Category: Big Ideas

Strontium Analysis in Archaeology

The analysis of strontium isotopes is an increasingly common method in the toolkit of archaeology. Strontium analysis helps archaeologists understand where people lived in the past. It can provide insight about whether people were immigrants to an area or whether products like meat and wool were traded over long distances. Strontium analysis is being used to question such conventional wisdom as how Hyksos “invaders” took over New Kingdom Egypt and whether the famously nomadic Scythians were really so nomadic. But how does strontium analysis provide these insights? How does it work? What can it do, and – importantly – what can it not do?

All isotope analysis of biological materials works by exploiting the fundamental fact of the food chain. The food, water, and even air that animals and plants consume have chemical links to their environmental conditions. For example, in the radiocarbon analysis of plants, the carbon isotopes reflect the composition of the air when the plant was alive, and the slow breakdown of these isotopes after the plant has died allows for the plant to be dated. In oxygen analysis, factors like altitude and precipitation patterns affect local hydrological cycles, leading to different ratios of oxygen isotopes in drinking water.

Strontium analysis relies on the way the chemicals in soil and water derive from bedrock. Strontium has similar properties to calcium, so it can be substituted for calcium when living organisms build tissues like bone and tooth enamel. Strontium in bedrock is released when the bedrock weathers into soil, or it can get leached into the water that flows through or around the bedrock. The strontium then enters the food chain as plants draw nutrients from the soil and water where they grow, and it gets incorporated into human and animal tissues as they eat the plants and the animals that have been feeding on the plants.

Figure 1 from Holt et al. 2021

Strontium analysis wouldn’t work if all the strontium in all the bedrocks were the same, but helpfully it isn’t. Bedrocks have different ratios of strontium isotopes in them. Chemically, isotopes are atoms of an element that have the same number of protons but different numbers of neutrons. As an analogy, you can think of elements as ice cream and isotopes as their different flavors. Mint chocolate chip and butter pecan are both ice cream, but if I gave you a bowl with three scoops of mint chocolate chip and one of butter pecan, you’d have no trouble telling how much of each flavor was in the bowl. The element strontium has four flavors (isotopes) that occur commonly in bedrocks. Archaeologists can separate the strontium from a sample, and by looking at how much there is of one flavor (the 87Sr isotope) versus another flavor (the 86Sr isotope), archaeologists can describe the nature of the strontium in an area – its 87/86 strontium ratio.

Understanding a place’s strontium ratio lets archaeologists think about whether various organic tissues could come from that area. All kinds of tissues can potentially be used for strontium analysis, but the most common are bone and tooth enamel. Tooth enamel is particularly useful because it’s resistant to absorbing more strontium from the soil where it was buried – which could mess up the results of the strontium analysis – and because it forms when an individual is a juvenile. This means it can be very helpful for identifying when a person grew up in a different place from the one where they were buried, which is great for archaeologists who want to understand issues like migration, nomadism, and trade networks in the past.

Strontium analysis does have limitations. Similar strontium ratios can be found in many geographical regions, so strontium analysis is better at identifying where a person was not from than it is for pinpointing where they were from. Because strontium analysis works by excluding possible places of origin, it’s most useful when applied alongside other isotope analyses that can exclude additional places of origin. Another challenge of strontium analysis is that it can be difficult to understand whether the strontium ratio of soils in the present is an accurate reflection of the strontium ratios of the past. Modern fertilizers and other soil treatments can affect strontium ratios, so archaeologists have to be careful in using modern comparisons for ancient individuals.

Properly applied*, however, strontium analysis can be a powerful tool for addressing many of the enduring questions we have about the past, such as understanding the nature of ancient diasporas or reconstructing pre-modern globalism. I look forward to many more studies like the fascinating examples cited above.

* A summary of Holt, Evans, and Madgwick. 2021. Strontium (87Sr/86Sr) mapping: A critical review of methods and approaches. Earth-Science Reviews 216: 103593.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 839517.​
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What Makes Us Resilient?

