One of the most widely used tools archaeologists have at their disposal to decipher where prehistoric humans lived and traveled is the element strontium. Because strontium isotope levels in remains match the concentrations in the surrounding landscape, scientists can track migrations. While the technique itself is sound, the baseline levels of strontium in different geographies may not reliably reflect ancient times as scientists have assumed.
Researchers from Aarhus University in Denmark published a report in Science Advances last week (March 13) showing that the data on strontium levels in soil and water used for these archaeological studies are not always accurate. With their new analysis, the team found that two Bronze Age human remains, Egtved Girl and the Skrydstrup Woman, who both died and were buried in Denmark, were not necessarily the international travelers that prior studies have suggested.
“Using strontium isotope ratios is a tremendously powerful tool, but the bottom line is that we need better sampling of the biosphere to create the strontium maps archaeologists use,” says Richard Madgwick, an archaeologist at Cardiff University in the United Kingdom who specializes in the analysis of animal and human remains and who was not involved in the work. “Every study should incorporate a bit of its own biosphere sampling to create higher resolution geographical maps of the strontium found in soil and water.”
Erik Thomsen and Rasmus Andreasen, both of Aarhus University, had been comparing the geological maps of Denmark to maps generated by other researchers of strontium levels in the country’s ground, based on sampling lakes and streams. “We noticed that the strontium maps were homogeneous [throughout Denmark] and that was strange because it did not reflect the underlying variation in the geological maps,” says Thomsen.
When 87rubidium, found in rock, decays, it forms the 87strontium isotope, which is distinct from the 86strontium isotope. Both forms of strontium are released into water sources where they are taken up by plants and also end up in the teeth, bones, and hair of animals and humans who consume the local water and food. Because little strontium is lost when it is transferred from the biosphere and into animals and humans, scientists can compare the ratios of the isotopes in the remains of an individual to the ratios found in the surrounding land. Archaeologists can determine where an ancient person traveled based on how the strontium ratios in the remains match the ratios of various locations.
“One day, Erik came into my office and showed me a piece of limestone,” says Andreasen. “‘I have an idea why there is a homogeneous strontium signal,’” Andreasen recalls Thomsen saying. It turns out that limestone contains a lot of strontium and is used heavily in modern day farming, which increases the amount of strontium in surface waters such as lakes. This means that strontium levels from farmed regions are higher than they were in prehistoric times, creating a potential mismatch between ancient remains and modern soil samples.
Andreasen and Thomsen did their own water sampling in Denmark comparing strontium levels from 84 locations including farmland where lime has been added to the soil and land that has never been used for farming. They found a clear distinction between the two types of samples, showing that the lime used for farming changes the strontium isotopes that are available for plants, animals, and humans to take up. The analysis suggests that the strontium ratios found on farmland do not accurately reflect the levels in the underlying rock that the prehistoric humans and animals were exposed to.
They also found that fertilizers, manure, animal feed, and pesticides also add small amounts of strontium into surrounding land and water.
The team then used its higher-resolution strontium isotope map to reassess the migration of Egtved Girl, a possible priestess, and Skrydstrup Woman, two well-known Bronze Age humans whose remains were found in the area where Andreasen and Thomsen conducted their sampling. While other researchers, using strontium ratios, had concluded that Egtved Girl had traveled between the village of Egtved in Denmark and a place far outside of Denmark and back again during the last two years of her life, the new strontium values were consistent with her having lived within 10 kilometers of her burial spot. They came to the same conclusion for the Skrydstrup Woman, who was thought to have traveled from somewhere in Europe to Denmark at the age of 12 or 13 and died there four years later. “There is nothing in the strontium data we obtained to suggest that these women came from far,” says Andreasen.
Another study published in the same issue of Science Advances, conducted by Madgwick and his colleagues, also used strontium mapping to analyze the remains of a large-scale celebration found at Stonehenge from the late Neolithic period, around 2,600 BCE. They also included four other isotopes including oxygen and sulfur. In this study, the remains of 131 pigs mostly came from nearby, but some of the pigs were herded from as far away as Scotland and Wales.
“Our research in Denmark shows that modern farming can obscure local variations but the researchers in the Stonehenge study used a large-scale strontium map of the United Kingdom,” says Andreasen. “It would be interesting to see if the local range inferred from the map of the United Kingdom is confirmed by analysis of pristine areas near the study sites around Stonehenge,” Andreasen, who was not involved in Madgwick’s study, tells The Scientist.
The results from the strontium study are relevant to any geography, because farming and other modern day uses of land is not unique to Denmark, according to Andreasen and Thomsen.
According to Harry Fokkens, an archaeologist at Leiden University in the Netherlands who was not involved in either work, the strontium isotope method itself is sound. “But the approach requires caution in the creation of the baseline strontium maps and in the interpretation of the results,” he writes in an email to The Scientist.
Based on the new results, Fokkens would like to see the use of nonmigratory rodent remains to determine the baseline strontium levels of a particular geography and then compare the human or animal remains to that baseline.
“The study from Thomsen and Andreasen is refreshing because it’s not a headline grabbing study but rather fundamental work that is key to our understanding of the archaeology,” says Madgwick. “I welcome more papers that give us these cautionary tales.”
E. Thomsen et al., “Agricultural lime disturbs natural strontium isotope variations: Implications for provenance and migration studies,” Science Advances, 5:eaav8083, 2019.