Welcome to the second video of the third module, where we will introduce you to the use of stable isotopes for the reconstruction of diet of past peoples. Don't be intimidated by the biogeochemistry involved. It's all pretty straightforward and we will explain it at an introductory level. Also, there is a list of additional sources on the website that you can consult, should you want more explanation. Let's begin with defining an isotope. Quite simply, what we are talking about are atoms containing a different number of neutrons. So think back to high school chemistry. Recall an atom is composed of three components. Electrons, which orbit the nucleus, and then protons and neutrons, which exist inside the nucleus. All elements have a specific number of electrons and protons that define them, and most atoms of that element will have the same number of neutrons as protons. So for example, carbon has six protons, and most carbon atoms will also have six neutrons. 6 plus 6 equals 12, so we call this atom carbon 12. But a small proportion of carbon atoms will have an extra neutron for a total of 7 neutrons. 6 plus 7 is 13, so we call this carbon-13. The same idea applies to nitrogen. Most nitrogen atoms have seven protons and seven neutrons, and thus are classified as nitrogen-14. But a small percentage have eight neutrons and thus are called nitrogen-15. And that's all an isotope is, easy. Atoms of the same elements, meaning they have the same number of protons and electrons, with a differing number of neutrons. Most elements of biological interest have more than one isotope including carbon and nitrogen, of course, but also hydrogen, oxygen, sulphur, calcium and strontium. In this research, we are primarily interested in the isotopes that do not decay over time, the so-called stable isotopes. In contrast to these stable isotopes there are radiogenic, or unstable isotopes such as carbon-14. Unstable isotopes decay according to a specific half life, a principle which is used for absolute dating purposes. One way to think of the difference is that the amount of a stable isotope in a tissue, say your bones or teeth, will remain the same over time. So whether it's measured 100, 1000, 10,000 years from now, as long as there's been no environmental degradation, the value will be the same as it was during life. The amount of an unstable isotope, however, will begin to decrease immediately after death. So how does the existence of these stable isotopes translate into something we can use to reconstruct what past people were eating? First, what's important to keep in mind about these isotopes is that chemically, they are the same. So carbon-12 and carbon-13 work the same way in biological processes and chemical reactions. But physically, they differ ever so slightly in their mass or their weight. So in the case of carbon, carbon-13 is slightly heavier than carbon-12. Small as it may be, the slight difference in mass results in differences in the amount of energy that is required to use them. So the heavier isotopes, carbon-13, nitrogen-15, will move and react a little bit more slowly and require a little bit more energy. Biological systems seek to maximize efficiency and whenever possible will discriminate against the heavier isotope in favor of the lighter isotope. This results in a difference between the global abundances of the different isotopes and the abundances that will form in different components of an ecosystem. So, for example, many plants, including wheat, rice, and barley, those that grow in more temperate parts of the world called C3 plants, after the photosynthetic pathway they use, will discriminate strongly against carbon-13. So relative to an international standard, this gives them a ratio of carbon-12 to carbon-13 atoms of -20 to -35 per mille. The ratio is denoted here by this symbol which is a Greek delta symbol. And per mille is like percent, except that it's in units 1,000 instead of 100. We use it because the abundance of the light to the heavier isotope is actually quite large. Then there's another group of plants called C4 plants, because they use a different photosynthetic pathway that doesn't discriminate as heavily against the heavier isotope. This results in ratios of carbon-12 to carbon-13 of -9 to -14 per mille. C4 plants include corn called maize, millet and sugarcane as well as other plants adapted to hot and arid environment. The ranges of these C3 and C4 plants do not overlap. The next step then is the demonstration that the stable isotope values of the food that you eat, that it becomes incorporated into your bodily tissues, into your bones and teeth, which is what we usually analyze in archaeology. But also into your hair and fingernails, skin and muscles, tissues that might be analyzed in living people or from recent forensic contexts. Each tissue will have its own offset. The difference between the stable isotope value of the food and the ratio that gets incorporated into the tissue. But this can be studied. It can be studied in different species and once known, taken into account. So the amount of