In my last post, I highlighted archaeological research that showed that strong communities – local-level groups that worked together and maintained social ties – helped past societies weather major disruptions, delivering better outcomes for human health and prosperity and even helping sustain institutions above the local level. These are promising results for us, pointing to at least one practical way we can combat the effects of the pandemic. Working to support and maintain our communities now will help us come out of this pandemic in the strongest way we can. But the researchers’ conclusions also raise an obvious question – what are local communities doing or providing that contributes to their larger societies being resilient to a crisis?

There are dozens of books on resiliency from the past decade alone, but most focus on recent societies and institutions. Resiliency is a popular topic in archaeology as well, but many studies of resilience in the past focus on just one culture or time period. Here I will highlight an excellent cross-cultural study by Peter N. Peregrine, an anthropologist and archaeologist at Lawrence University. Peregrine examined 33 archaeologically known societies that weathered 22 environmental crises to test two hypotheses: did either local participation in decision-making or rigid social norms help human societies weather environmental crises? These two strategies have both been identified as sources of resilience in contemporary societies, but archaeology can test their utility in a broader range of cases. Again, the strength of Peregrine’s study is that it analyzes completely unrelated societies across large spans of time and space. As Peregrine puts it: “If a predictor of social resilience to climate-related disasters can be identified and applies to societies of varying scales and complexities throughout human history, then there is good reason to believe that it can be used to create interventions applicable today.”

IMG_1553
Inside the water temple of S. Anastasia in Sardara, Sardinia – a site of communal ritual for the Bronze Age Nuragic Culture

Peregrine trained research assistants to code ancient societies that faced severe environmental crises according to where they fell on scales of “corporate-exclusionary” (level of participation in decision making) and “looseness-tightness” (enforcement of social norms). They also coded how resilient the societies were by evaluating seven variables – population, health and nutrition, conflict, household organization, village organization, regional organization, and communal ritual – both before and after the environmental crises occurred. You’ll notice how many of these variables are similar to those used by the LTVTP-NABO collaboration I highlighted in the previous post, reflecting the basic kinds of human well-being we hope to be able to maintain even in crisis situations.

Peregrine found a positive relationship between resilience to environmental crisis and high levels of participation in decision making*. When leadership was more fluid and open to input and action at local levels, societies were more resilient, maintaining higher levels of well-being throughout the crisis. Peregrine found the opposite for enforcement of social norms, however; societies with strict enforcement of social norms were less resilient to environmental crisis. For the diversity of cultures and over the long time scales studied by Peregrine, rigid codes of behavior were detrimental.

A pandemic doesn’t pose the same challenges as an environmental crisis, but the importance of Peregrine’s findings is still apparent. One factor in creating resilience is broad participation in local decision making and a willingness on the part of higher levels of government to listen to local voices. It makes sense that local people are the often the first to recognize how a crisis is affecting their particular community and that they are likely to have useful ideas for how the crisis needs to be handled to maintain well-being in their area. The current pandemic poses unprecedented hurdles for average citizens trying to participate in governance, but it is essential that we find ways. Making local decisions based on local input is key to maintaining our well-being.

 

* I prefer to feature open access research so everyone can read and evaluate the work for themselves, but if you happen to have access to academic journals, I recommend the following archaeological and historical study. It reaches conclusions similar to Peregrine’s regarding the role of local-level knowledge and participation in decision making for achieving sustainable soil use in case studies of ancient Mediterranean agriculture.

Butzer, K (2005) Environmental history in the Mediterranean world: cross-disciplinary investigation of cause-and-effect for degradation and soil erosion. Journal of Archaeological Science 32: 1773-1800.

 

 

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 839517.​
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Archaeology vs. COVID-19

Last week, I argued that archaeology has something useful to say in this time of Coronavirus. This may seem like a bold assertion, even to other academics. I wonder how many of my colleagues in economics, sociology, and psychology see archaeology as a social science like their own disciplines. I wonder how many people in general look at today’s problems – be they Coronavirus or climate change – and think “let’s ask the archaeologists.”

I’ll be the first to admit that my argument needs to be supported by evidence before anyone should take it seriously. So that’s what I intend to do: provide evidence that the broad human past gives us valuable insight into what we can and should do now. I’ll focus on open access studies that anyone can read, and I encourage everyone to read the originals and assess for yourselves whether these are good evidence for the contemporary relevance of archaeological insight.

