Stable Isotope Analysis in Archaeology

Stable Isotopes and How the Research Works

A plant growing through a wooden deck.
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Stable isotope analysis is a scientific technique which is used by archaeologists and other scholars to collect information from an animal's bones to identify the photosynthesis process of the plants it consumed during its lifetime. That information is enormously useful in a wide number of applications, from determining the dietary habits of ancient hominid ancestors to tracing the agricultural origins of seized cocaine and illegally poached rhinoceros horn. 

What are Stable Isotopes?

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 in our atmosphere exists in the form called Carbon-12; but the remaining one percent carbon is made up of two several slightly different forms of carbon, called Carbon-13 and Carbon-14. Carbon-12 (abbreviated 12C) has an atomic weight of 12, which is made up of 6 protons, 6 neutrons, and 6 electrons—the 6 electrons don't add anything to the atomic weight. Carbon-13 (13C) still has 6 protons and 6 electrons, but it has 7 neutrons. Carbon-14 (14C) has 6 protons and 8 neutrons, which is too heavy to hold together in a stable way, and it emits energy to get rid of the excess, which is why scientists call it "radioactive."

All three forms react the exact same way—if you combine carbon with oxygen you always get carbon dioxide, no matter how many neutrons there are. The 12C and 13C forms are stable—that is to say, they don’t change over time. Carbon-14, on the other hand, is not stable but instead decays at a known rate—because of that, we can use its remaining ratio to Carbon-13 to calculate radiocarbon dates, but that’s another issue entirely.

Inheriting Constant Ratios

The ratio of Carbon-12 to Carbon-13 is constant in the earth’s atmosphere. There are always one hundred 12C atoms to one 13C atom. During the process of photosynthesis, plants absorb the carbon atoms in the earth’s atmosphere, water, and soil, and store them in the cells of their leaves, fruits, nuts, and roots. But, the ratio of the forms of carbon gets altered as part of the photosynthesis process. 

During photosynthesis, plants alter the 100 12C/1 13C chemical ratio differently in different climatic regions. 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. Scientists categorize plants by the version of photosynthesis they use into groups called C3, C4, and CAM

Are You What You Have Eaten? 

The ratio of 12C/13C is hardwired into the plant’s cells, and—here’s the best part—as the cells get passed up the food chain (i.e., the roots, leaves, and fruit are eaten by animals and humans), the ratio of 12C to 13C remains virtually unchanged as it is in turn stored in the bones, teeth, and hair of the animals and humans.

In other words, if you can determine the ratio of 12C to 13C that is stored in an animal's bones, you can figure out whether the plants they ate used C4, C3, or CAM processes, and therefore, what the environment of the plants was like. In other words, assuming you eat locally, where you live is hardwired into your bones by what you eat. That measuring is accomplished by mass spectrometer analysis.

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. That research has led to a simply incredible diversity of human and animal dietary information.

Earliest Studies 

The very first archaeological application of stable isotope research was in the 1970s, by South African archaeologist Nikolaas van der Merwe, who was excavating at the African Iron Age site of Kgopolwe 3, one of several sites in the Transvaal Lowveld of South Africa, called Phalaborwa.

Van de Merwe found a human male skeleton in an ash heap that did not look like the other burials from the village. The skeleton was different, morphologically, from the other inhabitants of Phalaborwa, and he had been buried in a completely different manner than the typical villager. The man looked like a Khoisan; and Khoisans should not have been at Phalaborwa, who were ancestral Sotho tribesmen. Van der Merwe and his colleagues J. C. Vogel and Philip Rightmire decided to look at the chemical signature in his bones, and the initial results suggested that the man was a sorghum farmer from a Khoisan village who somehow had died at Kgopolwe 3.

Applying Stable Isotopes in Archaeology

The technique and results of the Phalaborwa study were discussed at a seminar at SUNY Binghamton where van der Merwe was teaching. At the time, SUNY was investigating Late Woodland burials, and together they decided it would be interesting to see if the addition of maize (American corn, a subtropical C4 domesticate) to the diet would be identifiable in people who formerly only had access to C3 plants: and it was. 

That study became the first published archaeological study applying stable isotope analysis, in 1977. They compared the stable carbon isotope ratios (13C/12C) in the collagen of human ribs from an Archaic (2500-2000 BCE) and an Early Woodland (400–100 BCE) archaeological site in New York (i.e., before corn arrived in the region) with the 13C/12C ratios in ribs from a Late Woodland (ca. 1000–1300 CE) and a Historic Period site (after corn arrived) from the same area. They were able to show that the chemical signatures in the ribs were an indication that the maize was not present in the early periods, but had become a staple food by the time of the Late Woodland.

Based on this demonstration and available evidence for the distribution of the stable carbon isotopes in nature, Vogel and van der Merwe suggested that the technique could be used to detect maize agriculture in the Woodlands and tropical forests of the Americas; determine the importance of marine foods in the diets of coastal communities; document changes in vegetation cover over time in savannas on the basis of browsing/grazing ratios of mixed-feeding herbivores; and possibly to determine origins in forensic investigations.

New Applications of Stable Isotope Research

Since 1977, applications of stable isotope analysis have exploded in number and breadth, using the stable isotope ratios of the light elements 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; and cattle dairying (milk residues in pottery), and mother’s 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 determining 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 fake $100 bills. 

Another example of isotopic fractionation that has a useful application involves rain, which contains the stable hydrogen isotopes 1H and 2H (deuterium) and the oxygen isotopes 16O and 18O. Water evaporates in large quantities at the equator and the water vapor disperses to the north and south. As the H2O falls back to earth, the heavy isotopes rain out first. By the time it falls as snow at the poles, the moisture is severely depleted in the heavy isotopes of hydrogen and oxygen. The global distribution of these isotopes in the rain (and in tap water) can be mapped and the origins of the consumers can be determined by isotopic analysis of hair. 

Sources and Recent Studies