New research reveals that two of the largest reservoirs at Tikal, an ancient Maya city in what is now northern Guatemala, were contaminated with high levels of mercury, phosphate and cyanobacteria known to produce deadly toxins.
Tikal is one of the largest archaeological sites and political, economic and military centers of the pre-Columbian Maya civilization.
The metropolis was inhabited from the 6th century BCE to the 10th century CE, and had a population of up to 90,000 people in its heyday.
“The conversion of Tikal’s central reservoirs from life-sustaining to sickness-inducing places would have both practically and symbolically helped to bring about the abandonment of this magnificent city,” said University of Cincinnati’s Professor David Lentz and colleagues.
“Previously, we found that the soils around Tikal during the 9th century CE were extremely fertile and traced the source to frequent volcanic eruptions that enriched the soil of the Yucatan Peninsula.”
“Archaeologists and anthropologists have been trying to figure out what happened to the Maya for 100 years.”
In the new study, the researchers examined layers of sediment dating back to the 9th century when Tikal was a flourishing city.
They analyzed samples form 10 water reservoirs and found toxic levels of mercury in two of them: the Temple and Palace Reservoirs.
“We traced back the pollution to a pigment the Maya used to adorn buildings, clayware and other goods,” the scientists said.
“During rainstorms, mercury in the pigment leached into the reservoirs where it settled in layers of sediment over the years.”
“But the former inhabitants of this city had ample potable water from nearby reservoirs that remained uncontaminated.”
Sediment from the Temple and Palace Reservoirs also showed evidence of cyanobacteria.
“Consuming this water, particularly during droughts, would have made people sick even if the water were boiled,” Dr. Lentz said.
“We found two types of blue-green algae that produce toxic chemicals. The bad thing about these is they’re resistant to boiling. It made water in these reservoirs toxic to drink.”
“The water would have looked nasty. It would have tasted nasty. There would have been these big algae blooms. Nobody would have wanted to drink that water,” said University of Cincinnati’s Dr. Kenneth Tankersley, co-author of the study.
But the authors found no evidence of the same pollutants in sediments from more distant reservoirs called Perdido and Corriental, which likely provided drinking water for city residents during the 9th century.
“We believe a combination of economic, political and social factors prompted people to leave the city and its adjacent farms. But the climate no doubt played a role, too,” Dr. Lentz said.
“They have a prolonged dry season. For part of the year, it’s rainy and wet. The rest of the year, it’s really dry with almost no rainfall. So they had a problem finding water.”
“One popular pigment used on plaster walls and in ceremonial burials was derived from cinnabar, a red-colored mineral composed of mercury sulfide that the Maya mined from a nearby volcanic feature known as the Todos Santos Formation,” Dr. Tankersley said.
“We ruled out another potential source of mercury — volcanic ash that fell across Central America during the frequent eruptions. The absence of mercury in other nearby reservoirs where ash would have fallen ruled out volcanoes as the culprit.”
“Instead, people were to blame. That means the mercury has to be anthropogenic,” he said.
With its bright red color, cinnabar was commonly used as a paint or pigment across Central America at the time.
“Color was important in the ancient Maya world. They used it in their murals. They painted the plaster red. They used it in burials and combined it with iron oxide to get different shades,” Dr, Tankersley said.
“We were able to find a mineral fingerprint that showed beyond a reasonable doubt that the mercury in the water originated from cinnabar.”