Understanding Carbon 14 dating

By Mary Hudson

The first of the three periods are the prehuman records, well-represented by the Latvis/Simpson Site in Little River. It provides an excellent record of fossils and sediments representing Aucilla life in the range of 30,000 years ago. The most important sites for the ARPP are those that feature the earliest human cultures. We have now identified at least five substantial Paleoindian sites, one or two in each of the three segments of the Aucilla River. Each Paleoindian site demands more carbon dates, meticulous documentation and thorough excavation. And thirdly, the ARPP has discovered several sites that represent human cultures and their environments after the terminal Pleistocene extinctions of the big mammals.

The interpretation of data in the field of archaeology is often subjected to intense scrutiny. And when the interpretation of a site directly depends upon its estimated antiquity, the methods by which its age was determined become crucial. The following discussion focuses on Carbon 14 dating, the most widely used method of age estimation in the field of archaeology.

Carbon 14 (hereafter C 14) was developed by the American chemist, Willard F. Libby at the University of Chicago in the 50's, for which he received the Nobel Prize in Chemistry in 1960. C 14 dating provided an accurate means of dating a wide variety of organic material in most archaeological sites, and indeed in most environments throughout the world. The method revolutionized scientists' ability to date the past. It freed archaeologists from trying to use artifacts as their only means of determining chronologies, and it allowed them for the first time to apply the same absolute time scale uniformly from region to region and continent to continent. Many older archaeological schemes were overturned with the advent of C 14 dating. Today it is possible to date sites, such as those studied by the ARPP, well back into the late Pleistocene with reliable and accurate chronologies.

The element carbon is abundant in nature, and is a basic building block of all living things. Like many elements, carbon exists in nature in several different isotopic forms. An isotopic form is an element with the same number of protons in its nucleus (and thus similar chemical behavior) but with a different atomic weight, due to a different number of neutrons in the nucleus. For example Carbon 12 (hereafter C 12), the most abundant isotope of carbon, has six protons and six neutrons in its nucleus. Its atomic number is six, and its atomic weight is 12. C 14 has two extra neutrons. C 12 accounts for 98.89% of all carbon on earth (including carbon dioxide in the atmosphere). Seven other isotopes make up the other 1.11% of the global carbon budget (see fig. 1). The abundance and stability of C 12 make it an ideal reference point for comparing with its unstable isotope C 14.

C 14 forms in the upper atmosphere when cosmic rays strike nitrogen. When nitrogen, with atomic number 7 and atomic weight of 14, is struck by a high energy neutron, it absorbs the neutron and emits a proton. This transforms it to a new element of atomic number 6, which, as we know, is carbon. But this carbon isotope has the atomic weight 14. Its two excess neutrons cause it to be very unstable, and it will eventually experience radioactive decay, changing back to the stable element nitrogen.

As C 14 circulates through the atmosphere, mostly as carbon dioxide, and is perhaps taken into the sea or transformed into plant tissue by photosynthesis, it behaves just the same as C 12. Over time, however, the number of unstable parent nuclei of C 14 decreases. This decay rate, as for other radioactive isotopes, is a constant, which can be measured in the laboratory. The rate of radiation of a given sample steadily reduces as the number of unstable nuclei steadily declines. That makes it convenient to measure the decay rate in terms of half-lives. The half-life of C 14 is 5,730 years. That is one of the reasons that C 14 dating is useful in archaeology, whereas potassium or uranium isotopes with much longer half-lives are used to date really ancient geological events that must be measured in millions or billions of years. The number of half lives that can be measured reaches practical limits at about nine or ten, when there is too little radioactive material left. Thus, dates derived from carbon samples can be carried back to about 50,000 years.

In recent years physical chemists working on carbon-dating have devised a new method of measuring C 14 decay. The TAMS method combines (in tandem) a particle accelerator and a mass-spectrometer (you can figure out the acronym from this sentence, if you wish). The spectrometer recognizes the energy and mass characteristics of any element, in this case C 14, and then submits the selected element to a particle accelerator where the decay particles are individually counted. This very precise method can count radioactivity from very small samples and does not bum the samples up, as with traditional dating methods. For example, ARPP used TAMS to date the oldest gourd seeds featured in the National Geographic Magazine.

The decision whether to use the older (beta counting) methods or the new TAMS method depends largely on the size and value of the sample to be tested. In general only a few milligrams of carbon are needed for TAMS dates, as compared to several grams of pure carbon for the older methods. Another advantage is that in a composite carbon sample, a peat bed for example , the TAMS method can date one individual particle at a time. On the other hand, TAMS dates cost two or three times as much as ordinary dates. And that is why (near the back of this issue) ARPP has solicited your help to afford some of these very special new carbon dates.

Despite the wonderful new world into which C 14 dating has brought us in the past 40 years, the method must be carefully integrated into the entire field operation. The carbon date is no better than the site stratigraphy from which it was sampled. Stratigraphy, the science of how strata accumulate in the earth's crust, forms the foundation of field archaeology. Dr. Vance Haynes' excavations and stratigraphic analyses of Blackwater Draw in 1983-84 and 1990-94 provide an outstanding model of good field archaeology. His procedures illustrate the intricate and important phasing of precise excavation, exquisite attention to sedimentary detail, and an abundance of C 14 dates cleverly placed throughout the fabric of the site. In his classic study of geoarchaeology at the Clovis Type Site in Blackwater Draw, New Mexico, Profess Haynes includes in his field strategy "core drilling, archaeological test excavations, stratigraphic profiling, sedimentary analysis, and radiocarbon dating" (Haynes, 1995).

The ARPP strives to emulate Professor Haynes in our underwater excavation of Paleoindian sites. We must continue to follow Hayne’s model for phase and detail of excavation, and we must acquire dates and more dates at every phase. We have had an interdisciplinary approach from the start. Our entire team, from scientists to graduate and undergraduate students, managers and supervisors, advisors, volunteers and political and financial supporters are all topnotch. Important too, is ARPP's dedication to education at all levels. We have what it takes to make this a world-class project.

Haynes, Vance. 1995. Geoarchaeology of Paleoenvironmental Change, Clovis Type Site, Blackwater Draw, New Mexico. Geoarchaeology, 1995 volume.