Strontium Isotopes: The New Hot Archaeology Trend

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?

Works Cited

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).

24 responses to “Strontium Isotopes: The New Hot Archaeology Trend

  1. Depending on the geographical area and what previous research has been done, strontium alone may not provide very much information. For example, take my post here: http://killgrove.blogspot.com/2010/01/more-and-more-migrants.html. Had I just looked at the y axis (Sr), I would have been able to identify only one person as an immigrant to Rome. But I added oxygen isotope analysis on the x axis – it’s not without its own complications, but it is an independent isotope system. Fortunately, in Italy, strontium varies roughly N-to-S and oxygen varies roughly E-to-W, so identifying immigrants becomes a bit easier.

    Still, in my dissertation (Migration and Mobility in Imperial Rome), I lay out the problems with each isotope system. Oxygen is affected even more than strontium is by diet. And in Rome, the craziness of the volcanic (young) geology on top of or interspersed with old geology, not to mention the fact that the Romans were importing the majority of their water from low-Sr sources into a high-Sr area, means that it becomes quite difficult to narrow down a “local” range.

    Strontium isotope analyses have been around in archaeology for about three decades now. I don’t know why it was the hot new thing at the SAAs – seems to me that the most interesting methodological papers came out in 1999-2005. Maybe Sr isotope analyses are just de rigeur now? I’d initially planned to do only Sr until I realized that I needed multiple isotope systems to make any sense of the data. I thought the hot new thing was to use two or more isotope systems – usually Sr with O, sometimes Sr with Pb, or a combo with S – as in all the latest articles by J.A. Evans and most of the stuff by Eckardt and Muldner too (the Brits are fairly well driving this trend).

    Also, I haven’t read Larsen in a while, but I wonder if the methodology behind Sr isotopes has changed in the last 12 years? I know it has with oxygen isotopes. Mostly, though, it sounds like Larsen is referring to Sr analysis from bone – and no one does that anymore. Sr isotopes in bone tell you where a person died (because groundwater leeches into the bone) – you know where the person died (presumably, since that’s where you dug the person up), and I’ve only seen dental enamel articles recently.

    • Thank you for the amazing comment. There are a lot of good points in there. I agree that Sr has become part of the archaeological toolkit, although it does seem that this needs to be analyzed a little deeper before use. I don’t know too much about the exact method of taking Sr and the analysis, or how much it has changed… perhaps this will make a good post for the future.

      • You’re definitely right. Since the geosciences department at UNC can do Sr on-site, some archaeo grad students have tried it out but then realized that Sr doesn’t “pinpoint” homeland the way we used to think it did. The main change I’ve seen recently in Sr – discussed at this year’s AAPAs in a poster session – is a move away from using M1 enamel (because it starts forming during breastfeeding, and the mother’s Sr signature can theoretically pass to her child, meaning M1s may reveal a person’s mother was an immigrant and the person himself was not) and a move towards using P3 or P4 enamel (which still forms in childhood, but well after weaning).

  2. A great article, thank you for providing some key information, and the helpful discussions in the comments.

    As a pre-masters student, what would yourself and Dr Killgrove recommend to read reading up to date information on the basics, and current theory on the use of light stable isotopes in archaeology?

    • Other than the basic bioarchaeology overview texts which all seem to have a quick review on the subject, I would check out “Biomolecular Archaeology” by T. Brown and K. Brown. It was recently published and might have more up to date information. Other than that I would just start looking at current articles like the three 2011 ones I mentioned in my works cited. These will give you a good idea of what anthropologists are currently using.

  3. Thank you for the prompt response! I’ll shall check out the Biomolecular Archaeology book, and shall hunt out some recent articles as well. Apologies for not grammar checking my comment before posting. Please keep up the great blog.
    Thanks,
    David.

  4. For light isotopes (C/N, O to some extent), the single most useful thing I found was Anne Katzenberg’s chapter in The Biological Anthropology of the Human Skeleton. For Sr, I mostly used Janet Montgomery’s PhD dissertation, “Lead and Strontium Isotope Compositions of Human Dental Tissues as an Indicator of Ancient Exposure and Population Dynamics.” But I had some very specific issues to contend with in regard to Sr (and needed to do lots of geological/statistical modelling).

    Dissertations are a great place to start if you want to learn the basics of isotope analysis. If you think about it, dissertations are usually written to be overly didactic (in order to explain the methods to committee members with no experience in the subject). You can, for example, skim my diss for info on C/N/O/Sr – including the background, techniques I used, and how I went about interpreting the data: http://www.unc.edu/~killgrov/Migration-Mobility-Imperial-Rome.html.

    Once you have a handle on the basics, then go read the latest articles – usually in J of Archaeo Sci or in Archaeometry. Most of them won’t make sense (and will seem like total magic – at least, they did to me) until you understand isotopes and isotope analysis a bit more in depth.

    Good luck!

  5. To add to Kristina comments ( I have just found this discussion) Strontium Sr isotopes need to be undertaken on tooth enamel not bone. We favour the M2 because it avoids the weaning signature and this is particularly important when doing oxygen analysis. The imporant thing about isotopes is that it is an “exclusive” technique. It cannot tell you where someone comes from, but it can tell you where they do not come from. This is because no signature is unique. Kristina is right when she says it can be confusing. The biggest problem is the lack of reference data to compare the tooth data with.

