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!
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.
Most research in archaeology happens in a lab. Despite the images of sweaty excavators and big hats that come to mind when “archaeology” is mentioned, the bulk of archaeology happens when the digging is done. It’s a truism among project directors that you plan three days in the lab for every one day in the field, but the essential work that goes on behind the scenes is largely invisible to the public.
I’m a zooarchaeologist – an archaeologist who studies animal remains – and I do most of my work in labs. Right now, I’m working at the Muséum national d’Histoire naturelle in Paris, where I study the bones of micromammals like mice and voles. These tiny remains were excavated from Bronze and Iron Age sites on the island of Sardinia (c. 1700-300 BCE), and despite their small size, they help me answer big questions about the cultures I study. Micromammals are sensitive to the environments around them. Different species have particular preferences for habitats and living conditions, which means that identifying the micromammals at a site is a way to reconstruct the site’s environment. And reconstructing the ancient environment is fundamental to understanding everything from past economies to climate change.
A typical day of zooarchaeology includes multiple projects. Today, I’m working on three. I begin the day by tackling a taphonomic analysis of the micromammal remains. Taphonomy is the study of how ancient bones are incorporated into archaeological sites, and it includes everything that happens to the bones after the animals die. You can imagine why taphonomy would be important for interpreting ancient bones. Let’s say, for example, that the ancient environment was swampy, so the local micromammals were adapted to wet terrain. But if there were grasslands nearby, an ancient owl could nest in the swamp but hunt in the grasslands, scattering bones of grassland species around its nest. A case like this will give you a confusing mixture of grassland and wetland species showing up together – so what was the ancient environment really like? A careful taphonomic analysis can sort out which species died at the site and which were brought there by predators, helping differentiate the immediate local conditions from the wider surroundings.
After a morning in front of the microscope recording taphonomic clues, I’m ready to move to my second project. This project uses geometric morphometrics – a kind of spatial statistics – to analyze the shape of micromammal teeth. The shape of the teeth shows genetic plasticity, meaning that it changes depending on which groups of micromammals bred with each other. Looking at the shape of ancient teeth is therefore a way of tracing population dynamics, and when the only way new micromammals get to an island is by sneaking onto ships, ancient micromammal interactions become a proxy for ancient human interactions.
I spend several hours taking images of the micromammal teeth. When I’ve captured images of all of the teeth, I’ll use specially developed software to compare the teeth with each other and with teeth from archaeological sites around the Mediterranean. Capturing the images takes up the major part of my day, but it’s only the beginning. Outlining each tooth so I can compare their shapes will take days. It’s a slow process, and I’ll work on it a little at a time after I return to the states.
I have just about an hour left in my day, so I decide to spend it studying. My third, long-term project is to do an environmental reconstruction for my site in Sardinia – Sa Conca Sa Cresia – which I excavated with my colleague Mauro Perra in 2009-2011. Even though we completed the excavations a while ago, I’ve only recently finished sorting the heavy fraction to remove the tiny micromammal bones. To prepare to do a complete environmental study, I first consult The Atlas of European Mammals to see which species are currently known to exist on the island. Then I visit the Muséum’s reference collections to familiarize myself with the characteristics of these species’ bones – and especially their highly diagnostic teeth.
By the end of the day, my brain is fried. I cover the microscopes, turn off the lights, and make my way to the metro line 7, then transfer to the 6. I’m in a bit of a daze, but it’s a good kind of exhaustion – similar to how muscles feel after a trip to the gym. It’s an exhaustion that lets me know I deserve to take the night off. And there’s no better place for a night off than Paris.
Archaeologists love their equipment. Most of us can tell you when and where we got our first trowel, when and where we got our last trowel, whether we prefer a leaf blade or a pointing blade or a margin blade, why we buy Marshalltown or WHS or Battiferro. And it isn’t just our trowels. We’re obsessed with our hand picks, our Leatherman tools, our GPS units, our water bottles, our boots, and – perhaps the one realistic thing in the Indiana Jones movies – our hats.
It doesn’t stop with field archaeology. Lab archaeologists are just as intense about their microscopes and stereo macroscopes, their sonic cleaners and deionized water, their bags and bottles and pipettes. I’m as bad as the next archaeologist when it comes to equipment, so when a magnifying lamp I ordered finally arrived on Wednesday, I was stoked.
My lab in Sardinia is still being set up. I buy a few new pieces of equipment each time I return, and this year’s addition of a magnifying lamp is a significant upgrade. It enables me to take some time for a second zooarchaeological project that has been on the back burner for a year now. That project is looking for mouse teeth.