My first piece of evidence is an article* by Michelle Hegmon and Matthew Peeples on behalf of the LTVTP-NABO collaboration. The Long- Term Vulnerability and Transformation Project (LTVTP) focuses on archaeological cases of social transformation in the arid and semi-arid United States Southwest and northern Mexico. The North Atlantic Biocultural Organization (NABO) focuses on cases that took place in the subarctic and arctic North Atlantic. Together, the LTVTP-NABO collaboration examined 18 examples of major social transformations as varied as the end of the Norse occupation of Greenland and the depopulation of the Mesa Verde region. The geographical, environmental, chronological, and cultural diversity of the cases studied suggests that any strong patterns probably hold true for human societies generally: that is, they point to specific ways that all human societies tend to respond to stress.

The researchers coded a broad set of variables, from institutional breakdown and depopulation to human securities**, migration, household organization, and changes in material culture (the archaeological catch-all term for “stuff”). The researchers then performed correspondence analyses among these different variables to identify meaningful relationships.

journal.pone.0208060.g003
Figure 3 from the cited article showing correspondence analysis of all human securities variables. Red and yellow cases have higher levels of breakdown.

Their results are fascinating and the paper is worth a detailed read, but some of their conclusions feel particularly relevant in the current crisis. First, there is a strong relationship between the breakdown of institutions and a decline in human securities. No one is an island: nobody thrives when we let our sustaining institutions crumble. How we get food, how we receive medical care, and how we keep interpersonal violence in check are all embedded in institutional systems. If those systems fail, we’re likely to suffer. We’re witnessing severe stresses on some of our systems right now as they struggle to keep up with the demands created by the current pandemic.

 

The hopeful part of the study is that the researchers identified communities and community security as strong predictors that social transformation would be less painful. If communities remained strong – if they didn’t disintegrate under the weight of social transformations – people experienced less food insecurity, less interpersonal violence, and less death. The researchers also found a positive feedback loop – strong communities could bolster institutional security, preventing or dampening the major institutional collapses that were found to be disastrous in the less fortunate cases. The authors conclude: “We must consider the people’s experiences because what happens at the local level can stabilize society, can augment people’s capabilities for contributing in positive ways, and thus can help avert disaster.”

I’m probably not the only one who’s found herself reaching out to friends she hasn’t talked to in years to check on how they’re doing in this crazy situation. It turns out this normal human impulse may have a practical benefit. Cross-cultural, cross-temporal evidence reminds us that our communities make us resilient. Strong communities protect and provide and care for us when larger institutions falter and can even prop up those institutions until they recover their footing. Community-building in the time of Coronavirus may not look like it used to, but it’s one of the smartest things we can do.

 

* Hegmon M, Peeples MA, on behalf of the LTVTP-NABO collaboration (2018) The human experience of social transformation: Insights from comparative archaeology. PLoS ONE 13(11): e0208060. https://doi.org/10.1371/journal.pone.0208060

** The researchers followed the United Nations definition of seven types of human securities to assess human security broadly: economic, food, health, environmental, personal, community, political.

 

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 839517.​
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Theory in Archaeology

The students in my Ancient Cities course recently encountered an insightful but challenging reading, a chapter in a book dense with that most hated of academic discourses: theory. Just say “theory” at someone – and I don’t mean only undergrads – and their eyes will glaze over as they visibly fight the urge to check their Twitter notifications.

Unless they’re like me. I love theory. Theory is my favorite thing in all of archaeology to teach. Understanding theory means deciphering the tangled logic puzzle of “how do we know what we know.” Understanding theory means making clear – and then criticizing – all the unstated thought processes that archaeologists go through in making sense of our information.

For example, take a string of numbers: 17351. You can observe the numbers written here, but if someone made you stop reading this post right now and asked you what those numbers meant, you’d probably say “who knows.”

But what if you kept reading and saw that I added some symbols in with the numbers: 17 x 3 = 51. Suddenly you can say something about the numbers. They’re a multiplication problem. There’s a relationship that makes the first three numbers equal to the second two numbers. The first two numbers should be considered as a unit, the third number is a unit on its own, and the last two numbers are also a unit.

We’ve applied theory to the numbers. Now we can say what they mean.

