed—perhaps by a flood or by people reusing them. Conclusions from radiocarbon dating need to be cross-checked with other techniques that, when available, might be written records, seed analyses to determine plant species changes, or soil layers.

Further complications emerge. Understanding of carbon-14’s decay rate continues to be refined and the air’s radioactive/nonradioactive carbon ratio has changed over time. Calculations must be calibrated for these factors, such as by using tree rings or materials of known (or assumed) age, leading to uncertainties and disagreements.

Samples can be contaminated through multiple materials from different time periods or from ocean-based material. The latter is a problem since water has a different radioactive/nonradioactive carbon ratio than air. Further measurements, calibrations, and calculations help to reduce these effects, leading to further disputes regarding the accuracy and precision of a sample’s reported age.

The reported age typically includes uncertainty of a few decades. At a reasonable confidence level, the sample’s age might be listed as 1,540 years old plus-or-minus 25 years, with a most likely age of 1,522 years old (and a range might be included). If a high confidence is desired, then the plus-or-minus might extend to 30 or more years.

Radiocarbon dating was developed in the 1940s and initially required grams of a sample for an analysis taking up to a month. Today, micrograms of a sample suffice to produce a result in less than an hour.

Steinar Solheim and colleagues at the University of Oslo’s Museum of Cultural History have been using radiocarbon dating to understand better when Norwegians shifted from mainly hunting, fishing, and gathering to including agriculture and domesticated animals. With charred wood and cereal grains from thousands of samples across hundreds of sites in southeastern Norway, they combined radiocarbon dating and other techniques to map in place and time how farming entered and spread across Norway.

Farming was definitely significant at or before 4,000 years ago. It might have begun up to 6,455 years ago, although most likely between 5,545 and 5,050 years ago. Farming was widespread in southeastern Norway from, at minimum, 2,400 years ago. Research continues to determine how early Norwegians thrived and survived from the land’s richness.

This article originally appeared in the June 2023 issue of The Norwegian American. To subscribe, visit SUBSCRIBE or call us at (206) 784-4617.