Yes, mouse teeth. Mouse teeth may sound insignificant, but these adorable, tiny fragments of ancient Rodentia are actually quite meaningful. Rodents are sensitive indicators of their local environments, and some species have particular relationships with humans that archaeologists use to understand how ancient people lived. In an ongoing research project that I’m pursuing in collaboration with the workgroup Mousetrack, run by Dr. Thomas Cucchi at the Muséum national d’Histoire naturelle, I’m using different mouse species to understand when exchange relationships developed between Sardinia and the cultures of the East Mediterranean. This is an important question because archaeologists debate whether the prehistoric cultures of Sardinia were more isolated or more connected to other Mediterranean groups.
I took my new lamp to the lab first thing on Thursday morning and set it up at a specially reserved desk. This desk is now my mouse study desk, where I do the painstaking work of sifting through what archaeologists call “heavy fraction.”
When archaeologists dig, they collect a sample of sediment from every important stratigraphic layer. This sample of sediment is then mixed with water and agitated in a process called “flotation.” Flotation can be done by hand in a bucket or with a variety of more or less sophisticated pump-and-barrel mechanisms, but the point is to get carbonized plant remains to rise to the surface. The floating material – which sometimes also includes fish scales and tiny bones – is called the “light fraction.” We collect it using an extremely fine mesh like chiffon and dry it carefully, preferably out of direct sunlight (if carbonized seeds dry too quickly, the difference between the dry surface and the wet interior can make them break).
The stuff that doesn’t float during the flotation process is also collected, usually in a slightly larger mesh with holes about 1 mm2. This is the heavy fraction, and it’s where we find lots of tiny animal bones as well as pottery, chipped stone, metal, glass – any heavy material that breaks into small pieces. The tiny artifacts that show up in heavy fraction can be fascinating. Beads and jewelry, nails, pins, and fragments of carved bone are all common. Food waste is also common, and evidence for small foods like eggs, fish, and sea urchins is often recovered only in the heavy fraction.
The heavy fraction I’m sorting comes from my excavation of the early Nuragic site Sa Conca Sa Cresia, located on the small plateau near Siddi, Sardinia, that also includes Sa Fogaia. I co-directed the excavation of Sa Conca Sa Cresia with Sardinian archaeologist Dr. Mauro Perra between 2009-2011. The excavation was very successful and our analysis of the resulting artifacts is ongoing – a good rule of thumb is that archaeologists expect to spend three days in the lab for every one day of excavation.
At first, sorting heavy fraction feels like doing excavation in miniature. It’s exciting to notice the shiny flakes of obsidian, the broken pottery, the piglet toes and lizard jaws and frog legs as they emerge from the tiny rocks that make up 99% of the heavy fraction. After a while, though, I start to get nervous. Where are the mouse teeth? Why haven’t I found one? Will I end up finding any at all, or will I just spend hours of precious research time looking for something that isn’t there? The problem, of course, is that there’s only one way to answer these questions, and that is to get in there and sort.
Realistically, I was rewarded pretty quickly with my first mouse tooth. And not just any tooth, a first molar of the mandible: the specific tooth that is most useful for my study. My heart leapt a little as I recognized the characteristic shape, and I picked it up gingerly and transferred it to a plastic specimen tray. I couldn’t say immediately what species the tooth belonged to – that requires careful cleaning and more powerful magnification – but the tooth was the right size to be one of the species I’m interested in.
It was an exciting find, and it re-energized me for a while. But after a couple of hours, the frustration crept back in. Then the concern. Obsessively, I sorted on.
When you’re invested in the outcome, sorting heavy fraction feels like playing a weird archaeological slot machine. I use flexible tweezers to spread out a small pile of sediment, then examine it minutely with the magnifying lamp. “C’mon, mouse tooth!” I say to myself as I flick past the ribs and toes and vertebrae that aren’t relevant for my study. There’s lots of what I don’t want, but is there anything I do want? No, nothing. I push this pile of sediment to the other side of my tray and start a new pile: “C’mon, mouse tooth…”
The hope is addictive. The possibility that each little pile of sediment might hold the tooth I’m looking for. The possibility that the tooth might belong to a significant species. The possibility that enough significant teeth will add up to a significant find, a meaningful advance in what we as humans know about our past. Students sorting heavy fraction for the first time usually put down their tweezers after fifteen minutes. “You do this all day?” they ask in disbelief. “Don’t you get bored???”