Archaeological theory works with artifacts like mathematical operations in a string of numbers. Theory structures the relationships among artifacts and other archaeological data, allowing archaeologists to understand how they interact. Without theory – and the application of theory, which we call method – archaeologists would endlessly collect old things without actually adding to our knowledge of the past. We would dig up artifacts, document architecture, maybe even identify some campfires and postholes, but we wouldn’t know what any of it meant. It would just be a string of numbers… er… artifacts.

The early days of European curiosity about the past were full of “strings” of artifacts – in reality, these were drawers, boxes, cabinets, and rooms that housed jumbled assemblages of ancient objects. Antiquaries1 of the late Renaissance and the Age of Enlightenment collected these objects – sometimes digging into barrows or other monuments to find them – and documented the existence of ancient sites. But they found themselves trying and failing to describe relationships among the sites and artifacts. They hadn’t developed any archaeological theory, so they couldn’t interpret what their finds meant. The problem was so acute that the Danish antiquary Rasmus Nyerup (1759-1829)2 complained that “Everything which has come down to us from heathendom is wrapped in a thick fog.”3

Fortunately, antiquaries began developing foundational theory that allowed them to start organizing finds. Ideas like the Three Age System – which was proposed in 1816 and divided European prehistory4 into Stone, Bronze, and Iron Ages based on the primary materials used for tools – allowed antiquaries to begin recognizing relationships among their materials. Other types of theory like the concepts of typology and seriation – the idea that artifacts that looked similar probably came from similar time periods and that these time periods could be arranged chronologically – moved the study of the past from antiquarian collecting into the budding discipline of archaeological explanation.

Great, you say. Antiquaries formulated theory, applied it to artifacts, established archaeology, and now we know what the past means. But not so fast – let’s return to that string of numbers. What if instead of making it a multiplication problem, I use different symbols: |1 x 7 – 3 – 5| = 1. This way of interpreting the numbers is equally consistent and so equally valid as the first.

Archaeology works like this, too. Often, multiple ways of applying theory give us internally consistent understandings of the past. What if I asked you for an ecological explanation of why I wore my orange sweater today? You might say I wore it because the weather turned cold and I wanted to stay warm. But what if I asked you for a postcolonial interpretation of why I wore my sweater? Then you might say it relates to my position within a historic system that channels global resources toward the west, making it possible for me to own a (probably excessive) number of warm sweaters. Both these ways of applying theory to my sweater produce perspectives that are true and useful for understanding my experience.

Now don’t get me wrong: the past is not all relative. Archaeologists can demonstrate that many ways of understanding the past are straight-up false, and it is not the case that every interpretation is as good as any another. For example, what if I ordered my numbers like this: 1 + 7 + 3 + 5 = 1. That’s simply wrong. Unlike my first two proposals, this interpretation of the numbers does not show a logical relationship. Or what if you claimed that I wore my orange sweater because I always wear orange on Wednesdays. Examining my previous Wednesday wardrobe choices would show you this is not an accurate explanation (I mean, last Wednesday I wore plaid!).

Even though not all understandings of the past are true, it is true that the past was as rich and varied as human life today, so it shouldn’t surprise us that multiple ways of understanding the past can be accurate and provide insight. And thanks to a long history of archaeological thinking, we’re not at a loss to distinguish the good explanations from the bad. Robust archaeological theory provides the tools we need to tell the difference between rigorous interpretations of the past and ones that don’t stand up to scrutiny.

  1. The term “antiquary” refers generally to a person who studies the past and is especially used to describe people who collected ancient artifacts and documented sites in the period before archaeology became a field.
  2. (2009). Nyerup, Rasmus. In The Concise Oxford Dictionary of Archaeology, 2nd edition (online). eISBN: 9780191727139 DOI: 10.1093/acref/9780199534043.001.0001
  3. Page 51 in Fagan, B. M., and Durani, N. (2016). A Brief History of Archaeology. Classical Times to the Twenty-First Century, 2nd edition. London and New York: Routledge. ISBN: 9781138657076
  4. The Three Age System was specifically applied to Danish prehistory when it was first proposed, but it was found to be broadly useful and adopted for European prehistory in general.
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