New research involving the University of Arizona, published in the journal Science Advances, overturns more than a century of thought about the source of turquoise used by ancient civilizations in Mesoamerica, the vast region that extends from central Mexico to Central America.

For more than 150 years, scholars have argued that the Aztec and Mixtec civilizations, which revered the precious, blue-green mineral, acquired it through import from the American Southwest. However, extensive geochemical analyses reveal that the true geologic source of Aztec and Mixtec turquoise lies within Mesoamerica.

Geochemist Alyson Thibodeau, who earned her doctorate in geosciences from the UA in 2012 and is now an assistant professor of earth sciences at Dickinson College in Pennsylvania, is lead author of the new research, which is based on work she did as a UA student alongside UA anthropologist David Killick, UA College of Science dean and geosciences professor Joaquin Ruiz, and collaborators from California State University at San Bernardino and the Museo del Templo Mayor in Mexico City.

The team measured the isotopic signatures of Mesoamerican turquoise artifacts associated with both the Aztecs and Mixtecs. These isotopic signatures function like fingerprints that can be used to determine the geologic origins of the turquoise.

Specifically, the researchers carried out analyses of lead and strontium isotopes on fragments of turquoise-encrusted mosaics, which are one of the most iconic forms of ancient Mesoamerican art. Their samples included dozens of turquoise mosaic tiles excavated from offerings within the Templo Mayor, the ceremonial and ritual center of the Aztec empire, which is located in present-day Mexico City. They also analyzed five tiles associated with Mixteca-style objects held by the Smithsonian's National Museum of the American Indian. The analyses revealed that turquoise artifacts had isotopic signatures consistent with geology of Mesoamerica, not the Southwestern United States.

"This work revises our understanding of these relatively rare objects and provides a new perspective on the availability of turquoise, which was a highly valued luxury resource in ancient Mesoamerica," Thibodeau said.

Turquoise figures prominently in Aztec poetry and rituals, and was used to make jewelry, shields, knife handles, mirrors and other objects belonging to high-status members of Aztec society, like rulers and priests.

"There's been a long preference for green and blue stones that goes back to the first millennium B.C.," said Killick, a professor in the UA School of Anthropology. "After about 900 A.D., turquoise became the preferred blue/green stone in the Southwestern U.S. and Mesoamerica."

While the new research points to Mesoamerica as the source of Aztec and Mixtec turquoise, exactly where the mineral came from hasn't been pinpointed, as no turquoise mines have been discovered in the area.

"There are very few known turquoise sources in Mesoamerica, which is why many archaeologists thought it came from the U.S.," Killick said. "But even though we can no longer find the source, it must once have existed and been mined out, because this turquoise bears the signature of a big block of the Earth's crust that wraps around present Mexico City."

The newly published work is the result of a decade-long collaboration between archaeologists and isotope geochemists to understand the nature of turquoise circulation and trade across southwestern North America. In earlier published research, Thibodeau showed that isotopic signatures could distinguish among turquoise deposits across the Southwestern U.S. and identified the geologic sources of turquoise artifacts from archaeological sites in Arizona and New Mexico.

Thibodeau said that the long-standing assumption that Mesoamerican civilizations imported turquoise from the Southwest had not been fully substantiated with evidence, and that the new geochemical measurements unveil a different story.

"These findings potentially reshape our understanding of both the nature and extent of long-distance contacts between Mesoamerican and Southwestern societies," Thibodeau said. "I hope this inspires people to be skeptical of claims."