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K. Kris Hirst

Stable Isotopes for Dummies

By October 24, 2006

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Stable isotope analysis has become among the most important types of study used in archaeology today---and in many other related fields as well. Applications of stable isotope analysis have focused on the light elements of hydrogen, carbon, nitrogen, oxygen and sulfur in human and animal bone (collagen and apatite), tooth enamel and hair, as well as in pottery residues (baked onto the surface or absorbed into the ceramic wall) to determine diets and water sources. Light stable isotope ratios (usually of carbon and nitrogen) have been used to investigate such dietary components as marine creatures (e.g. seals, fish and shellfish), various domesticated plants such as maize and millet; cattle dairying (milk residues in pottery), and motherís milk (age of weaning, detected in the tooth row). Dietary studies have been done on hominins from the present day to our ancient ancestors Homo habilis and the Australopithecines.

Other isotopic research has focused on the geographic origins of things. Various stable isotope ratios in combination, sometimes including the isotopes of heavy elements like strontium and lead, have been used to determine whether the residents of ancient cities were immigrants or were born locally; to trace the origins of poached ivory and rhino horn to break up smuggling rings; and to determine the agricultural origins of cocaine, heroin, and the cotton fiber used to make counterfeit $100 bills. The latest wrinkle involves the potential use of hydrogen and oxygen isotopes in hair to determine where a person has been lately, by comparing the measurements with an international database for rainfall and local water supplies.

How Stable Isotopes Work

All of the earth and its atmosphere is made up of atoms of different elements, such as oxygen, carbon, and nitrogen. Each of these elements has several forms, based on their atomic weight (the number of neutrons in each atom). For example, 99 percent of all carbon exists in the form called Carbon 12 (written as 12C): it has twelve neutrons in its nucleus. [Stable isotope researcher Eleanora Reber corrects me: "Carbon-12 has an atomic weight of 12, which is made up of 6 protons and 6 neutrons. The 6 electrons don't really count towards the weight, as they're so light. Carbon-13 still has 6 protons and 6 electrons, but has 7 neutrons, and Carbon-14 has 6 protons and 8 neutrons, which is basically too heavy to hold together in a stable way, so it is radioactive." Thanks to Kate for pointing out this discrepancy.]

But, one percent of the time, carbon exists in the form of Carbon 13 (13C), which has thirteen neutrons six protons and six electrons but seven neutrons in its nucleus. Both forms react the exact same way (if you combine carbon with oxygen you get carbon dioxide, no matter what the number of neutrons). In addition, both forms are stable, that is to say, they don't change over time. (There are other forms that do change, and a good thing too: Carbon 14 decays and we can use its ratio to Carbon 13 for radiocarbon dating, but that's another issue entirely).

Carbon in the Food Chain

The ratio of Carbon 12 to Carbon 13 is constant in earthís atmosphere. There are always 100 12C atoms to 1 13C atom. During the process of photosynthesis, plants absorb the carbon atoms in earthís atmosphere, water, and soil, and store them in the cells of their leaves, fruits, nuts, and roots. But as a result of the photosynthesis process, the ratio of the forms of carbon gets changed as it is being stored. The alteration of the chemical ratio is different for plants in different parts of the world. For example, plants that live in regions with lots of sun and little water have relatively fewer 12C atoms in their cells (compared to 13C) than do plants that live in forests or wetlands. This ratio is hardwired into the plantís cells, and, hereís the best part, as it gets passed up the food chain (i.e., the roots, leaves, and fruit are eaten by animals and humans), the ratio remains virtually unchanged.

In other words, if you can determine the ratio of 12C to 13C in an animal, you can figure out what kind of climate the plants it ate during its life came from. The measuring takes mass spectrometer analysis; but thatís another story.

Not Just Carbon

Carbon is not by a long shot the only element used by stable isotope researchers. Currently researchers are looking at measuring the ratios of stable isotopes of oxygen, nitrogen, strontium, hydrogen, sulfur, lead, and many other elements that are processed by plants and animals and lead a simply incredible diversity of human and animal dietary information.

The diagram of stable isotopes and the food chain was provided by Nora Reber of the University of North Carolina at Wilmington. A larger image and an explanation is provided in the Stable Isotopes and the Food Chain.


May 17, 2007 at 10:57 am
(1) Kate says:

HI. Sorry, I think the explanatio of carbon and the number of neutrons is wrong. Carbon 12 does not have 12 neutrons, it has 6 protons and 6 neutrons.

April 17, 2008 at 4:30 pm
(2) Kris Hirst says:

Oh, drat! I’ll check into this and see what I can find out. Thanks!


April 19, 2008 at 10:15 am
(3) Kris Hirst says:

Not being in any way shape or form a chemist (I’m the prototype ‘dummy’ of the title for this blog), I went to my friend Nora Reber for assistance with this, and she straightened me out. Thanks to Kate for pointing this out!


August 30, 2012 at 12:20 am
(4) JC Lerman says:

So, carbon 13 does not exist 99 percent of the time? You wrote:
”But, one percent of the time, carbon exists in the form of Carbon 13 (13C),”

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