Welcome to the second case study of the mobility and migration module. In this video we will learn about strontium isotopes and how they have been increasingly used to infer patterns of migration in past populations. Recall that video 3.2 reviews the basics about stable isotopes. What they are, how they work and how osteoarchaeologists can detect different isotopes. That video was focused on the isotopes that are commonly used for the reconstruction of diet. But as you will learn, there are also isotopes that are used to tell if a person moved from one geographic area to another in their lifetime. Perhaps the strontium isotope values of your bones and teeth would reveal that you had moved from one part of the world to another. You can tell us on the online discussion forum. Strontium is a trace element found in rocks which through weathering gets into water and soil. Which there by becomes incorporated into the tissues of plants and animals. There are four naturally occurring strontium isotopes present in different proportions as shown here. Osteoarchaeologists use the ratio of two of these, strontium 87 to strontium 86 for migration research. That's because the amount of strontium 87 varies in rocks according to the age of the rock. For example, very old rocks, such as granites and shales, have higher strontium 87 to 86 ratios than younger rocks. So the main type or types of rocks underlying where you live, referred to as bedrock, will give you a strontium isotope signature that varies from geographic areas composed of different bedrock. This figure shows the strontium-87 to 86 values of bedrock from the United States. Such maps are called isoscapes. Importantly, the world's oceans have a near constant strontium-87 to 86 ratio of 0.7092. Areas along the coast will have the same or at least a very similar value. Moving inland and away from coastal environments, bedrock strontium-87 to 86 ratios have been found to vary between 0.703 and 0.760. In 1985, Ericson was the first archeologist to demonstrate that the strontium-87 to 86 ratio in human bones and teeth reflects the strontium ratio of the geographic area that a person lived in while that tissue was being formed. One may ask how does strontium even get into our bones and teeth. Well, strontium has a molecular structure similar to calcium. Calcium, as you know, is a major component of the mineral of both bones and teeth. And quite simply a very small amount of strontium isotopes will replace calcium in hydroxyapatite. Now recall that tooth enamel forms during infancy and childhood. The last bit of your enamel in your third molars forms at 12 to 13 years of age. And after this, the chemical composition of your enamel does not change. So for your entire life, this enamel holds a record of your childhood home. In contrast, your bones as you know are always turning over, remodeling. And so, always changing and reflecting the most recent period of life. So the strontium-87 to 86 value of an adult's bone mineral will reflect their adult home. Different strontium-87 to 86 values in a person's enamel compared to their bone can therefore indicate a geographic change of residence from childhood to adulthood that is a migration event. Another common approach is to characterize the local bio-available range of these strontium isotope ratios and consider anyone who falls within two standard deviations of the average as local, and anyone who falls beyond this 95% range is non-local. This is shown in a figure here by Boric and Price, where a relatively high number of non-locals entered the Danube Gorge's region in the North Central Balkans around 8,200 years ago, which they interpret as evidence for early farmers entering the region. The non-local could come from an adjacent, close-by region that has different bedrock, or from hundreds or even thousands of kilometres away. Matching the non-local strontium isotope value to that of geographic regions with the same value can allow us to see what possible homelands they came from. And sometimes in combination with other isotopes, such as oxygen and sulphur, as well as clues from the skeleton itself and material culture items, we can deduce which homeland was most likely. Now the principle underlying the use of strontium isotopes to reconstruct migration in past populations is quite straight forward, but the reality is seldom as simple. And we have to keep a few things in mind that impact the feasibility of this kind of research. First, in order to use this method there has to be sufficient isotopic variation in the geographic area under study. There are large areas of the world underlain by the same bedrock or with bedrocks are very similar strontium isotope values, such that migration within and between this areas simply cannot be detected. Second, one's diet can influence if their strontium isotope values are useful to track migration. One example of this is the consumption of a lot of seafood. As regardless of where one lives, if they eat a lot of seafood they will have a strontium-87 to 86 value close to or of sea water. Another example concerns modern day and some select past groups and individuals who ate a lot of imported foods. If you eat a lot of non-locally grown food as is the case for many of us today in western developed nations, your strontium-87 to 86 value will not be reflective of where you live, but rather where your food came from. Third, the accuracy of the method of determining what the local strontium isotope value of a region is is important to consider. Archaeologists have characterized the strontium isotope value of an area in different ways including by direct measurements of exposed bedrock or soil; or by determining the value in plants; or from the bones of small mammals; or the shells of land snails, animals that can only roam over a small area. Each of these methods has it's own advantages and disadvantages that have to be kept in mind. And finally bone strontium ratios can be potentially altered due to chemical, physical