Yes, I get bored. But I stay hopeful.
After non-stop sorting for three days, I’m almost finished. I currently have eight new teeth, an average of less than one tooth for every liter of sediment. I’m hoping to find one more, but the sediment I’m working on now includes very little cultural material of any kind. There are only the rarest fragments of pottery, obsidian, or bone. Still, you never know. I always have hope.
Today is the seventh day of my bone study. I’ve been getting up at 5:30 am, arriving at the lab before the sun is above the horizon. The bakery across the street is the only building where the lights are on. I’ve worked seven to ten hours every day since I started last Saturday, and I’ve identified 1028 specimens so far, an average of 146 specimens per day.
When I plan a zooarchaeological study, I estimate that I can do 200 specimens in an eight-hour workday, 25 specimens per hour. Most of the time, I hit this rate. Sometimes I even exceed it. But what I forget is that this is the ideal rate, the rate I achieve after I’m settled into a study, with my database exactly how I want it, my reference materials in familiar locations on my laptop and work table, the measurements I need to take fresh in my mind. I forget that this is not my rate during the first week of the study.
Careful research takes time, and preparing a study is one of the most time-consuming parts. It took me this whole week of working with the specimens to get my database sorted out. I started designing the database before the bones were back in the lab. I made a list of everything I needed to record and how I would record it. It took a full day to create the database in Filemaker, then a second full day to edit and improve it. When I started identifying specimens on Saturday morning, the database included 111 fields. But in the past seven days, it has grown to 125 fields: working with the material has reminded me of important types of evidence that slipped my mind during the design phase. I was still adding fields to the database yesterday, and who knows – I may realize tomorrow there’s yet another field that should be added. But I’m crossing my fingers that I’ve finally hit the point of diminishing returns.
For every specimen I look at, there are 125 types of evidence I may have to record. I can usually eliminate many after a brief glance. For example, I don’t have to record the circumference of the shaft on a bone that doesn’t have a shaft. But some types of evidence take time to figure out. One of the hard ones is carnivore gnawing. Dogs and other carnivores often gnaw bones, and their teeth leave marks that are fairly easy to recognize on a fresh bone. But when that bone is also gnawed by rodents, then exposed to the elements causing cracking and flaking, then buried and etched by the twisting roots of plants, it becomes much harder to decide whether a particular pattern of indentations comes from the teeth of a carnivore or from some other type of modification, what bone specialists call taphonomic processes.
Teasing out taphonomic processes isn’t the only thing that takes time. Today, I had to investigate several bags that had been mislabeled. Figuring out their correct contextual information was a small feat of detective work. Fortunately, I keep good records, but the puzzle had me rereading the project notebook I kept back in 2009-2011 to see exactly which units my team was excavating on – for example – 14 June 2010. Needless to say, I did not average 25 specimens during those hours.
But these are the frustrating things that slowed me down. There were also exciting things. One of the great joys of archaeological research is the unexpected discoveries. As I examined my specimens for signs of taphonomic processes, I also noticed that several showed characteristic burnishing on the points and edges, a sign that they had been used as tools. I bagged and labeled each one and set them aside for a colleague who specializes in analyzing worked bone. Studying bone tool production is new in Nuragic archaeology, and the work my colleague will do to understand the worked bone industry at my site, Sa Conca Sa Cresia (Middle Bronze Age, c. 1700-1450 BCE), will result in only the second publication of its kind. As my colleague says, “È tutto da scoprire” – it’s all to be discovered.
So there are many reasons why research takes forever, but take forever it does. I’m a little nervous looking at the many bags of bones. Will I manage to identify every single one? At this point, I can’t say. But, like a good scientist, I took the time to design my study carefully. I will at least have a statistically significant sample – enough evidence to draw robust conclusions about the animal economy at my early Nuragic site.
I’m delighted to say I’m in the lab for my first full day of bone analysis. I worried that retrieving my materials from the museum where they were stored would take weeks, but our exceptional representatives at the Soprintendenza processed our permit in record time, and I collected the bones on Thursday. I spent yesterday morning finishing the database and yesterday afternoon finalizing my methodology. Now it’s Saturday morning, and here I am.
The beginning of a study is always intimidating. Even though I’m the one who excavated these bones, even though I know this archaeology inside and out, I look at the two crates filled with upwards of 5000 fragments and I think: “how will I ever…”
I know what’s waiting for me. I know there will be fragments I don’t recognize. I know there will be times when I turn a fragment over and over under the light, trying to decide if a series of round-ish dents are evidence of trampling or the marks of a dog’s teeth. I know there will be bones I need to reconstruct, bones I need to photograph, bones I need to set aside for further study. I know it will be a challenge.
