Human and animal skeletons provide unique insights into the past. Besides studying structures and DNA, archeologists use isotope signatures in teeth and bones to reveal information of ancient people and animals. Carbon, nitrogen and sulfur isotope ratio in bones can provide information on human and animal diets, as well as past climate changes, migration pathways and geographic origins.
Each isotope system tells a piece of the story.
Carbon is an element that gets absorbed by plants during photosynthesis. During this process, plants alter the isotope composition of carbon, depending on the climate where they grow. Plants that grow in sunny and dry regions, such as sugar cane and millet, have higher 13C/12C ratios compared to plants from forests and wetlands, such as wheat, rice, tubers, fruits and nuts. The carbon isotope composition of a plant remains unchanged when being eaten by animals and humans and being stored in their bones, teeth and hair. This implies that if you can determine the 13C/12C ratio stored in animal’s and human’s bone material, one can figure out what type of plants they consumed and thus what the environment of the plants was like. So, assuming you eat locally (what was likely true for most of our ancestors), your diet reveals where you live.
Nitrogen isotope composition variations of human and animal tissues are related to protein consumption. Nitrogen isotopes fractionate along the food chain. Plants have low 15N/14 ratios, whereas carnivores have high nitrogen isotope compositions. Herbivores fall in between. Marine mammals (seals, polar bears, whales) and piscivore fish (dolphins, otters) have even higher isotope nitrogen isotope compositions than carnivores and shellfish (clams, mussels, oysters, scallops). Therefore, nitrogen isotopes can perfectly distinguish between terrestrial vs. marine food sources. By determining the nitrogen isotope composition of human remains, one can determine whether they were fishers, marine mammal hunters, or farmers.
In recent studies, besides C and N isotopes, S isotopes are also used. This isotope system is useful to trace geographic origins of humans, as the sulfur isotope signature of plants primarily mirrors that of its geological surroundings.
The challenges for archeologists are the low amounts of C, N and S in the samples they are investigating. Especially the simultaneous analysis of all three isotope systems has been hampered by the accurate and precise analysis of small concentrations of sulfur (0.2 – 0.3 %). Therefore, traditional analysis of C, N and S isotope compositions was performed in two steps: firstly, the analysis of C and N in 0.5 – 1.0 mg of bone collagen, followed by the analysis of S in 10 – 15 mg of bone collagen. With the Thermo Scientific™ EA IsoLink™ IRMS System, this challenge has been overcome. The EA IsoLink IRMS system can measure all three stable isotopes in one a single sample drop of 1 mg of bone collagen.