or biological changes after death particularly from the soil, if the bone was buried, and the assessment of any such alteration must be done for each and every sample. But if all of these factors can be taken into account, which is often the case, strontium isotope analyses of past peoples can be a powerful method to reconstruct migration. Now to talk to us about one example from the Caribbean region is Dr Jason Laffoon is a postdoctoral researcher with Leiden University, and the Free University Amsterdam specializing in isotope bio-archaeology. Jason, thank you very much for joining us today. So to begin, I'd like to ask you to tell us about your research area, and what some of the main goals of this research are. >> Yes, thank you for having me. The research area is the Caribbean region which is consisted of series of island archipelagoes and the surrounding costal mainland of Central and South America. The earliest element of the islands of the Caribbean occurred for about 5,000 BC. But not very much known about this early inhabitants mostly because of issues of preservation. The lack of well preserved archaeological or osteological materials to study. My research primarily focuses on the later occupation of the islands of the Caribbean. What we call the late ceramic age that dates from approximately AD 500 to AD 1500. And the previous research in the region had revealed the widespread circulation of various aspects of material culture agent from ceramic styles to shared lithic resources, to similar manufacturing techniques. So we knew the people were probably moving from the mainland to the island, in between the islands and my research question focused on where exactly people were moving from and what were the patterns of migration over time. >> So in order to investigate the patterns of migration over time, you use strontium isotopes. As our users have been learning, the first step in such research is to characterize the strontium isotope variability of the region that you want to investigate. So can you tell us a bit how you undertook that task? >> Certainly. The method that we found that worked best in the Caribbean region was the bioavailable method. And that's where you characterize the variation for a site or population based on independent sampling of biological samples. >> Okay. >> So we focused out primarily on modern plants and archaeological and modern animal remains. And we analyzed more than 300 samples from all the different islands and island groups in the Caribbean region. >> Fantastic, so that's a very good base with which to start. The next step then being the analysis of the humans from these different archaeological sites in different regions and time periods. Can you tell us a bit about the sample size that you were able to look at, and also what tissue you analyzed? >> Certainly, in terms of the tissues, we focused solely on dental enamel. And that's for two primary reasons. Dental enamel is the most resistant to diagenic alteration. That's changes to the tissue based on being buried in the ground for so long. And we tried to develop a very large data set. So what we did is we focused on as many individuals as we could, and for my dissertation we analyzed more than 360 human individuals, from 26 different sites on 12 different islands. >> Amazing, very, very large sample size. So with all of that data, you must have been able to say quite a bit. If you could tell us, then, about some of the more remarkable or spectacular results that came out of this research. >> Yes, of course. So the first thing that we found that was one of the most interesting results actually was that, for every single population that we analyzed, we identified non-local immigrants. And the proportions of immigrants at each population were highly variable, ranging up to more than 30% for some populations. The second very interesting result that we found is that these non-local immigrants for each population were not all originating from the same place, and they displayed a high degree of isotopic diversity, indicating diverse origins for different immigrants. Other very interesting results included that there was no sex-based differences in immigration, so that an equal number of male and female immigrants proportionally over time. And that suggested that was there was no migration linked to post marital residents, such as patrilocal or matrilocal residents. For some populations but not others there were clear differences between local and non local immigrants in terms of both dietary practices and burial treatment. The overall chronological pattern that we observed was a decrease migration over time. And we think this is most likely linked to demographic variables such as increasing population densities and subsequent decrease in social distances between settlements. And lastly for the sole early colonial period of population that we analyzed, we identified several very long distance migrants, individuals originating from the Maya regions of Mesoamerica and from Western Africa. >> Wow, very interesting. Thank you very much for taking the time to share your research with us today. >> Thank you for having me. >> In this video, you learned about strontium isotope analyses of the mineral in teeth and bone as a method to reconstruct aspects of an individual's migratory history. With this method, it is in some cases possible to determine that an individual was born far away from where they died. You also heard about the strontium isotope research in the Caribbean, and how such data have improved our knowledge of the prehistoric colonization and migration of the area. Coming up next is the final case study for this module where you'll hear about how the genetic information contained in each and every one of us and in fossils going back hundreds, thousands, and even hundreds of thousands of years, has revealed a tremendous amount about our migration. Both at global and continental scales, as well as at smaller regional scales. See you soon.