  6. Hello,

    I as reading your very interesting posts and had a question about migration isotopy. All the papers I have come across give very detailed prep. steps, yet now seem to say what component (carbonate of phosphate) is being sampled for Sr. studies. Am I missing something here is it only phosphate that is used in for Sr? Can someone explain this a bit better pretty please.

    • Hey Steve, as far as I know both carbonate and phosphate are being used, although phosphate is usually less susceptible to diagenic (breakdown) processes. However, carbonate is more used when analyzing oxygen isotopes in teeth. I don’t know too much about the topic, but articles should specify which one they are using!

    • I think actually the carbonate (hydroxyapatite) is being isolated and measured for strontium isotopes, since Sr replaces Ca in minerals in the body such as hydroxyapatite. With oxygen isotopes, you can measure them from either carbonate or phosphate (and there are various equations that help relate one measurement to another). I could be wrong, though – the actual chemistry behind isotope analysis is not my primary research area. I’d suggest looking at Janet Montgomery’s PhD dissertation (reference in a thread above) for more info, as it’s very good and quite detailed on the procedure and the chemistry behind Sr isotope analysis.

      • Thank you both so much for your help. Might you permit me a follow-up question? So I have now been reading through the Sr studies and I am wondering, it seems tacitly obvious that Sr and O studies are really only used for a dichotomy type studies 1.) birth/early life residence and then 2.) late life/death residence, and all the stuff in between is a jumble of unknowns. Do these studies ever address the potential of looking at more than one movement? Can Sr and O do this (by this I mean would it be possible to say sample tooth, rib, iliac crest, femur to get a sort of mix of where people may have moved over their lifetime or do we get kind of washed out signal? I cannot seem to find any really critical papers on this topic. There seems to be a lot of critical issues in isotopic research and I was wondering if you knew of any good places to look for really thorough critiques of isotpic migration work? Any help would be greatly appreciated.

    • Steve, you can’t really look at bone for Sr/O isotope studies because of the issue of diagenesis (that is, the bone will tell you where someone died, which you already know because you dug them up). But you can definitely look at different teeth, as they form at different times. So Tracy Prowse (et al. 2007, AJPA) looked at changes from M1 to M3. But you could easily add in a bunch of other teeth to see how Sr/O vary at different ages. The problem with long-term studies of migration, as you suggest, is that after the 3rd molar forms, you’re kind of out of luck – I don’t think it’s currently possible to test different bones with different turnover rates for Sr/O because of diagenesis. But there are people who are microsampling teeth, to get a very fine resolution of migration or diet, and I think that analyzing dentine has potential (and less diagenesis than bone).

      As for critical articles, there’s not a lot for Sr/O, or at least not as much as there is for C/N because Sr/O is a much newer technology/method. A large group of Brits are doing quite a lot of isotope studies – Janet Montgomery, Jane Evans, Carolyn Chenery, Hella Eckardt, Gundula Muldner, etc. Check into their most recent articles.

      You can also look at Pollard et al. in AJPA early view called “These boots were made for walking.” It does a good job of outlining some of the issues with interpretation of Sr/O/C and will probably give you a bunch of references to follow.

  7. Hello once again,

    I have been combing my notes and the articles I have read and there seems to be two persistent questions that should be easily answerable yet I seem to have forgotten and/or cannot find with good equation and was hoping once again I might get a nudge in the right direction from this fantastic forum.

    My first question: In regard to the uptake of O and Sr into teeth and bone, how long is long enough when residing in a new residence to have the local signature incorporated (i.e. if I have a signature from Place A and then move to Place B, how long will the transition from A to B, in terms of O and Sr signatures in bone and tooth {assuming teeth are still active}, take?

    My second question, If you would allow me one more: Why do studies not more readily use hydrogen in their work? I see that H2/H1 has become increasingly common in studies on hair etc.. but there is not much in terms of paleomigration? Do you think this has to do with the significant exchangeable portion of H in collagen {as H is not in bioapatite correct?…} etc.. Why do people not use H more in tandem with O, do you think H would give any further useful information that O does not (it seems by and large they would be similar in the info. they provide, but perhaps I am missing something….).

    I feel like these are rudimentary question but perhaps someone can help me out all the same, pretty please, thanks.

  8. Steve – for your first question, you can theoretically see changes in Sr/O within the teeth at a very, very fine scale. That is, if you microsample the enamel, you can look at movement within the age range at which the tooth is forming (e.g., every few months in a first molar that forms from 0-3yo). I say “theoretically”, though, because I’ve only ever seen this done in zooarchaeological studies, not human studies (e.g., Pellegrini et al. 2008, Rapid Communications in Mass Spectrometry 22:1714-26). Bone is more difficult to do this with, partly because of the issue of diagenesis and partly because the turnover in bone is a much different process than the standard, accretional process of enamel formation. So I wouldn’t use bone to do this.

    Second, I don’t know why there’s a lack of H studies. There are a few sulphur studies, but those data tend to correlate well with Sr data, so I haven’t tried it yet. Maybe you could look at The Biomarker Guide (2005) for some additional information, if H isotopes aren’t in the Biological Anthropology of the Human Skeleton volume.

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