C4 plant food relative to C3 plant food that you eat is reflected in the ratio of carbon-12 to carbon-13 that we measure in your body. Whether from someone living today or someone living many thousands of years ago. A similar phenomenon exists for nitrogen, except instead of varying between plant types, the ratio of the heavier to lighter isotope varies according to trophic level. A trophic level is where something falls in the food chain. So for example, primary producers, which are the plants, are at the bottom. Herbivores, which eat only plants, exist one trophic level above this. Omnivores, which eat plants as well animals, exist one trophic level above this, and finally, carnivores exist at the top. Between each trophic level, we see a jump of about three per mille in the ratio of nitrogen-15 to 14. So as O'Connell and Hedges demonstrate, this is why, for example in humans, a vegan, someone who doesn't eat any animal products, will have a lower stable nitrogen isotope ratio than someone who does eat meat. As most human are omnivores, we typically fall between herbivores and carnivores. The proportion of meat to plant food can be inferred from someone's stable nitrogen isotope values when the values of the foods they might have been consuming are characterized. An additional common usage of stable carbon and nitrogen isotope values is to determine the amount of marine versus terrestrial food that people were eating. So basically how important sea food was in their diet. In terrestrial ecosystems, so on land, plants only have one source of carbon, and that's carbon dioxide. This has a carbon-13 to 12 ratio of minus 7 to 8 per mille. But in aquatic ecosystems, there are additional carbon sources, the main one being dissolved carbonate, which has a carbon-13 to 12 ratio of 0 per mille. Hence the stable carbon isotope values of plants and animals from terrestrial versus marine ecosystem are usually distinctive. With nitrogen, it's useful because in marine ecosystem usually there are more steps in the food chain. So think of tiny fish being eaten by a small fish, and then that small fish being eaten by a bigger fish, and that big fish being eaten by even bigger fish, etc. So it means top level carnivores such as seals and top carnivorous fish can have very high stable nitrogen isotope values up to and even above 20 per mille. The final aspect of stable isotope reconstruction of diet is about breastfeeding and weaning. If you think about it, an infant consuming mother's milk is essentially consuming her tissue, and this results in the infant being a trophic level above their mother. A lot of research in osteoarchaeology has used this phenomenon to estimate for how long different populations breastfed their infants and children. The weaning process begins once non breast milk foods are introduced. Usually at the latest by six months of age, as by then the infant needs additional sources of nutrition. It ends when nursing ceases completely. This process can take many years, as you will hear in the next video. Past populations have been found to have fully weaned their offspring from anywhere as early as one, to as late as six years of age, averaging around three years. When we conduct stable isotope analysis, osteoarcheologists have to think about what tissue to use to best answer the questions in which we are interested. If we use bone, we can look at the protein component, the collagen, or the mineral component, the hydroxyapatite. Research has shown that the collagen mostly reflects the protein component of one's diet. Whereas the hydroxyapatite reflects whole diet, including carbohydrates, fat, and protein. One unfortunate reality of stable isotope analyses of the mineral component is that there's no nitrogen in it. So we cannot measure that ratio. If we sample a tooth, we can look at the mineral in the outer enamel layer, or the underlying dentin layer, which contains both collagen and a mineral component. Also important to consider is the period of life that we capture. If we take a bone sample, say a piece from the shaft of a long bone or a rib, we are capturing the five to ten year period of a person's life before they died. And that's because as you've learned, bone is constantly turning over and remodeling. In contrast, teeth, once formed, do not remodel. So we capture the period of a person's life from when that tooth formed. As seen in this figure, the teeth form at different ages. So we know the age we are capturing. In this video, you've learned the basic principles of stable isotope analysis. Focusing specifically on carbon and nitrogen, as they are by far the most commonly used in our fields. There are other isotopes that are also used to reconstruct diet, including oxygen and sulphur. With this knowledge, you are now able to see examples of research and appreciate the tremendous amount of information we've gained via stable isotope analysis. Coming up next is an interesting case study on the Lake Baikal region of Siberia that will demonstrate the value of stable isotope research in osteoarchaeology. See you soon.
