An African strontium map sheds light on the origins of enslaved people

A little-known element is shedding light on the transatlantic slave trade. Researchers have assembled a map of strontium, a naturally occurring element, across sub-Saharan Africa. These data can be compared with strontium levels measured in human remains to more precisely pin down the geographic origins of individuals sold into slavery, the team reported December 30 in Nature Communications.

From the 15th to the 19th centuries, more than 12 million Africans were sold into slavery and sent to the Americas and Europe. Large port cities such as Lagos, Nigeria, and Luanda, Angola, were common points of departure from Africa, but the actual origins of most enslaved people — that is, where they were born and raised — were often lost to history. And while genetic evidence can reveal a person’s ancestry, it doesn’t pinpoint where someone grew up.

That’s where the metal strontium comes in. The geology of a place dictates its ratio of strontium isotopes, variations of the element with different atomic weights. Strontium is readily incorporated into living things and can be found throughout the human body. “It’s in everything and everyone,” says Vicky Oelze, a biological anthropologist at the University of California, Santa Cruz.

Researchers can therefore study strontium isotope ratios in plant or animal remains to better understand where an organism came from. “Each organism keeps the signature of its evolutionary environment,” says Lassané Toubga, an archaeologist at Université Joseph Ki-Zerbo in Ouagadougou, Burkina Faso.

But such analyses require many samples of soil, plant or animal remains to create strontium isotope ratio maps. Oelze, Toubga and colleagues spent more than a decade amassing nearly 900 environmental samples from 24 African countries and combined those measurements with other published data to create a strontium map of sub-Saharan Africa.

“This map represents the combined effort of over 100 collaborators from diverse disciplines, including archaeologists, botanists, zoologists and ecologists,” says study coauthor Xueye Wang, an archaeologist at Sichuan University in Chengdu, China.

The team focused on sub-Saharan Africa because of the area’s importance in fields from archaeology to conservation, where such data could be useful. Such a map might also help reveal details about the transatlantic slave trade, the researchers surmised, since most people sold into slavery came from sub-Saharan Africa.

To test that idea, the team collected published strontium isotope ratios from the dental remains of 10 enslaved people buried in Charleston, S.C., and Rio de Janeiro.

When the team compared those data with their map, they were able to glean details about these individuals not previously known from genetic analyses. For instance, two adult men named Daba and Ganda, interred in Charleston, were previously known to have generally West African ancestry, but the strontium analyses refined their geographic origins to southwestern Ivory Coast, southern Ghana or eastern Guinea.

Homing in on where a person came from is important to understanding their identity, Toubga says. “Determining the geographical origins of slaves makes it possible to identify the cultural and/or political groups to which they belonged.”

More environmental samples would help increase the spatial resolution of this map, says Murilo Bastos, a bioarchaeologist at the National Museum of Brazil in Rio de Janeiro who was not involved in the research. But this work is already impressive, he says. “It’s a big achievement.”