New chemical analysis of Aztec turquoise artifacts suggests the stone didn’t come from the Southwestern US as archaeologists have long thought, which raises questions about the scale of long-distance trade between the Aztecs and their northern neighbors.

For thousands of years, societies from Central America to the Southwestern US have prized turquoise for its unique blue-green color. Archaeologists and historians have assumed the Mesoamerican states got their turquoise via long-distance trade with people in the American Southwest. It made sense, because archaeologists have found macaw bones, cacao, and copper bells at sites around the Southwestern US—not in large quantities, but enough to show that at least some trade happened after 900 CE.

And there are still turquoise mines in the American Southwest in areas that have been mined since pre-Columbian times. Meanwhile, there are no turquoise mines in Mesoamerica, but there is an abundance of the stuff in Aztec artifacts. At the height of Aztec imperial power, known as the Late Postclassic period from 1430 to 1519, the stone was a favored ornament for ceremonial shields and sacrificial weapon hilts, mirrors, and jewelry. That includes items such as armbands, necklaces, earrings, and nose plugs worn by Aztec rulers, priests, and nobles. Turquoise—which the Aztecs called xihuitl—was so important that it had a role in Aztec cosmology and was often mentioned in poems.

A 16th-century Aztec document called the Codex Mendoza records tribute paid by conquered provinces to the Aztec capital. It mentions shipments of turquoise from three provinces: one in parts of the modern Mexican states of Guerrero and Puebla and one in the western part of modern-day Oaxaca, along with a third province in northern Veracruz, which at the time was the far northeastern corner of the Aztec Empire.

The logical assumption is that the Hohokam, Mogollon, and Puebloan peoples of the Southwestern US traded their local turquoise for goods from Mexico and farther south, so the provinces in the Codex Mendoza were importing the turquoise they sent as tribute. But it turns out that the chemical signatures in Aztec and Mixtec turquoise tiles don't match that of Southwestern US rocks; they actually match Mesoamerican rocks.

Rocks leave a signature

Turquoise forms near copper deposits when water laden with copper and aluminum filter through rock, which is why turquoise veins usually form at the edges of large copper deposits. So as the surrounding rocks weather, their particles end up contributing to turquoise formation, and the turquoise inherits its chemistry from those rocks. Geologists can get a rough idea of where a chunk of turquoise came from by comparing its chemistry to that of rock formations; specifically, they look for ratios of isotopes of the elements strontium and lead.

Geologist Alyson Thibodeau of Dickinson College and her colleagues tested the lead and strontium isotope ratios in 38 turquoise mosaic tiles, or tesserae, from buried offerings in the Sacred Precinct of Tenochtitlan, the political and ceremonial capital of the Aztec Empire. Most of the 1-5cm tiles were 15th-century offerings to the war and sun deity Huitzilopochtli, buried in the southern half of the Templo Mayor—an area that is symbolically linked with the god Xiuhtecuhtli, a fire deity also known as the Turquoise Lord. They also tested five tesserae from Mixtec mosaics, although archaeologists aren’t sure about their original context.

Alyson M. Thibodeau

Frances F. Berdan

Frances F. Berdan

Oliver Santana. Reproduced with permission from Editorial Rai´ces.

Frances F. Berdan

Frances F. Berdan

Frances F. Berdan

Generally, the ratio of strontium-87 to strontium-86 in the local rock decreases as you move south from Arizona to the state of Guerrero in west Mexico (formerly the Aztec province of Quiauhteopan mentioned in the Codex Mendoza). In southeast Arizona, that ratio is above 0.708, and in Guerrero it’s between 0.704 and 0.705. Most of the Aztec tesserae had strontium isotope ratios ranging from 0.705 to 0.706–much too low to have come from Arizona.

The lead isotope ratios in the Aztec tesserae did overlap with the ratios in Southwestern US turquoise deposits, but overall they were a much better match with those in the state of Michoacán, just to the west of the former Aztec capital. The isotope ratios could also be good fits with copper deposits in the states of Guerrero, Jalisco, or Veracruz—a list of possibilities that includes the provinces mentioned in the Codex Mendoza.

Rethinking pre-Columbian trade

That means pre-Columbian people probably weren’t transporting large quantities of turquoise south to Mexico and Central America as part of a far-flung trade network.

“Although the presence of cacao, macaws, etc. in the Southwest provides undeniable evidence of long-distance interaction, the volume of Mesoamerican items in the Southwest is not so great as to require the existence of large-scale exchange networks moving large quantities of materials between the two regions,” Thibodeau told Ars Technica. “Although perishable materials like cotton could have been traded south, it is also possible that there was no major flow of trade items from the Southwest to Mesoamerica.”

But there are still no known turquoise deposits in Mesoamerica. The explanation, say Thibodeau and her colleagues, is that turquoise deposits tend to be shallow, and they can be mined to depletion in short order. And there are copper deposits all over Mesoamerica. Thibodeau and her colleagues said the turquoise mines that supplied the Aztecs may have been mined out well before Europeans set foot on the continent. Without surviving turquoise deposits, however, the isotope ratios can tell geologists the general area a turquoise sample came from, but they can’t pinpoint a precise location.

“While our results clearly indicate the turquoise is of Mesoamerican origin, we cannot pinpoint the exact area(s) from which the mineral was derived,” said Thibodeau. “Thus, while it is possible the turquoise we analyzed derives from deposits in or near the provinces which gave turquoise tribute, it is also possible that some or all of the turquoise was mined in other parts of Mesoamerica (e.g., West Mexico).”

Thibodeau and her colleagues want to study turquoise samples from other Mesoamerican cultures—including the Toltec, the Maya, the Mixtec, and the Tarascan—to better understand how pre-Columbian economies mined, traded, and used turquoise.

“I hope this work begins a new discussion about—and is not the final word on—the role of turquoise in long-distance interactions between Southwestern and Mesoamerican civilizations,” she told Ars Technica.

Science Advances, 2018. DOI: 10.1126/sciadv.aas9370 (About DOIs).