Now that field collection is finished, the next stage of ZANBA can enter full swing – strontium extraction and analysis. The high level of organic material in the plant samples makes extracting the strontium a long process – longer than extracting it from the human or animal teeth that are often the target of archaeological strontium studies. The samples first have to sit in concentrated nitric acid to begin digesting the organic materials. After the nitric acid has fully reacted with the samples, I add a strong hydrogen peroxide solution to continue the digestion process. The process involves a lot of waiting – it can take more than a week for the chemicals to finish reacting with the samples.
The processing takes place in a clean lab under a fume hood, so the number of samples that can be digested at any one time is limited by space, supplies, and the other researchers who need to use the lab. Getting through all 126 samples is going to be a challenge!
Davide has finished collecting the plant samples for the strontium isotope base map, and his excellent work deserves serious recognition. Of the 137 collection points I had originally planned – spread out over the many peaks, valleys, hills, and plains of central Sardinia – Davide was able to sample 126 points. That’s 92% of the proposed samples, collected by navigating rough roads and heavy underbrush to reach remote locations in all kinds of weather, not to mention while staying on time and within budget. Congratulations, Davide, on work very well done!
In the end, our study area of approximately 6580 km2 was sampled at an overall density of about 1 point per 52 km2. This is quite a high density and will allow for the creation of a very representative domain map, in which each type of bedrock is characterized by a proportionally large number of samples. It will also allow us to begin exploring the utility of high-density domain mapping compared with maps produced by machine learning methods for interpreting archaeological remains. The interpretative possibilities are incredibly exciting!
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.
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.
It took longer than expected, but the first batch of plant samples is finally here! They had an arduous journey if the state of the box is anything to judge by, but the samples themselves are in good shape and they don’t show signs of mold. I can start the preparatory processing right away, and I should have them ready for chemical processing by the end of next week.
The first steps in processing the samples are physical. The samples need to be frozen for a few hours and then freeze-dried for a couple of days. Each point on the map is represented by a set of three samples, so once the samples in each set are freeze-dried, they’ll be ground up and mixed together to create one homogenized sample. The homogenized samples will then be processed chemically before they’re analyzed for strontium.
Twelve sets of samples are already in the freezer and will be moved to the freeze dryer by the end of the day. Exciting progress!
Great news for ZANBA – the first batch of botanical samples for isotope mapping central Sardinia is on its way to Cardiff! Davide has done an impressive job with the fieldwork, already collecting material for 31 of the 137 total points to be sampled.
Each of the 137 points is inside a particular lithological zone, with larger zones being represented by greater numbers of points. When Davide samples, he takes leaves from trees and large bushes: three different plants representing three different species (when possible) located within a 500 meter radius of each point. Taking multiple samples per point allows us to do homogenized sampling. In homogenized sampling, the leaves from the three samples will be freeze-dried, ground up, and carefully mixed to create a laboratory sample that captures the variability in the area around each point. Homogenized sampling helps us create a more representative isoscape – one that’s less negatively affected if any of the plants Davide samples happen to be outliers.
The samples should arrive sometime next week, and I can’t wait to get into the lab to start processing them!