Ancient Mayan reservoirs contained toxic contamination: study

The reservoirs in the heart of an ancient Mayan city were so contaminated with mercury and algae that the water was probably undrinkable.

Researchers from the University of Cincinnati found toxic levels of contamination in two central deposits in Tikal, an ancient Mayan city that dates back to the 3rd century BC. C. in what is now northern Guatemala.

UC’s findings suggest that droughts in the 9th century likely contributed to the depopulation and eventual abandonment of the city.

“Converting the central reservoirs of Tikal from places that sustain life to places that induce disease would have helped symbolically and practically to cause the abandonment of this magnificent city,” the study concluded.

A geochemical analysis found that two deposits closest to the city’s palace and temple contained toxic levels of mercury that UC researchers traced back to a pigment the Mayans used to adorn buildings, earthenware, and other property. During storms, mercury in the pigment seeped into deposits where it deposited in layers of sediment over the years.

But the ancient inhabitants of this city, who became famous for their imposing stone temples and architecture, had abundant drinking water from nearby reservoirs that remained uncontaminated, UC researchers found.

The study was published in the journal Nature. Scientific reports.

UC’s diverse team consisted of anthropologists, geographers, botanists, biologists, and chemists. They examined sediment layers dating back to the 9th century when Tikal was a flourishing city.

Previously, UC researchers discovered that the soils around Tikal during the 9th century were extremely fertile and traced the source of frequent volcanic eruptions that enriched the soil of the Yucatan peninsula.

“Archaeologists and anthropologists have been trying to figure out what happened to the Mayans for 100 years,” said David Lentz, a professor of biological sciences at UC and lead author of the study.

For the latest study, UC researchers took sediment samples from 10 deposits within the city and performed analysis on the ancient DNA found in the layered clay of four of them.

The sediment from the deposits closest to the central temple and palace of Tikal showed evidence of toxic algae called cyanobacteria. Consuming this water, particularly during droughts, would have made people sick even if the water was boiled, Lentz said.

“We found two types of blue-green algae that produce toxic chemicals. The downside of this is that they are resistant to boiling. It made the water in these reservoirs toxic to drink,” Lentz said.

UC researchers said it is possible but unlikely that the Mayans used these deposits for drinking, cooking or irrigation.

“The water would have looked unpleasant. It would have tasted unpleasant,” said Kenneth Tankersley, an associate professor of anthropology at the UC College of Arts and Sciences. “There would have been these big blooms of algae. No one would have wanted to drink that water.”

But the researchers found no evidence of the same contaminants in sediments from more distant deposits called Perdido and Corriental, which likely provided drinking water for city residents during the 9th century.

Today, Tikal is a national park and a UNESCO World Heritage site. The researchers believe that a combination of economic, political and social factors led people to leave the city and its adjacent farms. But the weather undoubtedly also played a role, Lentz said.

“They have a long dry season. During part of the year, it rains and wets. The rest of the year, it is really dry and it hardly rains. So they had trouble finding water,” Lentz said.

Co-author Trinity Hamilton, now an assistant professor of biology at the University of Minnesota, worked on the analysis of ancient DNA from algae that sank to the bottom of the reservoir and was buried by centuries of accumulated sediment.

“Typically, when we see a lot of cyanobacteria in fresh water, we think of harmful algal blooms that affect water quality,” said Hamilton.

Finding some deposits that were contaminated and others that were not suggests that the ancient Maya used them for different purposes, he said.

The reservoirs near the temple and palace would have likely been impressive landmarks, just like the reflective pool on the National Mall is today.

“It would have been a magnificent sight to see these brightly painted buildings reflected on the surface of these reservoirs,” said co-author Nicholas Dunning, chief of geography at the UC College of Arts and Sciences.

“The Mayan rulers claimed, among other things, the attribute of being able to control water. They had a special relationship with the rain gods,” Dunning said. “So the deposit would have been a pretty powerful symbol.”

UC Tankersley said a popular pigment used in plaster walls and ceremonial burials was derived from cinnabar, a red-colored mineral made up of mercury sulfide that the Mayans mined from a nearby volcanic feature known as the Todos Santos Formation.

Close examination of the reservoir sediment using a technique called energy-dispersive X-ray fluorescence spectrometry found that mercury did not leak into the water from the underlying bedrock. Similarly, Tankersley said, UC ruled out another potential source of mercury: volcanic ash that fell in Central America during frequent eruptions. The absence of mercury in other nearby deposits where the ashes would have fallen ruled out the volcanoes as the culprits.

Instead, Tankersley said, it was the people’s fault.

“That means that mercury has to be anthropogenic,” said Tankersley.

With its bright red color, cinnabar was commonly used as a paint or pigment throughout Central America at the time.

“Color was important in the ancient Mayan world. They used it on their murals. They painted the plaster red. They used it for burials and combined it with iron oxide to get different shades,” said Tankersley.

“We were able to find a mineral footprint that showed beyond a reasonable doubt that the mercury in the water originated from cinnabar,” he said.

Tankersley said ancient Mayan cities like Tikal continue to captivate researchers because of the ingenuity, cooperation and sophistication necessary to thrive in this extreme tropical land.

“When I look at the ancient Maya, I see very sophisticated people with a very rich culture,” said Tankersley.

The UC team is planning to return to the Yucatan peninsula to seek more answers about this remarkable period of human civilization.