I could stand here for hours, eyeing the bags of bones like I would the surface of a cold lake, but the best way forward is just to jump in.
So I do. And I’m right: on this first day, everything is a struggle. I realize I left several important fields out of my database, so now I have to add them. Several other fields aren’t in the right format and I have to change them. Several of the fields are too small to display the relevant information, so I have to make them bigger. Then I accidentally link one of the new fields to one of the old fields and can’t figure out how to unlink them, so I have to erase them both and start again.
It’s slow going, and the light isn’t great. I need to get a desk lamp. I need to organize my reference materials. I need to make a key for the various codes I’m using and write it on an index card. I need to find my photo scales.
I’m not even going to tell you how long it took to identify my first bag of bones. It’s too disheartening. What I will tell you – and myself – is that it gets better. The first day is awful. The second day is hard. The third day, however, is when I start to find a rhythm. And on the fourth day, I pick up the pace. There are many long days ahead of me, but the worst one is now behind me. I plan to celebrate with pizza.
My alarm went off at 4:15 am today. Work starts early at Tel Akko, and I like to run in the morning to wake myself up and collect my thoughts. When I open the lab at 5:30 am, I’m feeling alert and ready to meet the past. And it’s a good thing, too, because my tables are covered with piles of fragmented animal bones. It looks more like a mess than information.
I’m the zooarchaeologist at Tel Akko this year, and it’s my job to identify, record, and
interpret the animal remains recovered by the excavations. Animal remains are an extremely common find on archaeological projects, and they provide a wealth of information about diet, economy, environment, social status, mobility, and other aspects of ancient cultures that archaeologists work to understand. But getting from broken bits of bone to a reconstruction of something like ethnic differences in food choice is a complex and painstaking process.
How does zooarchaeological analysis begin? One bone at a time. I examine each bone or bone fragment for the shapes and features that would allow me to identify it to species – if I’m lucky – or as close to species as possible. All kinds of factors come into play when I make these identifications. For example, an astragalus (ankle bone) of a cow has basically the same shape as the astragalus of a sheep or goat, but of course it’s much bigger. Telling the difference between the astragali of sheep and goats is much more difficult – only a few parts of the bone are different, and it’s best if all of them are preserved for me to make a really secure identification. Often this can’t be done, and I’ll record a bone as “sheep or goat.” That particular bone won’t help me tell if
Akko ever developed an intensive wool-producing industry, but it will contribute to answering other questions, such as how food preferences at the site changed over time with the influence of new ethnic groups. Zooarchaeological analysis is many-layered, and the only way to get at all the questions we’d like to answer is by collecting a lot of data.
Every bone fragment provides some kind of data – even those that are unidentifiable. Unidentifiable bones can still show evidence of being chewed by carnivores, often an indication of the presence of dogs on the site. Similarly, many bone fragments preserve evidence of rodent gnawing. Some bones may preserve cut marks and help us understand ancient butchery practices (one of my favorite studies in zooarchaeology uses differences in butchering practices to look at inter-ethnic marriage in the ancient world*). Burned bone fragments can tell us about both cooking and trash disposal.
Even tiny bones are important sources of information, and some of the bones I study at Tel Akko at very tiny indeed. These are the bones recovered through the process of flotation – taking samples of excavated sediment and processing them with water to extract carbonized plant remains and other tiny finds. The resulting animal remains are often only a few millimeters in size, but they can be one of our most important sources of information for the use of marine resources and the presence of reptiles, amphibians, and small mammals like mice on the site. If you’re not sure what mice have to do with to understanding the past check out this excellent study.
Zooarchaeology takes a long time, and today I worked seven hours before lunch and still feel like I’ve barely made a dent. After lunch, I’ll attend a lecture by another of the specialists at the site and then spend two hours washing the new bones that have come in from the last few days of excavation. At this point in the season, it feels impossible. I have to take a deep breath and remind myself that no matter how daunting it looks, it will all get done in the end. And when it does, we’ll be thousands of fragments closer to understanding the past at Tel Akko.
* Gil J. Stein. 2012. Food Preparation, Social Context, and Ethnicity in a Prehistoric Mesopotamian Colony. In S. R. Graff and E. Rodríguez-Alegría (eds.), The Menial Art of Cooking: Archaeological Studies of Cooking and Food Preparation, pp. 47-63. University Press of Colorado, Boulder.