In every single bioarchaeological session at the Society for American Archaeology, their was at least one presentation or poster that included the use of strontium isotopes in their analysis. It is the new hot method, the buzz word of the discipline, and becoming increasingly frequent. It is being primarily used as a way to look at migration, as well as a method for determining local versus non-local patterns in diet, disease and burial patterns. When interpretations are made from strontium isotopes they are often presented in such a way that the biases and errors are unknown. Therefore, it is important that readers have an understanding of what strontium is, how it is being used, and the inherent problems with this method.
Strontium (Sr) is a type of isotope found in human bone that can be used to infer geographic location. Bentley (2006) notes that Sr isotopes serve as “geochemical signatures” that are used to discover where an individual has been. Sr isotopes are introduced into the body through geologic materials such as soil, which are taken into plants. Through this uptake, Sr becomes embedded into the local food chain. Sr replaces some of the calcium in the bone and tooth enamel. This means that by analyzing Sr in bones and teeth, we can assess where an individual has been. As Bentley (2006) argues that the key to this method is matching the isotopic signatures from the individual to the available signature at the hypothesized origin. Human Sr levels can be compared against faunal remains or snail shells as a way of discerning where the individual originated from.
However, this process is more involved and complicated than just matching Sr levels in teeth or bone to Sr levels in the soils or comparable faunal or snail remains. There is variation in the way that plants intake Sr, and trophic level (level in the food chain) can also be a factor. The consumption of high calcium foods can also change Sr levels. Diagenesis, chemical, physical or biological change in bone due to soil, can also cause changes in the levels of Sr (Larsen 1999). All of these factors must be considered when attempting to use Sr as a method for determining location.
Another problem which is less discussed, is the issue of what is being used for comparison against the human remains. Traditionally, archaeologists want to use remains of an animal from the predicted origin region that was fairly immobile such as domesticates and snails. One sample is taken from the remains and used as the Sr marker for that region. The problem with this is that we do not yet know the variation within a single individual, and whether certain areas may have different Sr levels. A study done by Deskaj (2011) showed that within a single snail shell there are drastically different Sr levels. Further study is needed to see the range of variation within single individuals.
Despite its problem, Sr isotopes are still a popular technique for determining mobility and migration in ancient populations. Knudson and colleagues (2004) used Sr levels to look at mobility between populations in Bolivia, Peru and Chile from 500 to 1000 CE. Archaeological evidence shows that there is trade between populations, but it was unknown whether this also coincided with the migration of people. By taking Sr isotopic readings from five locations across the study area they were able to determine whether individuals buried in the cemeteries were all of local origin or if there was mixture between the populations. Results of the comparison of the human remains against the Sr regional levels showed that there was migration of people within this region which may have aided in the circulation of artifacts (Knudson et al. 2004).
Sr isotopic analysis has become increasingly popular. Some more recent uses include Price and colleagues (2011) analysis of a Danish military cemetery from 10th c. CE shows that the army consisted of primarily foreigners. The sample included 48 burials from the fort, and Sr isotopes showed that none of the individuals were from Denmark, with Norway or the Slavic regions more likely. Another use is by Cucina and colleagues (2011) using Sr to understand migration patterns in the Yucatan. They used Pre-Hispanic sub-adult remains from three populations, and found that there was a high amount of movement that correlated with artifact distribution. Chenery and colleagues (2011) also used Sr to look at population diversity on the Roman Northern frontier. They compared Sr variability in human remains at urban centres to these Northern outposts. They found that there was higher Sr variability within the urban centres, suggesting that at frontier forts there was less inclusion of foreigners. Like the other reports, Chenery et al (2011) did find artifactual evidence of foreign influence, but there was no Sr variation to support movement of people.
What other uses have you seen for Strontium Isotopes? Do the potential answers they provide outweigh the biases?
Chenery, Eckardt and Müldner. 2011. Cosmopolitan Catterick? Isotopic evidence for population mobility on Rome’s Northern frontier . In Journal of Archaeological Science 38(7).
Cucina, Tiesler, Sosa and Neff. 2011. Trace Element Evidence for Foreigners at a Maya Port in Northern Yucatan. In Journal of Archaeological Science. In Press.
Price, Frei, Dobat, Lynnerup and Bennike. 2011. Who was in Harold Bluetooth’s army? Strontium isotope investigation of the cemetery at the Viking Age fortress at Trelleborg, Denmark. In Antiquity 85(328).
Knudson, Price, Buikstra and Blom. 2004. The Use of Strontium Isotope Analysis to Investigate Tiwanaku Migration and Mortuary Ritual in Bolivia and Peru. In Archaeometry 46: 5–18
Larsen. 1999. Bioarchaeology. Cambridge University Press: UK.
Deskaj. 2011. Presentation on Strontium Analysis in Albania. http://sylviadeskaj.wordpress.com/
Bentley. 2006. Strontium Isotopes: From Earth to Archaeological Skeleton. In Journal of Archaeological Method and Theory 13(3).